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IMAGE SENSING AND AQUISITION

SEMINAR REPORT

SUBMITTED IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

BACHELOR OF TECHNOLOGY

IN

ELECTRONICS AND COMMUNICATION ENGINEERING

BY

O.V.S SHASHANK RAM (12SS1A0446)

Department of Electronics and Communication Engineering

Jawaharlal Nehru Technological University Hyderabad

College of Engineering

Sultanpur, pulkal (M), Medak-502293 Telangana

2016

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Jawaharlal Nehru Technological University Hyderabad

College of Engineering Sultanpur, pulkal (M),Medak-502293 Telangana

Department of Electronics and Communication Engineering

CERTIFICATE

Date:

This is to certify that the seminar work entitled “IMAGE SENSING AND

AQUISITION” is a bonafide work carried out by “O.V.S SHASHANK RAM” bearing

Roll no.12SS1A0446 in partial fulfillment of the requirements of the degree of

BACHELOR OF TECHNOLOGY in ELECTRONICS & COMMUNICATION

ENGINEERING by the Jawaharlal Nehru Technological University, Hyderabad during

the academic year 2015-16.

The results embodied in this report have not been submitted to any other

University or Institution for the award of any degree or diploma.

------------------------- ---------------------

Mr. V. Rajanesh Mr. B. Prabhakar

Associate Professor Associate Professor

Guide Head of the Department

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Abstract

The inclusion of cameras in everything from cell phones to pens to children’s’

toys is possible because of the low cost and low power consumption of the imaging

arrays that form the core of the cameras. However, these arrays are low cost and low

power because they are CMOS-based; this allows for the devices to be made with the

same processes and facilities that are used to make memory and computer chips. Yet, the

continued surge in CMOS imager popularity goes beyond the lower cost to other factors

such as ability to integrate the sensors with electronics, and the ability to achieve fast,

customizable frame rates.

People have been using camera and film for more than 100 years, both for still

photography and movies. There is something magical about the process -- humans are

visual creatures, and a picture really does paint a thousand words for us!

In this report we will be discussing how an image is sensed and the acquitted,

what is a camera, how it works. Various image sensors are discussed and the differences

these sensors, the differences between the analog and digital image sensing is also

discussed in detail.

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Contents :

1. Introduction 1

2. Camera 2

3. History of Camera 3

4. Components of a Camera 6

5. Working of a Camera 13

6. Image Sensors 20

7. CCD vs CMOS 28

8. Anolog(Film) vs Digital 38

9. Conclusion 46

References 47

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Introduction:

Before any video or image processing can commence an image must be

captured by a camera and converted into a manageable entity. This is whole process is

termed as image sensing and acquisition.

Image Sensing and acquisition mainly deals with how the image is sensed and

then acquired as desired. This the first and foremost in the elements of digital image

processing. Only after this important step can we implement any processing on the

image.

Mostly the electronic device associated with image sensing and acquisition is

called as a Camera.

As a photographer I have always been interested in the camera as a medium.

Darwin’s writings on the evolution of the eye in his “On the Origin of Species” and

was struck (as many have been) by its remarkable similarity to the development of

photography. Nature did not create the eye fully formed—Darwin demonstrated how

this occurred over countless generations in a slow methodical process. It started with a

flat disk of light sensitive cells that could detect the presence of light but nothing

more. This disk began to dimple which grew deeper to form a cup. As the cup closed

over the opening formed an aperture which had enough power to resolve a dim fuzzy

image onto the back of the proto eye (still used today by the Nautaist). This cup filled

with mucus and an encapsulated lens formed over the opening. We now had the

compound eye, a very sharp and bright camera for seeing the world.

The analogy to human made cameras is not perfect. We did not progress

slowly by dimpling light sensitive paper until it eventually became a Nikon. But

cameras have evolved on their own trajectory. Humans were playing with pinhole

camera obscuras as written about in ancient Chinese and Greek text, and we had

simple lens camera lucida’s by the in the early renascence. So what was the first

human made camera? What was equivalent to the flat disk of light sensitive cells?

Many theorists have written about the analogy of Plato’s cave in which people trace

the shadows cast on the back wall of a cave. The first human made proto cameras

could have been present with the first proto humans who watched their shadows

dance across the cave walls cast from their fire pits.

Photography is undoubtedly one of the most important inventions in history --

it has truly transformed how people conceive of the world. Now we can "see" all sorts

of things that are actually many miles -- and years -- away from us. Photography lets

us capture moments in time and preserve them for years to come.

First let us discuss how a camera operates, the history of the camera

technology and as of how the name is originated.

Then we will be discussing what is an image sensor and different types of

image sensors and their basic operation and finally how an color image is formed.

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Camera:

With a rather gentle introduction, we ask ourselves what a camera really is,

and what its different components are. Chances are that you will already know some

of this, but going through it anyway will at least ensure that we have defined a

common vocabulary.

In the strictest sense, it is simply a device which can record light. It does so by

focusing light on a photosensitive surface. From this simple sentence, we can see the

three main parts of any camera.

The basic technology that makes all of this possible is fairly simple. A still film

camera is made of three basic elements: an optical element (the lens), a chemical

element (the film) and a mechanical element (the camera body itself). As we'll see,

the only trick to photography is calibrating and combining these elements in such a

way that they record a crisp, recognizable image.

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History of Camera:

1500 – Camera Obscura

The first pinhole camera (also called the Camera Obscura)

was invented by Alhazen (Ibn Al-Haytham).

Camera Obscura(Latin origin) implies a vaulted(closed)

room.Basically a pin hole camera should be in such a way

that light enters only through the hole.

1839-Daguerreotype Camera

The Daguerreotype Camera was announced by the

French Academy of Sciences. One of these inventions is

now the world’s most expensive cameras.

1840-First Patent

The first American patent issued in photography to

Alexander Wolcott for his camera.

1859-Panoramic camera

The panoramic camera patented by

Thomas Sutton.

1861-Stereoscope viewer

Oliver Wendell Holmes invents stereoscope

viewer.

1888

George Eastman patents Kodak roll-film

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camera. Eastman was a pioneer in photographic films usage. He also started

manufacturing paper films in 1885. His first Kodak box camera was very simple and

very cheap.

1900

First mass-marketed camera – the Brownie was

presented by Eastman. It was on sale until 1960s.

1900

The Raisecamera

(travel camera) was invented. Extreme light weight

and small dimensions when it is folded made this

photo camera the most desirable thing for landscape

photographers.

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1913/1914

The first 35mm still camera (also called

�candid� camera ) developed by Oskar

Barnack of German Leica Camera. Later it

became the standard for all film cameras.

1948

Edwin Land invented the Polaroid camera which

could take a picture and print it in about one

minute.

1960

EG&G develops extreme depth underwater

camera for U.S. Navy.

1978

Konica introduces the first point-and-shoot, autofocus camera Konica C35 AF. It was

named “Jasupin”.

1981

Sony demonstrates the Sony Mavica – the

world’s first digital electronic still camera.

1986

Fuji introduced the disposable camera. The

inventors also call this device “single-use cameras”.

1991

Kodak released the first professional digital camera

system (DCS) which was of a great use for

photojournalists. It was a modified Nikon F-3 camera

with a 1.3 megapixel sensor.

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1994-1996

The first digital cameras for the consumer-level market that worked with a home

computer via a serial cable were the Apple QuickTake 100 camera (February 17 ,

1994), the Kodak DC40 camera (March 28, 1995), the Casio QV-11 (with LCD

monitor, late 1995), and Sony’s Cyber-Shot Digital Still Camera (1996).

2000

In Japane Sharp’s J-SH04 introduced the world’s

first camera phone.

2005

The Canon EOS 5D is launched. This is first

consumer-priced full-frame digital SLR with a 24x36mm CMOS sensor.

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Components of Camera:

Camera

consists of :

1.Lens

2.Aperture

3.Shutter

4.Photo

Sensing

Element

5.Buffer

6.ISP

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Debriefed:

The photosensitive surface reacts to light through either a chemical process

(film) or an electric one (digital sensor). There are fundamental differences between

these two, which we will cover in a subsequently, but for now we can consider both of

them to be identical: they are a grid of several million tiny dots (pixels) and each can

remember how much light it received in a given period of time. There are three

important qualities to each sensor: resolution, size and what we can call “quality”.

Resolution is simply the number of pixels (it is slightly more complicated with

film, let’s forget about it for now). The more pixels you have, the more fine

grained details you can theoretically record. Any resolution above 2 or 3

megapixels (i.e. millions of pixels) will be enough for displaying on a screen,

but higher resolutions come into play for two important applications: printing

and cropping.

o In order to have a good reproduction quality, it is generally estimated

that between 240 and 300 pixels should be used for every inch of paper

(dots per inch, or dpi), which will give a natural limitation to the

biggest size one can print. For instance, a 6MP image of dimensions

2000×3000 pixels can be printed at a maximum size of 12.5×8.3″ at

240dpi (2000/240 = 8.3, 3000/240 = 12.5). It is possible to print bigger

by either lowering the dpi or artificially increasing the resolution, but

this will come at a serious loss of image quality. Having a higher

resolution allows you to print bigger.

o Cropping means reducing the size of an image by discarding pixels on

the sides. It is a very useful tool and can often improve composition or

remove unwanted elements from an image. However, it will also

decrease resolution (since you lose pixels), so how much cropping you

allow yourself will depend on the initial resolution, which you want to

be as high as possible. This is also what some cheaper cameras call

“digital zoom”, which use should be avoided as the plague, as the same

effect can very easily be reproduced in post-processing, and the loss of

image quality is often enormous.

The physical size of the sensor is very important and will have an impact on

many other parameters, most of which we will see in subsequent lessons: crop

factor, depth of field, high ISO noise, dynamic range are some of them. Bigger

sensors will also allow to have more widely spaced pixels (increasing image

quality) or more of them (increasing resolution). Bigger is almost always

better, and this is one of the main reasons that DSLRs (and medium format

cameras) produce much better images than compact cameras. In tomorrow’s

lesson, we will cover the different types of cameras in more details.

Finally, sensor quality is harder to quantify, but it refers to how well the sensor

reacts to difficult light conditions: either low light which will require to

increase ISO and for which we want the sensor to have as little noise as

possible, or high contrast, which will require a good dynamic range to be

recorded adequately.

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The lens is the second

component of any camera. It is an

optical device which takes scattered

light rays and focuses them neatly

on the sensor. Lenses are often

complex, with up to 15 different

optical elements serving different

roles. The quality of the glass and

the precision of the lens will be

extremely important in determining

how good the final image is.

Lenses must make

compromises, and a perfect all

around lens is physically

impossible to build. For this reason,

good lenses tend to be specialized

and having the ability to switch

them on your camera will prove

extremely useful.

Lenses usually come with cryptic sequences of symbols and numbers which

describe their specifications. Without going too much into details, let’s review some

of their characteristic:

Focal length refers roughly to the “zoom level”, or angle of view, of the lens.

It will have its own lesson in a few days, as it can be a surprisingly tricky

subject. A focal length is usually expressed in millimeters, and you should be

aware that the resulting angle of view actually depends on the size of the

sensor of the camera on which the lens is used (this is called the crop factor).

For this reason, we often give “35mm equivalent” focal lengths, which is the

focal length that would offer the same view on a 35mm camera (the historic

film SLR format) and allows us to make meaningful comparisons. If there is a

single length (e.g. 24mm), then the lens doesn’t zoom, and it is often called a

prime lens. If there are two numbers (e.g. 18-55mm), then you can use the lens

at any focal in that range. Compact cameras often don’t give focal lengths but

simply the range, for instance 8x. This means that the long end is 8 times

longer than the wide one, so the lens could for instance be a 18-144mm, or a

35-280mm, etc.

The aperture is a very important concept which we will talk about in much

detail later on. The aperture is an iris in the centre of the lens which can close

to increasingly small sizes, limiting the amount of light which gets on the

sensor. It is refered to as a f-number, for instance f/2.8. To make things worse,

it is quite counter-intuitive, as the smaller the number, the bigger the aperture!

For now, we don’t have to worry about this too much. The important number

on a lens is the maximal aperture, the lower the better. Professional zoom

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lenses often have f/2.8 maximal apertures, and cheaper consumer lenses have

ranges such as f/3.5-5.6, meaning that at the wide end, the maximum aperture

is f/3.5 and at the long end, it is f/5.6. Aperture can be closed to tiny levels,

usually at least f/22.

Lenses also need a focusing system. Nowadays, most lenses have an internal

motor which can be piloted by the camera: the autofocus. They also have a

ring to allow the photographer to focus manually. There are plenty of options

for autofocus motors as well, for instance hypersonic or silent ones.

Lenses are increasingly equiped with stabilisation systems (called VR by

Nikon, IS by Canon). They detect small movements, usually handshake, and

compensate for them by moving internally the optical elements in the opposite

direction. Though no magic pills, those systems tend to work very well and

allow to take sharp images at quite slow shutter speeds.

Finally, lenses can have all sorts of fancy options: apochromatic glass, nano-

coating, etc, designed to increase the quality of the final image. You probably

shouldn’t worry too much about those.

Finally, the body is the light tight box connecting the lens to the sensor, and

ordering everyone around. Though some film cameras are just that, black boxes, most

digital cameras are now small computers, sporting all sorts of features, often of

dubious usefulness. Let’s review some of the components found in most bodies:

The most

important is probably

the shutter. Think of it

as a curtain in front of

the sensor. When you

press the trigger, the

curtain opens, exposes

the sensor to light from

the lens, then closes

again after a very

precise amount of time,

often a tiny fraction of a

second. Most shutters

operate between 30

seconds and 1/4000s of

a second. That duration

(the shutter speed) is one of the three very important exposure factors, along

with aperture and ISO.

A light meter. As the name suggests, it measures the quantity of light and sets

the exposure accordingly. How much manual control you keep at this stage is

one of the most important questions in photography. There are different

metering modes, but except in very specific cases, using the most advanced,

most automated one (matrix metering on Nikon cameras) will provide the best

results.

A focus detector, used to drive the autofocus motor in the lens. There are two

competing technologies, contrast detection and phase detection, with at the

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moment an edge for the latter, which explains why DSLRs tend to focus faster

than compact cameras.

A way to store the image just created. Back in the days of film, this was just a

lever to advance the roll to the next unexposed frame. Now, it is a pipeline

which ends up in the memory card that the camera is using.

A way to frame. It can be a multitude of things, optical or electronic

viewfinder, LCD screen or even ground glass.

The optical component of

the camera is the lens. At

its simplest, a lens is just

a curved piece of glass or

plastic. Its job is to take

the beams of light

bouncing off of an object

and redirect them so they

come together to form

a real image -- an image

that looks just like the

scene in front of the lens.

But how can a

piece of glass do this?

The process is actually

very simple.

As light travels from one medium to another, it changes speed. Light travels more

quickly through air than it does through glass, so a lens slows it down.

When light waves enter a piece of glass at an angle, one part of the wave will

reach the glass before another and so will start slowing down first. This is something

like pushing a shopping

cart from pavement to

grass, at an angle. The

right wheel hits the grass

first and so slows down

while the left wheel is

still on the pavement.

Because the left wheel is

briefly moving more

quickly than the right

wheel, the shopping cart

turns to the right as it

moves onto the grass.

The effect on

light is the same -- as it

enters the glass at an

angle, it bends in one

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direction. It bends again when it exits the glass because parts of the light wave enter

the air and speed up before other parts of the wave. In a standard converging,

or convex lens, one or both sides of the glass curves out. This means rays of light

passing through will bend toward the center of the lens on entry. In adouble convex

lens, such as a magnifying glass, the light will bend when it exits as well as when it

enters.

This effectively reverses the path of light from an object. A light source -- say

a candle -- emits light in all directions. The rays of light all start at the same point --

the candle's flame -- and then are constantly diverging. A converging lens takes those

rays and redirects them so they are all converging back to one point. At the point

where the rays converge, you get a real image of the candle. In the next couple of

sections, we'll look at some of the variables that determine how this real image is

formed.

Cameras: Focus

We've seen that

a real image is formed by

light moving through a

convex lens. The nature of

this real image varies

depending on how

the light travels through the

lens. This light path depends

on two major factors:

The angle of the

light beam's entry into the

lens

The structure

of the lensThe angle of light

entry changes when you

move the object closer or

farther away from the lens.

You can see this in the

diagram below. The light

beams from the pencil point

enter the lens at a sharper

angle when the pencil is

closer to the lens and a more

obtuse angle when the

pencil is farther away. But

overall, the lens only bends

the light beam to a certain

total degree, no matter how

it enters. Consequently, light

beams that enter at a sharper

angle will exit at a more

obtuse angle, and vice versa.

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The total "bending angle" at any particular point on the lens remains constant.

As you can see, light beams from a closer point converge farther away from

the lens than light beams from a point that's farther away. In other words, the real

image of a closer object forms farther away from the lens than the real image from a

more distant object.

You can observe this phenomenon with a simple experiment. Light a candle in

the dark, and hold a magnifying glass between it and the wall. You will see an upside

down image of the candle on the wall. If the real image of the candle does not fall

directly on the wall, it will appear somewhat blurry. The light beams from a particular

point don't quite converge at this point. To focus the image, move the magnifying

glass closer or farther away from the candle.

This is what you're doing when you turn the lens of a camera to focus it --

you're moving it closer or farther away from the film surface. As you move the lens,

you can line up the focused real image of an object so it falls directly on the film

surface.

You now know that at any one point, a lens bends light beams to a certain total

degree, no matter the light beam's angle of entry. This total "bending angle" is

determined by the structure of the lens.

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Camera Lenses:

A standard

50 mm lens doesn't

significantly shrink

or magnify the

image.

In the last

section, we saw that

at any one point, a

lens bends light

beams to a certain

total degree, no

matter the light

beam's angle of entry.

This total "bending

angle" is determined

by the structure of the

lens.

A lens with a

rounder shape (a

center that extends

out farther) will have

a more acute bending angle. Basically, curving the lens out increases the distance

between different points on the lens. This increases the amount of time that one part

of the light wave is moving faster than another part, so the light makes a sharper turn.

Increasing the bending angle has an obvious effect. Light beams from a

particular point will converge at a point closer to the lens. In a lens with a flatter

shape, light beams will not turn as sharply. Consequently, the light beams will

converge farther away from the lens. To put it another way, the focused real image

forms farther away from the lens when the lens has a flatter surface.

Increasing the distance between the lens and the real image actually increases

the total size of the real image. If you think about it, this makes perfect sense. Think

of a projector: As you move the projector farther away from the screen, the image

becomes larger. To put it simply, the light beams keep spreading apart as they travel

toward the screen.

The same basic thing happens in a camera. As the distance between the lens

and the real image increases, the light beams spread out more, forming a larger real

image. But the size of the film stays constant. When you attach a very flat lens, it

projects a large real image but the film is only exposed to the middle part of it.

Basically, the lens zeroes in on the middle of the frame, magnifying a small section of

the scene in front of you. A rounder lens produces a smaller real image, so the film

surface sees a much wider area of the scene (at reduced magnification).

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Professional cameras let you attach different lenses so you can see the scene at

various magnifications. The magnification power of a lens is described by itsfocal

length. In cameras, the focal length is defined as the distance between the lens and the

real image of an object in the far distance (the moon for example). A higher focal

length number indicates a greater image magnification.

Different lenses are suited to different situations. If you're taking a picture of a

mountain range, you might want to use a telephoto lens, a lens with an especially

long focal length. This lens lets you zero in on specific elements in the distance, so

you can create tighter compositions. If you're taking a close-up portrait, you might use

a wide-angle lens. This lens has a much shorter focal length, so it shrinks the scene in

front of you. The entire face is exposed to the film even if the subject is only a foot

away from the camera. A standard 50 mm camera lens doesn't significantly magnify

or shrink the image, making it ideal for shooting objects that aren't especially close or

far away.

Lenses in the Lens

A camera lens is actually several lenses combined into one unit. A single

converging lens could form a real image on the film, but it would be warped by a

number of aberrations.

One of the most significant warping factors is that different colors of light

bend differently when moving through a lens. This chromatic aberrationessentially

produces an image where the colors are not lined up correctly.

Cameras compensate for this using several lenses made of different materials.

The lenses each handle colors differently, and when you combine them in a certain

way, the colors are realigned.

In a zoom lens, you can move different lens elements back and forth. By

changing the distance between particular lenses, you can adjust the magnification

power -- the focal length -- of the lens as a whole.

Cameras: Recording Light

The chemical component in a traditional camera is film. Essentially, when you

expose film to a real image, it makes a chemical record of the pattern of light.

It does this with a collection of tiny light-sensitive grains, spread out in a

chemical suspension on a strip of plastic. When exposed to light, the grains undergo a

chemical reaction.

Once the roll is finished, the film is developed -- it is exposed to other

chemicals, which react with the light-sensitive grains. In black and white film, the

developer chemicals darken the grains that were exposed to light. This produces a

negative, where lighter areas appear darker and darker areas appear lighter, which is

then converted into a positive image in printing.

Color film has three different layers of light-sensitive materials, which

respond, in turn, to red, green and blue. When the film is developed, these layers are

exposed to chemicals that dye the layers of film. When you overlay the color

information from all three layers, you get a full-color negative.

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For an in-depth description of this entire process, check out How Photographic

Film Works.

So far, we've looked at the basic idea of photography -- you create a real

image with a converging lens, and you record the light pattern of this real image on a

layer of light-sensitive material. Conceptually, this is all that's involved in taking a

picture. But to capture a clear image, you have to carefully control how everything

comes together.

Obviously, if you were to lay a piece of film on the ground and focus a real

image onto it with a converging lens, you wouldn't get any kind of usable picture. Out

in the open, every grain in the film would be completely exposed to light. And

without any contrasting unexposed areas, there's no picture.

To capture an image, you have to keep the film in complete darkness until it's

time to take the picture. Then, when you want to record an image, you let some light

in. At its most basic level, this is all the body of a camera is -- a sealed box with

a shutter that opens and closes between the lens and film. In fact, the term camera is

shortened from camera obscura, literally "dark room" in Latin.

For the picture to come out right, you have to precisely control how much

light hits the film. If you let too much light in, too many grains will react, and the

picture will appear washed out. If you don't let enough light hit the film, too few

grains will react, and the picture will be too dark. In the next section, we'll look at the

different camera mechanisms that let you adjust the exposure.

What's in a Name?

As it turns out, the term photography describes the photographic process quite

accurately. Sir John Herschel, a 19th century astronomer and one of the first

photographers, came up with the term in 1839. The term is a combination of two

Greek words -- photos meaning light and graphein meaning writing (or drawing). The

term camera comes from camera obscura, Latin for "dark room." The camera

obscura was actually invented hundreds of years before photography. A traditional

camera obscura was a dark room with light shining through a lens or tiny hole in the

wall. Light passed through the hole, forming an upside-down real image on the

opposite wall. This effect was very popular with artists, scientists and curious

spectators.

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Cameras: The Right Light

The plates in the iris diaphragm fold in on each other to shrink the

aperture and expand out to make it wider.

In the last section, we saw that you need to carefully control the film's

exposure to light, or your picture will come out too dark or too bright. So how do you

adjust this exposure level? You have to consider two major factors:

How much light is passing through the lens

How long the film is exposed

To increase or decrease the amount of light passing through the lens, you have

to change the size of the aperture -- the lens opening. This is the job of the iris

diaphragm, a series of overlapping metal plates that can fold in on each other or

expand out. Essentially, this mechanism works the same way as the iris in your eye --

it opens or closes in a circle, to shrink or expand the diameter of the lens. When the

lens is smaller, it captures less light, and when it is larger, it captures more light.

The length of exposure is determined by the shutter speed. Most SLR

cameras use a focal plane shutter. This mechanism is very simple -- it basically

consists of two "curtains" between the lens and the film. Before you take a picture, the

first curtain is closed, so the film won't be exposed to light. When you take the

picture, this curtain slides open. After a certain amount of time, the second curtain

slides in from the other side, to stop the exposure.

When you click the camera's shutter release, the first curtain slides open,

exposing the film. After a certain amount of time, the second shutter slides

closed, ending the exposure. The time delay is controlled by the camera's shutter

speed knob.

This simple action is controlled by a complex mass of gears, switches and

springs, like you might find inside a watch. When you hit the shutter button, it

releases a lever, which sets several gears in motion. You can tighten or loosen some

of the springs by turning the shutter speed knob. This adjusts the gear mechanism,

increasing or decreasing the delay between the first curtain opening and the second

curtain closing. When you set the knob to a very slow shutter speed, the shutter is

open for a very long time. When you set the knob to a very high speed, the second

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curtain follows directly behind the first curtain, so only a tiny slit of the film frame is

exposed at any one time.

The ideal exposure depends on the size of the light-sensitive grains in the film.

A larger grain is more likely to absorb light photons than a smaller grain. The size of

the grains is indicated by a film's speed, which is printed on the canister. Different

film speeds are suited to different types of photography -- 100 ISO film, for example,

is optimal for shots in bright sunlight, while 1600 film should only be used in

relatively low light.

Inside a manual

SLR camera, you'll

find an intricate puzzle

of gears and springs.

Click on each picture

for a high-resolution

close-up shot.

As you can see,

there's a lot involved in

getting the exposure

right -- you have to balance film speed, aperture size and shutter speed to fit the light

level in your shot. Manual SLR cameras have a built-in light meter to help you do

this. The main component of the light meter is a panel of semi-conductor light sensors

that are sensitive to light energy. These sensors express this light energy as electrical

energy, which the light meter system interprets based on the film and shutter speed.

Now, let's see how an SLR camera body directs the real image to the

viewfinder before you take the shot, and then directs it to the film when you press the

shutter button.

SLR Cameras vs. Point-and-Shoot

There are

two types of

consumer film

cameras on the

market -- SLR

cameras and

"point-and-shoot"

cameras. The

main difference is

how the

photographer sees

the scene. In a

point-and-shoot

camera, the

viewfinder is a

simple window

through the body of the camera. You don't see the real image formed by the camera

lens, but you get a rough idea of what is in view.

19

In an SLR camera, you see the actual real image that the film will see. If you

take the lens off of an SLR camera and look inside, you'll see how this works. The

camera has a slanted mirror positioned between the shutter and the lens, with a piece

of translucent glass and a prism positioned above it. This configuration works like a

periscope -- the real image bounces off the lower mirror on to the translucent glass,

which serves as a projection screen. The prism's job is to flip the image on the screen,

so it appears right side up again, and redirect it on to the viewfinder window.

When you click the shutter button, the camera quickly switches the mirror out

of the way, so the image is directed at the exposed film. The mirror is connected to

the shutter timer system, so it stays open as long as the shutter is open. This is why the

viewfinder is suddenly blacked out when you take a picture.

The

mirror in an

SLR camera

directs the

real image to

the

viewfinder.

When you hit

the shutter

button, the

mirror flips

up so the real image is projected onto the film.

In this sort of camera, the mirror and the translucent screen are set up so they

present the real image exactly as it will appear on the film. The advantage of this

design is that you can adjust the focus and compose the scene so you get exactly the

picture you want. For this reason, professional photographers typically use SLR

cameras.

These days, most SLR cameras are built with both manual and automatic

controls, and most point-and-shoot cameras are fully automatic. Conceptually,

automatic cameras are pretty much the same as fully manual models, but everything is

controlled by a central microprocessor instead of the user. The central microprocessor

receives information from the autofocus system and the light meter. Then it activates

several small motors, which adjust the lens and open and close the aperture. In

modern cameras, this a pretty advanced computer system.

Automatic point-and-shoot

camera use circuit boards and

electric motors, instead of gears

and springs.

In the next section, we'll

look at the other end of the

spectrum -- a camera design with no

20

complex machinery, no lens and barely any moving parts.

Throughout the history of photography, there have been hundreds of different

camera systems. But amazingly, all these designs -- from the simplest homemade box

camera to the newest digital camera -- combine the same basic elements: a lens

system to create the real image, a light-sensitive sensor to record the real image, and a

mechanical system to control how the real image is exposed to the sensor. And when

you get down to it, that's all there is to photography!

Types of Digital Image Sensors :

21

Working of Camera:

In the past twenty years, most of the major technological breakthroughs in consumer

electronics have really been part of one larger breakthrough. When you get down to

it, CDs, DVDs, HDTV, MP3s and DVRs are all built around the same basic process:

converting conventional analog information (represented by a fluctuating wave) into

digital information (represented by ones and zeros, or bits). This fundamental shift in

technology totally changed how we handle visual and audio information -- it

completely redefined what is possible.

The digital camera is one of the most remarkable instances of this shift

because it is so truly different from its predecessor.Conventional cameras depend

entirely on chemical and mechanical processes -- you don't even need electricity to

operate them. On the other hand, all digital cameras have a built-in computer, and all

of them record images electronically.

The new approach has been enormously successful. Since film still provides

better picture quality, digital cameras have not completely replaced conventional

cameras. But, as digital imaging technology has improved, digital cameras have

rapidly become more popular.

In this article, we'll find out exactly what's going on inside these amazing

digital-age devices.

Digital Camera Basics

Let's say you want to take a picture and e-mail it to a friend. To do this, you

need the image to be represented in the language that computers recognize -- bits and

bytes. Essentially, a digital image is just a long string of 1s and 0s that represent all

the tiny colored dots -- or pixels -- that collectively make up the image. (For

information on sampling and digital representations of data, see this explanation of

the digitization of sound waves. Digitizing light waves works in a similar way.)

If you want to get a picture into this form, you have two options:

You can take a photograph using a conventional film camera, process

the film chemically, print it onto photographic paper and then use adigital

scanner to sample the print (record the pattern of light as a series of pixel

values).

You can directly sample the original light that bounces off your subject,

immediately breaking that light pattern down into a series of pixel values -- in

other words, you can use a digital camera.

At its most basic level, this is all there is to a digital camera. Just like

a conventional camera, it has a series of lenses that focus light to create an image of a

scene. But instead of focusing this light onto a piece of film, it focuses it onto

a semiconductor device that records light electronically. A computer then breaks this

electronic information down into digital data. All the fun and interesting features of

digital cameras come as a direct result of this process.

In the next few sections, we'll find out exactly how the camera does all this.

22

Cool Facts

With a 3-megapixel camera, you can take a higher-resolution picture than

most computer monitors can display.

You can use your Web browser to view digital pictures taken using the JPEG

format.

The first consumer-oriented digital cameras were sold by Kodak and Apple in

1994.

In 1998, Sony inadvertently sold more than 700,000 camcorders with a limited

ability to see through clothes.

CCD and CMOS: Filmless Cameras

A CMOS image sensor

Instead of film, a digital camera has a sensor that converts light into electrical

charges.

The image sensor employed by most digital cameras is a charge coupled

device (CCD). Some cameras use complementary metal oxide

semiconductor (CMOS) technology instead. Both CCD and CMOS image sensors

convert light into electrons. If you've read How Solar Cells Work, you already

understand one of the pieces of technology used to perform the conversion. A

simplified way to think about these sensors is to think of a 2-D array of thousands or

millions of tiny solar cells.

Once the sensor converts the light into electrons, it reads the value

(accumulated charge) of each cell in the image. This is where the differences between

the two main sensor types kick in:

A CCD transports the charge across the chip and reads it at one corner of the

array. An analog-to-digital converter (ADC)then turns each pixel's value

into a digital value by measuring the amount of charge at each photosite and

converting that measurement to binary form.

CMOS devices use several transistors at each pixel to amplify and move the

charge using more traditional wires.

Differences between the two types of sensors lead to a number of pros and

cons:

23

A CCD sensor

PHOTO COURTESY DALSA

CCD sensors create high-quality, low-noise images. CMOS sensors are

generally more susceptible to noise.

Because each pixel on a CMOS sensor has several transistors located next to

it, the light sensitivity of a CMOS chip is lower. Many of the photons hit the

transistors instead of the photodiode.

CMOS sensors traditionally consume little power. CCDs, on the other hand,

use a process that consumes lots of power. CCDs consume as much as 100

times more power than an equivalent CMOS sensor.

CCD sensors have been mass produced for a longer period of time, so they are

more mature. They tend to have higher quality pixels, and more of them.

Although numerous differences exist between the two sensors, they both play

the same role in the camera -- they turn light into electricity. For the purpose of

understanding how a digital camera works, you can think of them as nearly identical

devices.

Digital Camera Resolution

The size of an image taken at different resolutions

PHOTO COURTESY MORGUEFILE

24

The amount of detail that the camera can capture is called the resolution, and

it is measured in pixels. The more pixels a camera has, the more detail it can capture

and the larger pictures can be without becoming blurry or "grainy."

Some typical resolutions include:

256x256 - Found on very cheap cameras, this resolution is so low that the

picture quality is almost always unacceptable. This is 65,000 total pixels.

640x480 - This is the low end on most "real" cameras. This resolution is ideal

for e-mailing pictures or posting pictures on a Web site.

1216x912 - This is a "megapixel" image size -- 1,109,000 total pixels -- good

for printing pictures.

1600x1200 - With almost 2 million total pixels, this is "high resolution." You

can print a 4x5 inch print taken at this resolution with the same quality that

you would get from a photo lab.

2240x1680 - Found on 4 megapixel cameras -- the current standard -- this

allows even larger printed photos, with good quality for prints up to 16x20

inches.

4064x2704 - A top-of-the-line digital camera with 11.1 megapixels takes

pictures at this resolution. At this setting, you can create 13.5x9 inch prints

with no loss of picture quality.

High-end consumer cameras can capture over 12 million pixels. Some

professional cameras support over 16 million pixels, or 20 million pixels for large-

format cameras. For comparison, Hewlett Packard estimates that the quality of 35mm

film is about 20 million pixels [ref].

Next, we'll look at how the camera adds color to these images.

How Many Pixels?

You may have noticed that the number of pixels and the maximum resolution

don't quite compute. For example, a 2.1-megapixel camera can produce images with a

resolution of 1600x1200, or 1,920,000 pixels. But "2.1 megapixel" means there

should be at least 2,100,000 pixels.

This isn't an error from rounding off or binary mathematical trickery. There is

a real discrepancy between these numbers because the CCD has to include circuitry

for the ADC to measure the charge. This circuitry is dyed black so that it doesn't

absorb light and distort the image.

25

Capturing Color

How the original (left) image is split in a beam splitter

Unfortunately, each photosite is colorblind. It only keeps track of the total

intensity of the light that strikes its surface. In order to get a full color image, most

sensors use filtering to look at the light in its three primary colors. Once the camera

records all three colors, it combines them to create the full spectrum.

There are several ways of recording the three colors in a digital camera. The

highest quality cameras use three separate sensors, each with a different filter.

A beam splitter directs light to the different sensors. Think of the light entering the

camera as water flowing through a pipe. Using a beam splitter would be like dividing

an identical amount of water into three different pipes. Each sensor gets an identical

look at the image; but because of the filters, each sensor only responds to one of the

primary colors.

The advantage of this method is that the camera records each of the three

colors at each pixel location. Unfortunately, cameras that use this method tend to be

bulky and expensive.

Another method is to rotate a series of red, blue and green filters in front of a

single sensor. The sensor records three separate images in rapid succession. This

method also provides information on all three colors at each pixel location; but since

the three images aren't taken at precisely the same moment, both the camera and the

target of the photo must remain stationary for all three readings. This isn't practical for

candid photography or handheld cameras.

Both of these methods work well for professional studio cameras, but they're

not necessarily practical for casual snapshots. Next, we'll look at filtering methods

that are more suited to small, efficient cameras.

26

Demosaicing Algorithms: Color Filtering

A more economical and

practical way to record the primary

colors is to permanently place a

filter called a color filter array over

each individual photosite. By

breaking up the sensor into a variety

of red, blue and green pixels, it is

possible to get enough information

in the general vicinity of each sensor

to make very accurate guesses about

the true color at that location. This

process of looking at the other pixels

in the neighborhood of a sensor and

making an educated guess is called

interpolation.

The most common pattern of filters is the Bayer filter pattern. This pattern

alternates a row of red and green filters with a row of blue and green filters. The

pixels are not evenly divided -- there are as many green pixels as there are blue and

red combined. This is because the human eye is not equally sensitive to all three

colors. It's necessary to include more information from the green pixels in order to

create an image that the eye will perceive as a "true color."

The advantages of this method are that only one sensor is required, and all the

color information (red, green and blue) is recorded at the same moment. That means

the camera can be smaller, cheaper, and useful in a wider variety of situations. The

raw output from a sensor with a Bayer filter is a mosaic of red, green and blue pixels

of different intensity.

Digital cameras use specialized demosaicing algorithms to convert this

mosaic into an equally sized mosaic of true colors. The key is that each colored pixel

can be used more than once. The true color of a single pixel can be determined by

averaging the values from the closest surrounding pixels.

Some single-sensor cameras use alternatives to the Bayer filter pattern. X3

technology, for example, embeds red, green and blue photodetectors in silicon. Some

of the more advanced cameras subtract values using the typesetting colors cyan,

yellow, green and magenta instead of blending red, green and blue. There is even a

method that uses two sensors. However, most consumer cameras on the market today

use a single sensor with alternating rows of green/red and green/blue filters.

Digital Camera Exposure and Focus

Just as with film, a digital camera has to control the amount of light that

reaches the sensor. The two components it uses to do this, the aperture and shutter

speed, are also present on conventional cameras.

Aperture: The size of the opening in the camera. The aperture is automatic in

most digital cameras, but some allow manual adjustment to give professionals

and hobbyists more control over the final image.

27

Shutter speed: The amount of time that light can pass through the aperture.

Unlike film, the light sensor in a digital camera can be reset electronically, so

digital cameras have a digital shutter rather than a mechanical shutter.

These two aspects work together to capture the amount of light needed to

make a good image. In photographic terms, they set the exposure of the sensor. You

can learn more about a camera's aperture and shutter speed in How Cameras Work.

In addition to controlling the amount of light, the camera has to adjust the

lenses to control how the light is focused on the sensor. In general, the lenses on

digital cameras are very similar to conventional camera lenses -- some digital cameras

can even use conventional lenses. Most use automatic focusing techniques, which you

can learn more about in the article How Autofocus Cameras Work.

The focal length, however, is one important difference between the lens of a

digital camera and the lens of a 35mm camera. The focal length is the distance

between the lens and the surface of the sensor. Sensors from different manufacturers

vary widely in size, but in general they're smaller than a piece of 35mm film. In order

to project the image onto a smaller sensor, the focal length is shortened by the same

proportion. For additional information on sensor sizes and comparisons to 35mm film,

you can visit the Photo.net Web site.

Focal length also determines the magnification, or zoom, when you look

through the camera. In 35mm cameras, a 50mm lens gives a natural view of the

subject. Increasing the focal length increases the magnification, and objects appear to

get closer. The reverse happens when decreasing the focal length. A zoom lens is any

lens that has an adjustable focal length, and digital cameras can

have optical or digital zoom -- some have both. Some cameras also havemacro

focusing capability, meaning that the camera can take pictures from very close to the

subject.

Digital cameras have one of four types of lenses:

Fixed-focus, fixed-zoom lenses - These are the kinds of lenses on disposable

and inexpensive film cameras -- inexpensive and great for snapshots, but fairly

limited.

Optical-zoom lenses with automatic focus - Similar to the lens on a video

camcorder, these have "wide" and "telephoto" options and automatic focus.

The camera may or may not support manual focus. These actually change the

focal length of the lens rather than just magnifying the information that hits

the sensor.

Digital zoom - With digital zoom, the camera takes pixels from the center of

the image sensor and interpolates them to make a full-sized image.

Depending on the resolution of the image and the sensor, this approach may

create a grainy or fuzzy image. You can manually do the same thing with

image processing software -- simply snap a picture, cut out the center and

magnify it.

Replaceable lens systems - These are similar to the replaceable lenses on a

35mm camera. Some digital cameras can use 35mm camera lenses.

28

Next, we'll learn about how the camera stores pictures and transfers them to a

computer.

Storing Digital Photos

A CompactFlash card

Most digital cameras have an LCD

screen, so you can view your picture right

away. This is one of the great advantages of a

digital camera -- you get immediate feedback

on what you capture. Of course, viewing the

image on your camera would lose its charm if

that's all you could do. You want to be able to load the picture into your computer or

send it directly to a printer. There are several ways to do this.

Early generations of digital cameras had fixed storage inside the camera. You

needed to connect the camera directly to a computer with cables to transfer the

images. Although most of today's cameras are capable of connecting

through serial,parallel, SCSI, USB or FireWire connections, they usually also use

some sort of removable storage device.

Digital cameras use a number of storage systems. These are like reusable,

digital film, and they use a caddy or card reader to transfer the data to a computer.

Many involve fixed or removable flash memory. Digital camera manufacturers often

develop their own proprietary flash memory devices,

including SmartMedia cards, CompactFlash cards and Memory Sticks. Some other

removable storage devices include:

Floppy disks

Hard disks, or microdrives

Writeable CDs and DVDs

No matter what type of storage they use, all digital cameras need lots of room

for pictures. They usually store images in one of two formats -- TIFF, which is

uncompressed, and JPEG, which is compressed, but some use RAW format. Most

cameras use the JPEG file format for storing pictures, and they sometimes offer

quality settings (such as medium or high). The following information will give you an

idea of the file sizes you might expect with different picture sizes.

640x480

TIFF (uncompressed) 1.0 MB

JPEG (high quality) 300 KB

JPEG (medium quality) 90 KB

800x600

TIFF (uncompressed) 1.5 MB

29

JPEG (high quality) 500 KB

JPEG (medium quality) 130 KB

1024x768

TIFF (uncompressed) 2.5 MB

JPEG (high quality) 800 KB

JPEG (medium quality) 200 KB

1600x1200

TIFF (uncompressed) 6.0 MB

JPEG (high quality) 1.7 MB

JPEG (medium quality) 420 KB

To make the most of their storage space, almost all digital cameras use some

sort of data compression to make the files smaller. Two features of digital images

make compression possible. One is repetition. The other is irrelevancy.

Imagine that throughout a given photo, certain patterns develop in the colors.

For example, if a blue sky takes up 30 percent of the photograph, you can be certain

that some shades of blue are going to be repeated over and over again. When

compression routines take advantage of patterns that repeat, there is no loss of

information and the image can be reconstructed exactly as it was recorded.

Unfortunately, this doesn't reduce files any more than 50 percent, and sometimes it

doesn't even come close to that level.

Irrelevancy is a trickier issue. A digital camera records more information than

the human eye can easily detect. Some compression routines take advantage of this

fact to throw away some of the more meaningless data.

Next, we'll tie it all together and see how a digital camera takes a picture.

CCD Camera Summary

It takes several steps for a digital camera to take a picture. Here's a review of

what happens in a CCD camera, from beginning to end:

You aim the camera at the subject and adjust the optical zoom to get closer or

farther away.

You press lightly on the shutter release.

The camera automatically focuses on the subject and takes a reading of the

available light.

The camera sets the aperture and shutter speed for optimal exposure.

You press the shutter release all the way.

The camera resets the CCD and exposes it to the light, building up an

electrical charge, until the shutter closes.

30

The ADC measures the charge and creates a digital signal that represents the

values of the charge at each pixel.

A processor interpolates the data from the different pixels to create natural

color. On many cameras, it is possible to see the output on the LCD at this

stage.

A processor may perform a preset level of compression on the data.

The information is stored in some form of memory device (probably a Flash

memory card).

A CCD Image Sensor:

Advent of CMOS technology in eighties led to the phenomenal growth in

semiconductor industry. Transistors have become smaller, faster, consume less power,

and are cheaper to manufacture. It is CMOS technology which has enabled very high

integration on the chips leading to modern high performance, miniaturized integrated

circuits.

Apart from the valuable contribution in miniaturization of integrated circuits, CMOS

technology found applications in sensing applications.

CMOS technology has been adopted to design sensors, especially in the field of

imaging. Due to the wide usage of CMOS based image sensors, CMOS sensors are

31

often considered to be a synonym of CMOS based image sensors and have emerged

as a competitor to CCD based image sensors.

Until recently, Charge Coupled Devices (CCDs) dominated most of the image sensing

systems, i.e., cameras, camcorders, etc. CCDs have been in use in astronomical

cameras, video camcorders and scanners. However of late, CMOS Imaging have

emerged as an alternative to CCD imagers and it also offers better features.

Subsequent sections will discuss both CCD and CMOS sensor based imagers, their

pros and cons, and also their applications. Further, other applications

of CMOS technology in the field of sensing will be discussed.

32

CMOS Vs CCD

Invention of CCD marked the end of vacuum tube imagers used in television cameras

as it overcame the disadvantages of vacuum tubes like chronic picture artifacts as lag

and burn-in, fragility of large glass tubes or the sensitivity to shock, vibration and

electromagnetic radiation, painstaking periodic alignment of tubes, etc. It also marked

the beginning of a new era in imaging systems and for decades, it enjoyed quality

advantages over the rival CMOS sensors. Wherever image quality was paramount,

CCDs were preferred, CMOS were used mainly in applications where small size and

low power were prime requirements.

With the technological development in CMOS technology, gap between CCD

and CMOS sensors has narrowed; CMOS sensors can also achieve competitive

quality. Choice amongst CCD and CMOS sensors has become increasingly difficult.

Both CCD and CMOS image sensors use large arrays of thousands (sometimes

millions) of photo-sites, commonly called pixels. Both carry out same steps.

1. Light-to-charge conversion

Incident light is directed by the microlens (a tiny lens placed over the pixel to increase

its effective size and thereby fill factor) onto the photo-sensitive area of each pixel

where it is converted into electrons that collect in a semiconductor "bucket."

The bigger the pixel, the more light it can collect. Thus, big pixel sensors work best

under low-light conditions. For the same number of pixels, bigger pixels results in

bigger chip, this means higher cost. Conversely, smaller pixels enable smaller chip

sizes and lower chip prices, as well as lower lens costs. But there are limitations on

33

pixel size reduction. Smaller pixels are less sensitive to light, the optics required to

resolve the pixels becomes expensive and requires expensive fabrication possesses.

2. Charge accumulation

As more light enters, more electrons accumulate into the bucket.

3. Transfer

Accumulated charge must be transferred to the signal conditioning and processing

circuitry.

4. Charge-to-voltage conversion

The accumulated charge must be output as the voltage signal.

5. Amplification

Voltage signal is then amplified before it is fed to the camera circuitry.Both CMOS

and CCD perform all these tasks; however the aspect in which they differ is the order

of execution of these tasks.

BRIEF ON CCD TECHNOLOGY

CCDs were first invented in 1969 as a way to store data using bubble memory. In

1974, the first imaging CCD was produced by Fairchild Electronics with a format of

100x100 pixels.

CCD imager consists of two main parts: color filter and pixel array

• Color filter

Micro-lenses funnel light onto the photo-sensitive part of each pixel. On their way,

the photons pass through a color filter array. The mosaic of these tiny filters captures

color information. Color filters enable separate measurement of the red (R), green (G)

and blue (B) photons. Color filter filters out wavelengths of unwanted colors and

allows only specific colors of light to pass through a pixel sensor. For this purpose,

each pixel is covered with a red, green and a blue filter according to a specific pattern,

like the Bayer CFA pattern.

Bayer filter uses the sub-

mosaic 2x 2 patterns with one

red, one blue and two green

filters. As human’s eye has

greater sensitivity for green

light, two green filters are

used.

34

• Pixel Array

The pixel array functions on the principle of the photoelectric effect and pixel sensors

are responsible for capturing the intensity of the light passing through. The light

intensity data is combined before being converted into an analog voltage signal,

which is outputted to an external circuit board to be further processed.

After conversion of incident light into electrons, electron charge is accumulated in the

same way as bucket stores water. The pixel charges are read using vertical and

horizontal shift registers which act as charge carriers.

CMOS SENSORS

A typical CMOS is an integrated circuit with an array of pixel sensors. In contrast to

CCD, each pixel sensor in CMOS sensors contains its own light sensor, an amplifier

and a pixel select switch. An analog-to-digital converter and other components critical

to the operation of the pixel sensors are located on the CMOS sensor.

The CMOS sensor contains four main parts: the color filters, the pixel array, the

digital controller, and the analog to digital convertor.

• Color Filter

Color filter is the same as was described in CCD based imager.

• Pixel Array

As in the case of CCD, function of the pixel array is to capture the intensity of the

light passing through. Each pixel sensor converts the sensitivity of the incoming light

to the voltage signal which is then fed to ADC for further processing

There are two types of architectures of Pixel sensors: Passive Pixel Sensor (PPS) &

Active Pixel Sensors (APS).

In Passive Pixel sensors, only one photo-detector (without any local

amplifier) per pixel is used, whereas in Active Pixel sensors, 3-4 transistors per pixel

are used. Passive Pixel sensors have smaller pixels and large fill

35

factor but they are slow and have low SNR. On the other hand, active pixel sensors

are fast, have good SNRs but larger pixels and low fill factor.

However, due to advancement of CMOS technology down to nm, pixel size/fill factor

is no longer a big issue and APS is the technology which is preferred and used in most

devices.

• ADC

The ADC takes the analog voltage signals from the pixel sensor array and converts

them into a digital signal.

• Digital Controller

The digital controller governs the functioning of the the CMOS sensor; it controls the

pixel array, ensures synchronism between all pixels, etc.

Operation of CMOS Sensors

a) Pixel sensor acts like a charge bucket; accumulates electron charges the same

way as water bucket stores water

b) Charge is converted to voltage & amplified at the pixel.

c) Individual CMOS microwire carry voltage from one pixel at a time, controlled

by the pixel select switch

d) To output video signal, following steps are followed

1. All pixel select switches are turned ON. This outputs voltage of each pixel to

column circuit.

2. Column select switches are turned ON from left to right. In this way, signal

voltages of each pixel in the same row are output in order.

3. This is repeated for all rows from the top to the bottom in order, signal

voltages of all pixels can be output from the top-left corner to the bottom-right corner

of the image sensor.

e) These signal voltages are output to the signal processor of the camera.

36

CMOS SENSOR TYPES

Difference between types of CMOS sensors is generally due to the number of

transistors (affecting fill factor) that are present for each pixel. A portion of the pixel

sensor that is actually sensitive to light is called fill factor.

a) Rolling Shutter type

This has got limited number of transistors and therefore has a high fill factor.

However, lines of pixels are exposed at different times and therefore, movement in

the target gives a distorted image.

b) Global Shutter type

The number of transistors is high in this case resulting in a low fill factor. But, all the

pixels are exposed at a time and thus the movement artifacts associated with rolling

shutter type sensors are removed.

CCD AND CMOS SENSORS: PROS AND CONS

1. Fabrication Process

CCD sensors use specialized fabrication that uses dedicated and costly manufacturing

processes, whereas CMOS sensors rely on standard CMOS technology (used for IC

fabrication like microprocessors, memory, etc.). As CMOS sensors can also integrate

required electronics on the same chip, CMOS sensors results in compact and cost

effective system

2. Dynamic Range

Dynamic range of CCD is roughly twice as that of CMOS sensor. This implies that if

better colour depth is required, CCDs are likely to offer better results. On the other

hand, CMOS are marginally more photosensitive.

37

3. Power Consumption

CMOS cameras have lower power consumption than CCDs but other CMOS circuitry

may require more power. Low end CMOS sensors have low power requirements, but

high speed CMOS cameras typically require more power than CCDs.

4. Noise

Two types of noise affect sensors’ performance: Temporal Noise and Fixed pattern

noise. Fixed pattern Noise is more in CMOS, compared to CCDs because charge is

converted to voltage at each pixel as compared to single point charge-voltage

conversion in CCDs. In terms of temporal noise, CMOS sensors are better as the

bandwidth of amplifiers at each pixel is lower than the output amplifier in case of

CCD.

5. Image Quality

Due to poor fill factor of CMOS, photosensitivity of CMOS sensors is poor in low

light conditions.

6. Uniformity of response

CCDs use single amplifier for all pixels and CMOS use separate amplifiers for each

pixel. Pixel-to-pixel amplification differences lead to non-uniformity. Response of

CCDs is pretty uniform.

7. Speed

CMOS sensors have higher speed due to the fact that it uses active pixels and ADCs

on same chip leading to lesser propagation delays.

8. Readout area

CMOS sensors allow any region or even multiple regions to be read off the sensor.

CCDs are limited by vertical scan read out

9. Smart functions

With the integration of signal processing circuitry on the CMOS sensor chip,

functions like auto gain control, auto exposure control etc., anti-jitter, image

compression, color encoding, motion tracking, etc. can be incorporated on-chip.

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10. Overexposure effect

Overexposure can cause smearing around over-exposed pixels. Smearing is caused by

spilling of charge into the shift register. Due to absence of shift registers in CMOS

sensors, they are immune to this effect.

A Cmos Image Sensor :

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Film vs. Digital: A Comparison of the Advantages and

Disadvantages

In a world in which photographs are primarily taken with digital image sensors, there

are a growing number of photographers who are newly interested in film formats of

the past. But why would anyone in our age of technological convenience still choose

to shoot with analog film?

To understand the advantages and disadvantages of each shooting practice, we are

comparing the different aspects of each’s image quality, along with the cost of usage.

If you have been thinking of tinkering with film photography, you have landed in the

right place.

Resolution

When it comes to both digital and analog formats, photographers want to

know that their efforts will result in sharp, high-resolution photographs. With digital

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image sensors, we determine resolution by counting the number of pixels within a

given area. Film does not have pixels, and thus an analysis of a film’s resolving power

is calculated through angular resolution. Both methods of measurement can be

correlated with each other and thus compared for equivalent resolution.

Just as different sensors produce different resolutions, different types of film

will also produce different resolutions. Roger N. Clark’s analysis of standard 35mm

film showcased that depending on the type of film used, the resolution fell between 4

and 16 million pixels. For example, Clark’s study noted that Fujifilm’s Provia 100

film produced a resolution around 7 MP while Fujifilm’s Velvia 50 produced a

resolution around 16 MP. Considering that entry cameras such as Nikon’s D3330

produce around 24 MP, 35mm film doesn’t have much of an advantage in this

scenario.

That being said, many professional photographers who shoot film opt to do so with

medium or large formats. According to research carried out by a team of four industry

experts, it was found that medium format film has a potential to capture a jaw-

dropping 400 MP photograph, however, after digital scanning, resulted in a resolution

of 50 to 80 MP. Another test, also conducted by Roger N. Clark, noted that larger

formats such as 4×5 inches can capture 200 MP equivalent photographs after being

scanned.

In short, that 35mm film camera that you picked up from the flea market may

not be able to outperform the latest digital cameras, but a medium format or large

format unit can deliver and exceed the same resolution of Phase One’s latest $40,000

camera system.

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Digital Noise / Film Grain :

The random appearance of small textures within a photograph may be referred

to as digital noise or film grain. With analog film, grain is the result of small chemical

particles that have not received enough light. Within digital image sensors, noise is

the result of unwanted signals created by the camera’s digital circuitry; this can be due

to excess heat or a sensor’s ability to handle unruly signals in the airwaves.

Increasing the ISO of a digital camera or selecting high-speed film will make

your photographs more susceptible to noise and grain. In most situations, noise is

unwanted in color photos; however, with black and white images, some artists view

the grain as adding character, and thus not as a negative point.

Testing by Magnetic Recording Technology Expert, Norman

Koren, showcased that digital photography has evolved to the point at which it has far

less noise than the equivalent available film speed. Of course, digital noise depends

on the sensor within a digital camera, so older units may not be as efficient.

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One last item to consider with noise/grain is that film may be a better medium

for capturing long exposure photographs. Image sensors must be operated at low

temperatures to avoid thermal noise, a process that can become difficult with

prolonged usage of the imaging circuitry. Film, on the other hand, does not have any

issues with overheating.

Dynamic Range

Once the almighty reason to shoot with analog film over digital, dynamic

range is no longer the huge debate it once was in the past. While the dynamic range of

an Image is a complex process that takes into account the sensor used, the type of file

compression, and other factors, digital is ultimately winning against analog film.

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A release by Kodak showcased that most film has around 13 stops of dynamic

range. Today’s modern digital cameras all average around 14 stops of dynamic range,

with high-end units such as the Nikon D810 reaching almost 15 stops. Film

continuous to deliver incredible dynamic range, but today’s digital technology can

easy match it.

Independent testing of dynamic range on film cameras, such as the tests conducted by

Roger N. Clark, showed that high-end digital cameras in 2005 began to show “huge

dynamic range compared to [scans of] either print or slide film”. Films used in the

testing included Kodak Gold 200 and Fujifilm FujiChrome Velvia.

In additional, many digital cameras take advantage of sequential shots and

HDR capabilities to create photographs with exceptional high dynamic range beyond

what is capable with film.

Film Speed

When it comes to shooting in low light conditions, digital image sensors easily

take the cake. Film can usually be found available in speeds between 100 and 3200,

although 6400 film does exist. Today’s digital camera systems can match the noise

produced by analog cameras in these ranges, as well as push their sensitivity many

stops higher. Consumer digital cameras such as Fujifilm’s X100T can simulate

sensitivities as high as ISO 51200 while professional Nikon systems, such as the D4s,

can shoot as high as ISO

409,600.

Digital cameras also

have the advantage of being

able to change film speeds

between individual

photographs. For most

common roll films used

today (135, 120, etc.), the

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ISO is kept constant across the entire roll. The exception is with large format cameras

that use one sheet at a time, and thus can be switched between shots.

Analog film can be pushed or pulled multiple stops when needed, but the

amount of contrast within the image is affected. Some photographers use this to their

advantage to create the ideal look they desire, but this method still does not allow

extremely high ISO speeds without impacting image tones.

Cost and Convenience

When it comes to cost and convenience, both digital and analog formats have

their advantages and disadvantages. Noting the number of photographs you take

within a given time, the urgency of needing an image available, and the type of

subjects you shoot, will help you choose between the two options.

Digital has a much more expensive up front cost and evolving technology

means you will most likely want to upgrade your equipment within a few years. For

those who demand instant access to their photographs, there is nothing faster and

more convenient than digital. When shooting high-speed action photography, there is

also no concern about running out of film; large memory cards can easilystore

hundreds or thousands of high-resolution photographs.

Analog is much more affordable up front, and you will most likely be able to

use your film body for decades to come, as genuine enhancements are to the film

itself. That being said, analog shooters will be spending a lot more money on film

rolls and development costs. There is the need to conserve film more carefully as

nothing can be just deleted as with digital and photos are not availably instantly. Most

available processing labs take at least 24 hours, if not a few days, to complete the

process. Sadly, one hour photography stores are a dying breed.

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Let’s say that you want a modern digital camera with resolution, dynamic

range, and grain equivalent to ISO 100 film. You may choose to pick up a Nikon

D3300 – an entry camera that checks off all the boxes. The initial purchase may cost

$500, but with a cheap memory card ($30) you can shoot unlimited photographs and

delete what you don’t need. You may then opt to upgrade your camera within a five-

year span for another $500.

If you were to pick up a decent film camera for $150 and then shoot 100 photographs

a month for a year, your total film costs would be around $260 (using Kodak Ektar

100 Pro) and your development costs would be around $370. Over a five-year span,

you may not want to upgrade your camera, but total development costs and film

would still amount around $3,200.

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Conclusion:

The digital revolution has caught up to film in many regards, killing many of

the arguments for film being better than its technological counterpart. However, the

most notable reason to shoot analog may be the resolution obtained from medium

format cameras. Not all explanations can be laid within technical comparisons though.

Many will argue that shooting analog is a more personal and enjoyable experience –

that decision, is completely up to you.

The inclusion of cameras in everything from cell phones to pens to children’s’

toys is possible because of the low cost and low power consumption of the imaging

arrays that form the core of the cameras. However, these arrays are low cost and low

power because they are CMOS-based; this allows for the devices to be made with the

same processes and facilities that are used to make memory and computer chips. Yet,

the continued surge in CMOS imager popularity goes beyond the lower cost to other

factors such as ability to integrate the sensors with electronics, and the ability to

achieve fast, customizable frame rates.

People have been using camera and film for more than 100 years, both for still

photography and movies. There is something magical about the process -- humans are

visual creatures, and a picture really does paint a thousand words for us!

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References :

1.Tom Harris-How Camera Works http://electronics.howstuffworks.com/camera.htm

2.Redditt Photo Class -http://www.r-photoclass.com/

3.Image Acquisition- Springer

4. -

http://electronics.howstuffworks.com/cameras-photography/digital/digital-camera.htm

5.Image Sensors-http://www.engineersgarage.com/articles/what-is-cmos-sensor

6.CCDvsCMOS-http://electronics.howstuffworks.com/cameras-

photography/digital/question362.htm/printable

7.Film vs Digital-http://petapixel.com/2015/05/26/film-vs-digital-a-comparison-of-

the-advantages-and-disadvantages/

8.E.R Fossum,"CMOS Image Sensors: Electronic Camera on a Chip,"IEDM,pp.1.3.1-

1.3.9,Dec.1995.

9.B. Ackland and A. Dickinson, “Camera on a Chip”, in ISSCC Dig.,Feb. 1996, pp.

22-25.

10.M. A. Schuster and G. Strull, "A monolithic mosaic of photon sensors for solid

state

imaging applications," in Electron Devices Meeting, 1965 International, 1965, pp. 20-

21.

11.R. Melen, "The tradeoffs in monolithic image sensors; MOS vs CCD," Electronics,

vol. 46,pp. 106-11, 1973.