what is light?

What is Light?

One of the greatest scientific mysteries of our time

Light acts both like a wave and a particle!

Dr Darren Reynolds

Faculty of Applied Sciences

In 1666 Newton discovered that white light is made up of all colours

Dr Darren Reynolds


What is Light?

Dr Darren Reynolds


What is Light?

About the same time a Dutch physicist and astronomer suggested that light consisted of waves.

Christiaan Huygens

(1629-1695)Dr Darren Reynolds


What is Light?

Maxwell’s Electromagnetic Theory broad spectrum of radiation ranging from cosmic rays (10 -14 nm) to radio waves (100 to 106 nm).

James Clerk Maxwell



What is Light?

According to Maxwell:

Light is an electromagnetic field which is characterised by both frequency,, and wavelength,

= c

Dr Darren Reynolds


What is Light?

The unit of frequency is Hertz (Hz) but its dimensions are S-1

The frequency is a fundamental characteristic

Dr Darren Reynolds


What is Light?

1900 Max Planck discovered E= hEnergy

Planck’s constant

Frequency

Max Planck



What is Light?

1905 Einstein used Planck’s work to propose that light is quantized

Albert Einstein

(1879-1955)

A ‘quantum’ is the amount of energy contained in a single photon.

Dr Darren Reynolds


What is Light?

400 500 600 700

Wavelength (nm)

Dr Darren Reynolds


What is Light?

What is Spectroscopy?

The analysis of the electromagnetic radiation emitted, absorbed or scattered by molecules and atoms

Dr Darren Reynolds


Interaction of Light with atoms and molecules

Three fundamental processes occur.

– Scattering

– Absorption

– Emission

Dr Darren Reynolds


Scattering

Why is the sky blue?

Dr Darren Reynolds


Scattering

Red Sunsets

Dr Darren Reynolds


Absorption

Why is grass green………..?

Dr Darren Reynolds


Absorption & Emission

Dr Darren Reynolds


Line and Band Spectra

Dr Darren Reynolds


Dr Darren Reynolds


Line and Band Spectra

Colour PerceptionColour PerceptionColour PerceptionColour PerceptionColour Perception

COLOUR PERCEPTION

proximal image

distal image

the eye- basics:

the average person can distinguish between

130 and 200 separate colours

there are only a few different types of photoreceptors

colour must be perceived through coding combinations among a few basic types of receptors


COLOUR PERCEPTION

the eye- basics:

the fact that you see colour means that the photoreceptors in the retina of the eye are able in some way to differentiate among the various wave frequencies of light

you do not see colour at night (rod vision) but you do see colour in daylight (cone vision)

- colour perception must occur in the cone cells


COLOUR PERCEPTION

the eye- basics:

the eye- basics:


COLOUR PERCEPTION

“yellow spot”

cross-sectional view

zoom (next slide)


COLOUR PERCEPTION

cross-section of the retina

the eye- basics:

the eye- basics:


COLOUR PERCEPTION

most mammalian species are dichromatic - only have middle (green) and short (blue) wavelength sensitive cones

conerods

primates (inc. humans) are trichromatic

birds, reptiles and fish are pentachromatic

humans have L-cones (red, maximally sensitive at 558 nm), M-cones (green, 531 nm) and S-cones (blue, 420 nm)

the eye- basics:


COLOUR PERCEPTION

in the fovea there are no rods, only cones

conerods

THERE ARE CONES IN THE REST OF THE RETINA, HOWEVER, ALONG WITH THE RODS

BUT

the distribution of L-, M- and S-cones is not homogeneous or the same as each other throughout the retina outside the fovea

the eye- basics:


COLOUR PERCEPTION

convey ability to see at night under conditions of very dim illumination

conerods

RODS are capable of being stimulated by a single photon

RODS

this very fine sensitivity is bought at a price:their response to light stimulation is much slower than that of a cone - signals may arrive as much as 1/10th second later that a simultaneous stimulation of a cone.

the eye- basics:


COLOUR PERCEPTION

in the human eye S-cones can be differentiated morphologically from L- and M-cones (but not L- from M-)

conerods

S-cones (blue) are at their LOWEST density in the middle of the fovea (5% of cones) and reach their max (15%) in the rest of the fovea. In the rest of retina they form only 8% of the cone population.

DISTRIBUTION OF CONES

L-cones (red) apparently constitute 33% of cones in the retina, M-cones (green) about 59% but in the fovea L- and M-cones constitute about 48% each

the eye- basics:


COLOUR PERCEPTIONconerods

• We have fewer S-cones (blue) than L- (red) and M-cones (green)• We can see colour in great detail at the fovea - but red and green predominantly• But we can still see colour in all other parts of the retina (except the blind spot)• We cannot see anything at the fovea in very dim light as there are no rods• When concentrating on intense patches of colour, blue receptors are likely to become exhausted before red and green

seeing colours

the eye -colour deficiencies:


COLOUR PERCEPTION

protanopiadeuteranopia

“normal” vision

Adding unit to form luminance

signal

Colour encoder to form

chrominance signal

Audio signal

Sync pulse

Composite signal


COLOUR PERCEPTION

lens

reflecting mirror

dichroic mirror

vidicon tube

colour production:

colour-television camera :

Light reflected from the scene being televised is focused by lenses and split by means of chroic (colour-separating) mirrors into three separate images, one in each of the three primary colours - blue green and red. Each beam of coloured light is then directed into one of three identical vidicon tubes. The pattern of light falling on a photoconductive layer within each tube causes a varying pattern of electrical resistance; as an electron beam scans the photoconductive area from behind, a varying electric current is induced in a circuit connected to the conductive layer. The pattern of dark and light in each primary-colour image is thus converted into one of three varying electrical signals. A black-and-white luminance (brightness) signal is created in the adding unit, by combining information from each of the three colour signals. A the same time, the colour encoder produces a single chrominance signal, which defines the hue and saturation of each primary colour. the luminance and chrominance signals are combined into a composite video signal. Prior to transmission, the audio signal is incorporated, together with a synchronisation pulse (‘sync pulse’), which ensures that the electron scanning in the receiving system matches that of the transmitting system.


COLOUR PERCEPTION

colour production:

colour-TV transmission:

Colour separator

Audio signal

frame pulse

Composite signalreceiverloudspeaker

Sync-pulse separator

line pulse

deflector coils

electron gunsluminance signal

chrominance signal

electron beams

TV screen


COLOUR PERCEPTION

colour production:

colour-television receiver:

The composite signal picked up by the receiving aerial is decoded, separating out the various constituent signals. The luminance signal controls the overall output of three electron guns in the cathode-ray tube of the receiving set, so determining the balance of light and shade in the final picture. The chrominance signal - now split into the three primary-colour signals - regulates the relative strength of each electron beam. The sync pulse, divided into line and frame components, controls the deflection of the beam across and down the screen.The television screen is coated with stripes of different phosphors, which glow red, blue or green when struck by electrons. Immediately behind the screen is a grille, or shadow mask, which contains many perforations. Travelling at slightly different angles as they pass through the perforations, the electron beams are caused to diverge before striking the screen, in such a way that the electrons from each gun can only reach phosphor stripes of the appropriate colour. Each image on the screen thus consists of stripes of varying brightness and colour that merge together to form the complete picture.


COLOUR PERCEPTION

colour production:

colour-TV reception:

No matter how multicoloured prints of slides may appear, they are made of only the three secondary colours arranged in layers. When you look at a photograph, light passes through the layers and combines to give full colour. Developing a print film produces a colour negative, while in a slide, a process called colour reversal forms a positive colour image on the film.

In a colour film each of three layers is similar to a black-and-white film, except the top layer is sensitive only to blue light, the middle layer to green and the bottom layer to red. The three layers detect the amounts of these colours in the image formed on the colour film by the camera lens.


COLOUR PERCEPTION

colour production:

colour-photography:

Film developer chemicals for colour films contain dye couplers, which attach dyes to the silver that forms in the emulsion during development. The silver is then dissolved, leaving a layer of dye. The top layer becomes yellow, the middle layer magenta and the bottom layer cyan.

It is then necessary to colour print the negative: if the middle and bottom layers have been exposed they will emit magenta and cyan light when the exposure light is shone. A dye emitting magenta light is absorbing green and emitting red and blue light . Cyan means red is absorbed and green and blue emitted. Therefore, if both these layers and emitting at the same time red and green will be absorbed by adjacent layers leaving only blue light, which is the colour printed.


COLOUR PERCEPTION

colour production:

colour-photography:

Colour has 3 attributes:• brightness• hue• saturation

roughly, the intensity of light

red, green, blue etc

amount of white a colourappears to contain

Black, greys and white are colours with zero saturation and no hue

achromatic colours


COLOUR PERCEPTION

colour:how to describe it:

YELLOWBLUE

RED

YELLOW-GREEN

GREEN

BLUE-GREEN

PURPLE

VIOLET

GREY

ORANGE

SATURATION

The colour circle - the psychological experience of hues

complementary colours on opposite sides of the circle saturation is

represented as distance from the centre


COLOUR PERCEPTION

colour:how to describe it:

Metameric pairs:

colours which appear identical to the eye but are produced by different wavelength compositions

such pairs demonstrate that the perceptual system cannot discriminate the component wavelengths that comprise a colourWHITE LIGHT

is made up of all the visible spectrum frequencies and yet no individual colours are distinguishable by the eye


COLOUR PERCEPTION

Colour mixing:

Colour mixing:

colours that look alike behave alike in mixing(the electromagnetic radiation composition doesn’t need to be considered)

Projecting 2 coloured lights adds them: blue + yellow lights will give grey

Using 2 coloured filters subtracts them: a blue filter will transmit blue and some green whereas a yellow filter transmits yellow and some green but will absorb any blue - together, one filter in front of the other, will give green (black if the filters were truly monochromatic)


COLOUR PERCEPTION

Psychology - perceiving

Painting - pigments

blue + yellow = grey [complementary]

blue + yellow = green


COLOUR PERCEPTION

colour perception:

Schopenhauer:

“All intuitive perception is intellectual…[without this] we would stop short at the mere sensation that might possibly have meaning in reference to the will as pain or comfort; but for the rest it would be a succession of states devoid of meaning, and nothing like knowledge. Intuitive perception, that is, knowledge of an object, first comes about through the understanding that refers every impression received by the body to its cause.”

the opening sentences of “Über das Sehn und die Farben: Eine Abhandlung (On Vision and Colours: An essay), pub. 1816


COLOUR PERCEPTION

colour perception:

this shouldn’t happen!

most objects are visible because of reflected light: they have no characteristic energy of their own and, therefore, strictly speaking, no colour of their own - they are always seen under variable conditions of illumination


COLOUR PERCEPTION

colour perception:

colour constancy:

However, under normal circumstances familiar objects are perceived with very little change in hue or saturation

colour constancy prevails

a breakdown in constancy can be induced by withholding information or providing false information

BUT we are very good at maintaining constancy, probably because outdoors illumination is so variable: changes from summer to winter, clear or cloudy days, noon or twilight

WE add stability to a changing environment

leaf & donkey sihouettes in same green material on white background illuminated in red light - use a colour disk to match each silhouette


COLOUR PERCEPTION

colour perception:

colour constancy:

colour constancy prevails

colour constancy is NOT divorced from other constancy mechanisms in visual perception - particularly size constancy:

WE add stability to a changing environment


COLOUR PERCEPTION

colour perception:

colour constancy:

proximal imagedistal image

eye

lenspupil image at

time 1same imageat time 2

1

2


COLOUR PERCEPTION

colour perception:

size constancy:

Objective colour is a misleading term - in colorimetry the observer is not acting like a well-calibrated machine

Of prime importance to colour perception is our past experience, both with familiar objects and with the effects of illumination


COLOUR PERCEPTION

colour perception:

objective-subjective colours:

Objective colour ...

Secondarily, the effect of alternating achromatic (black, grey or white) light is important

• flickering of a stationary light off and on• looking at a light through a rotating disk with sectors removed (like looking through a rotating fan) or• spinning a disk which is painted in an achromatic pattern

can be produced by:


COLOUR PERCEPTION

colour perception:


Secondarily, the effect of alternating achromatic light...

if the alternation is performed below the critical flicker frequency (where the stimulus is fused into a solid expanse of grey) hues are perceived

colours, often desaturated (washed out), can even be perceived in a close pattern of linesit is assumed that

small eye movements constantly alter the light reaching each colour receptor

Subjective colour :


COLOUR PERCEPTION

colour perception:


Benham’s top


COLOUR PERCEPTION

colour perception:

subjective colour:

it is presumed that all varieties of colour receptors in the eye are excited concurrently by the many wavelengths presented. When the alternation rate is appropriate one type of receptor may fire slightly in advance of another, allowing the hue associated with that type of receptor to be perceived

BENHAM'S TOP if rotated at slow speed will produce colours

the thin black curved lines appear as desaturated hues

even if observed in other than white light

the variation in persistence of receptor firing may also be a factor


COLOUR PERCEPTION

colour perception:

subjective colour:

Another form is based upon the effects of light which has preceded a stimulus in time

or light which surrounds a colour

TEMPORAL EFFECTS

AREAL EFFECTS


COLOUR PERCEPTION

colour perception:

subjective colour:

moving the surface further away seems to increase the size of the image

TEMPORAL EFFECTS

positive afterimages can appear projected upon a surface in the field of view when exposed to a brief intense flash of light

afterimages can exist in the absence of light and can be re-elicited after it has disappeared by altering the room illumination

they also seem to change hue over time - "the flight of colours"

recovery times for retinal receptors are thought to be the cause of the afterimage


COLOUR PERCEPTION

colour perception:

subjective colour:

are formed by fixating on a particular coloured object

negative afterimages

switching attention to another, even grey surface produces a negative afterimage

fixating on a red object will later result in a blue-green hue (complementary) and projected afterimage may mix with any other observed surface colour.

for example:


COLOUR PERCEPTION

TEMPORAL EFFECTS

colour perception:

subjective colour:

as the eye moves about it adapts to the average illumination

for example: a room was set up with grey walls only illuminated with intense red light

after 15 mins subjects were given a series of grey papers previously not seen

in the red illumination the papers could only reflect red light so it was predicted, from colorimetry, that all the samples should appear red

greygrey


COLOUR PERCEPTION

AREAL EFFECTS

colour perception:

subjective colour:

AREAL EFFECTS

CHROMATIC ADAPTATION

all grey papers lighter than the wall colour were perceived as red

papers the same as the walls were perceived as grey

the darkers ones as blue-green - the complementary colour to red

HOWEVER:


COLOUR PERCEPTION

colour perception:

subjective colour:

even though there were no blue-green wavelengths of light energy in the room

AREAL EFFECTS

CHROMATIC ADAPTATION


COLOUR PERCEPTION

colour perception:

subjective colour:

AREAL EFFECTS

grey curtains in a room painted yellow will take on a blue cast or tinge

Colours can be affected by surrounding hues

in order to appear grey they need to be tinged with yellow, the complementary colour of blue - simultaneous colour contrast is not time-dependent


COLOUR PERCEPTION

colour perception:

subjective colour:

A camera takes two black and white photos of a scene, one through a red filter and the other through a green filter. Two positive transparencies (OHPs) are produced, still in black and white

E.H. LAND

A striking example of subjective colours was popularised by E.H. Land (1959: Experiments in color vision. Scientific American 200:84-99).

the two OHPs are superimposed on a screen via two separate overhead projectors, one of which has a red filter for the red filter transparency and a corresponding green filter

A single image is thus seen on the screen


COLOUR PERCEPTION

colour perception:

subjective colour:

Then the green filter is removed

E.H. LAND

the scene is then seen in "full colour”

even though only red and white light is illuminating the screen which should produce shades of red from a highly saturated red through pink to an unsaturated white


COLOUR PERCEPTION

colour perception:

subjective colour:

E.H. LAND

green, yellow, blue - in fact, "full colour" - is reported

Nonetheless

the colours match the scene originally photographed well

especially if the photograph is of

familiar objectsrather than something academic - like a colour wheel or colour card


COLOUR PERCEPTION

colour perception:

subjective colour:

E.H. LANDREAL LIFE:

A whole range of subjective colour contrasts must be operating

simultaneous colour contrast from familiar "red" objects illuminated by red light to give blue-greens of other objects, chromatic adaptation working from "recognition" of reds and blue-greens.

such as:


COLOUR PERCEPTION

colour perception:

subjective colour:


COLOUR PERCEPTION

cross-section of the retina

the eye- basics:


COLOUR PERCEPTION

the eye- complexities:

However, rods and cones crosstalk

if the proximal image (on the retina) was “recording” the distal image (out there), it would be expected that nerve impulses would be dispatched to the brain as a series of independent cone and rod messages


COLOUR PERCEPTION


a rod may have as many as 5 junctions with cones and a cone might contact as many at 10 neighbouring rods. Cones also contact other cones, but interestingly, S-cones are relatively isolated in this respect.

i.e. there are less blue cones and they don’t seem to talk so much to other receptors


COLOUR PERCEPTION


1. Why do receptors crosstalk at all?

2. Given that crosstalk happens, how does the brain interpret images?

3. What do we “see” if there is no crosstalking?

questions are:

In the extreme, a Ganzfeld, a continuous, uniform, chromatic source fails as a stimulus; it was no longer perceived i.e. the mere presence of light is not sufficient for seeing but a stimulus change is necessary for a visual experience. The Ganzfeld creates a formless, depthless, unchanging visual experience and in many instances observers have reported bizarre hallucinations while under Ganzfeld conditions.

GANZFELD


COLOUR PERCEPTION

no colour perception:

no crosstalk?

what is light?

Documents

white light

retinathe eye basics

electromagnetic radiation

unit of frequency

electromagnetic field

sky blue

middle green

radio waves