new technical basis for optical experimentation part #4huhui/teaching/2014_summer/aere545/... ·...

Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!

HHui Huui HuDepartment of Aerospace Engineering, Iowa State University Department of Aerospace Engineering, Iowa State University

Ames, Iowa 50011, U.S.AAmes, Iowa 50011, U.S.A

Technical Basis for optical experimentationTechnical Basis for optical experimentationPart #4Part #4

AerEAerE 545 class notes #11545 class notes #11


Light sensing and recordingLight sensing and recording


LensesLenses

•• Focus length: Focus length: •• Depth of focus:Depth of focus:•• ff--numbers or focal ratio : is defined as the ratio of focal numbers or focal ratio : is defined as the ratio of focal

distance of the lens and its clear aperture diameter.distance of the lens and its clear aperture diameter.


Photo detectorPhoto detector

•• Photo detector is a device to convert light to an electric Photo detector is a device to convert light to an electric current through photo electric effect.current through photo electric effect.

•• Quantum efficiency: Quantum efficiency:

•• Noise: Noise: –– Shot noise: due to random fluctuation of the rate Shot noise: due to random fluctuation of the rate

of photon collection and back ground illuminationof photon collection and back ground illumination

–– Thermal noise: caused by amplification of current Thermal noise: caused by amplification of current inside the photo detector and by external inside the photo detector and by external amplifier.amplifier.

•• Dark current: the current produced by the photo Dark current: the current produced by the photo detector even in the absence of a desirable light detector even in the absence of a desirable light source.source.

•• Two kinds of photo detectors: Two kinds of photo detectors: –– Photomultiplier tubes (PMT) Photomultiplier tubes (PMT) –– photodiodes (PD) or photo electric cellsphotodiodes (PD) or photo electric cells

electrons emitted ofNumber :photons absorbed ofNumber :

p

e

p

eq

NN

NN

=η



•• Photomultiplier tubes (PMT) Photomultiplier tubes (PMT) –– Photocathode: absorbs photons Photocathode: absorbs photons

and emits electrons.and emits electrons.–– Dynodes: increase number of Dynodes: increase number of

photonsphotons–– Anodes: outputAnodes: output



•• photodiodes (PD) or photo electric cellsphotodiodes (PD) or photo electric cells–– PP--n junctions of semiconductors, n junctions of semiconductors,

commonly siliconcommonly silicon--silicon type.silicon type.–– High quantum efficiency High quantum efficiency –– But not internal amplification But not internal amplification


Linearity and Dynamic Range of a Digital CameraLinearity and Dynamic Range of a Digital Camera

•• Linearity:Linearity:–– Intensified CCD cameras usually need to Intensified CCD cameras usually need to

check its linearitycheck its linearity

•• Dynamic Range:Dynamic Range:–– The ratio between the fullThe ratio between the full--well capacity well capacity

and the dark current noise.and the dark current noise.–– For example, for a 8For example, for a 8--bit CCD camera, bit CCD camera,

maximum intensity is 2maximum intensity is 288=256, dark =256, dark current noise is about 25, then Dynamic current noise is about 25, then Dynamic range is about 10.range is about 10.

–– Available bits number:Available bits number:•• 8 bit, 16 bit, 24 bit8 bit, 16 bit, 24 bit

0

500

1000

1500

2000

2500

3000

3500

4000

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

y = a+bx+cx2+dx3

a=1.26, b=1.05, c=-5.36E-5, d=-1.67E-9Gain level 100%Gain level 90%Gain level 80%Gain level 70%y=x

single frame modeexposure time 300usf=2.8

input intensity

outp

ut in

tens

ity

Input Photon Input Photon


Interlaced CamerasInterlaced Cameras

• The fastest response time of human being for images is about ~ 15Hz• Video format:

– PAL (Phase Alternating Line ) format with frame rate of f=25Hz (sometimes in 50Hz). Used by U.K., Germany, Spain, Portugal, Italy, China, India, most of Africa, and the Middle East

– NTSC format: established by National Television Standards Committee (NTSC) with frame rate of f=30Hz. Used by U.S., Canada, Mexico, some parts of Central and South America, Japan, Taiwan, and Korea.

Even fieldEven field(2,4,6(2,4,6……640)640)

Old fieldOld field(1,3,5(1,3,5……639)639)

Even field

Odd field

16.6ms16.6ms

1 frameF=30Hz

480 pixels by 640 pixels480 pixels by 640 pixels

Interlaced cameraInterlaced camera

timetime


Progressive scan cameraProgressive scan camera

•• All image systems produce a clear image of the background All image systems produce a clear image of the background •• Jagged edges from motion with interlaced scan Jagged edges from motion with interlaced scan •• Motion blur caused by the lack of resolution in the 2CIF sample Motion blur caused by the lack of resolution in the 2CIF sample •• Only progressive scan makes it possible to identify the driverOnly progressive scan makes it possible to identify the driver


Mystery of flying rodsMystery of flying rods


Digital cameraDigital camera

•• CCD camera and Intensified CCD (ICCD) camera:CCD camera and Intensified CCD (ICCD) camera:

–– Spatial resolution: 1K by 1k, 4K by 4KSpatial resolution: 1K by 1k, 4K by 4K

–– Frame rate : 30 Hz, High speed camera 1khz ~10K Frame rate : 30 Hz, High speed camera 1khz ~10K hzhz–– In a CCD sensor, every pixel's charge is transferred through a vIn a CCD sensor, every pixel's charge is transferred through a very limited number ery limited number

of output nodes (often just one) to be converted to voltage, bufof output nodes (often just one) to be converted to voltage, buffered, and sent offfered, and sent off--chip as an analog signal. All of the pixel can be devoted to ligchip as an analog signal. All of the pixel can be devoted to light capture, and the ht capture, and the output's uniformity (a key factor in image quality) is high. output's uniformity (a key factor in image quality) is high.

•• CMOS (Complementary metalCMOS (Complementary metal--oxide semiconductor) camerasoxide semiconductor) cameras

–– In a CMOS sensor, each pixel has its own chargeIn a CMOS sensor, each pixel has its own charge--toto--voltage conversion, and the voltage conversion, and the sensor often also includes amplifiers, noisesensor often also includes amplifiers, noise--correction, and digitization circuits, so correction, and digitization circuits, so that the chip outputs digital bitsthat the chip outputs digital bits

–– These other functions increase the design complexity and reduce These other functions increase the design complexity and reduce the area the area available for light capture. With each pixel doing its own conveavailable for light capture. With each pixel doing its own conversion, uniformity is rsion, uniformity is lower. But the chip can be built to require less offlower. But the chip can be built to require less off--chip circuitry for basic operationchip circuitry for basic operation

•• In summary for CMOS camerasIn summary for CMOS cameras

–– Low costLow cost–– Operation versatilityOperation versatility

–– High speedHigh speed

–– Quality is not as high as CCD camerasQuality is not as high as CCD cameras

new technical basis for optical experimentation part #4huhui/teaching/2014_summer/aere545/... ·...

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