digital: converting light to data
TRANSCRIPT
SensorsSimplified
• Cameras’Sensors(andFilm)whereRaysofLightbecomePixels
ObjecBves
• PhotographicProcessesforDigitalCapture• TheFiveSimplifiedlayersofaCCD• TwobenefitsofusingtheCameraRaw
Reading:hHps://en.wikipedia.org/wiki/Charge-coupled_device
Digital:ConverBngLighttoData• CCD:Charge-CoupledDevice,adeviceforconverBngelectricalcharge,intoadigitalvalue
• Pixelsarerepresentedbycapacitors,whichconvertandstore(accumulates)incomingphotonsaselectroncharges
• WillardBoyleandGeorgeE.Smith,1969(WonaNoblePrizeinPhysicsin2009).
© 2015 Irfan Essa, Georgia Tech, All Rights Reserved
Digital: Converting Light to Data
CCD: Charge-Coupled Device, a device for converting electrical charge, into a digital value
Pixels are represented by capacitors, which convert and store incoming photons as electron charges
Willard Boyle and George E. Smith, 1969 (Won a Noble Prize in Physics in 2009). Smith and Boyle
commons.wikimedia.org
© 2015 Irfan Essa, Georgia Tech, All Rights Reserved
Digital: Converting Light to Data
CCD: Charge-Coupled Device, a device for converting electrical charge, into a digital value
Pixels are represented by capacitors, which convert and store incoming photons as electron charges
Willard Boyle and George E. Smith, 1969 (Won a Noble Prize in Physics in 2009). Smith and Boyle
commons.wikimedia.org
hHps://en.wikipedia.org/wiki/Charge-coupled_device
• Microlenses:CaptureLightanddirectittothelightsensiBveareasofthesensor.– AddedLenslayer
• HotMirror:SimpleFiltering.Letsvisiblelightpass,butreflectslightsintheinvisiblepartofthespectrum(dependsonkindoflighttocapture–e.g.,infrared,ultraviolet-UVcamera).AlsoprovidesAnB-aliasing.
• ColorFilter:BayerArrayseparatelightintoRGB.
• PhotoDiodes:ColorBlind.ButmeasureintensiBes,andenergyconvertedtoelectrons
• Deple8onLayer:WhereElectronArecollected(partofphotodiodeslayer)
Digital:ConverBngLighttoData
• MicrolensArray:CaptureLightanddirectittothelightsensiBveareasofthesensor.– AddedLenslayer
Digital:ConverBngLighttoData
DualMicrolensDesignsShrinksPixelSizeSonyCCD
HotMirror
• CutsIRlight.• Hotfilter,thatFiltersoutinfraredlighttomiBgatesensorsthataresensiBvetotheinfrareds(750nm1250),andcontaminatedcolors.– E.g.,Fires
• ColdFilter–ColdMirrorfiltersIRintothepahtway(morecommoninfrontoflens.
• BayerFilteronasensor
• Pixel:– squareof4colorsquares.
• Letsonly1colorlightthrough.– 3PaHernsoneforeachcolor
– CombinaBongivesusthefinalimage
© 2015 Irfan Essa, Georgia Tech, All Rights Reserved
Digital: Converting Light to Data
Images: Wikipedia
Bayer filter on a sensorCombine
BayerFilter
ConvertLighttoData
© 2015 Irfan Essa, Georgia Tech, All Rights Reserved
Raw input in Bayer mosaic format RGB color planes
Digital: Converting Light to Data
?
ConvertBayertoRGB
• R=200• G=170,180• B=153
ConvertBayertoRGB
• R=200• G=AVG(170,180)=175
• B=153
BayerFilter
½Green
1/4Blue
1/4Red
WiththismethodsensoronlycapturesRawcolordatafor1/3oftheimage.UseDemosaicinordertoesBmatetheremaining2/3oftheimage.
Demosaic• CombineBayerFilter&Interpola8ontoesBmateothercolorsfromtheincompletesamplebyBayerFilterMechansism
• BayerFilter
hHps://en.wikipedia.org/wiki/Demosaicing
hHp://www.csee.wvu.edu/~xinl/papers/demosaicing_survey.pdf
OtherFilters
• Foveon:stacksphotodiodestocapturealllightatasinglelocaBon.
• FujifilmSuperCCD(moreneighbors)àClaimsmoredatatoworkwithforinterpolaBon
cross over small amounts of other colors beyond those in their chromic make-up.This allows crossover through the green filter meaning that energies from the red andblue spectral areas will affect the green sites on the sensor. The second reason forhaving more green filters than red or blue is that blue is the noisiest channel andrestricting its use produces a cleaner image.
Another way to capture the color is via the Foveon X3® chip. This sensor usesa natural attribute of silicon to separate the colors. Depending on wavelength, a colorwill penetrate to varying depths into silicon, similar to what occurs with color film.This factor, along with the stacking of three layers of sites, allows the sensor to capturethe light’s color at each stack of photoreceptors. While theoretically there should beno crossover, there is some absorption of colors as the different wavelengths travelthrough succeeding sensitivity layers.
Sensors are also manufactured that do not have color capture filters or do notacquire color. These cameras are designed to produce monochromatic, black and white images or are used with tricolor photography. Using a monochro-matic sensor to produce tricolor photography, filters are placed in the light path to
36 s e n s o r s
Figure 3-5 Arrangement of filters for
photocells for a matrix or Bayer array.
Figure 3-6 and Figure 3-7 The Foveon X3®
chip stacks photodiodes to capture all light
at a single location. The sensitive areas vary
as shown in the colorized patent drawing.
ch03.qxd 12/4/04 02:42 PM Page 36
hHp://www.vividlight.com/arBcles/2514.htm
CameraRAWFileFormat
• Containsminimallyprocesseddatafromthesensor
• Imageencodedinadevice-dependentcolorspace• CapturesradiometriccharacterisBcsofthescene• Viewableimagefromthecamera'ssensordata• LikeaphotographicnegaBve– Hasawiderdynamicrangeorcolor;preservesmostoftheinformaBonofthecapturedimage