The Generic Sensor

Each photosite converts lightwave energy into photo-electrons

Pixels in the output image are a measure of the number of photo-electrons recorded at each photo site

Light wellswells

SensorsSensors

Problem:Problem:

Since the sensor only records light intensity, Since the sensor only records light intensity, we can’t differentiate between colors!we can’t differentiate between colors!

Thus, digital cameras only record in black Thus, digital cameras only record in black and whiteand white

Solution:Solution:

Color filtering, the most common form being Color filtering, the most common form being the Bayer filterthe Bayer filter

1) Designates individual photosites to be either red, green, or blue (RGB)

2) Respective color filters are placed over each photosite

3) Thus, only the light energy corresponding to that color’s wavelength reaches the sensor

4) Thus the sensor can interpret the energy recorded at that photosite to be a measure of that certain color

The Bayer Sensor

Light wellswells

SensorsSensors

• Color offset- light not recorded at the same spot-Demosaic algorithms required to interpolate missing data

• Discards 2/3 of light information-3x longer shutter speed, 3x larger aperture, 3x higher ISO

DrawbackDrawback

s of…s of…

Actual Image

Raw Bayer Output

Demosiaced Bayer Output

Linear Scanning

Scans a scene with RGB linear sensors

Gets full light information

Slow, only works for static subjecs

Alternative Color Filtering Technologies

Bayer-like filters with different colors

RGBC

CYGM

Same problems as Bayer filter

Multi-shot approach

Take three shots, each with different color filter

Gets full light information

Only works for static subjects, since a moving scene would change between filter switches

Alternative Color Filtering Technologies in DevelopmentFoveon sensor

Uses vertically stacked RGB sensors

Collects all light at each location

Three-sensor

Commonly known as “3CCD”

Prism splits light, 3 sensors to record R,G,B

Color Temperature

Light sources can be approximated as “black bodies” of a certain temperature

Daylight: similar to 5000K black body (even light across the spectrum)Incandescent light: lower temperature (more red light, less blue)Shade/daylight: higher temperature (more blue light, less red)

Human vision system makes corrections for different light sourcesCameras aren’t so good at this

<- What we see

What cameras see ->(interpreting all wavelengths equally)

White Balancing

Auto-white balanceWhite balance presetColor temperatureCustom white balanceRAW adjustment

Last resort: color balance the JPEG