The Canon EOS Digital Rebel XSi/450D Companion

CHAPTER 5

OVER AND UNDEREXPOSURE

90

FOOD FOR THOUGHT

WHERE COLOR COMES FROM

If you haven’t noticed already, the process I just described does not actually

describe anything about the color of an individual pixel. In other words, the only

thing the image sensor can determine from the photoelectric effect is how much

light struck an individual pixel. It can’t actually tell anything about the color of

the light that struck each pixel. A digital image sensor is inherently a luminance

or brightness sensor. In other words, it sees in black and white.

To achieve a color image, each pixel on the sensor is covered with a colored filter.

On the Rebel XSi, the filters are either red, green, or blue. As you may already

know, red, green, and blue are the primary colors of light. ose three colors can

be mixed together to form every other color.

e filters allow the image sensor to know how much of a particular color struck

each pixel at the time of exposure. Using complex algorithms, the camera is able

to analyze all of this information to interpolate the color of each individual pixel.

is interpolation process is called demosaicing, because the mosaic of primary

colors on the image sensor gets converted back into a full-color image.

If this sounds difficult and complicated, that’s because it is, and demosaicing al-

gorithms are some of the most important trade secrets in the camera industry. ...