The photodiodes by themselves are color blind. They can measure
only the intensity of light, not its color. So next, the concentrated light
passes through a color filter matrix situated above the layer of photo-
diodes. There is a separate filter—either blue, green, or red—for
each of the pixels. The most common design for the matrix is a Bayer
array. It patterns the three colors so that no two filters of the same
color are next to each other and so there are twice as many green fil-
ters as either red or blue. The reason for so many green is
because green light is in the center of the visible spectrum
and records more detail, which is more important than
color information in creating an image. It also com-
pensates for the photodiodes being less sensi-
tive to green light.When light strikes the top
layer—the P-layer—of silicon,
the light’s energy is trans-
ferred to electrons, creating a
negative charge. That nega-
tive charge and the positive
charge that was applied to
the N-layer before the image
capture began create an
electric field in which elec-
trons from the P-layer are
drawn by the positive force
into the diode’s depletion
area, a narrow strip separat-
ing the negative and positive
layers. The stronger the light
that hits a pixel, the more
electrons travel to the
depletion layer.5
Each photodiode continues to collect electrons as long as the
shutter is open unless it reaches its capacity limit, called its well.
Some photodiodes capture as many as 300,000 electrons
before their wells are full. If full diodes continue to receive elec-
trons,bloomingcan occur. Blooming is when the charges spill
over to neighboring pixels, causing them to be overexposed. To
protect against blooming, sensing devices are built with materi-
als between the cells that block and absorb excess electrons.6
CHAPTER 7 HOW LIGHT BECOMES DATA^103
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