SD1 Merrill Brochure

Not another monochrome sensor
with color filters
The image sensor in almost all digital
cameras, with the exception of Sigma's,
is a color filter array (CFA) sensor. The
image sensor itself is monochrome; it
detects light intensity, but not color. The
CFA overlaying the sensor's light-sensitive
photodiodes is a mosaic of red, green
and blue (RGB) filters in a checkerboard-
like grid. Therefore, each photosite,
corresponding to a single pixel, receives
just one of the three primary colors.
In this kind of system, a 15MP CFA sensor
allots 7,500,000 pixels to green light and
3,750,000 each to red and blue light,
respectively. Left as is, this checkerboard
pattern would create a strange image,
so a process called color interpolation is
used to blend in neighboring pixel color
information. For example, a green pixel
gets color information from adjacent blue
and red pixels, and so on.
CFA sensor relies
on eye's sensitivity to green
Why does a CFA sensor
assign twice as many
pixels to green as to red
or blue? Because the eye
is most sensitive to green
light (figure 1). Human
vision depends mostly on
wavelengths in the green
range to perceive fine detail
and luminance resolution.
The four charts on figure 2 all have
the same contrast. Notice how the green
background lets you detect fine detail
more easily. Most digital cameras take
advantage of this fact of human vision.
By capturing relatively more green
information, they can get by with much
less blue and red information.
Thanks to this clever solution, a mere
monochrome image sensor can be used to
deliver color images of high quality.
So, what's the problem?
Though it is true that our
eyes are most sensitive to
green luminance (sharpness
and contrast) information,
we also see blue and
red chrominance (color)
information. But the unequal
treatment afforded these
colors by a CFA sensor
causes a disparity between
the luminance resolution
and chrominance resolution
of the photographic images
it generates.
Unique, innovative direct image sensor
Unlike CFA sensors, the image sensor in
Sigma digital cameras uses three layers
of photodiodes to gather the entire red,
green and blue color information of light,
forming the world's one and only full
color capture system. Since introducing
its first-generation digital camera in 2002,
Sigma has maintained an unwavering
commitment to this unique technology.
The image sensor is made of silicon.
When struck by light, silicon absorbs
shorter wavelengths (blue) near its
surface and longer wavelengths (green,
then red) at deeper levels. The direct
image sensor takes advantage of this
fact, using three layers of photodiodes
to capture all colors of light at each pixel
location.
All pixels capture full RGB color
This means that a direct image sensor
having 15,000,000 pixel locations is
An image capture system you can believe in. Now, more than ever
figure 1
figure 2
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