Sensing Violet: The
Human Eye and Digital Cameras
Both the human eye and digital cameras use sensors that are
sensitive to blue, green, and red light, and yet
we and the cameras properly sense light of other colors. Of course,
what is happening is that the human and camera
systems use the output from more than one sensor type to determine the
wavelength of the light. What is surprising
however, is the fact that humans distinguish colors in and beyond the
blue region not with the the "blue" and adjacent
"green" sensors, but with the "blue" and "red" sensors.
Visible light possesses wavelengths from 380 nm, violet,
to about 700 nm, red, and contains the colors normally
named violet, indigo,
blue, green, yellow, orange, and red. The three types of color
sensors in the
human eye respond
most strongly to blue, green, and yellow light as shown
below, and they are usually referred to as the short, medium, and
long, S, M, and L receptors.
When
yellow light strikes the retina, both the M and S cone cells are nearly
equally stimulated and the ratio between the two signals
determines the color perceived. This information is coded and sent to
the brain on a channel called the red-green opponent channel.
Red
light more strongly stimulates the red-sensitive cone cells than the
green-sensitive cone cells, and the brain processes this ratio
as a
shade of red. Colors at the other end of the
spectrum similarly are coded by the ratio of the responses. This
time the blue-
predominantly responds, but the green-sensitive and red-sensitive cone
cells also respond somewhat. A signal deriving from the
blue-sensitive cone cells is sent to the brain on
the yellow-blue opponent channel. In addition, a signal deriving from
the red-
sensitive cone cells is also sent, this time over the red-green
opponent channel. The brain then interprets the combination of the two
signals as violet, blue, or cyan.
To
determine light's color in the violet-blue region of the spectrum either the red or green sensor would
suffice in addition to the blue
sensor if the shape of the response of the red or green sensor were
different from that of the blue sensor. One would expect that the
sensitivity of the red sensor to blue light would be considerably less
than that of the green sensor. Hence, one would expect that the
green sensor would be used. It turns out that in the blue region of the
spectrum the sensitivities of the red and green sensors are so
similar that it matters little which is used. Humans happen to use the
red sensor. How do we know that humans use the red sensor?
The simple demonstration is that individuals lacking red-sensitive cone
cells, protanopes, in addition
to
being insensitive to red light,
cannot distinguish violet, blue, or
cyan.
Digital cameras distinguish colors in about the same was as the human
eye. Most likely however, distinguishing colors at the blue
end of the spectrum utilizes the blue and green sensors rather than the
blue and red sensors used in humans. The ratio of the signals
from the pixels covered with red, green, and blue filters are used to
determine the color of the
light. As in humans, it is the combination
of the spectral sensitivities of the
filters and the algorithm used to interpret the ratios of the signal
strengths as
colors that determine the
camera's actual color response. Camera image files
in the raw format contain the individual pixel responses and also a
profile that tells
the raw conversion program what colors various ratios correspond to.
The presence of the profiles in the raw files allows different cameras
to use color filters with different spectral responses. It is no
surprise then, that the spectral responses of the filters used in
digital
cameras
differ from the pigments in the human eye. This is shown in the
figure above.
Although the human eye and digital cameras both use a three-color
sensor, there is no fundamental reason why a digital camera could not
use just two color filters. In order that all wavelengths of the
spectrum be distinguishable by such a camera, one filter must have a
sensitivity
peak at about 400 nm with decreasing sensitivity to increasing
wavelengths, all the way to about 650 nm. The other filter would need
to be
maximally sensitive at about 650 nm with decreasing sensitivity all the
way out to 400 nm. It would probably also be necessary that
wavelengths
below 400 nm and above 700 nm be excluded by additional filters.