Graphics Programs Reference
In-Depth Information
effect is not caused by the illuminant, but by the color filters in the digital camera or the photo-
sensitive layers in the film.
The problem of metamerism is greater with a digital camera than with a scanner. A scanner
only needs to understand specific spectral properties, for example those of film. A digital camera
tries to capture the spectral properties of all the objects in a scene—clearly a much more dif-
ficult task. One of the most obvious cases where metamerism is seen is a Grayscale image from
color printers. A profile built for tungsten lighting may produce a perfectly neutral Grayscale
image but when viewed in daylight, the Grayscale may change hue from light to dark.
Metamerism is a phenomenon that will be mentioned from time to time in future chapters since
output devices can often exhibit this effect.
Digital Camera Files
The vast majority of digital cameras in use today produce an image using
a sensor such as a CCD or CMOS chip to capture an image in a split
second. I point this out to differentiate these one-shot capture devices
from scanning cameras and multiple shot (three- or four-shot) cameras.
Scanning backs and multiple shot cameras capture their data differently
although the issues of profiling all cameras are similar. Scanning back
camera systems use a Trilinear CCD much like flatbed scanners. Some
actually have called them “scanners on a stick.” The digital cameras that
shoot an instant, single capture use a chip that actually doesn't “see” or
record color. CCDs and CMOS chips are monochromatic.
The way the color ultimately is produced is by using chips that have
a matrix of colored filters over each individual sensor as seen in Fig. 5-2.
One pixel captures the red information, the next one the green informa-
tion, and the next one the blue information. In essence, the camera
records a Grayscale file, which can produce a full color image when the
data is interpolated, a process called
demoasicing
. Assumptions are made
about adjacent colors on the chip from a photosensor that is seeing only
one color. It is truly amazing that this all works as well as it does.
I bring this all up, because at some point, our Grayscale data has to
be turned into an RGB color image. This Grayscale data, the full capture
information from the sensor, is often referred to as a
RAW
file. This RAW-
to-RGB conversion can be conducted on-the-fly using onboard camera
processors or with many cameras, done later by the user. Some cameras
allow the user to store the RAW data for this task, but some do not and
always process the RAW data into some flavor of RGB.
Input (Scene)-referred versus Output-referred Data
Photographers know that the world we live in and view is difficult to
record on film or even on a digital camera. Wouldn't it be wonderful if
we could capture all the color and tone on our digital cameras that we
can see with our own eyes! No need for fill-flash or fill cards, no need
