Graphics Programs Reference
In-Depth Information
values when defining a color in RGB or CMY (or using any three values).
This is incorrect. Since our aim is to keep the color-geek-speak to a
minimum, it's not important to know the differences in the various CIE
constructed color models, but rather to recognize that a color space such
as CIELAB is based on how we see color. What you should keep in mind
here is that using a set of three values, any color can be specified exactly
and mapped in three-dimensional space to show its location in reference
to all other colors. This can be useful! There are no capture or output
devices that directly reproduce CIELAB; however, this color space allows
us to translate any color from one device to another.
As we've seen, device-dependent color spaces are unique to the
device. Device-independent color spaces encompass all of human vision
and are not linked to a specific device. It is for this reason that values
provided in a device-independent color space is never ambiguous.
Although it is possible to take a document and convert it into LAB (as it
is named) within Photoshop, LAB is not an intuitive color space in which
to edit files. As we will see, there is little reason to do this.
Sidebar
White Point and Color Temperature:
We don't think of white as being a color. Nevertheless,
white is a color, and there are many different colors of white. Just look at two different sheets
of paper side by side and you'll likely see that even though both appear white, they aren't the
same. One sheet may appear brighter and cooler (bluish) or warmer (yellowish) than the other
sheet.
White point
describes the color of the brightest white of a particular device, such as a
printer or display or even light itself. In the preceding example, the white point of a printer
would be based upon the white of the paper used to produce a print. As another example, we
may have two monitors side by side, both emitting the brightest white they can (in this example,
R255/G/255/B255), yet one may produce a white that appears bluer than the other. This is due
to the difference in the white point of the two displays. We can measure and describe this white
point colorimetrically.
There are several ways to describe the color of the white point;
color temperature
is a term
most commonly understood by those in photography, although as you will see, it is not the best
way. Photographers have been taught for years that tungsten film has a color temperature of
3400 K (
Kelvin
; a unit of temperature). Lower Kelvin values appear more red, and as the Kelvin
values get higher, the color becomes more blue. In actuality, a color temperature is a
range
of
colors correlated to the temperature of a theoretical object known as a
blackbody radiator
.The
blackbody reflects no light and emits energy in shorter wavelengths as it is being heated.
Imagine a black cast iron pan on a very hot stove. As it is heated, it begins to glow dark red.
As the temperature increases, shorter wavelengths are emitted, causing the color of the light
emitted by the skillet to appear orange, then yellow-white, then blue-white. The tungsten fila-
ment of a light bulb behaves similarly to the blackbody and radiates energy in the form of light
because its temperature is so great (around 3200 K). If all light sources were true blackbodies,
Definition
Kelvin:
A unit of
absolute temperature. An
object at zero Kelvin has
no energy. This is often
referred to as absolute
zero (the coldest any
object can be).
