Graphics Reference
In-Depth Information
8.4
Colorimetry
The terms in the last section apply to
color response
and hence are terms of psychol-
ogy. There is a more scientific approach to analyzing color called colorimetry.
We begin with the question: What is light? Well, light is a form of energy,
electro-
magnetic energy
. Note that sometimes a distinction is made between
light
, that radia-
tion in proportion to its effectiveness in producing vision, and
visible radiant energy
,
which refers to all radiation in the visible range. Only part of all light is visible. Light
has wavelengths given in nanometers (nm), where 1 nanometer equals 10
-9
meters or,
equivalently, 10 angstroms. Visible light is the light with wavelengths in the 380 nm
to 780 nm range. For example, blue and green correspond to light with wavelengths
of 470 nm and 500 nm, respectively.
Here is some more terminology related to light and perceived colors:
Monochromatic light
:
This is light consisting of a single wavelength.
Spectral hues
:
These are the hues in monochromatic visible radiation
(those present in the sun's spectrum). The hues in a
rainbow are spectral hues.
Spectral colors
:
These are all the colors that are perceived to have a spec-
tral hue.
Monochromatic light has maximum saturation but may have an achromatic compo-
nent. We experience also
nonspectral
hues, for example, purple or purplish red, which
are not present in the sun's spectrum, and
nonspectral
colors.
Because perceived colors vary with illumination, one prefers to make comparisons
in daylight. But this must be specified
precisely
. In 1931 the Commission Interna-
tionale de l'clairage (CIE) defined the
CIE illuminants
. Colors can now be measured
by matching against standard samples under standardized viewing conditions using
colorimeters
. Here a photoelectric cell replaces the human eye. One takes a sample
color (the color one is trying to identify) and compares it to a mixture of standard
colors. The intensities of the latter are varied via three adjustments, the CIE tri-
stimulus values. The
tri-stimulus theory of light
is based on the assumption that there
exist three types of color sensitive cones at the center of eye which are most sensitive
to red, green, and blue. The eye is most sensitive to green.
Basically, one chooses three beams (a short, medium, and long wave length which
are typically the three
additive primary colors
red, green, and blue). Different colors
can then be produced by shining the three beams on a spot and varying the intensity
of each. The
chromaticity
of a color
C
is defined by a triple (x,y,z) of numbers speci-
fying these three intensities, or weights, for the color. Mathematically, one is repre-
senting the color
C
in the barycentric coordinates form
CRGB
=++
xyz
,
where
R
,
G
, and
B
represent the colors red, green, and blue, respectively, and x + y +
z = 1. The numbers x, y, and z would be called the
chromaticity values
of
C
. Figure
8.2(a) shows this in graphical form. The triangle is called Maxwell triangle chro-
