Biomedical Engineering Reference
In-Depth Information
The narrower the peak width is, the higher the saturation is. Brightness renders the
relative intensity of the reflection spectrum, strongly related to the response of our
vision system.
Practically, colors are specified in color spaces. Commonly used color spaces
include CIE
XYZ
and
xyZ
color spaces.
3
For the human eye, there are three
color-sensitive photoreceptors with sensitivity peaks located in short (420-440
nm), middle (530-540 nm), and long (560-580 nm) wavelengths. Three tristimulus
values are thus needed to describe a color since color spaces are 3D.
In the CIE
XYZ
color space, the tristimulus values are a set of values called
X
,
Y
,and
Z
. For a given illuminating source
D./
and the resulting reflection
spectrum
R./
of a color, they can be calculated by
Z
1
1
k
X
D
D./R./x./
d
;
0
Z
1
1
k
Y
D
D./R./y./
d
;
0
Z
1
1
k
Z
D
D./R./
z
./
d
;
(8.4)
0
where
x./
,
y./
,and
z
./
ar
e three CIE color matching functions [
30
]shownin
Fig.
8.6
a, and
k D
R
0
D./y./
d
is a normalization factor which ensures that an
object with
R./ D 1
yields the component
Y D 1
. In practical calculations, CIE
Illuminant D65 [
30
] is often used as the illuminating source
D./
, which matches
closely that of the sky daylight.
The CIE 1931
XYZ
is at the root of all colorimetry. In the CIE
XYZ
color
space, the
Y
parameter was deliberately designed as a measure of the brightness
of a color. The chromaticity of a color can thus be represented by the two derived
parameters
x
and
y
,definedby
X
X C Y
Y
X C Y
x D
C Z
;
D
C Z
;
(8.5)
and derived parameter
z
can be obtained from
z
D Z=.X CY CZ/ D 1x y
.The
derived color space specified by
x
,
y
,and
Y
is known as the CIE
xyY
color space
which is widely used in practice for color specification. In the CIE
xyY
color space,
the brightness of a color is given by
Y
, while
x
and
y
specify the color chromaticity
which can be obtained from the CIE color chromaticity diagram shown in Fig.
8.6
b.
3
The CIE
XYZ
color space was one of the first mathematically defined color spaces created,
according to the human visual system, by Commission Internationale de l'Eclairage (CIE) in 1931.
