Graphics Reference
In-Depth Information
where
W
is the spectral power distribution of the standard white light we're
using.
Suppose that the light
C
produces the same sensor responses as
X
X
+
Y
Y
+
Z
Z
.
(28.22)
In that case, we write
C
=
X
X
+
Y
Y
+
Z
Z
.
(28.23)
The CIE defines numbers that are independent of the overall brightness by
dividing through by
X
+
Y
+
Z
; doubling the incoming light doubles each of
X
,
Y
,
and
Z
, but also doubles their sum, so the quotients
X
X
+
Y
+
Z
,
x
=
(28.24)
Y
X
+
Y
+
Z
, and
y
=
(28.25)
Z
X
+
Y
+
Z
z
=
(28.26)
remain unchanged. Note that the sum
x
+
y
+
z
is always 1, so if we know
x
and
y
, we can compute
z
. Thus, the collection of intensity-independent colors can be
plotted on just the
xy
-plane; the result is the
CIE chromaticity diagram
shown
in Figure 28.18. Notice that
X
and
Y
were chosen so that the diagram is tangent
to the
x
- and
y
-axes.
Near the center of the “horseshoe” is
illuminant C,
which is a standard
reference “white,” based on daylight. Unfortunately, it doesn't correspond to
y
0.9
520
0.8
540
510
0.7
Green
560
0.6
500
580
0.5
Yellow
Cyan
0.4
600
0.3
490
Red
700
Blue
0.2
480
Purple
0.1
400
0.0
x
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Figure 28.18: The CIE chromaticity diagram. The boundary consists of chromaticities cor-
responding to monospectral lights of the given wavelengths, shown in nanometers. The dot
in the center is a standard “white” light called “illuminant C.”
