USL PCA-100 - Manual (Page 8)

- 7 -
PCA I NSTRUCTION M ANUAL
Ultra Stereo Labs, Inc.
The CIE also established a chromaticity diagram for displaying the region of
human color sensitivity (see figure 1). This diagram displays the hue and
saturation of colors in terms of the chromaticity coordinates, x (horizontal scale)
and y (vertical scale). Lightness is not indicated on this two dimensional chart.
The approximate center of the chart, (~x=.31 and y=.360), indicates no color,
thus no saturation. Saturation increases toward the outer limits of the curve.
For example, (~x=.45, y=.55) indicates a highly saturated yellow and (~x=.12
and y=.12) indicates a highly saturated blue.
This color system is often called the “Yxy” system, where Y is the lightness
or luminance and x,y denote the color chromaticity as referred to in the x,y
chromaticity diagram.
We use the Yxy system for reflected light, such as the light reflected from a
screen or wall or other reference surface. The luminance, Y is expressed in
foot - Lamberts or Candelas per square meter and the chromaticity in terms
of x and y.
Figure 5 shows the black body locus which indicates the relationship of color
temperature values to the chromaticity values x and y.
The following section lists some commonly used terms and definitions that are
applicable to making measurements of light and color.
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80
10000
4500
2500
1500
RED
BLUE
GREEN
Figure 5
Black body locus