Imatest Documentation

Imatest Documentation
dark
skin
7.
orange
13.
blue
19.
white
(.05)
What colors are the ColorChecker?
light
skin
8.
purplish
blue
14.
green
20.
neutral
8 (.23)
blue
sky
9.
moderate
red
15.
red
21.
neutral
6.5 (.44)
foliage
10.
purple
16.
yellow
22.
neutral
5 (.70)
blue
flower
11.
yellow
green
17.
magenta
23.
neutral
3.5 (1.05)
bluish
green
12.
orange
yellow
18.
cyan
24.
black
(1.50)
Before 2005 the ColorChecker came with insufficient information. Grayscale densities (for the bottom row) were printed on the back
of the chart. CIE xyY values (from the familiar 1931 xy chromaticity chart) were included, but these were for CIE illuminant C, whose
6774K color temperature is slightly different from standard 6550K color spaces. A set of RGB values with no specified color space
were also included. They should not be used. A ColorChecker purchased in October 2005 had better information: sRGB values and
CIE L*a*b* D50 (5000K) values. L*a*b* and RGB values for known color spaces are used in the Imatest analysis. (sRGB is the RGB
standard color space for website display.) xy chromaticity values (from the 1931 chart) are far from perceptually uniform, i.e.,
distances between points on the xy-plane are not proportional to perceptible differences between colors. (Green is greatly
exaggerated.) Color differences are better represented in the CIELAB color space, where L* is luminance, a* is color on a green-red
scale, and b* color on a blue-yellow scale. A distance of 1 between L*a*b* values represents the minimum perceptible difference
(just-noticeable difference, JND) between colors (for relatively unsaturated colors). For colors on the a*b* plane (neglecting L*), this
distance is expressed by the equation,
∆C* = ( (a2*-a1*) 2 + (b2*-b1*) 2 ) 1/2 ; where (...) 1/2 denotes square root.
More generally,
∆E*ab = ( (L2*-L1*) 2 + (a2*-a1*) 2 + (b2*-b1*) 2 ) 1/2 (includes L* differences)
Although ∆C and ∆E*ab (which are both Euclidian distances) are widely used to quantify color differences, they are not as accurate
as the CIE 1994 and CMC equations, which are discussed in detail in the Colorcheck Appendix.
Danny Pascale/Babelcolor's page on the Colorchecker contains everything you want to know about the chart. A great resource!
Colorchecker reference values
There are several sources, each slightly different. The values used by Imatest are shown below.
1. The GretagMacbeth website (now X-Rite) no longer has downloads available. The very best source is Danny
Pascale/BabelColor's RGB coordinates of the Macbeth ColorChecker is an outstanding resource that contains X-Rite's and
Pascale's most recent Colorchecker values (2005). They are very close, with mean ∆E =2. Pascale presents ColorChecker
values in xyY, L*a*b*, and a variety of color spaces, based on numerous measurements. He also presents procedures and
equations for data conversion. The L*a*b* values are for D50 illumination.
2. BruceLindbloom.com contains equations and tables for converting between various RGB color spaces as well as XYZ, xyY, and
CIELAB. Click on Math. His ColorChecker values are based on accurate spectrophotometer measurements of a single sample.
He has a useful ColorChecker calculator.
3. Bruce Fraser, Creativepro.com. Calibrating Camera Raw in Photoshop CS contains Bruce's ColorChecker L*a*b* values
from his book, Real World Color Management. I not sure what reference color temperature he uses. Allen Pacheco has
published a similar calibration technique.
The ColorChecker reference can be selected in Colorcheck. At present you may choose between references 1 (X-Rite) and 3
(Pascale), or you can read L*a*b* or xyY files. The default is the X-Rite L*a*b* values. The grayscale values are derived from the
definition of ISO speed for digital sensors, as described in the Colorcheck Appendix.
Photographing the target
Photograph the ColorChecker. The distance is not critical. There is no
need to fill the frame with the ColorChecker image. Filling the frame may
reduce accuracy if there is significant vignetting (light falloff due to the
lens system, which is often quite large in wide-angle lenses).
It may be useful to include other charts or scene elements that can affect
white balance. A ColorChecker image width of 500 to 1500 pixels is
sufficient for the Colorcheck noise analysis. That's under half of the
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