Colorcheck analyzes images of the widely-used GretagMacbethTM ColorChecker® for tonal response (using the six grayscale patches, which have an optical densities from 0.05 to 1.50; a range of 1.45 = 4.8 f-stops), noise (for the six grayscale patches), and color quality. Results for tonal response are similar to Imatest Q-13 program.
The GretagMacbeth ColorChecker is an 8x11" inch chart consisting of 24 patches with 18 familiar colors and six grayscale levels, white to black. None of the colors is completely saturated. The quality of the chart is very high— each patch is printed separately using carefully controlled pigments. Patches have a smooth matte surface. It is available from Adorama and other dealers. A 2.25"x3.25" mini ColorChecker is also available.
An ideal simulated ColorChecker is shown on the right for sRGB color space (the Windows/Internet standard). The optical densities (-log10(reflectivity)) of the grayscale patches on the bottom row are shown in parentheses. One color, 18. Cyan, is out of the sRGB gamut; hence it can't be reproduced perfectly on most monitors.
The example was photographed with the Canon EOS-10D at ISO 400, and RAW converted with Canon FVU using automatic color balance.
shows graphic results derived from the grayscale patches in the bottom row. The results include tonal response and noise.
|The upper left plot shows the average density of the grayscale patches (black curve) and first and second order density fits (dashed blue and green curves). The horizontal axis is the distance along the target. A portion of the patches themselves are shown just above the plot. The average density curve is useful for diagnosing uneven lighting: the tops of this curve will be flat if the lighting is even. The equations for the first and second order density fits are given in the Algorithm section, below. The second order fit accurately matches the patches; it is used to calculate the corrected ideal ColorChecker values in the third figure, below.||The upper right plot shows the density response of the colorchecker (gray squares) as well as the first and second order fits (dashed blue and green lines). The horizontal axis is Log Exposure (minus the target density), printed on the back of the ColorChecker.|
|The lower left plot shows the RMS noise for each patch: R, G, B, and Y (luminance). The average R, G, B, and Y noise for the gray zones (2-5) is also reported. RMS noise is expressed in percentage of the difference between the white and black patches.||The lower right plot shows the noise spectrum of patch 4 in the bottom row (middle gray). It contains hidden information about signal processing, for example, an unusually rapid rolloff of noise energy may indicate excessive noise reduction. The ISO speed and Exposure time from the EXIF data are displayed, if available.|
many open Figures
Figures can proliferate if you do a number of runs, especially SFR runs with multiple regions, and system performance suffers if too many Figures are open. You will need to manage them. Figures can be closed individually by clicking X on the upper right of the Figure or by any of the usual Windows techniques. You can close them all by clicking Close figures in the Imatest main window.
on Fgures during calculations
can confuse Matlab. Plots can appear on the wrong figure (usually distorted) or disappear altogether. Wait until all calculations are complete— until the Save or Imatest main window appears— before clicking on any Figures.
shows the density response, noise in f-stops (a relative measurement that corresponds to the workings of the eye), noise for the third Colorchecker row, which contains primary colors, and selected EXIF data.
|The upper left plot is the density response of the colorchecker (gray squares). It includes the first and second order fits (dashed blue and green lines). The horizontal axis is Log Exposure (minus the target density), printed on the back of the ColorChecker. Q-13 Stepchart provides a more detailed density response curve.||The upper right plot shows the noise in the
third colorchecker row, which contains the most strongly colored
patches: Blue, Green, Red, Yellow, Magenta, and Cyan. In certain
cameras noise may vary with the color. Problems may be apparent that
aren't visible in the gray patches.
|The lower left plot shows the R, G, B, and Y (luminance) RMS noise for as a function of Log Exposure each patch. RMS noise is expressed in f-stops, a relative measure that corresponds closely to the workings of the human eye. This measurement is described in detail in the Q-13 Stepchart tour. It is largest in the dark areas because the pixel spacing between f-stops is smallest.||The lower right region displays EXIF data, if available.|
illustrates the color error in the device-independent CIELAB color space, where a* is the horizontal axis and b* is the vertical axis. The small squares are the ideal (a*, b*) ColorChecker values, calculated by Danny Pascale of BabelColor. The large circles are the (a*, b*) values of the actual measured ColorChecker. The numbers near the squares or circles correspond to the numbers of the ColorChecker patches: 1-6 are in the top row, 7-12 are in the second row, and 13-18 are in the third row. The numbers for patches 19-24 (the bottom row) are omitted because their (a*, b*) values cluster around (0, 0).
The background of the chart shows expected colors (in monitor sRGB color space) for L* = 0.95. It presents a reasonably accurate picture of the hues associated with a* and b*. There is significant scatter in the results because the luminance (L) of the color patches (not displayed) varies considerably.
CIELAB (often shortened to LAB) was designed to be perceptually uniform, meaning that the perceived difference between colors is proportional to the distance between them, which is called delta-E. ( Delta-E = sqrt( (a2*-a1*)2 + (b2*-b1*)2 ) ). (The "-" in Delta-E is a hyphen, not a minus.) The smallest perceptible difference corresponds roughly to delta-E = 1. CIELAB isn't perfect in this regard. Delta-E for the minimum perceptible color difference increases with distance from the origin (a* = b* = 0). But it is much more uniform than the popular CIE xy chromaticity diagram.
Four values are reported on the upper right.
It will be difficult to assess these numbers until a large database of test results is available. I would characterize the mean value of 5.66 and sigma of 6.73 for the EOS-10D (above) as very good— perhaps excellent.
See the Links, below, for details on CIELAB color space.
is an image of the ColorChecker that allows you to compare the actual and ideal values. It also displays White Balance error using the bottom row (the grayscale patches).
The outer portion (1) of each patch contains the ColorChecker image as photographed, The inner portion contains the ideal ColorChecker values corrected for exposure (2) and uncorrected (3).
1. the outer region, is the patch as photographed. This corresponds to the circles in the L*a*b* color error plot, above.
2. the square in the center, is the ideal value of the patch, corrected for the luminance of the photographed chart. The correction is derived from the second order fit to the gray areas, described above. The average luminance of zones 1 and 2 shoud be close— zone 2 may be darker in some patches and lighter in others. Zones 2 and 3 correspond to the small squares in the L*a*b* color error plot, above.
3. the small rectangle to the right of the central square, is the ideal value of the patch, with no luminance correction. The luminance of zone 2 will be consistently lighter or darker than zone 3 for all patches, depending on the exposure.
In the above example, the ColorChecker image 1 is slightly underexposed; it is darker than the ideal uncorrected color 3. It is also darker than the corrected ideal color 2. This is a characteristic of this camera: it tends to darken oranges and lighten blues and cyans. This can be seen in the above right ColorChecker image, which is slightly overexposed. This image of the orange-yellow patch (row 2, column 6) has about the same luminance as the uncorrected ideal color 3, but is darker than the corrected ideal color. The uncorrected ideal value 3 in the two orange-yellow images above (the example on the left and the full chart on the right) are the same, but appear to be quite different because of the interaction of colors in the eye— an effect that should be familiar to artists and photographers.
This image provides a clear visual indication of color accuracy. The L*a*b* color error plot provides a quantitative indication.
The bottom of the third figure illustrates the White Balance error, which is quantified in three ways. (1) Saturation S in the HSV color model (burgundy). S can take on values between 0 for a perfect neutral gray and 1 for a totally saturated color. (2) Correlated color temperature error in Degrees Kelvin (blue), calculated from Color Science: Concepts and Methods, Quantitative Data and Formulae, by Günther Wyszecki and W. S. Stiles, pp. 2224-228. (2) Color temperature in Mireds (micro-reciproacal degrees) (blue), where Mireds = 106/(Degrees Kelvin) is a more perceptually uniform measure.
White Balance error tends to be most visible in the gray patches
in the bottom row). It is barely visible for S < 0.02. It is quite
for S > 0.10, particularly for the lighter gray patches.
|The image at the bottom shows the bottom row (the grayscale patches) with exaggerated white balance error, calculated by boosting saturation S using the curve on the left, keeping H and V constant. Low values of saturation are boosted by a factor of 4; the boost decreases as saturation increases. This image shows the White Balance error much more clearly than the unexaggerated chart. Keep it in mind that this image is worse than reality; small White Balance errors may not be obvious in actual photographs, especially in the vicinity of strongly saturated colors.|
When the Colorcheck calculations are complete, the Save results? dialog box appears. It allows you to choose which results to save and where to save them. The default is subdirectory Results in the data file directory. You can change to another existing directory, but new results directories must be created outside of Imatest— using a utility such as Windows Explorer. (This is a limitation of this version of Matlab.) The selections are saved between runs.
You can examine the output figures before you check or uncheck the boxes. After you click on Yes or No, the Imatest main window reappears.
The .CSV summary file contrains Excel .CSV output for the tone level, noise, and color calculations. It also contains some additional data, including EXIF data for JPEG files.