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The illuminant used to create L*a*b* reference files is almost always usually D50. The most widely used color spaces (sRGB and Adobe RGB) have D65 white points5points. A Bradford Transform is applied to convert the D50 file data to D65 color spaces. It is applied to spectrophotometer measurements for individual charts.
Color Reference files
Color CSV reference files files consist of the number of lines that the test chart has patches, with L*, a*, b* values on each line separated by spaces, commas (,), or semicolons (;). Example (first 3 lines of 24):
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If you have an Excel .CSV file with extra rows or columns, you can easily edit it Excel by selecting the key region (3 columns, 24 rows), copying it to a new file, and saving it in .CSV format. L*a*b* data is preferred to xyY data below because it is independent of white point color temperature, hence less error-prone.
Patch order
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Reference files read by Imatest are in row-major order. This means that the order for an m x n (m rows x n columns) chart is R1C1, R1C2, …, R1Cn, R2C1, R2C2, …, R2Cn, …, RmC1, RmC1, …, RmCn. Each row is entered before moving on to the next.
But some reference files available on the internet are in column-major order (R1C1, R2C1, …, RmCqn R1C2 R2C2, …, RnC2, …, R1Cn R2Cn, …, RmCn). They must be transformed into row-major order to be read correctly into Imatest. We show a method for doing this below.
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These are close, but not identical, to the values shown above, which were derived from a different source.
Density Reference Files
Density CSV reference files are much simpler, they contain one optical density value per line, going from least-dense (smallest) patch, up to the most dense patch.
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Load your image(s) into Imatest
Start your analysis
Proceed through settings & ROI selection
In the interactive results window, select Density CSV file (one value per line) in the Ref dropdown, as shown in the image below. This will open a browser for you to select your reference file.
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Color reference files, color temperature, and color spaces
The color reference for a test chart contains CIELAB (device-independent L*a*b*) values for each patch of the chart. It can be
obtained from default values built into Imatest or
read from a file, typically in CSV or CGATS format. The file can contain default values supplied by the chart manufacturer or values individually measured with a spectrophotometer.
Questions frequently arise regarding the color temperature used to (1) measure the patches for the reference file, (2) measure camera color accuracy from a chart image, and (sometimes forgotten), and (3) built into the color space used by the camera under test. (All interchangeable files have a color space associated with them, implicitly or explicitly.)
Patches are measured with a light source with a known color temperature, typically 5000K (D50). Color spaces have a illuminant white point. sRGB (the widely used internet Windows standard) has a 6500K (D65) illuminant white point and a D50 (~5000K) ambient (viewing environment) white point. Wikipedia does little to resolve the confusion, but the 6500K illuminant white point is the chief property of the file. A Bradford transform is typically applied to convert the 5000K measurement to the 6500K color space illuminant white point.
A frequent question is, “Is a special reference file needed for measuring charts with illuminants different from the color space white point (typically 6500K)?” The question is important because Imatest measurements (and indeed almost all camera operation) can be made with a wide variety of color temperatures, ranging from 2400K for incandescent lights to >10000K for some daylight conditions. The quick answer is “No”.
The standard reference should be used. The raw image is often transformed in the camera by an algorithm that estimates the illuminant color temperature, then applied a Color Correction Matrix (CCM) to the image to white balance and correct the colors. A CCM can also be applied to an uncorrected image by Imatest.
Converting a reference file from column-major to row-major
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To convert reference files from column-major order (frequently found in reference files on the internet) to row-major (the order used by Imatest), download the file and open it in Excel. Reference files often include extra data. Here is an example from the Image Engineering TE262 Universal Test Target file, used for evaluating scanners. The color pattern in the large UTT chart is shown on the right. (There are two of these in some printed charts.)
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