Color reference files read by Imatest can be CSV or CGATS format containing L*a*b* color and luminance reference data.
The reference illuminant is assumed to be the same as the selected color space (D65 for sRGB, etc.). This is at variance with current practice, where L*a*b* files are assumed to have D50 data. The new LAB D50 file (Imatest 4.4+) is recommended because it will transform the reference values using a Bradford Transform to move them to the selected color space’s white point. This is the recommended approach if you have spectrophotometer measurements for your individual chart.
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):
38.08, 12.09, 14.39
66.38, 13.22, 17.14
51.06, 0.38, -22.06
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.
Color reference files can also be derived from measured values from a camera you determine to be a “gold standard” by running Color/Tone Interactive, then pressing File, Save L*a*b* results as CSV reference (making sure to check CSV reference file of L*a*b* results…). This can be useful for manufacturing testing because the camera under test is compared with the attainable values of a golden sample rather than unattainable ideal values.
The CGATS file format is also supported.
Color/Tone Interactive General mxn charts (4×12 shown on the left). Any chart with an m x n grid can be analyzed. You must specify the grid size (click Settings, ISO speed & mxn chart settings to set the number of rows (m) and columns (n). You must also enter a CSV or CGATS file with reference patch settings (typically L*, a*, b* values, one set per line). If you have a spectrophotometer you can create a custom reference file (CGATS format) using techniques described in Measuring test chart patches.
In Color/Tone Interactive you can perform a number of operations that relate to the reference file using the File dropdown menu, shown on the right.
The operation of interest here is Copy L*a*b* reference to clipboard. This shows the currently selected reference, whether it’s a standard or custom file. You can also save it as a named file for future use as a reference— perhaps for a “gold standard”. Here are the first three values for the post-Nov 2014 X-Rite Colorchecker default values.
37.31 13.39 14.58
64.37 18.05 17.05
49.62 -1.162 -22.16
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.
Imatest will provide you with a density reference file when you purchase a 36-patch dynamic range or contrast resolution chart. You should always use this reference file while performing dynamic range measurements with multicharts or color-tone instead of using the default values.
Note that Copy density reference to clipboard is available in the File dropdown menu on the right.
- Color/Tone Interactive – Reference File
- Colorcheck Reference File (legacy module: Color/Tone is recommended for new work)
- Measuring Test Chart Patches with a Spectrophotometer for details about how to create a reference file using measurements from an instrument.