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The illuminant used to create L*a*b* reference files is usually D50. The most widely used color spaces (sRGB and Adobe RGB) have D65 white points. 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.
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Patch order
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: Reference files read by Imatest are should be inrow-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 }, i.e., patch data for each row is entered before moving on proceeding 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.
Using measured values as a reference: Color reference files can also be derived from measured values from a camera you determine to be a “gold standard” reference 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.
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