The Reference White in Adobe Photoshop Lab Mode

In an earlier post I explained that all CIELAB values are relative to a reference white, and one benefit of the reference white is the chromatic adaptation provided by dividing each CIE XYZ tristimulus value of the stimulus by the corresponding CIE XYZ tristimulus value for the reference white.

For this post I will focus on two example implementations of the CIELAB color space in digital color workflows and the reference white used in each example.

The first example implementation is color management with device profiles based on the International Color Consortium (ICC) specification. The reference white for every ICC profile is D50, and the corresponding CIE XYZ tristimulus values are X = 0.9642, Y = 1.0000, and Z = 0.8249. This information is provided in the description of the ICC Profile Connection Space (PCS) as guidance for interpreting PCS values that are encoded as CIELAB values. The following statement is a quote from ICC Specification ICC.1:2004-10:

So, in summary, the PCS is based on XYZ (or CIELAB) determined for a specific observer (CIE Standard 1931 Colorimetric Observer – often known colloquially as the 2 degree observer), relative to a specific illuminant (D50 – a chromatic adaptation transform is used if necessary), and measured with a specified measurement geometry (0°/45° or 45°/0°), for reflecting media.

Note that the ICC PCS values may be encoded as CIELAB values or CIE XYZ values, but the CIELAB encoding is preferred for output device profiles that characterize printing systems. The use of D50 for the reference white for CIELAB encoding in the ICC PCS makes the CIELAB values more suitable for printing systems than other white points (e.g., D65) because D50 is the standard illuminant for prepress proofing and matching press sheets to proofs. Therefore, the reference white for the ICC PCS matches the reference white universally used for evaluating color proofs and finished color prints.

The second example implementation is the Lab color mode in Adobe Photoshop. The Lab color mode is based on the CIELAB color space, and the reference white for the Lab color mode is D50 (confirmed by personal communication with a color scientist at Adobe). The Adobe Photoshop software has been widely used in graphic-arts prepress workflows since the early 1990’s (I began using Photoshop in 1993 in prepress applications); and Adobe was a founding member of the International Color Consortium when it was formed in 1993. Therefore, Adobe’s choice of D50 for the reference white for the Lab mode is consistent with Adobe’s support of the ICC profile specification, which I just described above, and the widespread use of Photoshop in graphic-arts prepress workflows where D50 is the standard illuminant for evaluating prints.

Both of these example implementations are relevant to digital color workflows used by photographers to produce photographic prints, and it should be clear that D50 is a good choice for the reference white when the CIELAB values are used in a workflow that ends with a print. But there is another component that is generally present in a digital color workflow, and that component is the color monitor on which the color image is evaluated before the image file is delivered to a printer.

In my next post I will present my position in the debate on the preferred white point for color-monitor calibration in a digital color workflow where an image on a color monitor will be compared to the same image on a print.

Post written by Parker Plaisted

References:
International Color Consortium, ICC Profile Format Specification. (http://www.color.org)

Note: I spent a significant amount of time looking for an official Adobe publication or document that would confirm the rumor that the reference white for the Photoshop Lab color mode is D50, but I could not find one. I then called a friend at Adobe who is one of the color scientists at Adobe who is familiar with this detail in the Lab color mode. He confirmed that D50 is the reference white for the Lab color mode in Adobe Photoshop.

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