Tag: CIELAB

CIE 1976 L*a*b* color space

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.

The CIELAB Reference White

One of the important factors in calculating color coordinates in the CIELAB color space is the reference white. The two primary inputs to the CIELAB equations are the set of CIE XYZ tristimulus values for the stimulus, or measured color, and the CIE XYZ tristimulus values for the reference white. The CIE does not identify a specific reference white for CIELAB, so any appropriate reference white may be used (e.g., D50). Therefore, it is important to state the reference white when using or reporting CIELAB color coordinates in order to avoid a misinterpretation of the CIELAB values.

The benefit gained from the reference white in the CIELAB equations 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 (i.e., X/Xn, Y/Yn, and Z/Zn where Xn, Yn, and Zn are the CIE XYZ tristimulus values for the reference white).

This chromatic adaptation in CIELAB based on the CIE XYZ tristimulus values is an approximation to the von Kries chromatic adaptation model, which is applied to the retinal cone responses, but it is less accurate than a proper von Kries adaptation. A proper von Kries chromatic adaptation adjustment would require a transformation from CIE XYZ tristimulus values to cone responses, scaling of the cone responses, and then a transformation of the scaled cone responses back to CIE XYZ tristimulus values. Applying the adaptation scaling directly to the CIE XYZ tristimulus values is easier to implement than a proper von Kries adaptation and provides adaptation results that were deemed sufficient by the CIE in 1976 for color difference calculations with the CIELAB equations.

A chromatic adaptation model provides a means to account for the observer’s visual adaptation to the illumination of a measured color. This is most apparent in the CIELAB color coordinates when the measured color is white. For example, when the CIE XYZ tristimulus values for the stimulus and the reference white are the same, then the CIELAB values are L* = 100, a* = 0, and b* = 0. Under this unique condition, when the CIE XYZ tristimulus values for the stimulus and the reference white are the same, the observer is expected to be visually adapted to the reference white.

This chromatic adaptation attribute of the CIELAB color space is the reason why it is important to state the reference white when using or reporting CIELAB color coordinates. Interpretation of any given color coordinates in the CIELAB color space is relative to the reference white that was used in the calculations of the CIELAB color coordinates.

Post written by Parker Plaisted

References:
G. Wyszecki and W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, John Wiley & Sons, New York, N.Y. (1986).

M. Fairchild, Color Appearance Models, Addison-Wesley, Reading, Massachusetts (1998).

Colorimetry, second edition. CIE Publication 15.2 (1986).

The Proper Notation for the CIELAB Color Space

The 1976 CIELAB color space is 3-dimensional with the dimensions labeled as L*, a*, and b*. Unfortunately, some implementations of the CIELAB color space in software applications have not followed the proper notation and have simply labeled the three dimensions as L, a, and b. To people outside of the color science community, this may appear to be a trivial difference that does not alter the meaning of the color space. But to people within the color science community, the difference is not trivial.

Here is a brief history of two of the many steps that led to the introduction of the 1976 CIELAB color space and the CIE L*, a*, and b* coordinates.

In 1948, Richard S. Hunter published a paper in which he described a photoelectric color-difference meter that would measure color and deliver 3 values to quantify the color. The axes in this three-dimensional, Cartesian coordinate system where labeled L, a, and b. This color space is known as the Hunter 1948 Lab color space, and the calculations for L, a, and b are based on measurement of the CIE 1931 XYZ tristimulus values with the photoelectric color-difference meter.

The Hunter 1948 Lab color space incorporates the opponent-colors theory—proposed by Ewald Hering in 1878—with the a-axis representing the redness or greenness of a color and the b-axis representing the yellowness or blueness of a color. The L-axis represents the perceived lightness of the color.

Building upon the Lab Cartesian coordinate system for a color space, Glasser, McKinney, Reilly, and Schnelle published a paper in 1958 in which they described a visually uniform color coordinate system and the use of cube-root functions to calculate the values for the three dimensions of the color space. They used L*, a*, and b* to denote the three dimensions. Although the notations and goals were similar, the equations for L*, a*, and b* had very little in common with the Hunter 1948 Lab equations.

There were several other sets of equations that were proposed for the determination of color differences by other scientists, but for the sake of brevity I will not go into those details. However, the existence and use of so many sets of equations prompted the CIE to develop and recommend one set of equations for the calculation of color differences. The result of this effort was the CIE 1976 L*a*b* color space in which the CIE accepted the opponent-colors theory, adopted the Lab approach to the notation for the three axes, and incorporated the cube-root approach proposed by Glasser et al. for the nonlinearity between physical energy measurements and perceptual responses. Recognizing the previous use of Lab for the Hunter 1948 Lab color space and the use of L*, a*, and b* by Glasser et al., the CIE denoted their uniform color space with their initials, the year, and the three axes: CIE 1976 L*a*b*. Some publications added another formality and enclosed the axes in parentheses: CIE 1976 (L*a*b*).

I hope from this explanation that you can see that the Lab notation refers to the Hunter 1948 Lab color space, and the L*a*b* notation refers to the equations proposed by Glasser et al. To avoid confusion, and disdain from color scientists, please use CIE 1976 L*a*b*, or the alternative CIELAB notation, when referring to the CIE 1976 (L*a*b*) uniform color space.

Post written by Parker Plaisted

References:
G. Wyszecki and W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, John Wiley & Sons, New York, N.Y. (1986).

Colorimetry, second edition. CIE Publication 15.2 (1986).

Hunter, R. S., Photoelectric Color-Difference Meter, J. Opt. Soc. Am. 38, 661 (1948).

Glasser, L. G., McKinney, A. H., Reilly, C. D., and Schnelle, P. D., Cube-Root Color Coordinate System, J. Opt. Soc. Am. 48, 736 (1958).

Hering, E., Zur Lehre vom Lichtsinne, Carl Gerold’s Sohn, Wien (1878).