Monitor Calibration: D65 White Point for Soft Proofing

If you buy a color computer monitor today, connect it to your computer and display a photographic image on it you will probably be happy with the appearance of that image on the monitor. The monitor will use LCD technology and have a native white point near the CIE D65 standard. Right out of the box, with no external calibration, the monitor will display photographic images that look good—not perfect, but good. This is very different from the “out of the box” experience for a color computer monitor 20 years ago.

Let me go back in time to describe a few issues that led to a debate on the “right” white point for monitor calibration and highlight some interesting research from Dr. Mark Fairchild on chromatic adaptation for soft proofing on computer monitors in prepress workflows. The results of Dr. Fairchild’s research are also valid for digital photography workflows.

Before we had color computer monitors we had color television sets. The technology that enabled color television was an impressive merger of electrical engineering and color science. In 1953, the National Television System Committee (NTSC) released a standard for color television that included a white point specified as CIE standard illuminant C, which had a correlated color temperature of 6774 K. The NTSC standard for television established CIE standard illuminant C as the preferred white point for images viewed on a color television, which was at that time based on cathode ray tube (CRT) technology.

Eleven years later, in 1964, the CIE recommended D65 as the main standard daylight illuminant, and the popularity of CIE standard illuminant C faded away. This shift to D65 was summarized by Wyszecki and Stiles: “In practice, illuminants B and C have already fallen into disuse in most applications. Instead, CIE standard illuminant D65 is now widely used as the representative of average daylight for colorimetry.” (p. 145)

Based on the recommendation from the CIE and other scientific research, D65—with a correlated color temperature of 6500 K before 1968—became the preferred white point for calibrated video systems including PAL and SECAM, which are analog encoding systems for color television that were implemented in the 1960s.

In the 1960s and 70s, calibrated video systems were synonymous with closed-loop systems. With the personal computer revolution in the 1980s, we began to see the color monitor as a component of a computer system that could be purchased separately and from a different vendor than the computer. In such a system, the color monitor was out of the color calibration loop.

In the 1990s desktop publishing gained acceptance as the computer hardware from Apple Computer and the software from Adobe Systems and other companies enabled professional quality results in prepress workflows. At this time, color monitors were based on CRT technology, and the native white point for a typical full-color CRT monitor was near a correlated color temperature of 9300 K. Therefore, color illustrations and photographic images seen on a full-color CRT monitor had a very strong blue color cast that was not visible in a printed version of the electronic file. The open-loop systems created by connecting the separate components left us with an image displayed on a CRT monitor that did not look like the image rendered in a print.

There was a strong desire, and economic incentive, to judge an image on a color monitor to reduce the time and cost of making prints for the same judgment. The concept of a “soft proof” viewed on a color monitor rather than a “hard proof” seen in a print quickly became a goal for people working in prepress workflows with personal computers. The color monitor was the weak link in the system that prevented an accurate soft proof. There was universal agreement among printing, prepress, and color science experts that the white point of the color monitor had to be calibrated to a lower color temperature to solve the problem, but there was disagreement on the choice of the best white point for monitor calibration.

Two white points were proposed: D65 and D50. The set of chromaticity coordinates for CIE standard illuminant D65 was the standard white point for CRT-based color video systems, with roots in color television. CIE standard illuminant D50 was the standard illuminant for viewing prepress proofs in a professional printing workflow.

On one side of the debate was the evidence that a CRT-based video system calibrated to a D65 white point delivered an image with whites that appeared white. On the other side of the debate was the set of standards and established practices where D50 was the specification for illuminating prints. And the divide between these two white points was significant because research on visual adaptation indicated that a D50 white and a D65 white were far enough apart to be visibly different when viewed side by side and in the same method of rendering (e.g., two side-by-side images on a color monitor).

There were two factors that established D50 as an anchor in this debate: 1) the people who wanted to calibrate the monitors for soft proofing were working in digital prepress workflows with the goal of preparing files for a printing press, and 2) the established and universally adopted standard for viewing proofs in the printing industry was not going to change to D65 to accommodate this new idea of soft proofing. Therefore, color monitors in a prepress workflow were destined to be calibrated to a D50 white point unless someone could show that an image on a print illuminated with D50 light looked like the same image displayed on a color monitor that had been calibrated to a D65 white point—a theory that was inconsistent with our basic understanding of colorimetry because D50 and D65 chromaticity coordinates are too far apart to achieve a visual match between two corresponding white fields when viewed side by side and in the same method of rendering.

To give you some perspective on this, the CIELAB coordinates of the CIE tristimulus values of D65, with a reference white of D50, are L*=100, a*=-2.4, and b*=-19.4. Therefore the Delta E between the CIE tristimulus values of D65 and D50 in CIELAB space with a reference white of D50 is 19.5. That is a very large Delta E number, which indicates a large visual difference.

Scientists around the world took up the soft proofing challenge and conducted research on chromatic adaptation to images displayed on a color CRT monitor in comparison to printed images displayed in a light booth under D50 illumination. The scientists quickly identified environmental factors that influenced chromatic adaptation when people viewed images on a color monitor (e.g., the ambient light in the room). But one of the most interesting factors was explained in an article written by Dr. Mark Fairchild at RIT. In this article, published by TAGA in 1992, Dr. Fairchild described sensory and cognitive mechanisms in chromatic adaptation. The sensory mechanisms are consistent with the science of colorimetry. The cognitive mechanisms explain how our knowledge influences our perception of color.

The cognitive mechanisms in chromatic adaptation enable an observer to discount the yellow tint cast by D50 illumination on white paper and see the paper as white. This explains why D50 illumination for contract proofs has worked very well for the printing industry for decades. Unfortunately, the cognitive mechanisms in chromatic adaptation do not deliver the same benefits for images viewed on a color CRT computer monitor. To quote Dr. Fairchild:

When hard-copy images are being viewed, the image is perceived as an object that is illuminated by the prevailing illumination. Thus both sensory mechanisms that respond to the spectral energy distribution of the stimulus and cognitive mechanisms that discount the “known” color of the light source are active. When a soft-copy display is being viewed, it cannot easily be interpreted as an illuminated object. Therefore there is no “known” illuminant color and only sensory mechanisms are active.

Dr. Fairchild also noted in his research that chromatic adaptation was incomplete for observers who viewed a white patch displayed on a computer monitor with chromaticity coordinates near CIE illuminant A (incandescent light). To the observers, the white patch on the computer monitor retained a yellow appearance. If the observers were able to fully adapt, the patch would have appeared achromatic after complete chromatic adaptation.

Scientific research has shown that D65 is a good white point for color displays, including televisions and color computer monitors. Research has also shown that a color computer monitor calibrated to a D50 white point would retain a yellow appearance—chromatic adaptation would not be complete.

The solution for soft proofing started to become clear. The printing industry would continue to use D50 illumination as the standard for viewing contract proofs. The graphic artists would use color CRT monitors calibrated to a D65 white point because the D65 white point would allow them to achieve complete chromatic adaptation. However, in order for this to work, the image on the monitor could not be directly compared to a print under D50 illumination in a side-by-side viewing environment. The observer would have to have time to fully adapt to each separate viewing environment.

Therefore, the right white point for monitor calibration is D65 in order for the viewer to achieve complete chromatic adaptation to the color monitor based on sensory mechanisms in human vision—cognitive mechanisms are not active. Since cognitive and sensory mechanisms are both active when a print is viewed, the viewer should not directly compare a print to an image on a computer monitor when the white point for the illumination of the print is different from the white point of the color monitor.

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)

M. D. Fairchild, “Chromatic adaptation to image displays,” TAGA 2, 803-824 (1992b).

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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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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).

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Monitor Calibration: 2-Degree or 10-Degree Observer Color-Matching Functions?

The CIE defined two sets of color-matching functions for use as standard observers. The first was the CIE 1931 Standard Colorimetric Observer. The second was the CIE 1964 Supplementary Standard Colorimetric Observer. The difference is based on the field of view used in the collection of the experimental data.

The CIE 1931 Standard Colorimetric Observer is based on experiments with matching fields of two degrees of angular subtense. The upper limit of 2 degrees was imposed to constrain the image to the fovea within each eye of each observer. The fovea contains a dense concentration of cones and no rods, and the cones are our color receptors. Therefore, the color-matching functions for the CIE 1931 Standard Colorimetric Observer describe cone vision within the fovea.

The CIE 1964 Supplementary Standard Colorimetric Observer is based on experiments with matching fields of ten degrees of angular subtense. This larger field extended the image beyond the fovea and gathered information about cone vision outside of the fovea. One of the goals was to determine if cone vision outside the fovea differed from cone vision within the fovea, so efforts were made to discount the central 2-degree field of view in the color-matching experiments of Stiles and Burch (1959) and Speranskaya (1959) which formed the basis for the CIE 1964 Supplementary Standard Colorimetric Observer. Since the larger field also included rods and the scientists sought to exclude rod vision from the experimental results, efforts were made to suppress the influence of the rods. To quote Wyszecki and Stiles, “Thus, the color-matching functions embodied in the CIE 1964 supplementary standard colorimetric observer aim to define the matching properties of the rod-suppressed retina for a large visual field.” In CIE Publication 15.2 (1986), the CIE recommends the use of the color-matching functions of the CIE 1964 Supplementary Standard Colorimetric Observer “whenever correlation with visual colour matching of fields of angular subtense greater than about 4 degrees at the eye of the observer is desired.”

Which set of color-matching functions is relevant for monitor calibration? From one perspective, the act of calibrating a color monitor involves measuring large color patches that are displayed on the screen of the monitor. The patches are clearly larger than a 4-degree field of view. In addition, we usually judge the whiteness (or coolness or warmness of white) of the monitor by looking at a relatively large white patch that is larger than a 4-degree field of view. However, we must keep in mind that monitor calibration is implemented to facilitate the evaluation and editing of photographic images, not large uniform patches of color, and the CIE 1931 Standard Colorimetric Observer is preferred for colorimetry applied to photographic images with detail that is small enough to fit within the 2-degree subtense. Furthermore, the CIE daylight locus is mapped on the CIE 1931 (x,y)-chromaticity diagram with chromaticity coordinates based on the CIE 1931 Standard Colorimetric Observer. Thus, the color-matching functions for the CIE 1931 Standard Colorimetric Observer (2-degree observer) are more appropriate for determining the CIE tristimulus values and chromaticity coordinates that are used for monitor calibration for a digital photography workflow.

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).

R. S. Berns, Billmeyer and Saltzman’s Principles of Color Technology, 3rd Edition, John Wiley & Sons, New York, N.Y. (2000).

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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).

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