CIELAB: The Basis of Color Measurement (Part 1 - CIE and XYZ)

Color measurement makes it possible to objectively record color and compare it reproducibly. One of the most important systems in colorimetry is CIELAB, developed based on the work of the CIE. This article describes the origins of the system, the role of the CIE, and the basis of the XYZ tristimulus values from which CIELAB is derived.

The Role of the CIE

The Commission Internationale de l'Eclairage (CIE) is the international commission that develops technical standards for light, vision, and color. In the early 20th century, the CIE investigated how colors could be numerically recorded based on additive color mixing. This led in 1931 to the CIE color system, the basis of modern color measurement (colorimetry).

The CIE defined a methodology whereby each color can be described with three numbers: X, Y, and Z. These tristimulus values form the basis for all subsequent color models, including CIELAB.

Color Matching Experiments

The original research consisted of a series of experiments:

• 17 subjects with normal color vision participated.
• Each subject looked at a stimulus of 2° visual angle consisting of two halves:
  • One half with a spectral color (single wavelength)
  • One half that had to be matched by mixing
• The match was made by mixing light from three primary wavelengths:
  • 700 nm (red)
  • 546.1 nm (green)
  • 435.8 nm (blue)

The goal was to achieve a visual match using different intensities of these three primaries.

The Problem of Negative Values

At certain wavelengths, subjects could only achieve a match when the spectral stimulus was made less saturated. In terms of the model, this meant that one of the primaries would need to have a negative intensity.

The implication:

• A measuring instrument would need six channels (positive and negative for R, G, and B).
• Practically, this was not feasible.

Solution: Imaginary Primaries X, Y, and Z

The CIE developed a transformation so that the primary functions are always positive. This led to the imaginary primaries:

• X
• Y
• Z

Characteristics:

• They are imaginary colors, not physically producible.
• They lie outside the triangle of actually realizable RGB light colors.
• They have a higher theoretical saturation than spectral colors.

With this definition, all color stimuli could be described without negative tristimulus values.

The corresponding functions are known as the:

• CIE 1931 2° Standard Observer
• Applicable to stimuli between 1° and 4° visual angle

For larger fields, the CIE later defined:

• CIE 1964 10° Standard Observer
• Different color matching functions due to variation in perception at larger visual fields

From XYZ to CIELAB (Continued in Later Parts)

XYZ forms the basis of CIELAB, a more perceptually uniform color space. Subsequent parts will cover:

• The development of CIELAB
• The L*, a*, b* axes
• Perceptual uniformity
• Color difference calculations such as ΔE

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