Colorimetric Evaluation of Color Coordinates and Color Differences According to DIN 6174 Homogeneous CIELAB Color Space

The German Standards Institute (DIN) published the standard “DIN 6174 Colorimetric evaluation of color coordinates and color differences according to the approximately homogeneous CIELAB color space”, which describes a method for the colorimetric calculation of color differences between object colors (typically surface colors such as paints, plastics, and textiles) using the CIELAB color space and the L*, a*, b* color difference formula.

In the color industry, color differences (DIN 5033-1 Colorimetry - Part 1: Basic terms of colorimetry) are increasingly described via color coordinates. Since these coordinates, which describe the magnitude of the perceived color difference between two samples (e.g., between a color target and a color match), are not derived from Euclidean distances within the standard colorimetric system, various empirical color distance formulas have been developed and used over the years.

This standard defines a method for calculating the L*, a*, b* color coordinates of the approximately uniform CIE 1976 (L*a*b*) color space (CIELAB color space) based on the X, Y, Z triple stimulus values of the visually non-uniform color space defined by the standard colorimetric system. It also describes how color differences, particularly object colors, can be quantitatively identified using color difference formulas. Such differences arise, for example, when colored products are affected by chemical or physical factors, when matching colors, or when a product's color is non-uniform. Identifying color differences can form the basis for agreed-upon color tolerances.

CIELAB color coordinates have become widely accepted in many ways within the industry as a way to represent colors, particularly in image processing.

The color difference formulas given here are used specifically for object colors (e.g., textiles, paint, plastics, glass, and printed documents). However, the formulas do not uniformly evaluate small differences in various areas of the color space; that is, color differences that are perceived as visually identical may have different corresponding CIELAB differences. Improved formulas that define the coordinates of the CIELAB color space more uniformly are given, for example, in the DIN 6176 standard or CIE 15 Colorimetry. These formulas were developed for the standard illuminant D65.

Due to different mathematical definitions, color differences determined based on different formulas cannot, as a rule, be converted linearly, and therefore the specific color difference formula used must always be given.

The CIELAB color space (officially named CIE 1976 (L*, a*, b*)) is the most widely used color space for measuring and specifying object colors (such as those in paints, plastics, textiles, and prints). Published in 1976 by the International Commission on Illumination (CIE), it is designed to be perceptually uniform, meaning that a numerical change in color coordinates corresponds to roughly the same amount of perceived color difference throughout the space.

CIELAB's goal is this:

Approximate perceptual uniformity (much better than XYZ or RGB)
Device-independent (human vision-based)
Calculating the total color difference using a simple Euclidean formula.
Serving as a reference field to connect instrumental measurements to visual perception.

The three coordinates mean the following:

Axis L* Range: Interval 0 to 100. As the value increases, the perceptual meaning changes: 0 = black, 100 = perfect white.
Axis a* Red-green axis: Range -128 to +127. As the value increases, the perceptual meaning becomes positive = red/fuchsia, negative = green.
Axis b* Yellow-blue axis: Range from -128 to +127. As the value increases, the perceptual meaning becomes positive = yellow, negative = blue.

Standard measurement conditions are as follows (the most common today):

Illuminator: Typical choice is D65 (daylight 6504 K). Standard reference: CIE 1964 or ISO 3664 standard.
Standard observer: Typical selection, 10 degrees (additional). Standard reference, CIE 1964 standard.
Geometry: Typical choice is d/8 degrees or 45/0. Standard reference: ISO 13655 and the older DIN 6174 standard.

While older devices used 2 degree observers and D65 or C, modern applications prefer 10 degrees + D65.

In CIELAB, the total color difference is simply the Euclidean distance.

The DIN 6174 standard was officially withdrawn in 2010 and replaced by the international standard “EN ISO/CIE 11664-4 Colorimetry - Part 4: CIE 1976 L*a*b* color space”. This new standard also uses the CIE 1976 CIELAB formula. However, the DIN 6174 standard is still frequently referenced in industrial contracts, technical specifications, and older literature in German-speaking countries (Germany, Austria, Switzerland).

The basic content of the DIN 6174 standard is as follows:

Color space: CIELAB (L*, a*, b*)
Illuminator: The standard illuminator D65 (daylight, 6504 K) is the default; other illuminators are allowed if specified.
Observer: 10-degree standard observer (additional observer).
Color difference formula: “E*ab = √[(“L*)2 + (“a*)2 + (“b*)2]
Measurement geometry: Mirrored component permitted, preferably 45/0 or 0/45, including d/8 (diffuse illumination/8-degree field of view).
Reference (white standard): Excellent reflective diffuser (L* = 100, a* = 0, b* = 0).
Tolerance interpretation: This standard does not define numerical fit/fail tolerances; it only provides the calculation method. Tolerances are agreed upon separately between the customer and the supplier.

Despite being withdrawn, the standard continues to be used practically in industry, and the following expressions are still encountered in technical specifications:

"Color tolerance according to DIN 6174 standard: 'E*ab ≤ 1,0'."
"Color tolerance according to DIN 6174 standard."
"Measured below D65/10 degrees according to DIN 6174 standard."

This is legally valid because the latest version of this standard is still publicly available and the calculation method is mathematically the same as the current ISO/CIE standards for "E*ab".

Consequently, the DIN 6174 standard is essentially a German adaptation of the original 1976 CIELAB color difference formula (“E*ab”). Although technically obsolete, its decades of historical use and contractual continuity make it the most frequently cited reference when defining simple total color difference limits in Central European industry.

When the expression “DIN 6174” appears in a specification, it always means: “Calculating the Euclidean distance in CIELAB space under D65/10 degrees”.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services to businesses in accordance with the "DIN 6174 Colorimetric evaluation of color coordinates and color differences according to the approximately homogeneous CIELAB color space" standard.