Color difference

Delta E, often written as dE or AE, is a measure of the perceived color difference, which is " equidistant " possible for all colors occur. The Delta stands as a sign of difference. This can work, dealing with colors, be quantified.

Equidistance

" Same color" to better achieve the same color stimulus, is difficult, especially with different materials and under different external conditions such as metamerism or because of the individuality of the viewer.

In operational practice, the specification of tolerances for instance in terms of delivery is common. When applied to color this means that a pinned color difference AE should be responsible for each hue by most people the same. For the construction of a color space perception is quantified in psychophysical experiments based on just noticeable differences. The goal is that areas of color varieties, their colors are just not yet distinguish, in the color space are spherical and independent of the start point of equal size. In the older xy chromaticity diagram ( " shoe sole " ) was not reached equidistance: David MacAdam stated tolerance ellipses whose direction and magnitude are from the locus -dependent. This condition was met for the first time * a * L b * color space fairly.

Color difference Delta E

The color difference is usually expressed as Delta E. In DIN 5033 Part 2, the term color distance is compared to the concept of color difference preferred. Opposite color difference, it stands for the quantified form. Each color actually occurring, at any of a equipment delivered or measured color, a color point can be assigned in a three dimensional space. This possibility is grounded in Grassmann's law.

The value of delta E between the color locations and is calculated according to ISO 12647 and ISO 13655 as the Euclidean distance:

Further developments of distance formulas

The Lab color space is not sufficient for practical applications equidistant. The saturated color differences are less experienced than can be expected, blue tones are mispriced. Thus, the calculation of the color difference was further developed with the Lab model of the CIE. The modifications to the Euclidean distance into account, inter alia, a better evaluation of the color distance in the blue region and the influence of ambient light, the adaptation to brightness, in a simple form. The influence of brightness perception ( lighter and darker tones) was revalued on color perception.

And as well as CIE94 and CIEDE2000 are the most common follow-up formulas that approach through some very complicated modifications of the CIELAB color difference formula in a visual equidistance better. The application of these models, however, is complicated because of the complex calculations, it took a long time until they could find a wider application. There are software packages for image analysis, color space transformation and the calibration of the devices that already use these newer recommended by the CIE Farbabstandsfomeln.

The model was developed primarily for the textile industry. It is used in this industry, because this industry features, such as the thread structure of tissue are taken into account by appropriate factors.

The new color space DIN99 is the alternative to the established formulas CIE94 and CIEDE2000. In its accuracy compared with the two color difference formula of the calculation is equal to a simple, the calculation of the color difference is the same. With sensation -design of the color space and the color locus thus otherwise described Euclidean distances are possible. Despite its great practical benefits and potential for cost savings is DIN99 in the American-dominated scientific community virtually unknown.

The old calculation from LAB coordinates from the 1980s is still widespread, although the shortcomings are well known. The argument that the transition to better color difference formulas is too expensive, does not observe the follow-up costs due to incorrect color difference evaluation.

Nevertheless, the determination of color differences is inadequate for some purposes, especially as the quality requirements are also rising. Thus, with CAM ( Color Appearance Models, models for color appearance) and IAM ( Image Appearance Models, models for image appearance) made ​​a new fundamental approach, which does not come from the calculation using color coordinates to color differences. The new model categories are based on the revised question: "How will a particular color or an image in the general context of the near and far an image". This further steps towards fairer equidistance perception of color spaces and the results of correction formulas are made ​​possible. The environment of the color observation should be included, such criteria are adaptation, HDR, simultaneous contrast, chromatic adaptation. Target are statements about how an image ( ie the interplay of colors) acts on the viewer.

Applications

Review of AE

For color differences AE Euclidean distances between the L * a * b * values ​​(or from the polar coordinates L * C * h * ) is usually given the named in the table below assessment.

In the CMC system, the value AE = 1, as " tolerable color deviation " listed. Since color spaces only in the vicinity of the spectrum locus are perceptually despite the improvements achieved, is an AE = 10 and higher are preferred to see a different color.

There are several studies to which minimal color difference from the ordinary viewer ( " usual user" ) can still be distinguished. As a result, ( somewhat dependent on the analysis purpose ) both AE = 1, and AE = 2.5 found. In visual matching tests also always adhering to the adaptation time plays a role that is affected by the exercise of the viewer to color differences. Human perception is practiced on color constancy, " trees and meadows are green." Nevertheless, can be given sufficient attention here shades of green yellow for up to blue green.

Such considerations are important for technical system, since in the course of the entire color management process (whole color reproduction workflow ) device-and system-related deviations from calibrations performed occur. The desired quality management of this process but there is a requirement that the " normal viewer " production-related color differences may not be noticeable. Technically normal deviations are in offset and rotogravure printing in color intervals of 2 ... 4 AE in desktop printers and similar output devices even higher deviations can occur.

The study by Stokes et al. states that error below 2.5 AE in real shots, which are considered to CRT displays are not visible.

In order to get useful results in the evaluation of color differences, the conditions must be precisely defined. These include, among other things, illuminance, with or without tolerance indication, time to brightness and color adaptation to the lighting and color or brightness of the near and far surroundings of the viewing field. For visual observations so-called Viewing Booth used to comply with the conditions.

Printing Techniques

With printed materials, distances between original and reproduction must be specified numerically, which is accepted and the sensation of a vast number of viewers. The goal is to obtain a numerical value in order to communicate over color distance and to be able to set the following in contracts for printing and reproduction services tolerances.

Certain values ​​for the allowable differences between test pressure ( Proof) and the printed result to be expected in the production run, set, such as the Print Media Standard.

Here, a AE = 1 a small, barely visible difference. A AE of 5 is clearly visible. Approximately this is a color difference between a gray as a 50% black component, and one having 55 % black. The distance of five Delta E units between the delivered print result and the desired color of the artwork is so bring the service provider normally a reprint or a discount, an economic loss.

For the practical application of the color measurement is the perceptive distance of importance to achieve a good match with the color information. However, the color distance measure also be used for other purposes, but the judged distance in all kinds of colors should be equal.

Metameric colors

The metamerism index indicates how far conditionally distinguish same color under different lighting. The measure here is Delta E of the investigated color at two fixed ( proper ) light types.

At body colors There are two forms of metamerism.

Two conditionally same color have the same color stimulus in a particular type of light, can not be distinguished by the eye, although their absorption spectra are different. One refers to this effect as metamerism. Without metamerism the representation of color images on screens with only three narrow-band primaries ( phosphors ) red, green and blue would not be possible.

Two colors look absolutely identical in all types of light equal, since the absorption spectrum is identical.

Farbrezeptierung

When color matching can be ascertained with the current color difference between color samples and standard recipe which adjustments must be made to improve the quality of the color pattern. Color matching in this sense is at all possible, since (body) colors can be numerically detected and quantified.

Dispersibility

The color difference is used as a comparative measure to assess the dispersibility of colored pigments can.

Aging processes

Colored products are subject to during its lifetime a change in color, as in the yellowing or by extreme environmental influences. This property can be tested by artificial aging and can be described by means of Delta E or evaluate statistically.

Color sensitivity

Color difference sensitivity differs from the above described concept of color distance.

Color difference is due to the color stimulus, The weighting of the color stimulus by the sensitivity functions of the pins.

The difference in color sensitivity, however, is directly based on the color stimulus, in the form of the electromagnetic spectrum. It describes the wavelength-dependent discrimination of the eye for adjacent color ways. In the areas of the pin maxima at about 450 nm, 500 nm and 600 nm, this is the greatest. Here differences between adjacent wavelengths can be perceived by only one nanometer.