ASTM D2244:14 pdf free download – Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates.
Unlike many previous color difference equations. C1E94 comes with a well defined set of conditions under which the equation will provide optimum results and departures from this set of conditions will cause the agreement between the visually evaluated color-difference and the computed color-difference to be significantly poorer. Those conditions are given in Table 1. The parameters k,. k(.. kH are the parametric factors that can be used to compensate for texture and other specimen presentation effects. These should not be used to introduce a commercial factor into the equation. For more information on the use of commercial factors in color tolerance equations. see Appendix X3. All the k values default to I in the absence of specific information or agreement between parties. The parameters S,. S, SH are used to perform the local distortion of CIELAB color space, again based on the position of the standard specimen in that space. They are computed using the following equations:
In Eq 24, the value of C is taken to be that of the standard specimen.
6.5 D1N99 Color Difference Equation—The publication in
1996 of the paper by Rohner and Rich (4) prompted the German standards institute (DIN) to further develop and standardize a modified version as a new color difference formula that globally models color space using logarithms of the CIELAB coordinates rather than the linear and hyperbolic functions of CMC and C1E94. The equations derived and documented in standard DIN 6176 provides an axes rotation and the logarithmic expansion of the new axes to match that of the spacing of the C1E94 color tolerance formula without the need to make the specimen identified as standard the source of the distortion of distances in the CIELAB color space. Also, as neither the tristimulus values XYZ nor the CIELAB axes are perceptual variables while the axes L*. C and /1* (lb arc correlates of the perceptions of lightness. chroma and hue, it seemed appropriate to scale the differences or distances in color space following the Weber-Fechner law of perception. This resulted in a formula which is easy to use and has equivalent performance to CMC or C1E94. It also eliminates the annoying reference-color based distortion of CIELAB. Thus computed color differences are based only on the Euclidean distance in the D1N99 space. The procedures for computing the D1N99 formula are.