Abney Effect - Chromaticity Diagrams

Chromaticity Diagrams

Chromaticity diagrams are two-dimensional diagrams that plot the projection of the International Commission on Illumination (CIE) XYZ color space onto the (x, y) plane. It is interesting to note that the X, Y, Z values (or tristimulus values) are simply used as weightings to create new colors from the primary colors, much in the same way that RGB is used for creating colors from primaries in televisions or photographs. The x and y values used to create the chromaticity diagram are created from the XYZ values by dividing X and Y by the sum of X,Y,Z. The chromaticity values that can then be plotted are dependent upon two values: dominant wavelength and saturation. Since luminous energy is not included, certain colors are left out of the diagram. For instance, brown, which is just a low-luminance mixture of orange and red, will not appear on the diagram.

The Abney Effect can be illustrated on chromaticity diagrams as well. If one adds white light to a monochromatic light one will obtain a straight line on the chromaticity diagram. We might imagine that the colors along such a line are all perceived as having the same hue. In reality, this does not hold true, and a hue shift is perceived. Correspondingly, if we plot colors that are perceived as having the same hue (and only differing in purity) we will obtain a curved line.

In chromaticity diagrams, a line that has constant hue must be curved, so that the Abney Effect is accounted for. The chromaticity diagrams that have been corrected for the Abney Effect are therefore excellent illustrations of the non-linear nature of the visual system. Also, the Abney Effect does not disallow any and all straight lines on chromaticity diagrams. One may mix two monochromatic lights and not see a shift in hue, thereby suggesting a straight line plot for the different levels of mixture would be appropriate on a chromaticity diagram.