Optimal and acceptable color ranges of display primaries for mobile applications

For many new display types, especially for mobile applications, the chromaticity coordinates of the primaries differ from those recommended by the EBU (European Broadcast Union). In order to identify optimal and acceptable ranges in which these primary colors may vary two studies were conducted: one related to the hue of the primaries [also reported in an earlier paper (Ref. 4)] and one related to their saturation. In the first part of the present study, observers set the saturation of one single primary (red, green, or blue) separately or of all three primaries simultaneously, to an optimum, for seven different (natural) images. In the second part, the same observers decreased the saturation until they perceived the image as "just acceptable" (defined as "having natural colors, given its content"). Both parts were conducted on an LCD monitor and repeated later with a different group of subjects on a CRT monitor. In this paper, the results of the current study are reported and they are combined with the results of the previous one. The results of the experiments show that for optimal image quality the saturation of the red and green primaries of a display must be at least 90% with respect to the EBU standard. For the blue primary it can be somewhat lower (at least about 70%). In the acceptance task, observers were the least tolerant in accepting a saturation reduction of the red primary (70%) and the green primary (60%) and they were almost twice as tolerant for the blue primary (35%). Ellipses were fitted to the results of the hue and saturation studies in 1976 CIE (u'v') color space such that display manufacturers can easily test whether the color reproduction of their displays is optimal, acceptable, or unacceptable with respect to reproducing natural colors.