System and method for determining relative luminosity of display chromatic channels
Original Publication Date: 2001-Nov-10
Included in the Prior Art Database: 2003-Jun-20
This invention is a method for determining the relative luminous effect (luminosity) of chromatic channels in a display, for a particular observer. Luminosity of a color is a function of both the radiometric output of the display and of the apparent lightness of that output to a particular observer. Because of both genetic and acquired (from disease or visual adaptation) color vision deficiencies and anomalies, and because effective spectral characteristics of displays may be altered by viewing throughf colored filters (e.g. sunglasses), luminosity of colors will vary from observer to observer (or among viewing situations). This invention provides data that can be acquired in seconds that can be used to calibrate the display or images displayed on it for a particular observer so that the apparent lightness, or luminosity of colors is known. It addresses issues of perceptual discriminability and legibility of display imagery and can be used, for example, to compare discriminability of pairs of colors (i.e. color contrasts) within the gamut of a display for a particular observer. Acquired color deficiencies accompanying vision-impairment are an especially strong determinant of such variations, and because people with vision impairment often function close to discrimination and legibility thresholds, this invention has particular importance in the areas of accessibility of display technology by visually-disabled people. It is also potentially useful in providing appropriate data to calibrate luminosity in data visualization software. The method involves allowing an observer to adjust the relative contribution of the chromatic channels by presenting on the display sums of pairs of oppositely-moving sinusoidal gratings (i.e. gratings with sinusoidal intensity profiles). The observer adjusts the relative contribution (amplitude) of each grating until that null motion is perceived. Each grating member of a pair is has colors which vary only within a single chromatic channel (e.g. the red channel of an LCD display); so for example, a red sinusoidal grating would have alternating red and black bars, with the amount of red being a sine function in a direction perpendicular to the orientation of the grating. Additionally each member of a pair moves at a constant speed, in a direction opposite to its fellow. For a typical LCD display with three channels, for example, the pairs of oppositely-moving gratings might be red-moving up summed with green-moving downward, green-moving up summed with blue-moving downward, and red-moving upward summed with blue moving downward. The appearance of such displays is of motion in the direction of the grating component whose color has a stronger luminous effect on the observer. By adjusting the ratio of intensities of the two channel's intensities to null the apparent motion, the observer provides an immediate measure of their relative luminosities. The relative luminosities of the color channels can then be used to compute the relative luminosities of any color in the gamut of the display. The sum of sinusoidal gratings (of a single color, and of equal amplitude) moving in opposite directions is known to be mathematically equivalent to a single sinusoidal grating whose contrast reverses (i.e. dark bars become bright bars, then the opposite, etc.) in time at the same temporal frequency as the motion of a single moving component. The nulling of motion of the oppositely moving gratings of this invention appears to be aided by this equivalence, in that, when nulled, the display does appear similar to a single contrast reversing grating. A perceptual luminance channel that responds to the relative (achromatic) luminosity of the monochrome gratings might underlie this phenomenon.