Digital Color

The computer monitor displays color using varying levels of RGB. (Generally 0-255 of each, but depending on the video card and monitor.) This corresponds to 256 = 28 levels per color (8 bits) and 16,777,216 = 224 (24 bits) total number of colors.Thus, 24 bit color has become the standard for “true color” reproduction and storage. Although 24 bits is standard, it doesn’t mean that the human eye perceives 24 bit color.

The human responses to different parts of the color gamut are not uniform. We have greater sensitivity to neutral colors than to saturated colors, lighter than to darker and to certain hues over others. The adaptive nature of perception makes us respond to relative differences in color rather than absolute ones. To replicate the full range of intensities

However it has been found that the number of discernible hues is about 200 and the number of gray levels is between 60 and 150, depending of level of background illumination. Thus, 24-bit color provides a sufficient number of colors but, because the perception is nonlinear, there is usually too much or too little information.

Color Constancy - Human vision can recognize objects under enormous variations in lighting conditions. Daylight varies from 1 lux (lumens/s) to over 100,000 in bright sunshine and in color from pink at dawn to orange at sunset, through the yellow of direct sunshine to the bluer light of the sky.

Indoors, artificial lighting is normally lacking in blue wavelengths, making objects appear more yellow. Yet an object is always recognizable, we perceive the same flower indoors as outdoors. A rose is a rose is a rose.

Equally we recognize brightness of a color under widely different levels of illumination. The factors influencing perception can be understood in terms of the light capture mechanism of the eye. The iris controls the amount of light entering the eye by adjusting its opening between 1.5 and 8 mm, like a camera aperture.

The ganglion cells also contribute to the “normalizing” of the perception of light intensity. The perception of an object is not just a “scan” of the visual field but a result of comparing it with its surroundings and a memory of what the object looks like.

Consequences of color constancy are the tendency to perceive any near white as neutral unless as compared with other near neutrals and the tendency for color perception to be influenced by surrounding colors.

Color Temperature - The exact color of “white” light is important, since if it tends towards one part of the spectrum, anything viewed in that light will have a color cast. The whiteness of a color is characterized in terms of its color temperature. The color temperature is defined in terms of the corresponding black body radiation intensity versus wavelength plot. The color temperature of unfiltered sunlight is approximately 5900 oK.

 

Standard color temperatures between 5000 oK and 7000 oK are used in the graphic arts. 5000 oK to is often used to define “white” light. 6500 oK is the default for Photoshop and is used in Europe

Other concepts are color rendering index and spectral power distribution. The color rendering index is a measure of how well a light source conforms to natural daylight on a scale from 0 to 100 (will define more precisely later). The spectral power curve is a plot of light intensity versus wavelength. A light source with a color temperature of 5000 oK and a spectral distribution similar to natural daylight is called a D50 illuminant. This combines color temperature and color rendering index.