Applied Color Science

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APPLIED COLOR SCIENCE

On This Page: • Physical Concepts • Psychological Concepts • Psychophysical Concepts • Psychophysics Topics

There are three distinct kinds of concepts relating to applied color perception: physical, psychological, and psychophysical. A lot of confusion can be avoided by knowing which kind we're talking about as we go along. While a few physical concepts occasionally crop up, and the user's perceptions are the goal of the display, most of the technical content of applied color perception is in the psychophysical category.

On this page we briefly describe the distinctions among the three types of concepts and give examples of each. We then provide annotated links to pages describing some psychophysics concepts.

A graph that displays the radiant brightness of different wavelengths of light (color). Reds have the ihighest radiant power, blue has the least readiant power with green in the middle. Physical Concepts

From a purely physical standpoint light is not colored. While there are physical optics dimensions (e.g., wavelength, frequency, quantum energy) that are closely connected with the effect of the light on the human visual system, the color is in the visual system, not in the light. Consequently, physical concepts show up in design of color graphics mostly in their subordinate role within the psychophysical concepts.

In purely physical terms, display primaries are described by their energy spectra, the radiant power at each wavelength of the light.

Psychological Color Concepts

Color is part of the user's mental response to the light entering the eyes from the display and its surroundings. The mental imagery stimulated by the display is private to the user, which poses challenges to scientific study. Debate about the structure of visual experience started early in the history of philosophy and continues to the present. For our immediate concern, applied color design, we can start with a few highly simplified attempts to describe the structure of color experience.

Most (but not all) color experience can be described within three dimensions. There are several alternative three-dimensional descriptions, but probably the most familiar is hue, saturation, and brightness.

Hue. Hue is the dimension of color experience that most people mean by the nontechnical term "color". It is described by the labels "red", "blue", "green", "magenta", etc.

Saturation. Saturation is the dimension of color experience which ranges from "colorless" through "pale" to "vivid". The perceived color can be described in terms of whether it appears closer to the "colorless" end or the purely chromatic end. Saturation increases from left to right in this figure.

Brightness. Brightness is the apparent amount of light, varying from "dim" to "bright". Brightness increases from bottom to top in this figure.

A display of a variety of saturation levels for one hue on a grey background.

Psychophysical Color Concepts

In color graphics design, as in other applied color disciplines, we are mainly interested in how to manipulate physical lights to produce desired perceptual responses in our users. This means we are mostly dealing with psychophysical concepts, concepts that bridge the gap between physical stimuli and psychological responses. Luminance and chromaticity, for example, are psychophysical concepts.

Applied color scientists and basic color scientists both use psychophysical concepts but they have different approaches. The applied science is engineering oriented and need only be concerned with reliable prediction and control. The basic science has the added task of explaining the underlying visual processes.

The psychophysical concepts used in applied work are necessarily approximations. To achieve a standard, shared technical description of the psychological effects of light stimuli, international organizations have developed a "standard observer". This is a hypothetical typical visual system that is described in terms of equations relating its quantitative visual responses to measurable physical statistics of light stimuli. The equations that define the standard observer are based on averages of laboratory measurements of the visual responses of real human subjects to particular light stimuli under particular viewing conditions.

The nature of the standard observer imposes at least two concerns for us to keep in mind when designing from psychophysical concepts:
1) Your users' visual systems will differ in varying degrees from the standard observer. The concepts have proven useful because most users are approximately like the standard observer, but some people are substantially different. 4-5% of the European population have anomolous color vision.
2) The context in which your users view the lights will usually differ from those under which the original standardizing data were collected. The perceived colors of two patches of light with exactly the same chromaticity coordinates and luminance will generally not exactly match if there are differences in the viewing contexts. Most of the important context differences that need to be considered are covered in these pages.

Psychophysics Color Topics

Luminance and Chromaticity. These are the quantities that define the color of physical lights for the standard observer.

Individual Differences in Color Vision. There are varying degrees and kinds of difference ranging from slight color anomalies to full dichromacy ("color blindness").

A sample chromaticity graph. Related to the Luminance and Chromaticity page.

Color Discrimination and Identification. The ability to tell colors apart and identify them depends on a number of graphic variables.

Demonstration of a color discrimination principle. Color discrimination is lost when the size of the colored item is reduced. Related to the Color Discrimination page.

Designing with Blue. The visual system's response to short-wave light has some special limitations that affect the usability of color graphics.

magenta on red	(magenta on red)
red on magenta	(red on magenta)
cyan on green	(cyan on green )
green on cyan	(green on cyan )

Masking by Patterns. The legibility of symbols can be affected by surrounding patterns.

Demonstration of different background patterns and their effect on the readability of text. Related to the Pattern Masking page.

Blinking, Flashing, and Temporal Response . Blinking and flashing can be used to increase salience beyond what color choices can produce.

Sample graph of a blinking stimulus that shows Luminace over time. Related to the Blinking and Flashing page.

Simultaneous and Successive Contrast. The color fields that surround a colored object and the color fields viewed prior to looking at the object can affect its color appearance. These phenomena are among the earliest studied in color science.

Sample image that displays a simultaneous contrast effect. Different saturated backgrounds can make a color sample look different. Related to the Simultaneous and Successive Contrast page.

Display Hardware and Software. Display hardware issues such as gamut, gamma, and calibration are related to visual psychophysics and affect design of color graphics.

Luminance Contrast in Color Graphics. Control of luminance contrast is a major concern in design of graphics. It is much more difficult in color graphics than in grayscale graphics.