#CHROMA DEFINITION SERIES#

To perceive colours of objects we must be able to make comparisons between areas of the visual field. If we were to view the world through a hole in a screen that appeared as a mere speck of light, we would be able to describe the hue, brightness, colourfulness and saturation of that light, but we could not tell if the darker, duller specks represented dark, dull objects in bright light, or bright, coloured objects in dim light. Illumination in Relation to Chroma & Saturation In other digital colour spaces and contexts so-called “saturation” can be a very different parameter, such as chroma or a relative chroma of some kind. The parameter called “Saturation” (S) in HSB colour space, used for example in the Adobe Photoshop colour picker, is a physical estimate of the saturation of an RGB colour relative to the maximum possible for its Hue Angle (H), and is based on a very simple calculation from its r, g and b components (inset, Fig. Areas displaying high saturation (R1-R3), moderate saturation (P1-P3) and zero saturation (W1-W3).

#CHROMA DEFINITION SERIES#

Within each series the light emitted by the areas has the same ratio of RGB components, varying only in brightness, and is seen respectively as relatively pure red light (high saturation, R1-R3), as white light (zero saturation, W1-W3), and as a fixed ratio of red and white light components (moderate saturation, P1-P3).įigure 1. Figure 1 illustrates three series of areas in each of which saturation should appear approximately constant. Saturation is the relative colourfulness of that light, independent of its brightness, or its freedom from whitishness, and is our perception of its relative energy imbalance through the spectrum, as detected by our visual receptors. Saturation is defined in terms of two attributes of the appearance of a single area, and thus like those attributes (colourfulness and brightness) depends entirely on the appearance of the light remitted by that area to the eye. Chroma is the “colourfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting” (17-139).

Saturation is the “colourfulness of an area judged in proportion to its brightness” (17-1136).

Colourfulness is the “attribute of a visual perception according to which the perceived colour of an area appears to be more or less chromatic” (17-233).

The distinction rests on an important difference between the colours of light reaching our eyes from the various parts of an object and the colour we see as belonging to the object itself.

The words chroma and saturation are often used interchangeably, but are defined as distinct concepts by the Commission Internationale de l’Eclairage (CIE), whose terminology is widely accepted as standard in science and technology.

Chroma definition