COLOUR PERCEPTION | INSTRUMENTATION
when different individuals are asked to match colours to samples. Local environmental factors and personal differences (age, memory, medication, even mood) among viewers seem to alter perception, which means we can’t be assured of accurate matches. In an industrial environment, this can cause problems such as production delays, material waste and quality control failures. As a consequence, business is increasingly turning to mathematical equations to specify colours and non-subjective measuring devices to ensure matching.
Mapping colour The CIE colour model, or CIE XYZ colour space, created back in 1931, is a mapping system that plots colours in a three-dimensional space using red, green, and blue values as the axes. Variants of this model include CIELAB, defined in 1976, where L refers to luminance, A the red/green axis, and B the blue/yellow axis. Yet another model, CIE L*C*h, factors in lightness, chroma, and hue. Measurement depends upon colorimeters or
spectrophotometers that provide digital descriptions of colours. For instance, the percentages of each of the three primary colours required to match a colour sample are referred to as tristimulus values. Tristimulus colorimeters are used in quality control applications.
Light and perception Colour in light, unlike colour in a pigment, depends on the spectral energy it contains. Objects that appear red reflect red energy while absorbing all others. Without the red energy, a normally “red” object will appear black. The colour of a light source can be described by
measuring the relative powers of various wavelengths. As this spectral power distribution (SPD) changes, so does the way light is reflected to our eyes, which affects the colours we perceive. Light sources are measured according to their
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Left: Light has a big influence on colour perception. The images show how the same colours appear in incandescent (top image) and cool white fluorescent (bottom image) illumination
ability to accurately reveal colours in comparison with natural lighting. This value, determined by the spectrum of the light source, is called a colour rendering index (CRI). The CRI for natural, outdoor light is 100. But natural light varies with the weather, time of
year, time of day, position of a building, and many other factors. Lighting designers can make adjustments through the careful selection of artificial light, while paint and textile colours can be chosen to offset characteristics of natural light. For instance, indirect northern light can make colours appear darker, so a designer might select brighter colours than for a southern exposure.
n Datacolor develops and supplies software, instruments (including a full line of spectrophotom- eters) and services to ensure accurate colour control of materials, products, and images. �
www.datacolor.com
PHOTO: DATACOLOR
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