Why do prints look different under other light sources and how can two colour samples look the same under one light but different under another. Metamerism and metameric failure are the terms used to describe these phenomena.

THE SHORT ANSWER

All materials reflect and absorb light by varying amounts. Under a given light source oil paints, acrylics paints and printing inks will all reflect light back to the viewer differently.

Add to the mix that different light sources will often have some form of colour bias, eg the old tungsten light bulb generally had a high red content, the potential for print colours to appear different in a particular circumstance is immense.

THE LONG ANSWER

In colorimetry, metamerism is the matching of apparent color of objects with different spectral power distributions. Colours that match this way are called metamers.

A spectral power distribution describes the proportion of total light emitted, transmitted, or reflected by a colour sample at every visible wavelength; it precisely defines the light from any physical stimulus. However, the human eye contains only three colour receptors (cone cells), which means that all colors are reduced to three sensory quantities, called the tristimulus values. Metamerism occurs because each type of cone responds to the cumulative energy from a broad range of wavelengths, so that different combinations of light across all wavelengths can produce an equivalent receptor response and the same tristimulus values or colour sensation.

Sources of metamerism

Metameric matches are quite common, especially in near neutral (grayed or whitish colours) or dark colours. As colours become lighter or more saturated, the range of possible metameric matches (different combinations of light wavelengths) becomes smaller, especially in surface colours.

Metameric matches made between two light sources provide the trichromatic basis of colorimetry. For any given light stimulus, regardless of the form of its spectral emittance curve, there always exists a unique mixture of three "primary" lights that when added together, or added to the stimulus, will exactly match it.

The basis for nearly all commercially available colour image reproduction processes such as photography, television, printing, and digital imaging, is the ability to make metameric colour matches.

Making metamerism matches using reflective materials is more complex. The appearance of surface colours is defined by the product of the spectral reflectance curve of the material and the spectral emittance curve of the light source shining on it. As a result, the colour of surfaces depends on the light source used to illuminate them.

Metameric failure

The term illuminant metameric failure is sometimes used to describe situations where two material samples match when viewed under one light source but not another. Most types of fluorescent lights produce an irregular or peaky spectral emittance curve, so that two materials under fluorescent light might not match, even though they are a metameric match to an incandescent "white" light source with a nearly flat or smooth emittance curve. Material colours that match under one source will often appear different under the other.

Normally, material attributes such as translucency, gloss or surface texture are not considered in colour matching. However geometric metameric failure can occur when two samples match when viewed from one angle, but then fail to match when viewed from a different angle. A common example is the colour variation that appears in pearlescent auto finishes or "metallic" paper; e.g., Kodak Endura Metallic, Fujicolor Crystal Archive Digital Pearl.

Observer metameric failure can occur because of differences in colour vision between observers. The common source of observer metameric failure is colour blindness, but it is also not uncommon among "normal" observers. In all cases, the proportion of long-wavelength-sensitive cones to medium-wavelength-sensitive cones in the retina, the profile of light sensitivity in each type of cone, and the amount of yellowing in the lens and macular pigment of the eye, differs from one person to the next. This alters the relative importance of different wavelengths in a spectral power distribution to each observer's colour perception. As a result, two spectrally dissimilar lights or surfaces may produce a colour match for one observer but fail to match when viewed by a second observer.

Finally, field-size metameric failure occurs because the relative proportions of the three cone types in the retina vary from the center of the visual field to the periphery, so that colours that match when viewed as very small, centrally fixated areas may appear different when presented as large colour areas. In many industrial applications, large field colour matches are used to define colour tolerances.

The difference in the spectral compositions of two metameric stimuli is often referred to as the degree of metamerism. The sensitivity of a metameric match to any changes in the spectral elements that form the colours depend on the degree of metamerism. Two stimuli with a high degree of metamerism are likely to be very sensitive to any changes in the illuminant, material composition, observer, field of view, etc.

The word metamerism is often incorrectly used to indicate a metameric failure rather than a match, or to describe a situation in which two colors are highly metameric, and hence the metameric match is easily degraded by a slight change in conditions, such as a change in illuminant.

Extra technical data courtesy of Wikipedia