Goniospectrophotometry

Two colours which appear identical when viewed at one angle, but which no longer match if the viewing angle is changed, are called geometric metamers.Causes of geometric metamerism are:

1.) Differences on the surface of the materials.
One of the reasons for this is differences between the surface structure of the two materials. Commonly one is shiny and smooth, the other dull and rough. Such materials can be made to look the same at one angle, but not at all angles.

Confusion results from materials exhibiting micro differences in the surface structure. The uniformity or homogeneity of the surface structures is another factor which may affect the relative colours when viewed at different angles. Thus, if roughness is not random, differences in the orientation may result in variations in relative colour differences with angle of viewing. E I Stears points out the importance of this variable in viewing irridescent fabrics, or when fluorescent dyes are used which enhance small differences.

Very thin films of materials with refractive characteristics different from the layer beneath can creat some confusing angular colour differences, which are difficult to control. This results in selective specular reflectance characteristics of metals. Hence the term bronzing used in the paint and printing industry to describe this effect.

2.) Differences inside materials.
Differences in the refractive characteristics of different phases inside an object can creat unusual colour effects as in thin film metallic paints. Even materials which have been exposed to weathering may exhibit metamerism because the mechanical structure may have changed.

Measurement of geometric metamerism.
Instruments which measure these phenomena are called goniospectrophotometers or goniocolorimeters. Reflectances have to be measured in full 3-D goniophotometric terms, i.e. with both angular illumination and full angular viewing which requires an elaborate instrument.