forty-five degree annular (45°a) geometry

irradiation of reflecting materials at 45° to the normal, from all azimuthal directions, simultaneously

NOTE In measuring the colours of reflecting samples by irradiating with this geometry, the effects of texture and directionality are minimized. This irradiation (influx) geometry may be achieved by the use of a small source and an elliptic ring reflector or other aspheric optics. This geometry is sometimes approximated by the use of a number of light sources in a ring or a number of fibre bundles illuminated by a single source and terminated in a ring. Such an approximation to annular geometry is called circumferential geometry, symbol "45°a".

See also CIE 15 Colorimetry (section 5)

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