| Definition: | irradiation of reflecting materials at 45° to the normal, from all azimuthal directions, simultaneously Note 1 to entry: In measuring the colours of reflecting samples by irradiating with the forty-five degree annular geometry, the effects of texture and directionality are minimized. The forty-five degree annular geometry can be achieved by the use of a small source and an elliptic ring reflector or other aspheric optics. Note 2 to entry: Forty-five degree annular geometry is sometimes approximated by the use of a number of 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", with notation "45°c".
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