The display of different intensities of light, reflected both specularly and diffusely from different parts of a surface exposed to the same incident light. High lustre is associated with gross differences of this kind, and empirical measurements of lustre depend on the ratio of the intensities of reflected light for specified angles of incidence and viewing.
Note: This definition makes these differences in intensity of light the key point, since these form the chief subjective impression on the observer of lustre. Both specular and diffuse light must be present together, for, if diffuse light only is present, the surface is matt, not lustrous, whereas, if specular light only is present, the surface is mirror-like, and again not lustrous. The phrase ‘exposed to the same incident light’ has been included to rule out shadow effects, which have no part in lustre proper. The general term ‘surface’ is intended to apply to fibres, yarns, and fabrics, and indeed to other surfaces, e.g., that of a pearl (through there the differently reflecting parts are very close together). In the second sentence of the definition, lustre is regarded as a positive function of the differences, the appropriate adjective of intensification being ‘high’
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The natural gloss or sheen characteristic of the fleeces of long-wool breeds
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