What Is the True Width and Height of the Specular Peak According the Level of Gloss?

Abstract

When evaluating the gloss of a surface, observers always look in and around the specular direction. Thus, the specular peak contains information connected with gloss. In order to progress in the comprehension and measurement of gloss, it could be therefore useful to study it. But this peak is narrow and not all measurement devices have the proper optical settings to access the peak. Using a dedicated goniospectrophotometer, we have measured the specular peak of a commercial gloss scale with samples varying from matt to full glossy. From these traceable and metrological measurements, we clarified the settings requested to access a reliable BRDF of the surface in the specular direction, according to its gloss level measured with a classical glossmeter. For gloss values higher than 60 GU, the angular resolution of the measurement setup should be lower than 0.05° to ensure proper BRDF measurement.

Physics and Measurement of Properties Linked to Appearance

“…The sharper the peak, the glossier the material is. To access gloss, it is necessary to measure the BRDF with very fine angular resolution and with very high dynamic range, to be able to analyse this sharp peak [6] [7]. But what's the point of covering the entire hemisphere?…”

The Measurement of Specular Gloss using a Conoscopic Goniospectrophotometer

“…In that case, the angular aperture of the source can be increased to obtain more incident flux on the sample. The diameter of the source pinhole (and therefore the source angular aperture) can be increased while keeping a low error on the specular peak measurement following the recommendations given in [9]: to measure the peak width with less than 5% of error, the aperture must be lower than 1° for matt samples (A), 0.66° for semi-matt samples (B) and 0.05° for glossy samples (C). It should be noted than 5% of error on the peak width corresponds to a lower error or the computed gloss value, as it is the total flux comprised in the rectangular aperture that matters.…”