Measurement of Differential Gloss Using a μ-Goniophotometer

Abstract

Differential gloss is the term used in print media to describe the condition where areas of a printed image, especially adjacent areas, appear to reflect light in different ways giving these areas varying gloss appearance. This phenomenon is quite common in dry toner electrophotographic imaging and some ink jet imaging technologies where the first surface reflection properties of the toner or ink and the substrate can be markedly different. Also, high density areas composed of multiple layers of toner can have substantially different specular reflection properties than low density areas composed of a sparse layer of toner through which areas of substrate remain visible. Differential gloss, while well known, is difficult to quantify in a meaningful way. Recently, a pilot experiment was conducted that indicated that it may be possible to quantify gloss artifacts using a device called a μ-goniophotometer. This device, previously described in detail in the literature, measures specular reflection at all angles, not just the one equal and opposite angle to the light source as is done with a traditional glossmeter. This device creates a curve or Bi-directional Reflectance Distribution Function (BRDF) for the sample measured. Because it also generates a series of these curves along the length of the sample, a quantification of the variability of the measured specular light, σ, is available. It was this variability that was found to relate to the degree of gloss artifact detected by observers, on average, in flat field images. And it is this variability that was posited to serve as a function of the degree of differential gloss apparent in an image. In the first steps toward testing this theory, experimentation was conducted involving the measurement of patches. The patches were created in conjunction with a differential gloss scaling experiment conducted by the W1.1 Committee on Perceptual Measurement of Gloss. Prints of three scenes were made on equipment exhibiting a range of differential gloss behavior. The prints were visually scaled by observers in several locations across the United States. To make comparative objective measurements, prints of the W1.1 Perceptual Gloss Measurement Committee's patch target were also made. An evaluation of the measured data generated using both a traditional glossmeter as well as the μ-goniophotometer relative to the visual data will be described.