Research Highlights: 22 June 2007

The Analysis of Gloss

Gloss is a familiar optical phenomenon, but the quantitative analysis of gloss remains a confusing problem. Traditional gloss meters provide insufficient information to fully characterize the phenomenon.

Research currently under way involves the development of instruments and analytical protocols that are cable of providing significantly more useful information about gloss than can be obtained with a traditional gloss meter. The instrument developed in this project is a relatively simple device that can be constructed on the laboratory bench with off the shelf components at a cost well below $10K. The project has made signficant project toward two primary goals: (A) relating the instrumental measurements to the fundamental material properties of printed images and (B) relating the instrumental measurements to the visual characteristics of gloss and gloss variation. Below is a description of the instrument and calibration to absolute material properties.

A complete analysis of gloss requires the measurement of specular light over a range of angles and also over spatial dimensions. An instrument capable of providing such measurements has been developed in this laboratory and is called a micro-goniophotometer. A schematic of the instrument is shown below. The instrument separates the specular and bulk diffuse components of the light and produces a separate image of each. The specular image, illustrated below, is scanned in software to produce a goniophotometric function (BRDF) of the specular light versus surface tilt angle. From this function, a variety of gloss metrics are extracted.

Details on the construction of the instrument and the interpretation of data from the instrument have been published. Complimentary copies are available below. The manuscripts are copyrighted and for your personal use only and should not be distributed further.

"A Micro-Goniophotometer and the Measurement of Print Gloss", J. Imag. Sci. & Technol, (48)485(2004).

"Color Properties of Specular Reflections", J. Imag. Sci & Technol, (50)228(2006).

"Interpretation of Gloss Meter Measurements", J. Imag. Sci & Technol, (50)567(2006).

"Analysis of Paper Gloss", J. Pulp. Paper Sci., (32)19(2006).

"Analysis of Print Gloss with a Calibrated Micro-Goniophotometer", J. Imag. Sci & Technol, in press (2007).

"Interpretation of the Results of a Micro-Goniophotometric Analysis", internal laboratory report.

The schematic diagram shown below illustrates the vertical scan of a specular image. The result is a scan of specular variation, and the scan can be used to extract a gloss noise power spectrum. In addition, the variation magnitude (standard deviation) can be measured and plotted as a function of the sample tilt angle. These analytical techniques provide a method for characterizing gloss granularity, and a report on this work is (J. Imag. Sci & Technol, (51)293(2007).

Development of analytical protocols has recently lead to the absolute calibration of the instrument. The area under the BRDF curve has been shown to be a measure of the specular reflectance factor, , of the material. A sample of black nylon was used as a reference sample (refractive index n =1.53, ref = 0.044), and the areas, A, of the BRDF were used to calculate the Fresnel specular reflectance factor, = (A/Aref) ref . By varying the angle of illumination, the reflectance factor, , can be measured as a function of the illumination angle, as shown below for a black printed sample from an electrophotographic printer. The data are shown as points, and the line is Fresnel's theory with optical constants n = 1.50 and = 0.00. Data for both s and p directions of polarization are shown. A preliminary report on this work will be presented at the IS&T NIP23 Conference in the Fall of 2007.

The value of was measured experimentally at = 0 for a wide variety of materials of known optical behavior. The measured value of are compared with the published values of in the graphs shown below.