Bench glossmeter

Abstract

The fundamentals of an image analysis method to measure surface brightness of various polished rock types are presented. The surface brightness was measured using a computer enhancement technique on illuminated polished rock surface images, taken by a high-resolution video camera. Brightness measurement equipment, which is based on an image analysis principle, was set up in accordance with related ASTM standards. The specimen surfaces were illuminated by a source generating a parallel light beam with a 60° incidence angle. The images of the surfaces were quantified with a video camera placed in the direction of the reflecting light beam with a 60° viewing angle and stored on the hard disk of a computer. A two-dimensional brightness matrix of the image was generated by converting the recorded image into black, white and gray colors using two different types of software. The brightness values measured with this method were divided into reflectance of a black calibration glass having a reflecting index of 1.567 at the same geometrical conditions. Thus, the brightness value of the illuminated surface was determined, and, moreover, the surface irregularities (or defects) causing scattered reflectance were determined based on the brightness matrix at the pixel level. The surface brightness measurements were made on various polished marble, limestone, travertine, onyx and magmatic rock samples and the data were analyzed using computer software to quantify the brightness index values. For each rock type, measurements were made at numerous points on the polished surfaces and isobrightness maps were prepared in order to determine the effect of mineralogical and petrographical factors on surface brightness. Surface brightness is affected negatively by the following factors: (a) porosity, (b) distinct crystal boundaries, (c) cleavages, (d) fillings in the micro-fractures, (e) obliqueness between the crystal orientation and the cutting plane. Additionally, increased mica content in the magmatic rocks resulted in a significant decrease of the reflection capacity of the surfaces. Smaller crystal size and dense crystallization have a brightness enhancing effect on the polished rock surfaces.