Measurement of polished rock surface brightness by image analysis method

The fundamentals of an image analysis method to measure surface brightness of various polished rock types are presented. The surface brightness was me asured using a computer enhancement technique on illuminated polished rock surface images, taken by a high-resolution video camera. Brightness measure ment equipment, which is based on an image analysis principle, was set up i n accordance with related ASTM standards. The specimen surfaces were illumi nated by a source generating a parallel light beam with a 60 degrees incide nce angle. The images of the surfaces were quantified with a video camera p laced in the direction of the reflecting light beam with a 60 degrees viewi ng angle and stored on the hard disk of a computer. A two-dimensional brigh tness matrix of the image was generated by converting the recorded image in to black, white and gray colors using two different types of software. The brightness values measured with this method were divided into reflectance o f a black calibration glass having a reflecting index of 1.567 at the same geometrical conditions. Thus, the brightness value of the illuminated surfa ce was determined, and, moreover, the surface irregularities (or defects) c ausing scattered reflectance were determined based on the brightness matrix at the pixel level. The surface brightness measurements were made on vario us polished marble, limestone, travertine, onyx and magmatic rock samples a nd the data were analyzed using computer software to quantify the brightnes s index values. For each rock type, measurements were made at numerous poin ts on the polished surfaces and isobrightness maps were prepared in order t o determine the effect of mineralogical and petrographical factors on surfa ce brightness. Surface brightness is affected negatively by the following f actors: (a) porosity, (b) distinct crystal boundaries, (c) cleavages, (d) f illings in the micro-fractures, (e) obliqueness between the crystal orienta tion and the cutting plane. Additionally, increased mica content in the mag matic rocks resulted in a significant decrease of the reflection capacity o f the surfaces. Smaller crystal size and dense crystallization have a brigh tness enhancing effect on the polished rock surfaces. (C) 2000 Elsevier Sci ence B.V. All rights reserved.