A system and method for measuring lubricant thickness and degradation, carbon wear and carbon thickness, and surface roughness of thin film magnetic disks at angles that are not substantially Brewster's angle of the thin film (carbon) protective overcoat. A focused optical light whose polarization can be switched between P or S polarization is incident at an angle to the surface of the thin film magnetic disk. The range of angles can be from zero degrees from normal to near Brewster's angle and from an angle greater than Brewster's angle to 90 degrees. This range of angles allows the easy measurement of the change in lubricant thickness due to the interaction of the thin film head, the absolute lubricant thickness and degradation of the lubricant. It also allows the measurement of changes in carbon thickness and the absolute carbon thickness. The surface roughness can also be measured at any of the angles specified above. The present invention utilizes a first and second histogram representing disk properties (1) at two different time periods, (2) at two different locations on the disk, or (3) on two different disks.