BRDF,—angle plots

C. Interpreting Scatter Data 1. BRDF Angle Plots... 

It is common practice to plot BRDF with respect to the angle from the specular beam, AO. If scatter is measured only in the PLIN, A0 = 0 —. ... 

Figure 5 shows the BRDF plot for two randomly polished mirrors. As explained previously, this is a plot of scattered intensity versus angle. The 0° point on the horizontal axis indicates the direction of the specular reflection. All other directions on the plot are relative to this direction (0, — 0j in Fig. 4). The vertical axis is the BRDF scatter (on a logarithmic scale). BRDF is typically plotted on a logarithmic scale because it changes over several orders of magnitude within a few degrees of the specular beam.

BRDF is often displayed in a log-log plot. That is, both the BRDF and the angle of scatter (horizontal axis) are plotted on log scales. There are two very good reasons for this. First, it allows a more detailed study of the scatter distribution close to the specular direction and, for a polished surface, this is where most of the light is scattered. Second, for surfaces produced by a random process, a log-log plot generally approximates a straight line. The slope of this plot is characteristic of the process that is used to produce the surface under test. Fig. 6 shows log-log plots for the same two mirrors as in Fig. 5. These mirrors are both produced by the same process. Although the slopes of the log-log plots are very similar, mirror 2 clearly has a rougher surface finish. Conversely, Fig. 7 shows the BRDF plots for two mirrors produced by quite different processes. Mirror... 

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