152 Pages

Intensity Reflection Models

Fig. 21. The base-characteristics converge on the anti-solar point.

We can no longer simply integrate. It is easy, however, to solve the above differen­ tial equation for z by separating terms:

and so

Finally, note that the phase angle g is no longer constant. This has to be taken into account when calculating //£ from the measured image intensity. On the whole, the process is still very simple. The paths of integration are predetermined straight lines in the image radiating from the anti-solar point. At each point we measure the image intensity, determining which value of І/E gives rise to this image intensity. Then we calculate the corresponding slope along the straight line and take a small step. Repeating the process for all lines crossing the image we obtain the surface elevation at all points in the image. The same result can be obtained by a very complex algebraic method [6].