Details

In the first three snapshots, the spheres vary in diffuse reflectivity.

Snapshot 1: no specular reflectivity

Snapshot 2: adds specular reflectivity at the same moderate exponent as in the main output graphic, but with both diffuse and specular reflectivity increased so that the total intensity of light reflected from polygons facing the light sources is generously larger than 1

Snapshot 3: a high specular exponent with the reflectivities decreased to keep all reflected light intensities less than 1

Snapshots 4-6: The spheres vary in specular rather than diffuse reflectivity. The diffuse reflectivity is absent or at a very low level so that the effect of varying the specular exponent is emphasized. Note that a specular exponent of 1 gives reflectivity somewhat similar to diffuse reflectivity, but not quite the same because the reflected intensity depends on the difference between the viewing angle and the specular reflection angle rather than the angle between the incident light and the surface normal.

The lightness control crudely approximates the Munsell color system’s vertical axis; see the Wikipedia articles on lightness and relative luminance. The RGB values are manipulated in a simple way (see the source code), so that the perimeter sphere’s colors are fully saturated at lightness 1/2, smoothly decrease to black at lightness 0, and increase to white at lightness 1.