Three Lustrous Material Panels, 36" x 36"
These three panels are the same size, same design, even same size squares. They serve to illustrate the differences between three lustrous surfaces; lustrous concrete (here terracotta-colored concrete), brushed metal (blue brushed aluminum), and wood veneers (Etimoe, or African Rosewood).
One can see three bright diagonal areas angling upward to the left in both the Etimoe on the right and the blue aluminum in the center. When I did the concrete, I did the exact same layout for the mold, forgetting that the inside mold surface is reversed. The result is that the three bright diagonal areas angle upward to the right.
Most materials have surfaces which are non-lustrous. Light which strikes the surface is scattered, or redirected, away in more or less a uniform 180 degree spread. This is technically an isotropic (uniform) light redistribution. Lustrous materials have a surface condition which causes light to be redirected more strongly in one direction than others, and is anisotropic, or non-uniform.
The panels above serve to illustrate refractive and reflective luster. The wood has surface cells that are semi-transparent, looking like tea-colored glass under a microscope. Light enters from one direction, and can only emerge from the cells according to principles governed by the index of refraction, like a camera lens. Because the cells are not all uniform, curving somewhat as they grow, the lens effect is somewhat inexact, and the luster areas are relatively wide.
The brushed aluminum has a surface that was sanded (brushed) in one direction to give a uniform surface, which in reality is a series of linear scratches. For purposes of this discussion they may be considered a multiplicity of very long, very shallow mirrors. For this reason the luster area is very confined, the brightness quite intense in the squares in the middle of the light bands and quickly falling off at any distance from those squares.
The lustrous concrete is an example of a lustrous opaque material. This may be a new type of luster, differing from both the mirror effect of the metal and the refraction of the wood cells. It is caused by articulating the surface in a series of ridges and grooves at particular angular relationships with each other. The luster from this technique (patent pending) is broader and softer than either the metal or the wood.