The Lustre Primer
Lustre is the quality of light a surface gives back — the difference between a mirror, a pearl and a piece of chalk.
When light meets a surface, three things can happen: absorption, transmission and reflection. Lustre concerns only the third, but concerns it completely — how much light returns, in what directions, and whether it carries an image with it.
The governing rule is the law of reflection, known since antiquity: the angle of incidence equals the angle of reflection. On a surface smooth at the scale of a wavelength — for visible light, well under half a micrometre of roughness — every ray obeys it in concert, parallel light stays parallel, and the surface can hold a reflection. This is specular reflection.
On narrow screens, swipe or scroll the plate sideways.
Roughen the surface and the law still holds for every ray, but each microscopic facet points a different way, so the rays scatter. The image dissolves into a directionless glow: diffuse reflection, the optical signature of matte.
Real materials almost never sit at either extreme. A satin varnish is a diffuse base carrying a narrow specular lobe; a worn tabletop shows patches of each. The vocabulary of everyday shine — gloss, sheen, haze, matte — is a set of names for positions in this mixture.
It matters because lustre carries information that colour cannot. The same pigment reads deeper under gloss and flatter under matte; a damp patch, a polished step, a fingerprint on glass are all detected by changed reflection. Learning to read lustre is learning to read surfaces.
Further reading