Veiling luminance

Also called disability glare, glare veil, veiling glare, contrast loss

Veiling luminance is the even glow that scattered light lays over everything in your field of view, and it blinds you not by being bright but by shrinking the difference in brightness between an object and its background.

What is veiling luminance?

Light that has been scattered does not form an image. It arrives from every direction at once and adds an even layer of brightness on top of the scene. The International Lighting Vocabulary defines veiling luminance as the luminance that superimposes on the retinal image and reduces the contrast by stray light in the eye, and it defines the result separately as disability glare: glare that impairs the vision of objects without necessarily causing discomfort.

The vocabulary definition is about stray light inside the eye. Off-road the same word gets used for the same effect produced outside it, by dust and fog scattering your own beam back at you before it ever reaches your cornea. The mechanism differs, the arithmetic does not: a veil is a veil, and both kinds are usually present at once.

The distinction matters because disability glare is measurable and does not depend on how you feel about it. You can be entirely comfortable and still unable to see.

Why it blinds you

Vision runs on contrast, not on brightness. You see a rock because it is a different luminance from the ground behind it, and below a certain difference your visual system cannot separate them at all.

A veil adds the same amount to both. The rock still reflects exactly what it did, the ground still reflects exactly what it did, and the ratio between them collapses. Add enough veil and a well-lit rock becomes invisible without anything about the rock or the lamp changing.

The veil does not hide the object. It hides the edge between the object and everything else, which is the only thing you were ever seeing.

How much difference it makes

Bullough and Rea put numbers on it in 2016, tracing a hundred million rays through simulated fog. Shielding a light source so it stopped throwing light where drivers were not looking raised a target's contrast from 0.13 to 0.14. Giving the same source a proper ten degree beam raised it to 0.26. Their visual performance measure went from 0.328 to 0.647 against a threshold of roughly 0.8 for comfortable driving.

Same lamp, same output, roughly twice the useful vision, purely from controlling where the light went. Their model is a roadside beacon rather than a vehicle lamp, so the mechanism transfers and the exact figures do not.