Backscatter

Also called glare back, light blowback, reflection off dust

Backscatter is light from your own lamps hitting airborne dust, fog or snow and bouncing back toward you, and it is why a badly placed light can leave you seeing less than you would with none.

What is backscatter?

Clean air is invisible. Air with something in it is not. Every dust particle, fog droplet and snowflake your beam touches scatters some of that light in every direction, and a share of it comes back at you. That returning light does not carry any information about the trail. It just arrives at your eyes as brightness.

The amount of it depends on how much lit air is between you and what you are trying to see. Which is why two variables, and only two, govern it: how wide the beam is, because a wide beam lights more air, and how high the lamp sits relative to your eyes, because that decides whether the lit air falls inside your line of sight or below it.

Fog and dust scatter what the International Lighting Vocabulary calls non-selectively: the particles are far larger than a wavelength of light, so the scattering properties do not vary much with colour. That is why fog looks white rather than blue, and it is why the fix for backscatter is geometric rather than chromatic.

Backscatter is what happens in the air. Veiling luminance is what it does to your eye. Two halves of the same problem.

Why more light makes it worse

Because the scatter scales with the light you put into the air, and the object you are looking at only reflects what reaches it. Past a certain point, adding brightness adds more to the veil than it adds to the target. Bullough and Rea found exactly that: high beams at 100,000 candela improved hazard visibility in every fog condition they modelled except the thickest, where the target became harder to see than it had been.

This is the part that surprises people who buy lights by lumen count. In bad air, where the light goes matters more than how much of it there is.

What controls it

Position, and this has been the recommendation for a very long time. NCHRP Report 95, the Highway Research Board's 1970 review of driving in fog, summarised the research of the previous decade in one sentence: headlights should be mounted as far as possible from the driver's line of sight and aimed so as not to illuminate the fog directly in front of the vehicle.

The example it gives is the one that should interest anyone building a truck. Such schemes, it says, are now used with some success on trucks and buses where headlights, fog lights, are mounted near the ground and the driver's seat is very high. A lifted vehicle is that geometry by accident. The question is whether you keep it or spend it.

In practice: mount wide beams low, below your eye line, so the brightly lit air near the truck sits under your sightline rather than in it. Keep tight beams for height, because a narrow beam lights very little air and what it lights is a thin column that stays clear of your eyes until it is a long way ahead. Use lamps with a sharp upper cutoff, which is the optical control that stops a wide beam throwing light above its own horizon. And fit a switch, because the fix for backscatter in the worst conditions is turning the lamp off.