100 lx at 10 ft becomes 25 lx at 20 ft in this model, because (10 / 20)² = 0.25. Is that the pair of distances you care about?
Tool
Light at a distance
The estimate treats the lamp as a point source. Illuminance follows E ∝ 1/d², so E2 = E1 × (d1 / d2)². Distances can be feet or meters, as long as both distances use the same unit. The model is a teaching ratio, not laboratory photometry of a reflector or projector. A real headlamp does not behave like a bare point.
This figure is an educational estimate. It is not a product listing, not DOT or SAE certification, and not legal advice on aftermarket lamps. Aiming and wattage do not make a lamp street-legal. Check the vehicle manual and local law.
When this estimate fits
- You want a point-source ratio, not a headlamp candela map.
- You already have a lux figure at one distance, and you want the same model at another distance.
- You accept that a reflector can keep a beam brighter at range than 1/d² predicts.
Worked numbers from this formula
100 lx at 25 ft becomes 6.25 lx at 100 ft. The model falls quickly. A headlamp beam is not this bare source.
Double the distance and the inverse-square model keeps one quarter of the lux. That sentence is the whole method.
Questions
Is this how far I can see at night?
No. Seeing distance depends on aim, beam pattern, road reflectance, and your eyes. This page only scales a lux number with 1/d².
Why not use candela from a lab report?
This site does not store photometric files. The calculator stays with the point-source assumption written on the page.
Can I enter miles?
No. Both distances are in feet. Convert miles to feet first if you need a long span for a teaching example.
Open the aim calculator if you care about drop on a wall, not lux at range.