The formula
E = I / d² ; tilted surface: E = (I / d²) × cos θ
- E
- illuminance on the surface (lux)
- I
- luminous intensity of the source (candela)
- d
- distance from source to surface (metres)
- θ
- angle between the light and the surface normal
Worked example
A 1200 cd lamp 2 m above a bench, and the same lamp lighting a surface tilted 30°.
- Directly below: E = 1200 / 2² = 1200 / 4 = 300 lux
- Tilted 30°: E = 300 × cos 30° = 300 × 0.866
300 lux head-on, dropping to 260 lux on the tilted surface.
Where you'll use it
Illumination engineering questions and practical lighting layout. The inverse square is brutal: doubling the mounting height quarters the light on the floor, which is why high bays need far more powerful lamps.