Uniformity describes how evenly light is spread across a space. It is the minimum illuminance divided by the average (or sometimes minimum over maximum).
What the terms mean
| Symbol | Meaning |
|---|---|
| U | Uniformity ratio (0 to 1) |
| E₋ | Minimum illuminance, in lux |
| Eₐᵛᶫ | Average illuminance, in lux |
Worked example
If the darkest spot reads 200 lux and the average is 400 lux, U = 0.5.
See the Illuminance Uniformity Calculator.
Lighting uniformity ratio
Uniformity describes how evenly light is spread across a space, and it often matters more than the peak level for comfort and safety. It’s expressed as a ratio — commonly average-to-minimum or maximum-to-minimum illuminance. A lower ratio means more even light: an average-to-minimum near 1.5:1 is good for careful tasks, while 3:1 or 4:1 may be acceptable for general or outdoor areas. A high ratio signals bright pools with dark gaps between them.
To improve uniformity, reduce fixture spacing relative to mounting height, use wider beam distributions, and overlap the light from adjacent fixtures. In outdoor and parking design especially, specifying a maximum uniformity ratio — not just an average level — is what prevents the unsafe dark spots an average figure alone can hide.