A 0–100 figure for how much visible light a paint or surface reflects, used in decorating and building design.
Essentially the luminance factor Y of the surface under a stated illuminant, expressed as a percentage. It is used for daylighting calculations and for visual-contrast guidance between surfaces such as walls, floors and doors.
LRV is not the same as the WCAG relative luminance of a colour's HEX value, and a HEX value cannot tell you a real paint's LRV.
A white ceiling paint might be around LRV 85; a charcoal around 10.
Room brightness and accessible contrast between surfaces are specified in LRV difference, not in colour names.
LRV is the luminance factor of a surface: its luminance compared with that of a perfect white diffuser lit and viewed the same way, as a percentage. It is obtained from a spectrophotometer reading by weighting the reflectance curve with the eye's sensitivity under a stated illuminant, which is the Y tristimulus value. In the UK the test method is BS 8493, and Approved Document M treats two surfaces as visually contrasting when their values differ by more than 30 points. Paint makers print the figure on colour cards and data sheets.
| Neutral surface | CIELAB L* | Luminance factor Y |
|---|---|---|
| L* 20 #303030 | 20 | 3.0% |
| L* 50 #777777 | 50 | 18.4% |
| L* 70 #ababab | 70 | 40.7% |
| L* 85 #d4d4d4 | 85 | 66.0% |
| L* 95 #f1f1f1 | 95 | 87.6% |
A surface that looks mid-grey, L* 50, reflects 18.4% of the light, so its reflectance value is about 18, not 50. A pale wall at L* 85 is at 66.0%. Small visual steps near white are large steps in reflected light, which is why choosing a slightly deeper off-white can cost a room a noticeable amount of daylight.
How this was calculated: Y is obtained by inverting the CIELAB lightness formula for a neutral surface. The swatches are screen renderings of those neutrals; a paint's real figure comes from measuring the paint.
A door, its frame and the wall around it are specified in three tasteful, similar greys.
People with low vision cannot find the door. The check is the difference in LRV between adjacent surfaces, and a scheme can meet it with colours of the same hue if their depths are far enough apart.
A north-facing corridor is repainted from an off-white to a fashionable mid-tone.
Each bounce of light off the walls now returns far less, and the drop compounds across several reflections. The space needs more artificial light to reach the level it had.