What is Level of Illumination

Horace He

Last Updated: August 14, 2023

What is Level of Illumination

Level of illumination is the measurement or calculation of the amount of light present in a specific area, particularly in roadway lighting situations. It is a factor in determining the visibility and safety of roadways. The level of illumination can be determined by calculating either the illuminance levels or the luminance levels, or both. Illuminance is the amount of light that hits the pavement surface and is measured in foot-candles (US customary units) or lux (SI units). On the other hand, luminance refers to the measurement of the reflected light from the pavement surface that is visible to the motorist’s eye. It is measured in candela per square meter (cd/m2). Calculating luminance requires knowledge of the reflectance characteristics of the pavement and the use of lighting design software.

The level and uniformity of illuminance or luminance along a highway depend on various factors, including the lumen output of the light source, luminaire distribution, mounting height, luminaire position, pavement reflectance, and pole spacing and arrangement. Different installation arrangements, such as a few high-output light sources or a greater number of low-output sources, can achieve the same average level of illumination. To ensure accurate lighting calculations, the proper light loss factor (LLF) should be also taken into consideration. The LLF is a depreciation factor applied to the calculated initial average illuminance or luminance.