Illuminance refers to the amount of light from a natural or artificial light source that falls on a surface. It is usually used to describe the usable light , regardless of the total brightness of the light source.
- When a book is placed on a table, different levels of illuminance can be observed depending on whether the sky is overcast, the time of day, or whether the surface is indirectly lit.
- Illuminance is a measure of the amount of light that falls on a surface per unit area. It is determined by the intensity of the light source and the distance from the light source to the surface, but is independent of the characteristics of the surface it strikes, such as its colour or reflectivity.
- Illuminance is a measure of the quantity of light that falls on a surface, but it does not provide information about the spectral composition or other characteristics of the light. Other measures such as spectral power distribution, colour temperature, and colour rendering index can be used to describe other qualities of light beyond illuminance.
- The brightness of a light source does not necessarily correspond with its illuminance. For example, a 10-watt light bulb placed next to a person reading a book can provide enough illuminance, while a 1000-watt light bulb located far away may not provide enough light to read by.
- Illuminance is typically measured in units of lux (lx).