Illuminance is defined as what quantity?

Illuminance is defined as what quantity?

• A. The amount of light that touches a surface per unit area, measured in lux
• B. The total light emitted by a source, measured in lumens
• C. The brightness of a surface, measured in candela per square meter
• D. The energy of light per unit area, measured in watts per square meter

Answer: (A)

Illuminance is the amount of light falling onto a surface per unit area, and it is measured in lux. This means illuminance tells you how much light actually reaches or touches a surface, regardless of the surface’s reflectance. It equals the luminous flux (in lumens) divided by the area (in square meters), so one lux equals one lumen per square meter. The other quantities describe different things: total light emitted by a source is luminous flux (lumens); the brightness of a surface as seen by the eye is luminance (candela per square meter); and energy of light per unit area in watts per square meter is irradiance (radiant flux density, not weighted by human eye response).

Illuminance is the amount of light falling onto a surface per unit area, and it is measured in lux. This means illuminance tells you how much light actually reaches or touches a surface, regardless of the surface’s reflectance. It equals the luminous flux (in lumens) divided by the area (in square meters), so one lux equals one lumen per square meter.

The other quantities describe different things: total light emitted by a source is luminous flux (lumens); the brightness of a surface as seen by the eye is luminance (candela per square meter); and energy of light per unit area in watts per square meter is irradiance (radiant flux density, not weighted by human eye response).