Illuminance is defined as a measure of the amount of light falling on a surface. Which statement best describes illuminance?

• A. The density of the luminous flux incident on a surface
• B. The total luminous flux emitted by a light source
• C. The luminous intensity of a point source
• D. The reflectance of a surface

Answer: (A)

Illuminance measures how much light is falling onto a surface per unit area. It is the density of luminous flux incident on that surface, expressed in lux (lumens per square meter). This distinguishes it from the total luminous flux emitted by a source (the overall light output in lumens), the luminous intensity of a point source (light per unit solid angle, in candelas), and the reflectance of a surface (the fraction of incident light that is reflected). So the statement describing illuminance as the density of luminous flux incident on a surface is the correct one.