If dΦ is the luminous flux incident normally on an area dA, then Illuminance is given as

• A. Φ × A
• B. dA ⁄ dΦ
• C. dΦ − dA
• D. dΦ ⁄ dA

Answer: (D)

Luminous flux measures the total radiant power reaching a surface, but without regard to surface position: the power might be concentrated on only part of the surface or spread evenly across it.
Irradiance (Ee) or illuminance (Ev) is the density of incident radiant flux or luminous flux at a point on a surface and is defined as radiant flux or luminous flux per unit area, as given by
E = dφ/dA
where (dφ) is the radiant flux or luminous flux incident on an element dA of the surface containing the point. The unit of irradiance is Wm−2, and that of illuminance is lux