Illuminance is the total luminous flux incident per unit area. It refers to the amount of incident light.

Note that the expression for the amount of light "reflected" from a surface is luminance.

In SI derived units, it is measured in lux (lx) or candela steradian / metre squared

SI light units

SI light units edit  (http://www.wordiq.com/definition/Template:SI_light_units)
Quantity	SI unit	Symbol	Notes
Luminous energy	joule	J
Luminous flux	lumen or ( candela · steradian )	lm	also called Luminous power
Luminous intensity	candela	cd
Luminance	candela / square metre	cd/m2	also called Luminosity
Illuminance	lux or (lumen / square metre)	lx
Luminous efficacy	lumens per watt	lm/w