The strength of light falling on a certain area is called illuminance.

Here's a breakdown:

* Illuminance is a measure of the amount of light falling on a surface.

* It is measured in lux (lx), where 1 lux is equal to 1 lumen per square meter (lm/m²).

* Lumen is a unit of luminous flux, which is the total amount of visible light emitted by a source.

So, higher illuminance means more light is falling on the surface, making it brighter.