How to get a magnitude of illuminance

Hello folks,

Could anyone tell me how to calculate the illuminance magnitude at a point on a flat wall, which is shown in a fig. as I1 ?

Only one point source light is just front of a camera.
A Distance between the light and the point is d.
Illuminance at a corresponding pixel on a rendering image is I2.
There is neither fog nor rain, etc.

Pointers would be also appreciated.

Thank you.


This is actually pretty simple :slight_smile:

Just apply the pythagoras theorem:


Since angle “a” is 90deg, only the distance from the camera to the wall is needed. the line segment “d” can be calculated then as the square root of the sum of the squares of the distances of “c to a” and “a to b”.
Once you have the length of “d”, a normal inverse square falloff calculation applies to get the resulting value.

PS: In the 3D space you would of course have to use the 3D version of pythagoras…