# Calculating Light Fall-Off from a Large 'Soft' Source

I'm a working cinematographer from Australia. One of the first properties of light 'law's' we are taught early in our career is the inverse square law. So an object double the distance receives a quarter of the energy.

This however is from a point source, or - "the inverse square rule is often still a useful approximation; when the size of the light source is less than one-fifth of the distance to the subject, the calculation error is less than 1%"

My question is how would one calculate light fall-off from a source substantially larger than the subject.

For example I recently rigged 50 Par64 Cans (A PAR Can is an 1kw globe with a PAR reflector). I rigged them in a row spanning 12 metres with a layer of Light Grid directly in front (a diffusion material) this source was 14 metres in width and 2 metres in height. How could one accurate calculate light fall-off from said source.

My current approximation is using the inverse square law from the original source while taking into consideration the -0.8 Stop of light loss from the diffusion material (almost halving the total output).

Thanks Gabriel