[magicalhippo]

This is more generally called absorbance[1] and stems from the Beer-Lambert law[2].

Been too long since I've done computer graphics, but you made me think about implementing transparency pass using absorbance instead.

[1]: https://en.wikipedia.org/wiki/Absorbance#Absorbance_of_a_mat...

[2]: https://en.wikipedia.org/wiki/Transmittance#Beer%E2%80%93Lam...