Discussions of photographic materials usually use density instead of opacity. Density is a logarithmic value equal to -log10(transmission), where transmission is the amount of light passed through (or reflected) on a scale of 0 to 1.

So to give some examples, a material with 100% transmission would have a density of 0, a material with 10% transmission would have a density of 1.0, and a material with 1% transmission would have a density of 2.0.

Density values have some advantages, including:

- They can be combined with simple addition

- Compared to transmission or opacity values, they correspond more closely to human visual perception