Average Scene Reflection

Why is 18% assumed to be the average scene reflection value? Well, it's a curious story.

Let's start with the ISO exposure equation.

$\frac{N^2}{t}=\frac{LS}{K}=\frac{ES}{C}$.

According to Wikipedia, a typical value of K is 12.5 for a reflected-light meter while a typical value of C is 250 for cosine-responding light meters with a flat receptor. For an ideal diffusely reflection surface, the relation between illuminance an luminance is π, which gives the reflectance formula: $R=\frac{\pi L}{E}$.

The assumed average scene reflectance is therefore $R=\frac{\pi K}{C}$, which yields 0.157 using the default values for K and C.

So again, where does the 18% value coming from?

A document titled "Exposure Tools" from Kodak’s website once provided the following explanation.

A standard definition of "average scene reflectance" allows camera and meter manufacturers to build equipment that provides consistent results. ANSI standard PH2.12 explicitly specifies 16% average scene reflectance as the metering constant for incident and reflection meters. Kodak studio photographers preferred 18% reflectance, one-sixth of a stop more than 16%, because it provided better results.

I can no longer find that document on Kodak's website, but in pratice, an exposure difference of one-sixth of a stop is negligible.

So, to answer the initial question: in the ISO/ANSI system the average scene reflection is 16%, but Kodak decided it should be 18%. In German this is called the normative Kraft des Faktischen.

Next question: why was 16% assumed to be the average scene reflection value?

To be continued.