Here and throughout these books we will use the term albedo (Latin for "whiteness"). Albedo is a precise technical term in some disciplines, but in all cases it is used to define some form of fractional reflectance. Albedo will vary with material color and (as we will later implement for glass materials) can also vary with incident viewing direction (the direction of the incoming ray).
Lambertian (diffuse) reflectance can either always scatter and attenuate light according to its reflectance R
, or it can sometimes scatter (with probability 1−R) with no attenuation (where a ray that isn't scattered is just absorbed into the material). It could also be a mixture of both those strategies. We will choose to always scatter, so implementing Lambertian materials becomes a simple task