Probability theory is an obsolete term for the probability distribution of an unobserved variable. Today is called inferential statistics, the method of inverse probability, Bayesian probability. Modern terms given a probability distribution p on an unobserved variable θ for an observable quantity x, were until the middle part of the 20th century in use. The inverse probability problem was in the experimental sciences from experimental data.
Albedo is calculated for albedo for a single angle of incidence