Modern mathematical description of the two-stream approximation is given in several books.

There is a surprisingly large number of applications of the two-stream approximations, including variants such as Kubelka-Munk approximation.

Two-stream approximation is commonly used in parameterizations of radiative transport in global circulation models and in weather forecasting models such as WRF.

The two-stream approximation comes in many variants, including Eddington approximation, Modified Eddington, Quadrature, Hemispheric constant models.

Two-stream approximation of the radiative transfer equation is an approximation of the radiative transfer equation in which radiation is propagating in only two discrete directions.

The two-stream approximation is the simplest approximation which can be used to explain common observation inexplicable by single-scattering arguments, such as the brightness and color of the clear sky, the brightness of clouds, the whiteness of a glass of milk, the darkening of sand upon wetting.