Christiansen filters are usually made of grains of transparent materials, such as glass, immersed in a fluid with matching RI. If the RI of the grains matches that of the fluid, the spectral transmittance is maximum. It decreases with increasing difference between the refractive indices. Since a formula due to Raman and Shelyubskii, which is widely used in the literature to describe the spectral transmittance of such filters, has no scientific basis, a new theory is developed taking into account reflection and refraction at the boundaries between grains and fluid. The theory is applied to spheres distributed in the immersion fluid. In this case, adjustable parameters are not required. For grains of arbitrary shape, one needs a statistical description of the boundaries and their orientation. An adjustable parameter is introduced to account for such effects.
