The theory of operation is based on the wave nature of light and Huygens' Principle (See also Diffraction). Designing the diffractive pattern for a beam splitter follows the same principle as a diffraction grating, with a repetitive pattern etched on the surface of a substrate. The depth of the etching pattern is roughly on the order of the wavelength of light in the application, with an adjustment factor related to the substrate's index of refraction. The etching pattern is composed of "periods" –identical sub-pattern units that repeat cyclically. The width d of the period is related to the separation angle θ between output beams according to the grating equation: