Angle of incidence is defined to be the angle between the direction of propagation of incoming light and a line perpendicular to the component surface.
The performance of most thin film coatings varies with angle of incidence. Specifically, the wavelength at which peak reflectivity or transmission occurs may shift with angle of incidence, and polarization dependent effects may also become manifest. This should be taken into account when using optics in which the angle of incidence varies over the surface. This happens when components are placed in converging or diverging beams, and with radiused optics, such as lenses.
Some ARO coatings, such as polarizers, must be tilt tuned in order to optimize performance. In other words, the angle of incidence of each individual unit must be adjusted within a small range in order to achieve specified performance.