Manufacturers of optical coatings and components

Thin film coatings rely on the fact that electromagnetic energy (e.g., light) has wave properties, and therefore exhibits interference effects.  Light waves that are exactly in phase with each other undergo constructive interference.  Light waves that are 180° (p) out of phase with each other undergo destructive interference.  For intermediate cases (0 < f < p), the total amplitude is given by the vector resultant and the intensity is the square of the amplitude.  Thin film coatings use these properties to manipulate light incident on the film to produce a wide range of desired effects.

Briefly, when light is incident on a thin film, some of the light will be reflected from each of the interfaces, while the remainder is transmitted.  For example, for a two-layer coating on glass, there is an interface between the glass and the first coating layer, the first coating layer and the second coating layer and the second coating layer and air.  At each of these three interfaces, some of the light is reflected and some is transmitted.  Now, for any given wavelength, the total intensity reflected depends on the amount of constructive interference between the light reflected from each of the three interfaces.  If all three reflected waves destructively interfere, then the total reflectance for that wavelength will be zero. 

In the absence of absorption or scatter, which are typically very small for most thin film coated optics, the conservation of energy requires that the sum of the reflected and transmitted beam intensities is equal to the incident intensity.  That is,

                            I(l) = T(l) + R(l)

where I(l) is the incident intensity at wavelength l, T(l) is the transmitted intensity at wavelength l and R(l) is the reflected intensity at wavelength l.  So, for this is example, if R(l) is zero, T(l) will be 100%.

In practice, the reflected waves will not destructively interfere over all wavelengths, so the coating will only exhibit the desired properties over a limited wavelength range.  Desired properties can be extended to wider ranges (with accompanying performance trade-offs) with the addition of more layers.

Optical thin films are designed to control the phase of the energy reflected from each interface over a given range of wavelengths and incident angles by careful selection of the index of refraction of each layer, the thickness of each layer, and the film structure of each layer.  Therefore, optical thin films typically employ many layers consisting of two or more different materials of varying thickness throughout the film.  Calculating the theoretical response of the film as a function of incident angle and wavelength is straightforward, but involves tedious matrix algebra and a lot of computation time.  Newport Thin Film Laboratory uses fast, state-of-the-art microcomputers to design optical thin films.