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A Fabry-Perot interferometer is constructed of two very flat, partially reflecting mirrors held parallel to one another at a fixed distance. Interference occurs among the multiple reflections leading to the condition that wavelengths that exactly divide the spacing between the mirrors by an integer are transmitted very efficiently and all other wavelengths are reflected. Thus if the plates are held fixed at a separation of 10 μm, then radiation at 10, 5, 3.333, … μm will be transmitted. Note that these wavelengths are equally spaced in energy according to the relationship E=hc/l, where l is the wavelength of the light and h and c are Planck’s constant and the speed of light, respectively. This particular FPI technique makes use of these multiple passbands to increase the measurement signal and the resulting signal to noise ratio.
A Fabry-Perot can be tuned to transmit different wavelengths by changing the (optical) spacing between the mirrors. This is commonly done by employing piezo-electric transducers to translate the mirrors by very small distances, while maintaining the very precise parallelism between them. Fixed gap Fabry-Perots can be tuned by tilting, which changes the effective path length between the plates; by using the thermal expansion and contraction of the spacers between the mirrors; and by changing the composition or pressure of the gas that fills the space between the plates, which alters the index of refraction thereby changing the optical separation. Finally, Fabry-Perots can be constructed using a solid substrate of fused silica or optical quality glass onto which reflective coatings are deposited. These devices can be angle tuned or temperature tuned.