All-dielectric filters require transmissive optical monitoring. Other methods can be used successfully for AR coatings and edge filters of most types. The basic optical system consists of a collimated beam from a light source emitting in the spectral area of the desired filter that (usually) passes through the substrate and out of the vacuum into a spectrometer. The detector can be a photodiode or a phototube. When a layer is complete, the signal is processed by a computer program to automatically stop the deposition. Alternatively, the signal is fed to a meter or a lock-in amplifier, and a vacuum system operator observes the signal and manually cuts the layers. There are a number of automatic deposition systems available. Ten years ago I used the Leybold system satisfactorily for quarterwaves; a quartz crystal monitor controlled the rate of deposition. The process could use either method as the primary control for any layer. Resolution of the spectrometer must be at least 70% of the bandwidth of the desired filter and collimation of the light beam must be such that the angles through the substrate do not degrade the signal. The beam diameter should be as small as possible so that any nonuniformity of the filter is within the bandwidth of the monochrometer. If any of these factors are not adhered to, the signal will not follow the computed monitor chart.
If only quarterwave layers are used, some of the layers in the filter design will not change transmission value appreciably, but these layers are necessary for the filter's final shape. See Fig. 8.1.