This chapter deals with the functional characteristics, or performance, of typical thin film filters, and how one measures this performance. The measurement techniques must be up to the task that is required of them. For example, it is not acceptable to measure the transmittance of a narrow bandpass filter with a spectro-photometer whose resolution is less than that of the filter. This and other such problems are covered in this chapter. For further information, the reader is directed to the excellent book on photometry by Walsh (1958).
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