Edge filter which block one spectral rgion and pass an adjacent region are normally constructed with periodic stacks of high nad low index layers of equal quarter wave optical thickness at the center wavelength of the blcked band. A preliminary and a subsequent aperiodic structure of several layers which minimize the eflectance in the passband are usually needed to transition from the effectiven index of the substrae to the effective index of the periodic structure and from the structure to the final medium. This paper addresses the estimation of the possibilities and limitations of these antireflection or matching layers to reduce the refeections before the coating is actually designed. Equations are givne for the estimation of the average rflectance in the pasband as a function of the number of layers and the width of the passband for both short and long wavelength pass filters and for passgbands on both sides of a "minus filter".
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