Abstract: A method of designing a thin film optical bandpass filter, for transmitting light in a narrow band of wavelengths centered about a bandpass wavelength, begins by defining a first quarter-wave stack, including a first plurality of layers each having an optical thickness equal to one fourth the bandpass wavelength. A second quarter-wave stack includes a second plurality of layers each having an optical thickness equal to one fourth the bandpass wavelength. Next, a cavity layer is defined with an optical thickness equal to an integral multiple of one half the bandpass wavelength, to be interposed between the first quarter-wave stack and the second quarter-wave stack. The cavity layer is then replaced with a partitioned cavity, including at least one layer of a first optical material with a relatively high index of rfraction and at least one layer of a second optical material with a relatively low index of refraction.