Patent ID: 7319560

Claim:
A tunable optical filter for transmitting light in a band of wavelengths centered about a bandpass wavelength that is tunable over a second wider band, comprising: first and second reflectors held in substantially parallel alignment and separated by a variable air gap; a partitioned cavity including a first dielectric layer on the first reflector, the variable air gap and a second dielectric layer on the second reflector, said partitioned cavity having an effective optical thickness substantially equal to an integral multiple of one half the bandpass wavelength and having an effective refractive index (n eff ) greater than two, said first and second dielectric layers each having an optical thickness less than one fourth of the shortest wavelength within the second wider band; and a tuning mechanism for moving at least one of said reflectors to vary the air gap and tune the bandpass wavelength, said filter having an associated transmission spectrum which shifts to shorter wavelengths as the angle of incidence (θ) of light impinging on the filter increases from normal incidence, said partitioned cavity arranged such that the magnitude of said shift for a given θ is less than it would be for a filter having a cavity consisting solely of an air gap.