device for compensating for the thermal effects and other effects of liquid crystal in a standard filter (10). a bandwidth of the filter (48) by adjusting the amplitude of a control signal (44, 46) to ac power applied to the electrodes (22, 24) located on each side of the liquid crystal filter (26).an optical detector (36) detects the intensity of the light (30) from an input beam (34) to the narrow band that is passed to the detector.an electrical circuit (40) determines the amplitude of the component of the polar light intensity, which is twice the frequency of the alternating current control signal, the amplitude frequency doubled), and adjusts the amplitude of the control signal (48) in response to the amplitude. in order to reduce the frequency and amplitude to frequency and close to zero.
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