Light filter with automatic regulationof optical transmission

摘要

The invention relates to improvement in a light filter with automatic regulation of the optical transmission. The filter comprises a filter element (1) with a liquid crystal layer located between two transparent plates (2) and assigned polarizers. The optical transmission of the light filter is varied by applying a variable electric voltage. A first optical sensor (4) is arranged behind the filter element (1) in the radiation emitting direction, and a second optical sensor (8) is arranged beside or in front of the filter element (1) in the radiation emitting direction. The two sensors are connected to a regulating circuit (6), which comprises a subtraction circuit (15) which subtracts the signal generated by the second optical sensor (8) from the signal of the first optical sensor (4), or the signal of the first optical sensor (4) from the signal of the second optical sensor (8), so that the output signal of the regulating circuit is at least approximately proportional to the amount of visible light. As the second optical sensor (8) a sensor sensitive only to infrared radiation, may be used or, if this is also sensitive to other radiation, an infrared band-pass filter may be placed in front of it.