Optical filter for protective welding lens assemblies

Claim 1

An optical filter in a protective welding lens assembly having a predetermined field of vision, comprising two parallel electro-optical cells, each of which covers substantially the entire field of vision of the protective welding lens assembly and each of which has a light transmission which is dependent on the voltage applied across two transparent electrodes included in the respective cell and having intermediate layers of liquid crystals, one of said cells being light-transmitting when voltage is applied and light-absorbing when voltage is not applied, the other of said cells being light-transmitting when substantially no voltage is applied and light-absorbing when voltage is applied, wherein at least a first of said cells is nematic with an admixture of dye molecules with anisotropic light absorption, the optical filter further comprising a contrast-enhancing filter sheet which also covers said entire field of vision of the protective welding lens assembly, said filter sheet being disposed in the ray path of the cells and allowing transmission of visible light within a wave-length range in which the first cell is absorbing light in its absorbing condition, and said filter sheet preventing transmission of light within a wavelength range within which the first cell is transmitting light in its absorbing condition, said optical filter having an electric circuit for controlling the transmission of the optical filter by selecting voltages for applying across the electro-optical cells, said electric circuit having a timing circuit for applying a voltage of a short duration which is higher than the subsequent working voltage to said other cell when the optical filter is caused to switch from the transmitting to the absorbing condition, the electric circuit further comprising a high-pass filter and a photodetector whose output serves a voltage supply for said electric circuit and, via said high-pass filter, is connected to the trigger input of the timing circuit for triggering the timing circuit when the photodetector receives light modulated by frequencies above the limit frequency of the high-pass filter.