UV low pass filter

摘要

The low pass filter for use in the UV spectrum comprises a dielectric stack which ensures an anti-reflective function for wavelengths ( lambda ) greater than a critical value ( lambda c). The critical value is less than 300 nm. Beneath these layers there is a metal base layer ensuring a reflection function for wavelengths greater than the critical value of wavelength ( lambda c). The combination formed in this way does not require a very thick dielectric stack - a thickness between a hundred nanometres and several micrometres if sufficient. The metal forming the base may be an alkali metal, or an alkali earth, so that the cut-off wavelength lies within the UV spectrum. One or more layers of material having the required anti-reflective properties may be used, and the layers may alternate with two different materials.