The Construction of Interference Filters for the Transmission of Light of Specified Wavelengths

Optical interference filters are composed of thin films deposited on a glass base by volatilization in a high vacuum. The narrowness of the transmission bands characteristic of these filters demands a high degree of accuracy in the regulation of the thickness of the films if maximum transmission is to occur at a specified wavelength. A method of observing the optical thickness of transparent films during deposition is described and it is concluded that its accuracy is sufficient, for instance, to enable the centre of a fourth-order transmission band to be placed within 20 A. of any wavelength in the visible spectrum.