Optical Systems for Use at Low Light Levels

The development of image intensifier tubes and their application in instruments for operation under low light-level conditions created the need for optics which require specific properties. Recognition of the problems involved when combining a lens with a tube results in general requirements for the lens, some of which are different from those in standard lens design. Refined basic lens configurations and new systems have been introduced in this field, e.g. refractive and catadioptric lenses. Some examples of their performance are shown. In addition relay lenses having an extremely high speed have been realized for applications mainly for x-ray image intensifier systems. Single-tube instruments with a bi-ocular eyepiece are shown to have many advantages over the double-tube binocular.