This arrangement corresponds to a classical form of monochromator and can be considered as an optical system which images the input slit on the output slit and at the same time selects a narrow wavelength band. For such a monochromator can be replaced in equivalent terms, if viewed as an element in a larger system, by a simple slit through which only one wavelength band passes. Consider now, by way of illustration, an optical imaging system of the kind used as a conventional camera objective. In the invention there are two imaging systems. Secondly, there is the further inner imaging system which images the rays incident on the input slit on the output slit, in this process accepting only a narrow wavelength band.
Experience shows that there is a considerable range within which neither effect is noticeable.
In other words, it can be said that the inner optical system receives light rays from each object point which have an angular relationship to each other within a range from slightly divergent to slightly convergent. Due to the use of a dispersive means such as a diffraction grating or prism it is possible to select a required narrow wavelength band with great precision, as is the case with the classical spectroheliograph. As explained, the image monochromator of the present invention can be constructed as a much simpler mechanical instrument than the spectroheliograph having synchronously moving slits. The advent of modern image intensifier tubes makes it possible to use the image monochromator of the invention for a wide range of applications. Image intensifier tubes with gains of the order of 100,000 are now available, and which can thus match the light reduction in such a monochromator.