These web pages are a slightly updated revival of the original KOJAC web pages, that don’t exist anymore on the web site of the Applied Optics Group of IMT (formerly at the University of Neuchâtel, now at EPFL).
If imaging quality is often aimed in priority in optical systems, there may be some cases where minimising the energy losses is also a critical issue. In some systems, oblique incoming beams are simply suppressed or severely weakened by vignetting.
We present here the concept of the field lens, which is used for example in microscopes or in Microlens Projection Litography (MPL). The system we use here is a simple telecentric lens doublet, used for a 1 to 1 imaging. The field lens lens is placed in the image focal plane of the first lens, and is imaging the aperture of the first lens onto the second lens.
When the object point is moved away from the axis, more and more rays pass beside the second lens, i.e. are lost. If the field lens is used, then those rays are bended back into the second lens, and the final image contains the maximum energy. Nevertheless, the imaging properties of the original system are not influenced by the field lens.
The field lens presents a strong interest when light propagates within a tube-like system. It allows the use maximum energy, which can be a critical issue in many applications.
However, some precautions are to be taken with field lenses. They should have no dust and scratches, because otherwise these will become visible in the final image.