An Optical Analogue Fourier Transformer

The design and construction of a simple optical analogue Fourier transform computer is described. The instrument which uses MoirB fringes to form a variable spatial frequency filter is intended to produce approximate spectra from interferograms in about five minutes. Results obtained with the instrument are shown. ime to time a number of analogue Fourier transform computers have been described, principally for crystallographic studies [I]. The increasing use of the Michelson and other Fourier spectrometers has led to the consideration of a wide range of ways of performing the transformation from interferogram to spectrum. Whilst there is little doubt that numerical transformation using a general purpose digital computer is the most versatile method currently available there are circumstances in which a special purpose instrument like the one we are about to describe may be of considerable value, since it can produce spectra within minutes of the interferograms being obtained. This makes the operation of a Fourier spectrometer in locations and laboratories which do not have rapid and direct access to a digital computer a simpler and more convenient process. It seems to us that a useful analogue system should satisfy three criteria. Firstly, it should operate with an accuracy comparable with that of the spectrometer with which it is to be used. Clearly the greater its accuracy the less often it will be necessary to resort to digital computation. Secondly, it should be fairly simple to operate and should permit the utilisation of such facilities as apodisation which are normally available with a digital system. Finally, it is desirable that it should be both reliable and readily adaptable to operate with existing Fourier spectrometers. The system that we have designed and constructed uses MoirC fringes already well known to users of the Michelson spectrometer as a position measuring device to take Fourier transforms. The principle of the system, which was first outlinedby Lohmann, [2] is shown in figure 1. PROJECTION CONDENSER LENS LENS LIGHT BOX CL~T OUT GR~DS FORMING DETECTOR MASK MOIRE FRINGES (Photomult~pliertube)