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This technique can obviously be usefully applied to the study of a variety of patterns 
of geomorphological interest. The examples given in this section relate to some 
experiments by the author on a complex dune pattern in Tunisia and on a crevasse 
pattern of a part of the Karlinger Kees/Glacier in Austria. The equipment used in 
these experiments is not the Laserscan produced in the USA in 1964 but the Dudif 
(double diffraction filter) developed independently by the Royal Shell Research and 
Production Laboratory, Rijswijk (ZH), the Netherlands in 1964/1965, and described 
by GEURTS and van WIJNGAARDEN (1965). The author is indebted to this labora 
tory for the assistance received from the staff in the execution of the experiments 
and for subsequently donating the equipment to ITC. For background information on 
the theory of refraction the reader is referred to BORN and WOLF (1959) and for 
details about Fourier transforms to the papers by DUFFIEUX (1946) and HOPKINS 
(1953). 
The Dudif equipment used by the author comprises: 
1. 5 milliwatt He-Ne (Philips) laser (wave length 6328 %) which acts as a light 
source, 
2. An optical system, that includes a condenser serving for widening the diameter 
of the beam that measures only 0. 5 mm when leaving the laser tube, 
3. Two transparent plates between which the cut-out of the aerial photograph is 
immersed in oil, 
4. A range of interchangeable pie-slice filters through which a narrower or 
broader sector of the laser beam can be suppressed. Since the filters can be 
rotated, this sector can be placed in such a way as to coincide with the orien 
tation of selected (terrain) features on the immersed image, 
5. A viewing and recording system consisting of a 35 mm reflex camera with 
which the Fourier spectrum to the filtered image can be photographed after 
a visual inspection by means of a television camera and a monitor placed 
behind the camera. 
The first example relates to directional optical filtering and shows a rather complex 
pattern of wavy dune ridges in Tunisia that are predominantly of the transverse type. 
A fairly thin sand sheet covers the surface also between the ridges and a finer- 
textured, more humid layer occurs underneath. The groundwater level is at rather 
shallow depth. Most of the ridges stretch diagonally across the photo and are 
formed under the influence of the dominant winds which blow at right angles to the 
ridges slightly obliquely from the top to the bottom of the picture. A series of 
roughly parallel ridges of larger dimensions and stretching vertically across the 
photo is noteworthy. The irregularities of their shapes suggest their being longi 
tudinal dunes re-worked by the wind.