Description of the microdensitometer. 
The microdensitometer used in this study was develop- 
ped at the University of Louvain to meet the needs in remote sen- 
sing. A brief description of the instrument is given hereafter. 
A more detailed account of it can be found elsewhere (1). 
The instrument (called MIDAS) is automatized and 
programmable and thus highly versatile. It discerns 256 gray levels 
(8 bits) with the resolution of 0,01 point of optical density (the 
range is thus 2,5 points of density). The spatial resolution 
achieves 20 um easily which is not a technical limit for the in- 
trument. Accuracies are comparable to resolutions as proved by 
tests with targets and calibrated absorbing strips. finally the 
acquisition rate is 10 measurements per seconde. 
The system(see figure below) is based on the explo- 
ration of elementary zones of the transparent cliché by the beam 
of a cathode ray tube (flying spot scanner principle). The beam is 
focalized on the cliché by three interchangeable objectives. Ac- 
cordingly, a shift of one step of the spot on its screen yields 
an interval (resolution) of 20, 40 of 80 um on the cliché. Dis- 
placements between adjacent zones are obtained by means of a 
X-Y table supporting the document and actuated by stepping motors. 
At the output of the cliché, the beam is picked up by a photomulti- 
plier the electrical signal of which is logarithmically converted 
in a digital value giving thus the optical density of the measu- 
red pixel. This value is finally accumulated on a magnetic band. 
The measuring process as described above is comple- 
tely automatic owing to a calculator (Hewlett - Packard 9815) 
coupled to the densitometer via the universal GPIB bus. By modi- 
fying the program of the calculator, the user can adapt the mea- 
suring protocol (namely the formats) to his needs. 
This calculator has not only a mission of manage- 
ment but also a mission of data processing. These data are ‘after- 
wards displayed on a printer (for example in the shape of signs 
plot) or reported on a magnetic band for eventually more complex 
treatments by computer. 
Application to the determination of urban perimeters. 
The parts of the satellite cliché corresponding to 
the selected towns are swept by the densitometer. The data are then 
transcribed on the printer in the shape of a signs plot covering 
the urban zone the density of which is lower than a given thres- 
hold (the towns are seen light on the cliché). 
(1) B. Mollet: "Microdensitomètre pour l'analyse automatique de 
clichés, to be presented at the Symposium International sur 
les équipements de photogrammétrie analytique et de télédétection. 
Paris, september 1978.