WORKING GROUP 1 
WELANDER 
35 
The T-function from the air 
The test method outlined by the author implies that a narrow bright line on 
a dark background is photographed from the air. In this manner, a slit image as 
used in the laboratory method above, is received. This image contains all the 
factors affecting the quality of the aerial photograph, and it may also be trans 
formed to a T-function as explained above. In principle the test contrast of the 
bright line is unimportant here. 
The method used is based on an infinitely narrow slit. The width of the 
bright line, however, is 8 cm. Consequently a special T-function for the object 
must be calculated. As is clear from fig. 6, the effect of this function is compa 
ratively small. 
The dark background used had a diffuse density value of 1.4, while two 
different densities of the bright line were tested, viz 1.0 and 0.6. The density 
differences obtained are well within the brightness range of a normal terrain 
for aerial photography, and the background corresponds, for example, to 
the reflectance of a pine forest. Consequently, conditions as in practice are 
obtained, and so the prerequisites for the T-function of the slit image to con 
stitute a quality criterion for the aerial photograph are available. 
The validity control of the test method 
The aerial photography of the bright line was carried out by the Geographical 
Survey Office under the conditions described above. 
The evaluation of all aerial photographs was performed by the Institute of 
Optical Research. The micro-photometer measurements of the negatives were 
carried out by Hendeberg [11] according to a method published in 1960. 
The calculations were performed in a Mercury digital computer. The result 
is shown in fig. 7. The theoretical T-function, determined by multiplying 
the individual factors affecting the aerial photograph under the same con 
ditions, according to fig. 6, is also copied. As shown, the two curves agree 
very well. 
The next step was a comparison between the theoretical T-function for the 
image motion alone, and the same function obtained in practice from the air. 
According to the principle outlined by the author, it is necessary to make use of 
two aircraft. The bright line is painted on the wing of the one aircraft and 
photography is then carried out from the other. The two planes are flying at 
a velocity of 268 km/h with a difference in altitude of 1,500 m, partly in the same 
and partly in opposite directions. In the first case the velocity is zero km/h 
and in the second case 536 km/h. The two T-functions were calculated in the 
manner described above. Then the curves were divided frequency by fre 
quency and the final result is the T-function for the image motion alone at the 
altitude of 1,500 m and the exposure time 1/280 sec. Also a curve for the 
velocity 249 km/h was obtained. The agreement between the experimental 
functions and the theoretical ones is fairly good, as shown in fig. 8. This study