Common frequencies in aerial photographs
Eight years ago the Geographical Survey Office began to carry out photography of test targets from the air.
Low contrast targets of Foucault- and Howlett-type were used as well as targets with continuously variable
contrast similar the one described by Washer (7). With propeller-aircraft altitudes up to 3 000 m have been
used, Here the Foucault-lines ought to be of the length about 4 m. The graphs in fig 8 and 9 show among others,
how the resolving limit does increase with increasing altitude. Further photography from jet-aircraft bas been
carried out from altitudes up to 10 000 m. Here the length of the Foucault-lines has reached 14 m. With an ol-
der lens and the test contrast 0.7 the resolving limit 30 1/mm vas obtained. However, this value is not compa-
able with the results in fig 8 and 9. Though a modern lens likely will give some higher values, frequencies
above 40 à 50 1/mra seem not to be considered in the transfer function calculations.
The transfer function for the aerial photographic system .
By multiplying the different transfer functions, the resultant function for the entire system may be calculated.
As an example the graphs in fig 10 and 11 show the resultant curves for the lenses under actual cenditions in
Sweden according to the table. The remission value for the ground is presumed to be 6 %. From fig 10 is among
others apparent, that lenses I and II are superior to III for the altitude 10 000 m. Further fig ll shows that lens
I and II are equivalent for extreme low-altitude photography, because here the effect of image motion is decisi-
vely. For example lens II — with the smaller contrast transmission — does give a higher response for 1/750 sec
than lens I for 1/250 sec.
Definition and detectability CT
The size of the details on the ground, which are on the edge of detectability, is important, When magnification
and illumination are optimized for viewing photographs, the minimum detectable resolution has been suggested
at the modulation of about 0.04. But in the last few years the contrast sensitivity of the eye has been closer exa- NS
mined. The degree of the object-contrast, that may be detected by the eye, is a function of the size and form of SS
the object and the light intensity of adaption. According to Rosenbruch (8) the curve in fig 12 should be represen- X
tative for the contrast sensitivity of the eye without enlargment for specified conditions, Thus the difference bet- =
ween plotting in stereoinstruments under various magnification and viewing without magnification must be consi- .
dered in estimating the quality of aerial photographs. 4
The correlation between the ''definition' and the contrast transfer function for the photographic system has re-
cently been investigated by Higgins, Lamberts and Wolfe (9). Similar investigations should be of importance in
the photogrammetry, but here also the effect of granularity must be of great importance, The accuracy in photo- 50
interpretation and in photogrammetric measurements, expressed as standard errors, ought to be connected with
the image quality, expressed among others as contrast transfer functions. As a link in such an investigation the
Geographical Survey Office at the suggestion of dr Ingelstam and in cooperation with the Institute of Optical Rese-
arch has photographed a light ramp in the night from the altitude of 1 500 m. The transfer function has been de-
termined by scanning the negative in a micro photometer. At this the entire effect of the lens, the emulsion, the
linear image motion, the vibration and the atmosphere is obtained in practical work. This investigation is going
on.
When it comes to the question of accuracy in photointerpretation and in photogrammetric measurements it should
perhaps be suitably to continue the investigation in collaboration with the 1.5. P. Commissions I, IV and VII. 0
REFERENCES
1, Ingelstam, E. 1959 a, Photographische Korrespondenz, 3. Sonderheft.
1959 b, Optik aller Wellenlängen, Akademi-Verlag, Berlin. C
2. Djurle, E. 1955, Teknisk tidskrift 85, 533 (in Swedish).
3. Ingelstam, E. - Djurle, E. - Sjógren, B. 1956, Journal of Optical Society of America 46, 707.
4 Hendeberg, L.O. 1960 a, Arkiv Fysik 16, 417,
1960 b, Arkiv Fysik 16, 457,
B, Scott, R. M. "Contrast rendition as a design tool." Photographic Science
and Engineering, vol. 3, 1959:3.
6. Carman, P.D, and Carruthers, R.A.F. "Brightness of fine details in air photography."
Journal of Optical Society of America 1951:5,
7. Washer, F.E. "Resolving power of airplane-camera lenses,' National Bureau
of Standards, Cirkular 526, 1954.
8, Rosenbruch, K. -J, "Die Kontrastempfindlichkeit des Auges als Beitrag zur Frage
der Gütebewertung optischer Bilder,'' Mitteilung aus der
Physikalisch-Technischen Bundesanstalt. Optik 16, 1959:3.
9. Higgins, G.C., Lamberts, R. L. and "Validation of sine-wave analysis for photographic systems,"
Wolfe, R.N. Communication No, 2028 from Kodak Research Laboratories
1959,
Focus
