The differential brightness measurement and the exposure system of aerial photography
Because the aero photographic platform generally is a high-speed flying machine, it is important to calculate image displacement and select the matching of optimum exposure so that exposure can be done at a short shutter speed.
Fig.3 The relation curve
Resolution will be reduced in using large aperture while reducing shutter time so as to reduce image displacement will rely on the application of large aperture. It is a feasible method to
select higher sensivity and better film and use image displacement compensation camera. brightness ration
It is quite necessary to add the same band filter on camera and sensor so as to eliminate atomosphere scattering effects. The real multiplier of filter is calculated by the following formula :
Kf=KI. AV . H
The formula of exposure using mean brightness is:
The formula of exposure calcultion using minimum brightness is:
Kf = practical filter multifiler
AV - the Vignetting coefficient of camera
n-the correction of film color sensitivity
K-aperture number
t--- shutter coefficient
B - object mean brightness
B small-minimum mean brightness
S-Sensitivity
C.C. correction constant
The determination of differential mean brightness,
What the automatic exposure control for most aerial presently uses is the method of integral determination object brightness, the objects of high brightness and low brightness within sensor visual angle are mixed up, the photographed comprehensive output is a mean value of weighting. In terms of the resolution characteristic curve D//= Do+0.85; and the whole brightness range will be in the part of straight line of characteristic curve, i.e. within D0.4 1.6 However the proportion of object brightness can not be measured by integral light measuring, and the proportion and measurement value of bright object and dark object can not be reflected. As a result, the development r-value cannot be selected in accordance with the brightness proportion. It will cause deviation in the following three conditions: While the object brightness ratio is especially great, the development r is fixed, and the corresponded latitude of optimum density range is also fixed, at this moment the density of high brightness part will be over D1.6, and the part of low density will be less than D0.4. When the to high brightness object is great and the low brightness object is smaller, the correspondent density of integral mean brightness at this moment is 1.0 the object of low brightness part will be below 0.4 and the exposure is insufficient. While the low brightness are is great and the high brightness area is smaller (e.g. open space in forest) , the part of high brightness will appear over great density and overexposure. The worst situation is that as water surface appears mirror reflection, or snow mountains, small pieces of clouds, white sandy beach show up, the other object density will be reduced or even film scrapped by over powerful reflection value.
Another method to determine exposure by minimum brightness is on the basis of SD0+0.3 as exposure so that the point density comes to Do+0.3. The accuracy and reliability of this method is dependent on the minimum brightness which can be measured within format, and it should avoid to product a great change for the minimum brightness jumping in the photographic process, especially several large area bright objects continuously come out or the minimum brightness has not been collected, so that exposure will appear.
In order to overcome the weakness of the above two methods, a system differential mean brightness exposure is designed. The system involves 30 silicon photoconductive elements and 30 lines will be scanned on each sheet and 900 points in total. After removing the overlage brightness automatically, 30 maximum values and 30 minimum values will be taken to extract the mean maximum brightness and the mean minimum brightness, and to calculate the brightness proportion of object and r-value. Through the changes of development r value the density range and exposure latitude can be accordant with the brightness proportion of object, even if the whole piece of object is bright, it also can be image-formed at optimum resolution about D1.0, the resolution of photographic results can be improved and enhanced in overall.
r = DD / DlogB
DD = Select the suitable density difference of photographic film,
Dlogarithm of object brightness B
