The differential brightness measurement and the exposure system of aerial photography
Yang, Qiu Feng, Zhan Cuie, Zengyun
The Research Institute of Surveying and Mapping,
National Bareau of Surveying and Mapping,
Beijing, China
Niu Puning, Gao Po
The General Aviation Co.of China
Abstract
This paper analyzes the exposure control principle in the process of aerial photography. Acquiring the optimum exposure is a key to improve the image quality of photography .The method of using differential brightness measurement and mean differential brightness control exposure has been advanced through analyzing the strong and weak points of various exposure light measuring methods and by means of sensor filter and microcomputer. The affected factors of all related aspects, the different mathematical models of calculation and exposure control or the method of calculation, diagram have been analyzed, which provides the way for realizing this system.
Introduction
In the process of black / white and color aerial photography, the ground object brightness range of aerial photography is changeable, if the spectral radiation brightness range of object can be recorded linearly on sensitized material and the optimum space frequency response can be got, to a considerable degree, it depends on the acquirement of optimum exposure and the development of optimum condition. Controlling this condition will become the important research project of the present aero photography, too.
Owing to photography being carried out through atmosphere, such a changeable medium under the condition of dynamic flying, its object brightness range will be changed with terrain, flying height, the angle of solar altitude, atmospheric transparency, vegetation and the spectral characteristic of object.
In order to correctly determine its object brightness and brightness range, it is necessary to adopt real - time, multi-points and light measurement for new differential brightness measurement and exposure system.
On the basis of application of modern photograph camera and film, automatic developing equipment and sensitized instruments, in determining the effective sensitivity of photographic film, the aerial objects can be scanned by silicon linear array sensor of craft carrier and differential light measurement can be done with more than enough points so as to measure object brightness and brightness ratio. Calculating exposure and developing contrast coefficient is in controlling the developing process. Image displacement shall be calculated so that the matching of optimum aperture shutter can be selected and differential mean brightness can be utilized to control exposure, all of which will enable the air photographers to control and acquire better image quality.
The principle of acquiring optimal exposure
The quality of photographic image is manifested on the property of record, resolution and measurement for detailed objects. The record property shows that the object spectral energy is in density mode for linear expression ability.
The resolution and measurable property show the capability of resolution and measurement for minimum adjacent object, including the geometric and physical characteristics of objects.
The experiments prove that the size of image grain formed on the same sensitized material is mainly dependent on the forming latent image center. Therefore, it is very important to obtain the optimum exposure. From Fig. 1 it can be seen that, the corresponded density value of maximum resolution Rmax is about D=1.0, , the corresponded
DlogH of its Rmax 0.8 is the optimum exposure rangew. In order to enable all the objects of various brightness range to be recorded within optimum density range (D0.4~1.6), different r value is needed, in the developing process so as to get different exposure latitude
D Long H and to obtain different brightness ratio for object so that all of these can be within the density range of optimum resolution for sensitized material. Fig.2 and Fig 3 show the relations of the maximum resolution and contrast as well as latitutude, object brightness proportion.
Fig.1 Exposure and resolution curve
Fig.2 The characteristic curve of film 1022P
