Towards the end of the World War film cameras were developed which were capable of carrying rolls of film giving ioo exposures 18 x 24 centimetres in size. The great advantage which cameras of this kind have over other cameras is, of course, the light weight of the material used. In the year 1918 entirely automatic film cameras were made, the changing of the film and the expo sure being carried out by means of a motor which might be either electrical or driven by the wind. (See Plate V., fig. I.) Automatic film cameras are now used almost exclusively,- in view of their superiority over plate cameras.
From the photographic point of view the most important phenomenon encountered when working from a great height, as in an aeroplane, is the scattering of the light by the atmosphere, an effect which is generally known as haze. Since this scattering is greater for shorter wave-lengths of light, it can be eliminated to a considerable extent by the removal of the blue light and by the use of only longer wave-lengths. In order to accomplish this, the materials used are generally sensitive to red light, and filters can be used with satisfactory results as to exposure ; under such conditions great penetration through hazy atmosphere is possible. Excellent photographs have been taken from the greatest height reached by an aeroplane, and by balloon from the stratosphere.
While aerial photography is an immense aid to the military both for the detection of enemy operations and also for the preparation of maps, its application to precision surveying is subject to limitations. It is not possible to note with sufficient accuracy the height and angle of the aerial camera for a map to be made without correction for errors, and consequently aerial surveying requires the provision of bases of known position on the ground which can be included and used to scale and correct the photograph. This limits its use in surveying (q.v.) since such measured bases are not always available. Nevertheless there are many purposes for which aerial surveying is very well adapted. A fire survey of a city can be made with sufficient accuracy by aerial photography at less than one-tenth of the cost of a corre sponding ground survey. This makes it possible to repeat the fire surveys of rapidly growing cities at much more frequent intervals than could be done otherwise. Surveys of lakes and forests can be carried out by the aeroplane rapidly and with sufficient accuracy for many different purposes. (See ARCHAEOLOGY ; SURVEYING.) Radiography. (See RADIOTHERAPY ; X-RAYS, NATURE OF.) In this branch of photography very special conditions of work are involved. From its introduction in 1896, when X-rays were dis covered by Röntgen, the field of radiography—almost entirely in connection with its medical application—has grown until at the present time approximately as many negatives are taken by means of X-rays as are made in portrait studies.
The earlier radiographs were made on plates of the same type as those used for portraiture; improvements were made by the introduction of large quantities of silver salt into the emulsion, while later attempts were made to load the emulsion by the addition of salts of other metals in such a way that the absorption of the X-rays would be increased and high sensitiveness obtained. There is reason to believe that these attempts were based on an incorrect understanding of the laws of the absorption of the X-rays. A considerable advance in the technique was made when double-coated film was introduced, a special X-ray emulsion being coated on both sides of the film, so that the image was formed half on the front and half on the back. It is customary to use such films in combination with one or more, generally two so called intensifying screens. These intensifying screens consist of a layer containing calcium tungstate coated on a suitable sup port. The calcium tungstate fluoresces under the influence of the X-rays, transforming a portion of the X-rays into light which is active photographically. The general method of working, therefore, is to expose the film in a cassette ; that is, a holder in which the film is pressed into contact with an intensifying screen on each side.
One of the great difficulties in obtaining radiographs of good quality, especially when photographing through portions of the body of considerable thickness, is the presence of scattered radia tion or secondary X-rays pro duced by the scattering of the primary X-rays in the tissues.
Measurements have shown that the scattered radiation generally accounts for as much as three fourths of the entire density of a radiograph taken through the body, and since this scattered ra diation does not contribute to the formation of the image, it natur ally produces a great lowering of contrast and general loss of detail and quality. The best method of eliminating this is the use of what is known as the "Potter Bucky" diaphragm, which consists of a grid formed of strips of lead foil pointing toward the source of the X-rays. This dia phragm is placed between the subject photographed and the film. In order to prevent its forming shadows, the diaphragm is moved during exposure, and under these conditions the strips of lead cut out much of the scattered radiation, which is not proceed ing from the source of X-rays but is scattered in all directions, and thus enable the image to be formed to a much greater extent by the direct X-rays coming from the focal point. (See fig. 5.) For dental work the film is supplied in special packages to be held inside the mouth, usually with a backing of lead foil to stop secondary radiation and to enable the package to be moulded to the shape of the mouth. Dental radiography has been extended very rapidly, and radiographs of the teeth are among the most valuable guides to the operations of the dental surgeon.