Survey photographs are normally taken in black and white, on panchromatic film. But in recent years other types of film have been used, notably those sensitive to the infrared wavelengths. Infrared radiation penetrates haze much better than visible light, which makes photographs taken from high altitude much clearer. In addition, the amount of infrared radiation emitted by an object changes with its temperature. This makes it possible, when using a scanning image, to distinguish between warm and cold water, for example, so that the discharges from factories into rivers can be monitored to make sure they are not overheating the water. The amount of heat absorbed from the Sun by living and dead vegetation is different, so the state of a field or forest can easily be seen at a glance. Infrared pictures cannot be printed in infrared, so false-color film is generally used. This renders infrared as red, red as green and green as blue. It is insensitive to blue light. Another device, the air-scan thermograph, uses an electronic scanner similar to a television camera to record infrared radiation only, ignoring visible colors altogether.More detailed information can be obtained with the multiband camera, a device which takes nine simultaneous pictures of the same scene. It is loaded with nine combinations of film and color filters.
Pictures can also be taken with side-looking airborne radar (slar for short) which has the advantage that it works in complete darkness or fog. This makes it particularly suitable for military use. Unfortunately, the quality of the picture is not very good, though it is steadily being improved.
Photographs have been taken from the air almost since the beginnings of photography itself. The first aerial photograph known was taken from the basket of a balloon over France in 1856. By the 1880s, photographs were being taken from balloons, kites and even rockets in the course of experiments in Europe, and in 1909 from aircraft, both in France and America.
The French were early pioneers in both aviation and photography. At the outbreak of World War I they already had some aerial photographs taken in peacetime of the very places the German Army was invading.
The French photographs provided the inspiration for better aerial photography. J. T. C. Moore Brabazon (later Lord Brabazon), a keen photographer in charge of the British Army’s air reconnaissance team, experimented with old-fashioned bellows cameras but found them useless. They could not be kept still in the slipstream of an aircraft flying at 80 mph (130 km/h). So he designed a camera suitable for inserting in the floor of an airplane – the first purpose-built aerial camera.
Moore-Brabazon also introduced the use of a stereoscope to view pairs of pictures taken from slightly different points, giving an exaggeratedly three-dimensional effect that allowed the heights of objects taken from above to be measured.
At the end of World War I, the newly developed techniques of aerial photography were applied to peaceful uses – though a certain amount of spying still continued. The main applications were map making and surveying, but there were other uses.
In Canada, forests being grown for timber were photographed from the air as early as 1921. This was an ideal way of checking on trees.
Another use of aerial photography was discovered in the U.S. in the 1920s and 1930s. The Agricultural Production and Marketing Board regularly photographed farms from the air to check what crops were being grown. In this way, they were able not only to compile statistics, but also to detect false claims made by farmers for the subsidies that were paid for growing certain crops. This unusual peacetime spying made them extremely unpopular.
By 1938, it had become obvious to everyone that Germany was preparing for war. In Britain, the Royal Air Force commissioned the brilliant Australian aerial photographer Sidney Cotton to get as many pictures of military installations as he could without attracting attention.
Cotton had been taking aerial photographs since the early 1920s. His method of tackling the job was most ingenious. First of all, he used RAF funds to buy a Lockheed Electra, a fast civil airplane.
He modified this by installing three cameras under the floor; they were hidden by a close-fitting sliding panel when not in use. He also arranged for a stream of warm air to be blown into the camera compartment inside the airplane. The ventilation prevented the camera from fogging up or freezing at high altitudes and low temperatures, a problem that had dogged aerial photographers for years. Regular aerial photography proved vital during World War II, when Sidney Cotton was given the job of organizing and running the first British photographic reconnaissance unit. Frequent pictures enabled a reference file on a place to be built up, so that troop movements, new buildings and unusual events could all be observed. One early success of the unit was the detection of the V1 and V2 sites. Subsequent bombing of the sites forced the Germans to use smaller, movable installations.