To create a photographic image of known properties with highly reliable information, the user cannot tolerate any break in the continuity of information about his aerial film. He must have a complete history of the environmental conditions to which the film has been exposed, from the moment the emulsion is coated through storage, shipment, and completion of processing. This consideration must include the environmental conditions in both the aircraft and the laboratory. The exact sensito-metric characteristics of the emulsion are a function of the total environment to which it has been exposed as well as its manufacturing formulations. Each manufacturer will furnish a report as to the sensitometric and physical characteristics of each type of film to be manufactured. Experience has told us that this is generally an excellent guideline. It should not be considered absolute fact, however.

The manufacturer will usually use very exacting techniques and highly competent personnel in formulating an emulsion. It is then coated to the film base and cut into rolls. Each emulsion formulation is called a batch. For some usually unexplain-able reason, each batch differs slightly from other batches of the same emulsion made under the same conditions. Hence, the user, planning to make exacting geoscientific measurements must conduct his own analyses of all films as soon as they are received. It therefore makes good sense to insist that all film in a specific shipment come from the same batch (that is, with the same emulsion number).

The user has a responsibility here also. He should secure his film in as large an order as practical and have adequate storage space with temperatures no higher that 15 °C and preferably 0 °C or below.

Film will not greatly change in sensitometric response if stored frozen, even for long periods.

Upon receipt of a film shipment, the photographic scientist should make his own comprehensive analysis of its sensitometric characteristics. In some cases, he may call for storage of the film under certain temperature conditions for a specific time to shift its spectral characteristics to a desired condition. In other cases, he may require a change in exposure or a specific filter to be used. He may also modify the film processing techniques. In certain cases, he may reject the shipment as unusable for its intended purpose.

To accomplish such detailed investigations of aerial films, the photographic scientist must have accurate equipment. An accurate sensitometer, a controlled processing capability, and a densitometer are necessities. Other equipment can make his investigations even more detailed and accurate. The sensitometer permits him to expose emulsion samples to exact amounts of light in contact with an accurately calibrated density scale. The controlled processing usually includes accurately formulated chemicals, precise temperature regulation, precision timing, and accurate and thorough chemical agitation. The densitometer permits the scientist to determine accurately the response characteristics of the film.

When the film is removed from storage prior to an aerial photographic mission, the photographic scientist repeats his tests to insure that the film has not changed in sensitivity. He also places an exposed sensitometric density scale on the loading end of the film (occasionally both leading and trailing ends). During an actual photographic mission, the film should be subjected to a normal environment. Cold air in a high-flying aircraft will not degrade the film, but temperatures in excess of 35 °C can cause serious problems (depending on the type of film, the duration of the condition, and the humidity).

Upon return of the film to the laboratory, the photographic scientist will place another sensitometric density scale on the film. He may also remove an unexposed portion of the film, place a sensitometric density scale on it, process it, and determine if there has been any image degradation during the mission. Also he should simultaneously expose a sensitometric density scale on a laboratory controlled film from the same emulsion batch.

The resulting information will permit him either to alter the processing technique or to detect a change in the sensitometric characteristics significant enough to be brought to the attention of the geoscientists using the film.

The important message here is that quality control is mandatory for a quality product. These procedures will Vary with each user and will evolve as your program develops. No matter how rudimentary or sophisticated your surveys are, you should always insist on the most comprehensive quality control practices commensurate with the desired end product.

Photographic Processing Of Aerial Films

The third link in the chain of aerial photography is the all-important work within the photographic laboratory. In this phase of workmanship, quality control and a comprehensive knowledge of photographic science are an absolute necessity. Poor work, incomplete records, or substandard quality control can totally negate the very expensive aircraft operations which are performed to secure a precision photographic record.