Many factors must be considered before an aerial film is selected. These factors include:

(1) Intended Use Of End Product

(a) Prior to selecting a specific film, the user must survey the technical literature of the various manufacturers to determine which film will best do the intended job. Of great importance is the firm's sensitivity to light in the selected spectral region. In a film of wide spectral latitude, this is commonly called film speed and is measured in units of DIN or ASA. Speed is particularly important when low-transmission filters are to be introduced into the optical path.

(b) The granularity of the film must be considered. It usually bears a direct relationship to film speed. Slower or insensitive films are usually very fine grained. Fast, very sensitive films are usually coarse grained. There is little to be gained by using very fine-grained, highly resolving films unless your lens system is capable of producing a high-resolution image. As an example, it would be useless to have a film capable of recording 250 line pairs per millimeter. You would be making a great sacrifice in film speed with no possible gain in information.

(c) The physical properties of the film are another consideration. The film base material is of major importance. If you are considering making precise photogrammetric measurements, polyester-base films are highly recommended. It is also important to consider film thickness to insure proper passage through the camera system as well as the processing system.

(2) Technical Competence Of The Manufacturer

This factor is of the greatest importance to your aerial surveys and will have a most profound effect upon your photographic results and the ultimate value of your programs. Remembering that your own specialized situations and experience will be your major guides, the following questions must be carefully considered:

(a) Do the available products meet your requirements ?

(b) Does the manufacturer furnish detailed information on the physical and sensitometric properties of the film?

(c) Do your own studies and evaluations indicate that his information is reliable?

(d) Does he have specialized technicians and/or photographic scientists available to assist you with special problems and requirements?

(e) Do you find that a specific product yields essentially the same test results from one emulsion coating to the next?

(f) Will he help you analyze your water supply and chemicals and furnish recommendations based on his findings?

(g) Does he ship his product from his place of manufacture to your delivery point under conditions which will not cause degradation of sensitometric properties? Some films are exceptionally critical. Your Customs Office facilities must also be considered, if you import the film.

(h) Does he have a competent research staff which can help you?

(i) Is he easily accessible by cable, telephone, or letter? And does he respond to communications prompdy ?

(3) Product Availability

There are a few manufacturers who advertise a wide variety of film products. Your problem may be in securing them. You should consider a manufacturer who keeps sufficient stock levels at strategic locations so that you can secure film in a reasonable time period. Some will manufacture a film only after sufficient orders are received to justify a profitable sale. In such cases your source is unreliable, particularly if you have a seasonal application. In some cases a manufacturer will prepare a very specialized film if you are willing to order a sufficient quantity. An example is Eastman Kodak S.O. (special order) films.

(4) Level Of Laboratory Competence

You should secure testing samples of various films before you undertake ^ny large-scale project. Your photographic scientists should conduct a series of studies to determine the sensitometric characteristics of each emulsion. Test films exposed in aircraft should then be processed in your laboratory. Processing should meet preestablished standards and goals. You should not consider using any film which cannot be processed and reproduced (if required) to meet the standards of your photographic scientists or user-geoscientists. If you do not secure reliable and repeatable results, you will find that you are furnishing your geoscientists with erroneous materials, and hence erroneous deductions and conclusions will result.

(5) Film Cost

There is one consideration in product selection which is valid only when all those that I have cited are absolutely equal, and that is the cost of the film. You will quickly discover that the cost of film is a very small portion of the cost of your program. Savings on the cost of a product which may be inferior can place your program efforts and results in serious jeopardy.