Reflected infrared photography:
Exposure

Precise film speeds cannot be quoted for infrared emulsions, and since exposure meters are not sensitive to infrared there is little point in using them. Some workers calibrate their exposure meters by placing a deep red filter over the sensor and then conducting tests to establish the "effective" film speed using this system (this involves calculating the relative speeds of the red versus the infrared). Care must be taken with this approach as the relationship between the actinic value of visible light and the infrared content varies considerably with different times of the day in daylight.

Infrared films are now remarkably sensitive. When used in daylight with the Wratten 88A filter one can expect exposures of the order of 1/125th sec. at f/8 for most of the black-and-white films, or with a moderately powered electronic flash unit, exposures of f/32 of f/22 are common-place for close-up biomedical work. Infrared Ektachrome is also very fast with an effective speed rating of ISO 400 when used with the Wratten 12 filter and processed in E6 chemistry. This falls to 200 ISO when processing in the AR-5 chemistry. (Kodak state that the effective film speed in E6 is only 200 ISO but in practice the authors have found that in both daylight and under studio flash conditions it is one stop faster).

The following table gives some idea of the relative sensitivity of the commonly used black-and-white-film and filter combinations:

The real advice is bracket, bracket, bracket - two stops either way in half stops is usually necessary. The half stops are important and will often save the image.

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Processing black-and-white infrared films >