Friday, June 27, 2008

Special Films

Instant photography, as popularised by Polaroid, uses a special type of camera and film that automates and integrates development, without the need of further equipment or chemicals. This process is carried out immediately after exposure, as opposed to regular film, which is developed afterwards and requires additional chemicals. See instant film.

Films can be made to record non-visible ultraviolet (UV) and infrared (IR) radiation. These films generally require special equipment; for example, most photographic lenses are made of glass and will therefore filter out most ultraviolet light. Instead, expensive lenses made of quartz must be used. Infrared films may be shot in standard cameras using an infrared band- or long-pass filter, although the infrared focal point must be compensated for.

Exposure and focusing are difficult when using UV or IR film with a camera and lens designed for visible light. The ISO standard for film speed only applies to visible light, so visual-spectrum light meters are nearly useless. Film manufacturers can supply suggested equivalent film speeds under different conditions, and recommend heavy bracketing. e.g with a certain filter, assume ISO 25 under daylight and ISO 64 under tungsten lighting.

This allows a light meter to be used to estimate an exposure. The focal point for IR is slightly father away from the camera than visible light, and UV slightly closer; this must be compensated for when focussing. Apochromatic lenses are sometimes recommended due to their improved focusing across the spectrum.

Film optimized for sensing X-ray radiation is commonly used for medical imaging by placing the subject between the film and a source of X-rays, without a lens, as if a translucent object were imaged by being placed between a light source and standard film.

Film optimized for sensing X-rays and for gamma rays is sometimes used for radiation dosimetry and personal monitoring.

Film has a number of disadvantages as a scientific detector: it is difficult to calibrate for photometry, it is not re-usable, it requires careful handling (including temperature and humidity control) for best calibration, and the film must physically be returned to the laboratory and processed.

Against this, photographic film can be made with a higher spatial resolution than any other type of imaging detector, and, because of its logarithmic response to light, has a wider dynamic range than most digital detectors. For example, Agfa 10E56 holographic film has a resolution of over 4,000 lines/mm—equivalent to a pixel size of 0.125 micrometres—and an active dynamic range of over five orders of magnitude in brightness, compared to typical scientific CCDs that might have pixels of about 10 micrometres and a dynamic range of 3-4 orders of magnitude.

Special films are used for the long exposures required by astrophotography.

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