PHOTOCELL TYPES
There are two general categories of photocells available in cameras and meters. The photovoltaic or "self-generating" photocell creates a voltage when exposed to light. As the light intensifies, more current is produced and in this way the meter indicates an exposure setting according to the available light. The current in these cells may or may not be boosted for use in low light conditions through the use of a battery. The second type of photocell, the photoconductive or photoresistive cell, operates in a somewhat different manner. When exposed to light the cell doesn't generate electricity but rather changes its normal resistance to the flow of electricity. As light intensity increases resistance to the electricity drops and more current flows through the system. These types of cells require a battery as they do not generate a current of their own.
Another way to compare photocells is according to the material from which they are made. This can effect the exposure reading that the meter recommends, because the meter reading is effected by the types of light to which the meter is sensitive and this is determined by the minerals composing the photocell. The graph displays the sensitivity ranges of films and meters in comparison to that of the human eye.
Sensitivity Ranges of Films and Meters
Wavelengths in nanometers
300 400 500 600 700 800 900
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ultraviolet blue yellow red infrared
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human vision
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everday, "panchromatic" films
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infrared films
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selenium photocells
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cadmium-sulfide photocells
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silicon photocells
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gallium-arsenide photocells
The ranges presented here must, of course, be considered average sensitivities and cannot be considered exact for any particular meter because photocells are frequently metered or otherwise modified to help bring them more in line with human vision or film sensitivity.
SELENIUM -- since selenium cells are photovoltaic, meters employing selenium don't require a battery. The photocell itself generates a small electrical current when exposed to light. There are advantages to using a meter which doesn't require a battery. First, these meters are frequently smaller and lighter that those which need a battery. Another advantage is evident in cold weather. As mentioned earlier, batteries become sluggish or useless under cold conditions but the selenium meter's battery free design allows it to continue to operate; It has become the photocell of choice under arctic conditions.
Just as every silver lining has a cloud, so the selenium meter has its limitations. In order to generate an adequate current this type of cell needs relatively bright illumination. As the sun sets, so does the usefulness of the selenium meter. Under dimly lit conditions, it becomes very sluggish and quickly useless. In addition, the selenium cell is sensitive to ultraviolet light, and in situations where a lot of UV light exist, for example at high altitude, the meter can easily over-respond to this invisible light and suggest an inaccurate exposure based on the ultraviolet light. Lastly, under very bright conditions, such as sunlit snow scenes, the selenium meter has a tendency to read a little low. While they are not perfect, the selenium meter is inexpensive and will operate where other meters fear to tread.
CADMIUM-SULFIDE -- commonly abbreviated Cds, this photoresistive cell opened the doors of existing light photography to everyone. It's battery dependent design allowed photographers for the first time to enter low-light conditions and get accurate readings. Currently, the Cds cell is the most commonly used photocell in cameras.
The Cds cell is not without its share of problems however. First, although it does not have as great a hypersensitivity to the UV end of the light spectrum as the selenium cell, it does have a hypersensitivity to the red end of the spectrum. Although some Cds meters are filtered to compensate for this oversensitivity, scenes that contain greater amounts of red or infrared illumination, such as tungsten lights, can lead to the meter's recommending an inaccurate exposure to compensate for what it assumes is an overabundance of light.
Perhaps more importantly is the Cds problem commonly referred to as "memory". If the cell is first pointed at a very bright object, and then at a dark, it tends to "remember" the first reading and the needles or LED's respond sluggishly when pointed at a darker object. The brighter the first scene and the darker the second, the more sluggish the meter becomes. If enough time isn't given for the meter to adjust, inaccurate exposures will result. Obviously it's important to give the meter enough time to adjust, sometimes a few minutes are required. The opposite problem can occur when the meter is stored for long periods of time; The Cds cell becomes hypersensitized to light and recommends exposures to reduce what it perceives as excess light.
SILICONE -- silicon photocells are photovoltaic cells and generate a small current but are normally associated with a battery in order to boost the sensitivity range. As a result, much like Cds cells, the silicon cell gives excellent low-light responsiveness. Additionally, because they respond to light more than 1,000 times faster than the Cds cell, they are immune from the "memory" problems which plagued their predecessor.
As you might expect, however, silicon cells are not without limitations of their own. They are somewhat hypersensitive to the red end of the spectrum, as seen in the graph, and consequently can give inaccurate readings in situations where red and infrared light provide a significant proportion of the illumination. Some manufacturers compensate for this by filtering the cell, as mentioned earlier.
GALLIUM ARSENIDE PHOSPHIDE -- like the silicon cell, the GAP cell is battery dependent and responds to light over 1,000 times faster than the Cds cell. It has the added advantage of not being hypersensitive to the red end of the spectrum. But, it is more expensive, and currently, it is available in only a few cameras.
Cameras and meters are currently available using each of the four types of photocells mentioned. All are useful under certain circumstances and it is important to know which type of photocell you are using in order to appreciate its limitations and avoid unwanted, improper exposures. Typically the cell type is specified in the owner's manual of the camera or meter but if not, try to find out from a camera shop or directly from the manufacturer.
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