Photometry
Photometry, the measurement of the intensity of light and the illumination it produces. Instruments for this purpose, called photometers, are used by laboratory scientists to study light; by astronomers to measure the brightness of distant stars; by photographers to judge what lens and shutter settings to use; and by engineers to determine what kinds of lighting fixtures are needed in various buildings.
The fundamental unit of photometry is the candela, which is a measure of light intensity, or candlepower. The visible radiation (light) emitted by a light source is referred to as luminous flux and is expressed in units called lumens. The total luminous flux of a light source with a uniform intensity of one candela in all directions is about 12.6 lumens. (A typical 100-watt light bulb emits about 1,700 lumens.)
The illumination, or illuminance, of a surface is a measure of the light it receives per unit of surface area. The illumination from a point source of light varies according to the inverse square law, which states that illumination varies as the inverse of the square of the distance to the light source. For example, if an object is moved from one location to another twice as far from the source of light, the illumination on the object will be one fourth as great.
A lux is the illumination produced on a surface that is everywhere one meter from a point source of light with an intensity of one candela. Expressed in another way, one lux is the illumination produced by one lumen of luminous flux on a surface of one square meter. A footcandle is the illumination of one lumen on a surface of one square foot. One lux is equal to about 0.09 footcandles.
Comparison PhotometersComparison photometers measure the strength of light by comparing it with a standard light (one of known intensity). In the bench photometer, of which there are several types, the light to be measured and a standard light are mounted on opposite ends of a bench. Between them is a device that allows the observer to compare the amount of light given off by each source. This device is moved until it is at a point where the two sources produce the same amount of illumination. The candlepower of the unknown source is then calculated. Comparison photometers are used mainly by laboratory scientists to study light.
Photoelectric PhotometersPhotoelectric photometers contain light-sensitive devices that convert light into electricity or undergo changes in electrical resistance when exposed to light. These devices are called photoelectric cells, or photocells. Two important types of photoelectric cells are photovoltaic cells and photoconductive, or photoresistor, cells. The photovoltaic cell generates an electric current when light falls on it. The photoconductive cell undergoes changes in electrical resistance when exposed to light and regulates the flow of current that is supplied by a battery or other electric power source. In both types of cells the strength of the current varies with the amount of illumination. Intensity of light is determined by measuring the current with a galvanometer, microammeter, or other electric meter.
Because the photoelectric photometer is more accurate and easier to use, it has largely replaced the comparison photometer. The exposure meter used by photographers is a common type of photoelectric photometer. Some of the photoelectric photometers used in astronomy can measure light from very faint sources. The general practice at observatories is to make photometric measurements of a few stars in an area and then photograph the area. On the photograph, images of measured stars are used as standards with which to compare brightness of other stars.