Measuring brightness once took nothing more than paper and a grease spot
Put a spot of oil on a sheet of paper and it turns slightly translucent. Hold it between two lamps and slide it along until the spot seems to vanish from both sides: now each face receives the same light. Before electronics, measuring brightness depended on tricks like this, because the human eye is good at judging when two surfaces match.
A photometer measures quantities of visible light, such as how much a lamp gives out or how brightly a surface is lit. The early instruments all relied on comparison with a standard source and a simple law: illumination weakens with the square of the distance. Once two lights were balanced, their positions revealed how much stronger one was. A lamp that had to be twice as far away to match another was four times as intense.
By 1861 three designs were common. Rumford's shadow photometer set the two lights casting shadows of the same object side by side on paper and moved them until the shadows looked equally dark. Ritchie's photometer was a small box, six or eight inches long, holding a white paper wedge; the eye peered in through a tube at the top and saw both slopes at once, each lit by one lamp. A third method, thought the most precise, relied on the point at which a second light completely wiped out the shadow cast by the first.
Modern photometers swap the eye for electronics, turning light into an electric current with a photoresistor, photodiode or photomultiplier. The most sensitive count individual photons, which only works when light is faint; in orbit, photon counters study X-rays and far ultraviolet, where sources are dim and light behaves more like particles. The most familiar photometer sits inside your camera. It once came as a separate exposure meter, but modern ones read different parts of the scene and let software pick the best exposure.
In laboratories, photometers shine light of a chosen wavelength through a sample to work out how concentrated a coloured solution is, use infrared to probe molecular structure, and measure metals by the light they absorb in a very hot flame. Luminometers, an especially sensitive kind, measure the glow of luminescence and fluorescence in biology and chemistry.
Source: Photometer
