Light Meters

The intensity of visible light is measured by means of a light meter or illumination meter. It is tempting to suppose that it’s easy to make this kind of meter by connecting a milliammeter to a solar (photovoltaic) cell. As things work out, this is a good way to construct an inexpensive light meter (following figure). More sophisticated devices use dc amplifiers, similar to the type found in a FETVM, to enhance sensitivity and to allow for several different ranges of readings.

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A simple light meter. A microammeter can be substituted for the milliammeter if greater sensitivity is required.

One problem with this design is that solar cells are not sensitive to light at exactly the same wavelengths as human eyes. This can be overcome by placing a colored filter in front of the solar cell, so that the solar cell becomes sensitive to the same wavelengths, in the same proportions, as
human eyes. Another problem is calibrating the meter. This must usually be done at the factory, in standard illumination units such as lumens or candela.

With appropriate modification, meters such as the one in above figure can be used to measure infrared (IR) or ultraviolet (UV) intensity. Various specialized photovoltaic cells have peak sensitivity at nonvisible wavelengths, including IR and UV.