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58 Measuring devices
3-12 A VU meter. The heavy
scale is usually red,
indicating high risk of
audio distortion.
3-13 A sound-level meter.
Light meters
Light intensity is measured by means of a light meter or illumination meter. You might
think that it’s easy to make this kind of meter by connecting a milliammeter to a solar
(photovoltaic) cell. And this is, in fact, a good way to construct an inexpensive light me-
ter (Fig. 3-14). More sophisticated devices might use dc amplifiers to enhance sensitiv-
ity and to allow for several different ranges of readings.
3-14 A simple light meter.
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 wave-
lengths, in the same proportions, as human eyes. Another problem is calibrating the
meter. This must usually be done at the factory, in units such as lumens or candela. It’s
not important that you know the precise definitions of these units in electricity and
electronics.
