Light meters

Light intensity is measured by means of a light meter. You may think that it is easy to make this kind of meter by connecting a milliammeter to a solar cell. And this is,  a good manner to construct an inexpensive light meter. More sophisticated devices might use direct current amplifiers to enhance sensitivity and to allow for the various different ranges of readings.

Figure: A simple light meter.

One difficulty with this design is that solar cells are not light sensitive at exactly the same wavelengths as human eyes. This can be conquer by placing a colored filter in front of solar cell, so that solar cell becomes sensitive to same wave- lengths, in same proportions, as human eyes. The other problem is calibrating the meter. This should be usually be done at factory, in units like lumens or candela. It not important to know the precise definitions of these units in electricity and electronics.

At times, meters such as the one in figure given to us are used to measure infrared or ultraviolet intensity. Different types of photovoltaic cells have very high sensitivity at different wavelengths. Filters are used to block out wavelengths which you do not want the meter to detect.