In most silicon solar cells, the short-circuit currrent (ISC) is linearly proportional to the light intensity so that measuring the cell output current with a multimeter gives a good measure of the light intensity. It is often more convient to measure voltage and this is achieved by adding a shunt across the cell. In the figure below the IV curve of a resistor is plotted along with that of a solar cell. The legend shows the amount of illuminaton across the cell.
