Light intensity measurements are made using a radiometer (an instrument that
transfers light energy into an electrical current). Some radiometers have the ability to
measure both black and white light, while others require a separate sensor for each
measurement. Whichever type is used, the sensing area should be clean and free of
any materials that could reduce or obstruct light reaching the sensor. Radiometers are
relatively unstable instruments and readings often change considerable over time.
Therefore, they should be calibrated at least every six months.