In addition to the Thermal sensors described above, Photodiode sensors are used for low powers from picowatts up to hundreds of milliwatts and as high as 3W. A Photodiode sensor is a semiconductor device that produces a current proportional to light intensity and has a high degree of linearity over a large range of light 

power levels – from fractions of a nanowatt to about 2mW. Above that light level, corresponding to a current of about 1mA, the electron density in the Photodiode becomes too great and its efficiency is reduced causing saturation and a lower reading. Most Ophir PD sensors have a built-in filter that reduces the light level on the detector and allows measurement up to 3W without saturation.