Photodiodes are usually used to measure the power and energy of low power laser beams. Can photodiodes be used to measure low power broadband light sources?
Laser measurement of high-power broadband light sources can easily be measured using standard thermal or pyroelectric sensors. Pyroelectric and thermal sensors generate an electrical current proportional to absorbed heat whether the light creating the heat is broadband or of a single wavelength. Low power sources are usually measured with photodiode sensors. These sensors have a wide variation in sensitivity as a function of wavelength and are usually calibrated to measure a particular wavelength.
When the user measures a narrow wavelength source, he selects the wavelength on the laser power meter the photodiode is attached to and the correct calibration factor is selected to give the correct reading. However, energy/power of low power broadband light sources can not be measured in this way, because sensitivity of photodiodes varies strongly with wavelength and since the light source is broadband, there is no unique sensitivity factor to use.
In view of the above, Ophir has designed a special sensor to measure the power/energy measurement of a source with a spectral spread of wavelengths, the PD300-BB. The PD300-BB has a filter mounted on top of the photodiode with opposite characteristics from the photodiode. It is most transmissive to wavelengths to which the photodiode is least sensitive and least transmissive to those wavelengths that the photodiode is most sensitive. The output of the sensor is the product of the photodiode response and the filter response, so the sensor response is approximately flat, i.e. equally sensitive to all wavelengths.
Below is a graph of the corrected sensitivity of the PD300-BB
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