What if you need to measure very low power laser beams, say on the order of nanowatts, but with wavelengths in the mid or far IR region? This leaves you in a quandary:
The low power would require a photodiode type sensor; however, photodiodes have a limited spectral range. Silicon works out to about 1100nm, while Germanium or InGaAs can go farther out to 1700 nm or 1800 nm. Using specialty photodiodes made of exotic materials is usually not a practical alternative.
Thermal sensors with broadband absorbers can handle the wavelength part of the problem, but even the most sensitive models would not help when it comes to measuring powers as low as nW. Ophir’s most sensitive thermal sensor, the 3A, can measure down to 10uW – impressive for a thermal sensor, but still not a solution in this example.
So we have a gap that looks like this: