How to Measure Broadband Spectral Sources

Broadband spectral sources, such as arc lamps or gas-discharge lamps are still used in many lighting applications due to their high efficiency and other unique characteristics.

Lasers are measured with laser power meters.  But how can you measure a broadband spectral light source?

Thermal Sensors

In most cases, a thermal head is the logical choice.  It has a relatively high damage threshold and a broadband spectral response.  Depending on the gas used for the lamp, it will output wavelength in a broad spectrum.  That means it needs to be measured with a device that is wavelength-independent.

Thermal sensors are very close to that ideal.

But what about low power lamps?  Thermal heads can measure powers in the tens of mW, for the special 3A sensor down to tens of μW.  But what about lower power?

PD300-BB

In that case, you will have to use a photodiode detector.

The problem is that photodiodes’ response varies far more greatly with wavelength.  This is fine for measuring single-wavelength lasers, but will make measuring broadband lamps quite difficult, if not impossible.

For that reason, Ophir developed a special filter that renders its photodiode PD300 sensor nearly flat.  We call it the PD300-BB (BB for “broadband”).

Last Resort

In the unique case where your lamp is both low-power and UV, neither of these solutions will work.

(The PD300-BB sensor can only measure from 430 nm and higher.)

Fear not!  Not all hope is lost.

If you know the wavelength spectrum emitted by your gas lamp, you can calculate the power.  We can send you the spectral response curve of our photodiode sensors.  Now take the two spectral graphs and multiply them to get your overall calibration factor.

This is not an exact science, and susceptible to error, but if it’s all you’ve got, it should work.

Good luck!

Of course, if you have a particular challenge, it’s always best to let us know.  You never know when the solution will be easier than you thought!