Monday, June 2, 2014

Use an oscilloscope to collect optical spectral data

I'm using my new Tek oscilloscope to collect data from a DIY spectrometer.  The scope is in X-Y mode with infinite persistence.  The X axis is controlled by the rotational position of the diffraction grating in the spectrometer, and the Y axis is controlled by the photomultiplier tube, which is detecting the spectra.  In this video, I discuss the scope setup, and in future videos, I will talk more about the spectrometer, and how to use it.


5 comments:

Damien Miller said...

Nice - I've just recently bought a PMT module myself and was planning on making a spectrophotometer too. I'd really like to see how you arranged the optical side.

Are you using any output circuitry on the PMT or is the anode connected directly to the scope? I briefly saw close to a 1kV out of my Hamamatsu module when I accidentally turned the lights on when it was at full gain (something that I believe can permanently damage the tube, though mine seems to have fortunately survived) - your output voltage seems much lower.

fluffy said...

I'd be interested to see a bunch of different light sources, particularly with regards to the IR and UV portions of the spectrum. Also, have you tried calibrating it with a monochromatic light source such as a laser diode?

Ben Krasnow said...

Damien, the PMT output (anode) acts very much like a current source, so an easy way to convert the output current into a voltage is to put a resistor between the anode and system ground. Since the cathode is at a negative potential (and emitting electrons), the anode will be pulled negative when said electrons impinge on it, thus the output voltage will be negative. I used a 100K resistor, so that 100uA of output current will equal 10V. The scope has a high input impedance of at least 1M, so it can read this 100K source directly. If you have nothing connected the anode (eg near infinite resistance), even tiny nanoamp currents could make high voltages appear there. You must have an output load.

Jeff said...

Why did you use a PMT instead of a solid state detector? Do PMT's have flatter frequency response?

Ben Krasnow said...

Jeff, I believe PMTs are still the most sensitive optical detectors of all modern tech. They are very low-noise, and often do not require an external amplifier at all, thus eliminating another source of noise. Unfortunately, PMTs do not have a flat response -- they are sometimes made to be very responsive to a particular phosphor.

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