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I previously worked on Virtual Reality and other hardware at Valve.  I currently work at Google[x].

Prior to starting at Valve, I built computer peripherals such as keyboards, mice, and joysticks that were designed to be used inside MRI machines.  My company, Mag Design and Engineering, sold these devices directly to researchers at academic institutions who used them to publish scientific papers in peer-reviewed journals.

After work, I spend time on many different types of projects that usually involve circuit design, machining, material selection, and general fabrication/hacking.  My favorite place to be is my home workshop.

ben dot krasnow at gmail




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.


  1. 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.

  2. 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?

  3. 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.

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

  5. 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.