Teardown, Upgrade and Experiments with a Verity Visible Wavelength Monochromator

In this episode, Shahriar upgrades a Verity visible wavelength monochromator model EP200Mmd to be able to perform automatic scans. The instrument is retrofitted with a stepper motor and a microcontroller which performs wavelength scanning between 225nm to 875nm. The signal from the internal photomultiplier is sampled by the microcontroller and wirelessly sent to the computer. Matlab is used to plot the wavelength response.

The principle operation of a monochromator is thoroughly reviewed starting with the grating mirror behavior. The operation of the photomultiplier is also described. Various mechanical and electronic of the upgraded system is also shown. The presented slides can be found here.

5 comments

  1. JIm Miller says:

    Thanks for the great monochromator presentation. Your video has inspired me to restart my own monochromator project shelved some time ago. It is also based on a Verity EP200Mmd. It is a great little “mono” for sure. I am using an Arduino (don’t remember what you used) to drive the stepper motor, digitize the data, etc. All is working very well. The stepper is running and data is flowing into the PC. However, I can’t figure out how you made those beautiful color plots. I know this project is old news for you since you did the video over a year ago. However, if you can point me in the right direction on plotting the data I would really appreciate it. Thanks for the project inspiration and any help you can give me. Keep up the great website!

  2. Ben Truscott says:

    Thank you so much for all your interesting videos! I always enjoy watching them. You did a great job with this monochromator.

    About the spectra you observe: in general, alkali metal photocathodes have much higher sensitivity in the UV and the blue than in the IR (the response goes to practically nothing above 850nm, though it depends on the specific phosphor). It seems to me that the intensity of the incandescent light should continue smoothly upwards as you go further to the IR (the peak of this bulb emission when run at low power is probably ~1500nm, with a filament temperature of ~2300K). That you don’t see this, but rather the secondary peak, is (I think) due to two things: the roll-off of the PMT sensitivity, combined with the second-order reflection from the grating. In other words, the second hump is a compressed and “red-shifted” copy of the spectrum as measured in first order. You would have to beware of this effect if you want to do an intensity calibration, because it’s easy to confuse out-of-band stray light with real signal.

    Are you in need of any slits or ND filters for the system? I can make these quite easily and would be happy to contribute something to say thank you for all your hard work making these videos!

  3. HeyTom says:

    Might be cool to see if the color of an incandescent bulb changes when dimmed.

    By the way, your videos are great.

  4. Jim Adams says:

    Wow, another very nice instructional tutorial. Thanks!!!

  5. macona says:

    Very cool, I have one of these same monochromaters as well as a SPEX scanning monochrometer that I would like to get working. Any chance of posting the code for it? Have you thought of adding limit switches for homing?

    Another option to connect the stepper to the thimble of the micrometer is to use the screw hole in the center and a small spline, that way the motor can remain stationary.

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