Electronics > Metrology

[S] Small and cheap spectrometer calibration source?

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Since this is my first post to this section, I should introduce myself. I am a freelance electronics engineer. My work mostly revolves around microcontroller applications for various fields, e.g. medical and lab diagnostics. A couple of years ago, forum member branadic introduced me to EEVBLOG. He did this whilst showing great patience in discussing the basics of metrology via email and telephone to a person whose normal work considers 0.1% accuracy as the ACME of precision.

Last year I worked with Hamamatsu sensors and came across their C12880MA. I liked the device and decided to treat myself by adding a small spectrometer to my lab. I could have bought their eval board, but rather chose to roll my own, using an AD7980 ADC plus STM32F4. The design works quite nicely, at the moment I am writing the Windows software.

Today's question is about a cheap calibration source for the Hamamatsu C12880MA spectrometer module. Although the spectral range of this sensor is given with 340..850nm, even in the visible range the sensitivity varies by about 30%.
My current approach is to use white LEDs and compare their spectrum to the sensor output. I expect quite a bit of variation between devices, though.

I see several ways to tackle this issue:
- could anybody point me towards a small and cheap calibration source?
- would anybody with a calibrated spectrometer be so kind as to run an analysis on a couple of LEDs provided by me? (C512A-WNN / SPMWHT541MP5WATMS0, both available from Digikey)

As far as accuracy is concerned - I have no idea what is possible and at which cost. Anything better than 10% should be great, I guess.

Thanks in advance!

Edit210430: Link to the sensor: https://www.hamamatsu.com/eu/en/product/type/C12880MA/index.html
Edit210510: It seems that I haven't specified my problem clear enough for readers not familiar with the C12880MA: each module comes with spectral calibration data. However, the sensitivity data is only given as a "typical" curve in the datasheet. So I am looking for a fairly simple broad emission source to calibrate the spectral sensitivity. The spectral resolution of the sensor is only 15..12nm. After a look at the actual datasheet, my request might not appear that insane after all.

Not sure whether it is relevant, but in my experience, white LEDs (the blue LED with yellow phosphor variant) show very much variance between individual parts of the same type. It may be even that the relative intensity between 'yellow peak' and 'blue peak' is completely different between individual parts.
For intensity calibration, why take a halogen bulb? There is a temperature marked on the box, which is quite accurate. At least that was suggested to me by Ocean Insight for calibration of one of their spectrometers.

In 2016 i also made an effort to calibrate our spectrometer from aseq-instruments. They made a 500 to 1100 nm spectrometer for us, useful for checking pulse oximetry probes (LEDs at 660 nm and 880 nm)
In the end i found a standard neon indicator bulb useful for wavelength calibration and i cross-checked with an Oriel calibration lamp we have with a 6060 supply. I found that the original calibration had a scale error and was off up to 15 nm. Or the instrument was hit during shipment, who knows.
For intensity calibration i used daylight spectra that i captured with a small hole camera in front of the fiber. I remember that i had to repeat those measurements several days until i had a reasonably blue sky with similar to expected results. I found skylight spectra in publications on the web. The CCD sensitivity curve that i received from the seller of the spectrometer was helpful, too (published by Thorlabs).

Regards, Dieter

Since you like to "roll your own," have you considered using chemical actinometry to calibrate intensity?

It's been a long time for me, but that is what I did in the late 60's as needed for my graduate research.  I used ferrioxalate as the detector and phenanthroline as the indicator.  There are lots of options, here's a short review: https://www.nature.com/articles/s41598-018-23735-2

--- Quote --- loc. cit.
A widely accepted actinometer is the Hatchard-Parker actinometer (ferrioxalate) which is characterized by a quantum yield higher than 0.96. It involves the reduction of K3Fe(C2O4)3ยท3H2O in the presence of light, and the resulting Fe2+ ions are quantified by the complexation with 1,10-phenanthroline.
--- End quote ---

Wikipedia ("actinometry") also has some references.

Perhaps these may provoke some thinking along these lines. See attachment and link:

https://iopscience.iop.org/article/10.1088/0143-0807/31/4/022 (sorry it's behind a paywall)



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