amplitude calibration of spectrometer? - Page 1
Forum section: electronics / general

author: JBeale
author_rank: Frequent Contributor
timestamp: July 01, 2024, 03:33:17 am
content: Not sure if this is more electronics or physics, but I know there are smart people here, so...I got a used VIS-NIR spectrometer. It was quick, cheap and easy to calibrate the wavelengths, using well-known emission lines from a neon bulb and compact-fluorescent bulb. What seems far more difficult is any sort of amplitude calibration, even to say 10% or 20% accuracy. I don't care about absolute numbers like counts/mW/nm, just a relative amplitude sensitivity curve would be fine.AFAIK there are various ways to do this. You can just send your device to a calibration house, which is quite expensive and I would presume limited to certain brands of instruments they work with.The cheapest calibrated light sources seem to start above $500. (There's a supposedly-calibrated tungsten-based source on AliExpress around $220, but the spectrum they show looks very far from what I'd expect for such a source.) From what I have read, standard uncalibrated tungsten-filament bulbs apparently have a quite wide range of variation in output spectral shape, with some bulbs showing nearly "greybody" (flat) spectra, instead of blackbody output.I took a detour into color-temperature sensors, trying two different kinds but neither one shows me anything like the expected result on my tungsten bulb. There's one thing I'm sure of, which is the CCT (correlated color temperature) of a tungsten-filament bulb should increase with higher input voltages and currents, as it visibly changes from dim orange-red up to bright and more nearly white, but my TI OPT4048 color-temp sensor consistently shows the reverse! Fail.Also you could just point the spectrometer at the sun. The solar spectrum in _space_ is well known, but unfortunately even at noon on a clear day, there seems to be a broad range of variation due to atmospheric attenuation from dust and aerosols, so I don't know what real accuracy is possible here.I have an (uncalibrated) monochromator, so if I had a detector with known response curve, I could compare that with the spectrometer reading at separate points across the spectrum, to find the calibration. Many photodiodes do have a datasheet spectral responsivity plot but they typically look suspiciously simple and rounded-off, almost like an interpolated hand-sketch and not much like accurately measured unit data. I've never seen error bars or min-max limits on those graphs, which I presume only show "typical". I have also seen some calibrated photodiodes, but starting above $500 each.If I had an actual flat-response detector, I could do the same thing without reference to the sensitivity curve, and measuring the temperature rise of a blackbody absorber is supposed to do that. The bolometer-type thermopile or pyroelectric optical detectors (eg. laser power meters) I've found start around $100 on the cheapest sites, but the ones under around $500 appear unspecified and questionable in accuracy.Anyone ever try to do this, and/or have any advice?

author: jjoonathan
author_rank: Frequent Contributor
timestamp: July 01, 2024, 05:42:51 pm
content: My instrumentation lives almost entirely on the other side of the THz gap, but I have used two successful and one unsuccessful strategy to calibrate my power sensors:1. Tried to slip it onto the calibration cart at work. Turns out that yearly calibration does double duty as yearly inventory (in hindsight: duh) so they caught me, but I got out of there before they could figure out what the consequences should be so there weren't any2. Walked up to the calibration truck, knocked on the door, and asked if they did walk-up calibrations, but made it clear that I wasn't after a certificate just EEPROM coefficients. The price wound up being a six-pack of beer (twice) and twenty bucks (once). YMMV.3. I finally paid up for a good bolometer (or "thermal power sensor" in RF instrumentation terms) that, in combination with a network analyzer (for the equivalent of gray-body correction), let me transfer a voltage calibration over to RF. Or, rather, verify it. Everything lined up well within uncertainty. Very satisfying.

author: JBeale
author_rank: Frequent Contributor
timestamp: July 01, 2024, 09:46:30 pm
content: Thanks for those notes, very interesting!If there is such a thing as a visible/NIR spectrometer calibration truck, that would be an amazing thing to find. Unfortunately I think the calls for such service are few and far between.Many electrical measurements seem reasonably accessible at 0.1 percent, and frequency in many ranges is available at parts-per-million and better. Still, optical power across the visible spectrum seems a much more tricky thing to calibrate, even to 1%.Thorlabs sells a calibrated plain photodiode around $200, leaving the readout circuit up to you, and their stated measurement uncertainty is +/- 5%.https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=2822There's a $440 photodiode with a datasheet listing only +/- 7% spectral match "typical" which may still make no guarantee at all about limits.https://www.digikey.com/en/products/detail/osi-optoelectronics-inc/PIN-10DF/12503750Maybe the bolometer / thermopile / pyroelectric power meter is the best thing. They are still apparently not more flat than several percentage points across the visible spectrum. But I suppose energy that isn't absorbed must be reflected, meaning I might be able to measure that from specular and diffuse reflectance. An uncalibrated photodiode could work ok, because that only needs to be a ratio measurement so all I need is a beamsplitter with known properties, which I can also measure directly, so long as my detector is linear.

author: fmashockie
author_rank: Frequent Contributor
timestamp: July 05, 2024, 02:06:03 pm
content: As someone who works as a lab instrumentation engineer in a biotech company, calibrating spectrophotometers can be tricky business because each manufacturer has their own means to do so. So I would start there. Get yourself a manual for the instrument you have. Depending on how old the equipment is, the manual might have a section for how to calibrate it. If it is newer, there's a chance the manufacturer will keep that calibration procedure proprietary (which is absurd). Reach out to manufacturer if you don't have a manual to see if they can offer a calibration procedure and/or what calibration standards they use.For example, I calibrate spectrophotometers that are designed to read 96/384 plates (tissue culture plates). The manufacturer provides a plate with optic standards built into it for photo precision, photo accuracy, wavelength accuracy, and wavelength precision. Comes with a certificate with values and tolerances for each.So I would start with reaching out to manufacturer if you can.