I need a cheap spectrometer for under $100. I have been looking at the Little Garden Spectrometer since its dirt cheap for how good it is. But I saw in a review on it in another forum that getting the thing properly calibrated was impossible for the reviewer, even after calibration using 436nm and 546nm from mercury vapor didn't get it properly calibrated. So I want to know if it's just the reviewer getting unlucky or if it's a problem. And of course I want to know if there are better options, and the software matters.

Well I have one and in fact it has more than enough resolution for a toy class instrument. Wavelength calibration is okay over visible range using a couple of known mercury lines, though its bundled software can not compensate for slight scale non-linearity when you are interested in infrared. Spectral sensitivity is wavy and it is rather hard to calibrate this out. May be all this can be solved with better SW like spectragryph, I did not have enough time to play. Overall, good value for $60, very useful to quick ID lamp spectra (but no good for any quantitative measurements except peak wavelengths).

See a last example by Laurens here: https://www.lighting-gallery.net/index.php?topic=17744.0


If you need a *really* calibrated instrument, sure nothing fits for $100, you need to splurge for $300-350 approximately and get one of that handheld Chinese meters. Lower resolution overall, but real values for lm, CRI, color coordinates etc.

Well, if it allows data export (like csv or so), you can do postprocessing calibration for the spectral sensitivity in e.g. Excell, at least relative. As an ethalon you may use incandescent lamp, the blackbody is quite well defined and easily reproducible...
And the temperature you may calibrate by measuring hot (measuring volts across the lamp, current into it) and cold resistance (a regular Ohm-meter) and look for resistance vs temperature tables for tungsten...

With Theremino you can export the wavelenght-intensity data in 5nm steps. That's how you can do the CRI and color temperature measurements with the Little Garden (in theory). It comes with some XLSX files that do all of that fully automatically for you if you paste in the exported data.

The little garden spectrometer is a nice thing to have. It is good enough to be able to figure out which fluorescent materials or arc chemistry are used in lamps. Consider it like you would consider a €10 multimeter.
It has been doing everything it promised to do. No issues with calibration here. Perhaps it is 1 or 2nm off, but that doesn't really matter for 'our' purpose.

One of the downsides is limited dynamic range. In my most recent post about the Osram daylight deluxe lamp, i was fumbling around with it a bit before realizing the green mercury spectral line was so strong that to get that one in view at its full height, you have to accept the continuous phosphor spectrum gets a lot smaller. If you're not interested in the discharge lines but only in the phosphor, you can of course choose to allow the discharge lines to clip with no consequence for the non-clipped part of the spectrum.

It is also not very sensitive. If you want to do reflectivity measurements your main light source needs to be ridiculously powerful. It is good for all lamps though, with the exception of the weakest indicator lights. To do those kinds of measurements, you can calibrate it in theremino to interpret your light source (halogen preferred) as the baseline).

Finally, it doesn't come with the proper diffusing filter (cosine correction filter) - just a strip of diffuse paper. Without a diffusing filter, your spectrum will get stronger or weaker on the blue or red side of the spectrum depending on at what angle you point the spectrometer at the light source. A point source or a flood source also makes it behave differently. This is normal for all diffraction grating based spectrometers. Putting the strip in front of the opening dramatically drops sensitivity to the pont that you have to put it really close to whatever lamp you're taking a measurement from. For fluorescents i typically use it without because they give a fairly diffuse light that comes in over a wide angle by default.
This is not a problem for distinguishing between different lamp chemistries, but it is a problem for doing CRI or CCT measurements.


Here is a video about improving it significantly: https://www.youtube.com/watch?v=43f2U8kHp70

One of the great benefits is that you are not bound to some phone app. I've seen many more expensive ones rely on phones. That means that in a couple years the software will disappear from the internet/app store/whatever, and in a couple more the phone you use with it will break and you're left with a useless device.
This thing, on the other hand, is a bog standard USB webcam with the bayer filter scraped off, and it will work with any computer from the past 15 years provided it can run Theremino or another video processing software.
This part is what made moe *not* buy one of the fancier spectrometers of a couple hundred euro.

It seems that these spectrometers would be very hard for me to use. The fact that they need to be calibrated, and I don't know how to do this...

@dor123 this spectrometer can be calibrated using the 436nm and 546nm mercury peaks from a fluorescent or mercury vapor bulb, which is done in the software. For the price this thing is about what I would expect for $60, but rather good for the price.

How good are homemade spectrometers? I saw a video where they took a webcam, removed the IR filter, glued a dvd next to it, and made a small slit in the box.

With those home made ones you would have to do/program all, even the basic calibration youself. Plus the DVD/CD would never be as quality grating as a purpose made thing. On the other hand the grating sheets are sold on sites like aliexpress also, I would expect better results with those than the CD/DVD. Mainly the contrast (so how much light leakage is there into wrong direction; in theory it could be calibrated, but it makes all the processing even more complex).
So I would expect even a cheap- (even toy-)grade but otherwise properly made spectrometer will very likely perform better than a home made one.

The big thing about the webcam in the Little Garden is that the maker removes the bayer filter from the webcam. This increases sensitivity, linearity and resolution a lot.
That's not just something you pull out of the thing - it is bonded to the sensor.
It will work without that operation, but you might lose a smooth sensitivity over the full frequency range.

But most cameras I opened had the filter glued above the module to the plastic casing. I was able to remove it easily

OK well overall the Little Garden Spectrometer seems like the best option for it's price. Definitely not the best spectrometer, but for the price it's about all your gonna get.

But most cameras I opened had the filter glued above the module to the plastic casing. I was able to remove it easily

Are you sure you aren't confusing the Bayer filter with an infrared/ultraviolet filter?

The bayer filter contains the RGB microdots that assign red/green/blue light to each pixel triplet, to make a color picture. It cannot be mounted separately from the sensor, because of the micrometer level accuracy you need in mounting it. Each microscopically small filter needs to correspond exactly with one pixel on the sensor.