Question: [SOLVED] Why is my image of the spectrum extremely "slim"? DVD+RW is not good as a diffraction grating!

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jaksch asked on July 21, 2018 10:27
194 | 0 answers | #16762

A friend gave my a spectrometer kit (of unknown origin) and I'm trying to get it to work. I use DVD-RW grating at 65° (lines horizontally), camera at 45° and a distance to the slit of 200mm. I adjusted the distance camera <--> grating, so that the spectrum is in the middle of the captured image.

I'm getting a very narrow image as you can see below. I can get it even more narrow when I adjust the focus really carefully and adapt the exposure. This makes it hard to choose a good cross section. Light source here is a phone LED.

image description

Things I noticed:

  • When I adjust the focus, s.t. the image appears sharp, the focus is not around 200mm as suggested, but far beyond (more than infinity). When I adjust to 200mm, the image is blurry (e.g. the LED typical gap between blue and green gets blurred).
  • For some reason the slit included in the kit is vertical. However, in a dark room with a point light source, I can reproduce the image above without _any _slit, so I don't think that's the reason. But why would they design it that way??

I will try to produce a CFL spectrum later today, to see whether I can still get a sharp spectrum. I'm glad about any suggestions, on what could be wrong with my setup.

Here's more pictures:

image description

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I build a quick and dirty horizontal slit (approx 1mm wide) and indeed the image is more broad now. However, it is very dark. Below is a picture that uses a 6W LED lamp at 10cm distance. I cranked the exposure all the way up using Guvcview and set the focus to 200mm.

Suggestions highly appreciated :)


Switched the DVD-RW for DVD+R and tadaa works like a charm.

So sorry for the kinda pointless topic, but maybe the info that DVD+RW does not work is helpful for someone.

Still wondering why the kit had a vertical slit though ...


This is really helpful, yes! Great sleuthing!

This kit looks really interesting! It's not one of ours, but seems possibly based on one of ours, the #dsk3 -- i'd be interested to learn more!

You might do a search for notes by @stoft 're the plab v3 model. There are drawings that he produced and excellent discussions. The plab v3 mounts the slit horizontally and the grating is mounted directly onto camera lens housings. Your camera appears to have 6 ir leds for night illumination which should be disabled. I have built about 9 of these with various mods. Stef

Haha @warren thanks :)

I agree, it's very similar to the plab v3! Maybe someone drew the laser cutting and the vertical slit was just a mistake.

My local electronics supplier doesn't sell CFLs anymore 🤦‍♂️, so I'm now waiting for a shipment, before I can go on with testing. But here's an LED spectrum:


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