Public Lab Research note

shorter spectrometer geometry with two slits?

by mathew | October 09, 2014 03:04 09 Oct 03:04 | #11254 | #11254

What I want to do

answer two questions: Can straylight be eliminated in a simple spectrometer setup through using a double slit? Can a double slit reduce the length of the spectrometer?

My attempt and results

I took the block that comes with the desktop spectrometry kit and cut two grooves in it cross-wise.


then i aligned two slits in the front and parallel groove in the block, and attached a DVD slice to the angle, and a webcam mounted on a block parallel to the DVD: IMG_20141008_183921.jpg


Then I put the whole thing in a small box for some light-proofing: IMG_20141008_185848.jpg

Questions and next steps

Waving it around there is definitely no stray light and it gives a clear spectrum. The .18mm slits are a little small for capturing much light from fluorescence spectra. I'm going to trade them out for some laser cut slits tomorrow and try again.

Why I'm interested

it would be easier to mount a spectrometer on a smartphone and other devices if it were smaller and more rigid.


It might help you if you look into angles of reflection, angles of refraction & absorption...

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Yes, this is a good topic. I did some work on this; maybe it will help.

First, the 'size' of the spectrometer (the PLab type) is mostly decided by the camera (w/grating) and the slit -- where the camera is supposed to be focused on the slit. These small webcams cannot effectively focus below about 4-in and the best I could do while obtaining a resonably sharp focus (and the best resolution for a given slit width) was about 6 inches. [ Note: The PLab instructions (the one's I received) are actually wrong for the electrical box housing design as the focus is far out past the distance to the slit.

Next, yes, ambient light is potentially a sever factor and the "double-slit" does help. However, be aware that it is actually NOT a double-slit (it's just a convenient naming) but it is one slit plus one or more "baffles". The camera cannot focus on two sequential slits and, if in fact you really did have 2 slits, there would be distortion as a result. If the 'chambers' between the slit and the baffle is made of VERY non-reflective material, and the camera 'chamber' is VERY non reflective it does help to eliminate significant 'fuzzyness' in the camera image.

I have previously noted this in discussions and it would be really good to have a 2nd generation PLab spectrometer designed with a slit and a baffle with VERY good non-reflective internal material. (Note, the input "port" on the present PLab electrical box housing is actually quite reflective and does add ambient noise even though the black paper box inside is fairly dark. This is mostly because the reflection angles are fairly shallow relative to the incoming light.)

Cheers, Dave

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I think that small commercial spectrometers typically use a collimating lens to get light coming through the slit parallel prior to going through the grating (Ben used this approach here: If you go to a 3d printed housing, then having a lens holder integrated into the design should be fairly easy and that might help cut down on the stray light without losing all your intensity.

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