DSK 3.0 - Cannot see anything beyond 750 nm

[Nicolò Canever]

Hello everyone! ncanever here. 

I recently purchased the Desktop Spectrometry Kit 3.0; after playing around for a while (super fun stuff!), I wanted to try out some NIR spectroscopy, so I promptly removed the IR filter from the camera and tried acquiring some spectra, but I'm having a problem: I can't acquire anything beyond ~750 nm in my spectra. This seems to be an issue with the geometry of the instrumental setup, since longer wavelengths appear to be outside the frame of the camera feedback. In other words, the "rainbow" appears to be too much shifted to the right, so that most of the NIR range of diffracted light falls outside of the shot. I tried reassembling the spectrometer a few times to see if I could make a bigger portion of the spectrum fit inside the shot, but the best I could do is about 760 nm, as seen in this spectrum: http://spectralworkbench.org/spectrums/56374

Does anybody have any ideas on how to solve this? thanks in advance.

[Jeffrey Warren]

Hi! Can you upload a raw image from your spec's camera, so we can see where the spectrum lies? 

I wonder if you could shift the angle of the camera on the camera block -- try, for example, wedging something under one edge of the velcro -- to see if that centers the spectrum more usefully for you. If so, you could make a more permanent adjustment. 

Thanks and please share what you do!

--
Post to this group at plots-sp...@googlegroups.com
 
Public Lab mailing lists (http://publiclab.org/lists) are great for discussion, but to get attribution, open source your work, and make it easy for others to find and cite your contributions, please publish your work at http://publiclab.org
---
You received this message because you are subscribed to the Google Groups "plots-spectrometry" group.
To unsubscribe from this group and stop receiving emails from it, send an email to plots-spectrome...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.

[Nicolò Canever]

I found that lowering the position of the entry slit as much as I could helped me gain some more nanometers, as I have reached about 780 now (see here: http://spectralworkbench.org/spectrums/56395). Here's the raw spectrum from that same acquisition: 

As you can see, there is still a lot of margin between the first blue peak and the left edge of the frame, so it can still be improved. I'm going to try modifying the camera angle as you suggested and see if that works - will update the results soon.

[Nicolò Canever]

Success! I followed your advice and modified the angle of the camera block. I did so by attaching the camera module a little lower on the wooden block, so that the bottom part of the chip would stick out a little and lie on the bench, keeping the block tilted at a slightly less steep angle than the normal 45°; I had to use some hot glue to hold the module firmly on the block, since the double-sided tape I used before was not strong enough. 

Doing so definitely seemed to move the spectrum in the right direction of the camera feed, but then I noticed another problem: the remaining part of the spectrum was being blocked by the top part of the diffraction grating support/"grating angle" (this was easily diagnosed by looking at the camera feed with the lid open); I solved this simply by cutting a bigger hole on the support; I had to use hot glue again to attach the piece of DVD-R to the support, because I noticed that otherwise the binder clip would be sitting too high, and the entire piece would have bent when closing the spectrometer lid.

After these modifications the spectral range of the spectrometer was increased notably, with my best result being within 350 and 850 nm, as seen here. I feel that tweaking it a little more could give even better results, but for now I feel satisfied enough with what I obtained so far.

Thanks for the help!

[Chris Fastie]

The blue color of the peaks you captured around 800 nm to 850 nm suggests that they might be the beginning of the second order diffraction pattern. Fluorescent bulbs do not have much emission at wavelengths longer than the peaks at 707 nm to 712 nm, at least after they have warmed up.

So your blue peaks are probably not from the infrared emissions from a CFL. The entire spectrum from a diffraction grating repeats ad infinitum, so you are probably seeing the 405 nm and 436 nm lines again. To learn the range of NIR that the web cam is sensitive to you will have to use a different type of bulb that emits past 712 nm. But beware when a signal is captured near 800 nm to 950 nm. It's probably not what it appears to be.

Chris

[Nicolò Canever]

Interesting, I did not know that. You're probably right, those last two peaks look like second order diffractions. On the other hand, I acquired another spectrum from a different CFL lamp, which shows a number of peaks between 700 and 850 nm that look quite different from those in the visible range. What about those? I think that these could be actual NIR emission lines from the lamp.

Spectrum's here: http://spectralworkbench.org/spectrums/56457

[Chris Fastie]

That's an intriguing spectrum. It is possible that you have captured emissions from the CFL before it has warmed up. When a CFL is still very cold, right after it is powered on, there are strong peaks in the near infrared:

More info about this here: http://physics.ucsd.edu/do-the-math/2012/05/spectral-extravaganza-the-ultimate-light/ (about halfway down the page).

To test whether you are seeing the NIR emissions from a cold lamp, watch the spectrum as the bulb warms up to see if the NIR lines disappear. If they do, they were CFL NIR peaks. If they persist when the lamp is warm, they might be something else.

Chris