HI, Luke! Great post. What paper did you use for the sleeve? We chose the black paper as it's absorbent in the near infrared range, but do you think the extra light is entering through cracks and joints in the enclosure, or through the paper itself? How bright was your light such that it got into the case? How bright must light be before it begins entering the chamber?

We'd love to see a picture of the sleeve -- its design may answer some of these questions, along with an exact description of the type of material.

It's great to see the ASD FieldSpec data too -- is it so smooth because of a longer exposure, with time-averaging to reduce noise? Or is it otherwise smoothed? Tests on an Ocean Optics device seemed similarly noisy to our device: http://publiclab.org/notes/warren/09-23-2014/oil-fluorescence-scanning-on-an-ocean-optics-spectrometer so I'm curious why it's so smooth in yours.

If the ASD is calibrated, it'd be really terrific to get a baseline sensitivity graph for our webcams with a full-spectrum light. They may not be totally exposure linear (though @stoft has more data on this) but just knowing how sensitive our type of webcam is to each wavelength would be a good thing to know, and we could in theory build in a correction to Spectral Workbench. I wonder how much it varies camera to camera. @stoft - do you have thoughts on this -- maybe I'm overlooking something?

@warren Thanks! The sleeve was just a piece of corrugated cardboard, about 3 mm thick, folded around the enclosure and secured with some tape. I’m afraid I can’t provide a photo right now as I’m away from home.

I believe light is entering through the paper itself. It’s quite thin, particularly on the lid where there’s only one layer, so there is likely to be some transmittance. When looking at the webcam image itself I was sometimes able to discern the shadow of my fingers!

The experiments were carried out under bright sunlight, so this may not be so much of an issue if other sources of light are used, but it definitely is for field spectroscopy. Having said this, even under indoor lighting I did notice the black areas of the webcam image appeared darker when the sleeve was used. This probably needs some further experimentation.

In terms of the ASD data, they are the average of three samples - each time you take a measurement, the average of a specified number of samples is produced to reduce noise as you mention. You may be correct about a longer integration time too - this is automatically determined when the device is optimised for the current illumination conditions. Dark current measurements are also obtained during optimisation using an electro-mechanical shutter. They are automatically subtracted from subsequent measurements. I'm not familiar with the Ocean Optics device, but if dark current subtraction wasn’t undertaken this might provide some further noise reduction?

The ASD is calibrated, although getting access to it would be subject to approval - I’d need to have chat with some people in the department. The ASD data presented here were actually collected as part of some work for my master's degree.

We would love some input on selecting a better material for the spec v3 -- it's complex since not every black cardstock is actually black in the near-infrared range. Would you have any interest in testing out some different materials that we could still die-cut and print on?

One additional suggestion has been to use a water-resistant paper or thin plastic. I'm wondering if we could make a water-resistant "sleeve" to slide the spectrometer into. Would it be enough to have a sleeve with open ends? Or would it have to have an end flap as well? Could we improve both water resistance and opacity at once?

Hmm, I just did a test in (admittedly cloudy) sunlight vs. closing the blinds, and covering the device with a thick blanket, and didn't see a big difference. It doesn't seem likely to me that light is leaking in through this big dark folded-over blanket. How does this compare to your noise?

This has had 10 pixel rows of data smoothed to try to reduce noise:

Hmm, the embed isn't working in comments. I'm going to post a quick note:

http://publiclab.org/notes/warren/07-13-2015/testing-if-light-leaks-through-the-walls-of-the-pl-spectrometer-v3

@warren Have posted a response in your note with some webcam images to try and illustrate what is going on at my end. In terms of sleeve design, mine is in fact open ended (please excuse how rudimentary it looks):

I think a water-resistant sleeve is a great idea, the critical thing in my opinion is that the material used is thick enough to prevent any substantial transmittance.

Great, thanks. I think we could relatively easily find a heavier material to make this work. Would you be able to post the picture of your sleeve plus your suggestion here, where we collect input for the next revision? https://github.com/publiclab/spectrometer3/issues

Thanks for your excellent empirical work and suggestion.

Hi, we're compiling input here: https://github.com/publiclab/spectrometer3/issues/25 for an October redesign. Thanks!