Question: Does the spectrometer webcam have an IR filter?

Yes it does have a filter! But if you use the Lego spectrometer at #lego, you can use a Raspberry Pi Noir camera with the Raspberry Pi version and it will not have a filter. 

On Fri, Oct 26, 2018 at 4:22 PM \<notifications@publiclab.org> wrote:

Public Lab contributor MarkE just asked a question entitled ' Does the spectrometer webcam have an IR filter?':

Help them by posting an answer here: https://publiclab.org/questions/MarkE/10-26-2018/does-the-spectrometer-webcam-have-an-ir-filter

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I am investigating the possibility of using the spectrometer to measure THC and CBD content in cannabis. The idea is to use a dye like Fast Blue BB to couple to the THC and CBD. They produce slightly different colors so I am hoping to at least get a measure of the THC:CBD ratio. The colors are in the deep red spectrum, plus there may be some absorption lines in the NIR (unknown at this point). Hence my question regarding the webcam that is currently part of the spectrometer kit.

Additional background: fast blue BB is used to develop thin-layer chromatography plates for analyzing cannabinoids. However, the procedure requires additional consumables, one of which is a mixture of chlorinated hydrocarbons (chloroform and 1,2-dichloroethane). Chlorinated hydrocarbons are health and environmental hazards so I want an alternative. In addition, the chromatography approach is pretty subjective -- you're looking at the size of the color spots -- so there's a lot of room for error.

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Thank you for your reply. At this point I am trying to minimize the cost of developing this approach, so I want to stick with the webcam in the kit. Examination of the webcam indicates it uses a pinhole as a lens, and has an AR-coated window that might include the IR filter. I'm not adverse to modifying the webcam -- I'm a retired electrical/electronics engineer so I'm familiar with ESD-preventing measures. Do you know if it is possible to remove the IR filter?

I suppose one negative aspect of removing the window is that dust could enter the camera and lodge on the sensor -- but the diffraction grating would act as a dust-blocker so no harm there.

Thanks! Mark

If you look at the Microscope page, you'll see some teardowns of the webcam - the lens is small, but not a pinhole! https://publiclab.org/micro

I think my main worry is that the IR cut filter may be somewhere in the lens cylinder, stacked between other lens bits and pieces. It's not clearly visible from either end of the lens screw. But if you're careful about disassembling it and not losing or flipping or reordering any of the parts, it might be removable! I'd love to know more if you try it out -- and we do sell just the webcams in the store if you damage yours.

Please post pictures if you do this!

It's good to know you have the webcams as a store item. That frees me up to try some things. I'll take notes/photos and post results. Mark

I have some additional information regarding the webcam and conversion to IR. Examination of the lens revealed a flat glass window. I think it is the IR filter. I have attached a photo taken from my stereo microscope showing the reflection off the surface of the filter:

Also, examination of the back of the lens assembly revealed an odd lens. It is not smoothly convex -- the central portion is lower than the middle (it becomes concave). I tried taking a photo of the feature but it's not easy to capture. The strongest indication of this is a ring of scratches at the high point of the lens. Here's a photo showing what I'm talking about:

The ring of scratches can be seen just outside the reflections of the LED ring illuminator. I also confirmed that the central portion was lower by focusing back and forth between the center and high point.

I also did some measurements using a digital caliper to determine if the IR filter could be accessed by milling or drilling a hole in the lens body. The lens assembly screws into a molded piece that is mounted on the camera board. The overall length of the lens assembly is 18mm, and the IR filter is located 13.33mm below the front edge of the lens assembly. This places the filter 4.67mm from the back side of the assembly, smack in the middle of the threads used to mount the lens to the board.

I could be wrong, but I think the lens (with its concave/convex surface profile) probably is defective. I've never seen a camera lens with this configuration. A mix of positive and negative FL lenses, yes, but not BOTH on a common surface! This, plus the fact that removing the IR filter would likely make it difficult or impossible to screw the assembly back onto the board, prompted me to find a replacement lens assembly. I found one with a 4.5mm lens, IR-ready, on ebay for less than $20. The lens mount appears to be a very common size -- 12mm diameter with .5mm threads -- so there are many choices available. I may have to modify the camera case to accommodate the replacement lens, but that is relatively straightforward if one has a lathe or mill (I have both).

Whoa, very interesting!

OK, one thing to consider is that the lens mount is an M12, or very close -- so the lenses you can buy here should fit in the same screw mount: https://store.publiclab.org/collections/diy-infrared-photography

as will many other lenses where removing the filter will be easier. Maybe this will help a bit?

I should have inquired before I ordered the 4.5mm lens. I guessed at the OEM lens' focal length by focusing a remote light source, and it looked to be around 5mm. Hopefully it won't affect the spectrometer's resolution much.

I just received the 4.5mm lens. Unfortunately, it appears that the webcam lens is NOT the standard 12 x .5mm. The lens does not screw into the on-board holder.

Additional measurements of the original webcam lens indicate that its threads have a .6mm pitch, not .5mm. I determined this by measuring the change in distance between the front of the lens and circuit board when I turned the lens one revolution.

If it weren't for the fact that the lens is defective, at this point I would just use the camera as-is. To prove my assertion about the lens, here's an image I took with the camera. The object (my computer screen) was about the same distance the spectrometer slit is from the camera:

Whoa! So what is your next step? We might have an extra webcam lens (the type that shipped with it) to see if you can confirm it's defective by comparing to another...? Happy to send you one!

Ooh, I'm just catching up on this thread, but this is really interesting! I have a bunch of the webcams similar to the one we're selling as an upgrade option at home (some that are in various stages of being pulled apart), and many of them seem to have the IR filter outside of the lens barrel--- I'll take a look at some of them this weekend to see if I can figure out which is which. The webcams we're selling as upgrades have a standard mount that should allow for use of different lenses (these can be purchased separately and are pretty affordable) so it's possible destroy a lens here and there without having to replace the whole camera each time (the upgrade cams can take a little bit of a beating). I'll see what I can figure out over the long weekend and update here next week. Really interested to see how this project evolves!

Warren, I really appreciate your offer to provide a replacement lens! If you do have a spare, my address is: 5655 Kenny St. Lake Oswego OR (USA) 97035. If you want me to pay shipping, just let me know how to do that.

Bronwen, I did find a couple of Pi cameras here: https://store.publiclab.org/collections/diy-infrared-photography. Are they what you are referring to?

Thanks, Mark

Warren, Bronwen,

Amanda at publiclab sales has indicated she will send me a replacement webcam, so you don't need to send me a replacement lens. I really do appreciate your support!

Hopefully my next posting(s) will be more result-rich!!

Regards Mark

oh super. Keep us updated!

On Fri, Nov 9, 2018 at 4:45 PM \<notifications@publiclab.org> wrote:

Hi! There's been a response to a discussion you're involved in. You can reply to this email or visit this link:

https://publiclab.org/questions/MarkE/10-26-2018/does-the-spectrometer-webcam-have-an-ir-filter#answer-0-comment-21198

MarkE wrote:

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Warren, Bronwen, Amanda at publiclab sales has indicated she will send me a replacement webcam, so you don't need to send me a replacement lens. I really do appreciate your support! Hopefully my next posting(s) will be more result-rich!! Regards Mark

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