Spectral filters on SEEK or similar cheap microbolometer cameras - Page 1
Forum section: electronics / general

author: pseudotexan
author_rank: Newbie
timestamp: November 11, 2018, 04:50:10 pm
content: Hi all!Been lurking this forum for some time, mostly reading up about the great work everyone's done with reverse engineering and interfacing with low-cost thermal cameras. My thanks!My interest is in using them to image the beam profile of lasers in the 3-5 um wavelength range (I am a photonics researcher). I also will use them to align beams into different optical structures such as waveguides. VOx is generally sensitive even down to 2 um, so it is a good candidate detector technology for something much cheaper than MCT or InSb sensors. We do have an InSb camera, but it's not always available and I want to see if I can get away with a much cheaper device so that we can implement it in multiple experiments. I've also seen some commercial products like the DataRay beam profiler system (VOx-type), but again that's like $10k.I picked up a SEEK CompactXR and have been messing around with it a bit. Some initial testing: I have a 3.39 um HeNe infrared laser with about 1.2 mW output power, and the SEEK easily sees it on the sensor, even if I add 30 dB of optical attenuation. This is still true even if I remove the chalcogenide lens and focus the beam onto the sensor externally. So ~1 uW sensitivity over a few hundred um^2 area is not too bad to start. I will add some photos when I get back to work on Tuesday if that is of interest to anyone.However, I have reason to expect that much better sensitivity might be possible. For example, FLIR uses band-pass filters in the 7.5-13 um range, presumably coated into a window atop the microbolometer die (correct me if I'm wrong of course). This is to maximize SNR for the intended sensing region. If the SEEK cameras have a similar filtering arrangement, then I am missing out on most of the sensitivity in the mid-IR!So my question is: does anyone know if there is any spectral filter (other than the lens, since I can easily remove that) on the SEEK cameras? I am happy to look into it if it hasn't been figured out yet. I have a couple XRs and one CompactPRO, so I could experiment on one of the XRs and see what can be done.I have access to plasma etching equipment, so I could potentially mill down the top surface of the window by a few microns and see what that does to the responsivity.If anyone has insight as to where the spectral filters typically go on other thermal cameras, that'd also be helpful.

author: Fraser
author_rank: Super Contributor
timestamp: November 11, 2018, 05:55:21 pm
content: It is my understanding that two factors effect the microbolometer sensitivity (or lack of) to MWIR1. The Windows in front of the pixels is AR coated for Longwave on the outside surface.2. The reflector distance behind the pixel is tuned to 1/4 Wavelength mid-band.If you remove the AR coating from the microbolometer window you create higher losses at the interface. This may still be acceptable in your case though. You can do nothing about the pixel to reflelctor tuning but again this may not be an issue in your scenario.The window material on the SEEK microbolometer is Silicon so suited to Mediumwave transmission.There is still a very small hard vacuum within the SEEK microbolometer so be careful that whatever method you use to remove the AR coating does not compromise the capsule holding the vacuum. The Read Out IC is is CMOS and static sensitive so avoid high voltages around the microbolometer.Fraser

author: Hydron
author_rank: Super Contributor
timestamp: November 11, 2018, 08:50:40 pm
content: Quote from: Fraser on November 11, 2018, 05:55:21 pmWould mechanical lapping be an alternative to plasma etching to remove the AR coating? I believe that surface roughness requirements are relaxed compared to visible wavelength optics, though you'd want to avoid defects big enough be imaged given the proximity to the focal plane.

author: pseudotexan
author_rank: Newbie
timestamp: November 11, 2018, 09:47:28 pm
content: Thanks for the replies, Hydron and Fraser.Good call about the high voltages.Quote from: Hydron on November 11, 2018, 08:50:40 pmLapping is a very good idea, I think that would be relatively easy, if I can protect the board well enough from slurry or residue. the MB die seems to have good clearance off the PCB so I can probably 3D print a cage around it to keep it tidy.