337.1 nm transmission of microscope slides

To extend the UV sensitivity of commercial image sensors, the front cover glass can sometimes be removed. 

A company even offers this as a service: https://www.eureca.de/files/glasscoverremoval_2019-09-24.pdf

The cover glass is most likely N-BK7 with an AR coating for the visible range. Here's a microscope slide demonstrating that it is highly transmissive even at 337.1 nm light provided by a N2 laser:

Apeman Action Camera goes IR (Part 3: Zoom)

A 5-50mm zoom lens (or D14 flange type sold in the EU) would be a neat addition to an action camera otherwise stuck with an ultra-wide lens (140° FOV or more):

1. 5MP resolution lens, 1/2.7 inch, optical glass and metal process.
2. Lens is vari-focal 5-50mm manual focus and zoom, about 15-120 meters' view.
3. Lens has IR CUT ,it can be used in the day-time as light changed.(IRIS will be open and close)
4. Lens is Motorized Lens(Motorized zoom and auto focus with two high precision motos),it  should be is used with drive PCB.

1. Because the product has a short back focal length, the purchase of this product will be accompanied by a D14 mount IRCUT.
2. The back focal length of this product is special. If you need to match it yourself, please note that the height of the lens mount is 10mm±0.3mm.
3. Please make sure that the size of the lens fits your camera housing.

Mount:             D14
Resolution:       2-5 Megapixel(Full support)
Image Form      1/2.7"
Focal Length:    5.0-50mm
MBF:                4.56mm
BFL:                 6.46mm
FLANGE BFL:    10.53mm
IRIS:                 IR CUT
M.O.D.:             1m
Weight:             53g
Dimension:        Φ30x57.7mm
Field Angle DxHxV(°):       58.6 °x49.5°x27.7°(wide) / 8.4 °x7.4   °x4.2  °(tele) (1/2.8")
Operating temperature:    -20°C ~+ 60°C

Package Included: 1X 1/3" 4mp 5-50mm M14/D14 CCTV IR Lens

Ok, so here's the problem:

The mechanical back focal length is too short when you want to fit a filter assembly between the lens and the sensor:

Be it due to the lens being fast, of a design goal to reduce chief ray angle at the outer edge (see https://www.qualitymag.com/articles/95873-design-considerations-for-new-high-resolution-and-frame-rate-cmos-sensors) - M12 lenses end up being as close to 3.5mm away form the sensor. The 4.56mm dimenion is what ultimately makes the zoom lens unusable for our project.

Apeman Action Camera goes IR (Part 2: A80)

A80 Teardown

The newer A80 with QHD at 30 fps and 4K24p capabiltiy (20MP still frames) has a lens shroud that is tucked under the front cover (left), which needs careful persuasion for its clips to release without breaking. The short sides appeared to be the best to attack first.

Next, the lens shroud (right) can be removed.

The structural plate (bottom) makes the thread locking adhesive hard to access, but it can be removed by removing all 7 screws. Note there are 3 different types of screws.

It is recommended to un-solder the power button wires to prevent wire breakage, otherwise the structural plate is always in the way.

To further improve access, the WiFi module can be peeled off the internal walls, and its flat flex cable can be unplugged from the mainboard.

The lens in the A80 was fixed with a clear adhesive around the top of the threads. An Ersa 0102CDLF04 soldering iron tip at 150-160°C was pushed into the joint makes it soft and crumbly. If the lens does not unscrew after the the glue bond has been broken all around the top end of the lens holder, the camera body can be covered with 2-3 layers of Al foil, and 120-140°C hot air can be blown against the aluminium lens housing to pre-heat it to 60-80°C, after which another attempt to unscrew it can be made. It is recommended to protect the plastic enclosure from coming into contact with the soldering iron, as this will easily melt into and ruin the plastic.

What looked like uncured adhesive or grease on the M12x0.5 threads was removed with mineral spirits.
More time was spent to clear the threads with the fine soldering tip.

The VIS bandpass filter is removed by carefully holding the lens in an SMD reworking fixture, application of hot air (120-140°C) and gently pushing against the filter glass on the rear. When the glue is sufficiently softened, it will start rotating or lift off.

Subsequently, the lens, WiFi module and battery were re-installed and functionality was checked with webcamoid. With a rough adjusted focus, the camera can be fully re-assembled.

Infrared Filter

Next, a Proxxon 28846 diamond cutting disc is needed:

40.5mm filters were easy to procure. With the glass having a 37.5mm OD, some trimming was required. A ring was painted onto the filter (Edding 750 paint marker, white), and a 1.75mm offset was scribed with calipers.
After complete drying, the filter glass was trimmed down to the scribed line with continuous water flushing (the usual PPE applies: elastomeric respirator or well-fitting FFP3 mask, safety goggles or face shield).

Stock window on the left, IR720 / HWB720 filter on the right. Its thickness of ~1mm matches that of the cover glass.

The Finished Product

Addendum

The Apeman A80 is said to use a Sony IMX078CQK sensor (capable of 60 fps at 1920x1080 and 42 fps at its native 4072x3044 (12.4 MP)) and Novatek N96660 chipset . From a product brief one can glean the spectral sensitivity. Unfortunately, this 2009 era sensor for consumer electronics isn't fully characterized to 1000 nm or beyond

When comparing to Sony IMX307 , a similar spectral response of R, G and B channels presents itself, giving a cursory idea what the response in the IR could be. The R channel is closest to the equivalent monochrome sensor response. From 850 nm on, most of the spectral sensitivity characteristics of Sony sensors look very similar.
With an HWB720 filter installed, two channels in the IR can be extracted.
With the further assumption that the camera can be configured such that only a lesser amount of non-linearities is introduced, R and B channels can be further processed.
It seems that B selectively recevies 800-1000 nm light, whereas R minus B primarily detects 700-800 nm.

With ffmpeg, one can extract monochrome video as a function of input channels. Example (extracting y, u, v channels as monochrome files using extractplanes:

ffmpeg -i video.avi -filter_complex 'extractplanes=y+u+v[y][u][v]' -map '[y]' y.avi...