Public Lab Research note


Mobius IR conversion

by cfastie |

Read more: publiclab.org/n/10342


Above: The Mobius ActionCam opens with two screws. One more set screw and you can expose the IR filter.

To convert a Mobius ActionCam so it captures near infrared photos requires opening the camera and removing the IR block filter. Of the half dozen cameras I have converted, the Mobius is by far the easiest to open, but removing the IR filter is really hard. What I had read and heard about the Mobius didn't prepare me for the job. There are good instructions for opening and refocusing the camera, but I did not find good instructions for removing the IR filter.

cable_clamp.jpg
There is no reason to unclamp the ribbon cable or pull the battery plug (top right).

Steps:

  1. Loosen two screws to open clam shell case.
  2. Scratch line across the sensor unit and the lens to mark focus point.
  3. Loosen set screw.
  4. Pry up sensor/lens unit.
  5. Unscrew lens ~12.4 turns to remove it.
  6. Place camera in safe place.
  7. Find the IR block filter on the rear end of the lens and bust it to smithereens.
  8. Clean rear lens element of glass fragments.
  9. Screw lens back on sensor unit and align focus scratches.
  10. Replace sensor/lens unit in case.
  11. Tighten set screw (or not).
  12. Close case and tighten two screws.

Two small Phillips head screws open the case, exposing the sensor/lens unit and the tiny set screw that must be loosened. The threaded lens will be removed by unscrewing it from the sensor unit. Loosen the set screw before you lift the sensor/lens unit from the case to reduce stress on the ribbon cable. Some Mobiius cameras apparently ship with an Allen wrench for this set screw, but mine didn't. When the set screw is loose (just a little so it can't fall out), the sensor/lens unit can be pried up (see illustration here).

MobiusConvertLens.jpg
Scratch a line across the sensor unit and lens to mark the focus point.

Without yanking on the ribbon cable, unscrew the lens and count the number of complete turns (mine was 12.4). Now the lens can be worked on and the rest of the camera can be put in a safe place. The reddish IR block filter is glued to the rear end of the lens unit. The only way I could remove it was to break it into many shards of glass and lots of glass dust. The dust mostly fell onto the rear lens element and would not blow off. My camel's hair brush just smeared something on the lens, so I had to clean it with lens cleaner. It was the messiest IR block filter removal I have done.

MobTest-399-21.jpg
The rear of the lens after the IR block filter was removed. Note the sharp edges of broken glass which made cleaning the lens an annoying task.

After I screwed the lens back into the sensor unit, I did not tighten the set screw all the way. This allows the lens to be focused without opening the camera. The factory focus point is great for anything about seven feet or farther from the camera. To photograph plants up close, the lens can be rotated counterclockwise just a little. If the set screw is not too tight, this can be done using the notches on the front lens housing that just show when the camera is assembled. You will want to scratch the front of the lens so you can see where the factory focus point is.

MobiusLensFocus.jpg
The scratch at six o'clock on the front of the lens is at the factory focus point. By rotating the lens a few degrees counterclockwise, the lens will focus at about three feet (yellow arrow) or at about seven inches (green arrow). The lens can be rotated with a sharp object inserted into one of the notches around the lens housing.

I now have a full spectrum Mobius camera which takes really weird photos. Using the Mobius Filter-O-Matic, I can try different filters to learn how to make the best plant health images.


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21 Comments

Nice writeup, thanks Chris. I like the tip on focusing, hadn't tried that.


Ok-- I have the ROSCO Fire filter and several others in my swatch booklet. Going to run tests tomorrow, and try digging into the white balance presets.


Here is Mathew's PDF on conversion, as well, for reference: mobius_instructions.pdf


" I can try different filters to learn how to make the best plant health images."

Which filters have you found that make the best plant health images?

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The only filter I tried that worked was Wratten 25A. Two blue filters worked poorly. @mathew has tried Rosco #19 which seems to work well. I don't remember any other research notes about comparisons among other filters. I eventually installed a Wratten 25A filter internally, then replaced it with a glass IR block filter to see if I could have a normal RGB camera again. That didn't work (the photos were way blurry) and I have not gotten back to troubleshooting.


Would acetone or some other solvent dissolve the glue holding on the IR blocker so that it can be removed without shattering it? Is it known what type of glue the manufacturer used to install it?

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I tried acetone, ethanol, and hot air, none moved the glue.


Just a friendly warning for those of you looking to attempt this conversion, I did this last week, it wasn't too hard to smash the ir blocker out but I'm now left with a lens that seems to suffer from focus creep. If I connect the camera to my webcam software and focus it sharply then tighten the set screw, as soon as the lens is put back in the case and the case screws tightened it is rendered out of focus. This leads me to believe that by knocking the ir blocker out something inside the remaining lens elements have been loosened. I have checked the set screw and it remains tight.

Has anybody else experienced this? I now need to buy a new lens and repeat the process but I'm obviously hesitant to do so.

Thanks for your replies!

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Hmm, i once saw this with a different camera, where the filter was helping to pressure the lens into place, and removing it let the lens rock back and forth. Does holding the camera upwards or downwards make a difference? Does it rattle when you shake it?

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theowallis, Did you add a new filter to replace the IR block filter? Where did you put it?

Do you think it is the act of tightening the set screw that moves something and loses your careful focus? Or the act of closing the case?

I did not fully tighten the set screw, and then did fine focus adjustments after everything was put back together. That seemed to work well.

It seems unlikely that you broke something (other than the old IR block filter) or that the old filter has a mechanical influence on focus.

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Nothing rattles when I shake the lens but reassembling the case (or any movement/vibration) alters the focus. I haven't installed a new filter yet. Tightening the set screw doesn't affect the focus if this is done with minimal movement of the lens which says to me that something inside the lens assembly is loose. Tapping the back of the lens housing alters the focus too. This is a very gradual shit so the more you tap, the more out of focus the image becomes. Tapping the lens housing in the opposite direction has the reverse effect.

This bothers me as I've build a quadcopter on which I am planning to use the ir camera. As you guys probably know, as much as you isolate the camera from the quad's vibrations, the camera will still inevitably receive some, thereby causing the focus to shift and rendering it effectively useless. I'd also be interested to hear if anybody has experienced these same issues with the camera even if the ir blocker hasn't been removed.

@cfastie Thanks for your response as I see you're the one who posted this modification. If you haven't experienced this issue then maybe I've just been unlucky in executing the whole modification but I'm sure you can understand why I'm reluctant to repeat it. Who knows, it may be fine the second time around. What I'd like to see is these lenses manufactured sans ir blocker.

Some pictures of my quad because everybody likes looking at photos!

Thanks again for your responses.

BlackHawk.jpg

BlackHawk2.jpg


The sensor is screwed in straight to the lens assembly, so the issue must be some other focus hardware, or a loose piece inside the lense itself.
I didn't think there was any image stabilization or electronic focus systems... you've got me stumped.


Yeah I'm stumped too. As you said, I can only think it's a loose element inside the remaining lens assembly. I've ordered another lens to repeat the process and will post the results.


My second lens arrived today and I reluctantly performed the same procedure (with a slight difference) on it. I went out and bought a glass cutter to score the ir blocker so there wasn't nearly as much force required to break and remove it. It turns out I was over zealous in knocking the ir blocker out of the first lens and this resulted in the shattering of the second piece of glass located 3mm below the ir blocker. This piece of glass below the ir blocker is obviously integral to keeping all the lens elements in alignment.

So I have now tested the new lens in the camera and it is tack sharp no matter how much I tap it. Happy days! The old lens is on the left.

Thanks for your help on this and I hope it helps somebody else to not make the same mistake.

20140808_152806.jpg


You can remove the ir filter on the mobius by using a heat gun to melt the glue and screwing the unit out, you can then hit it with heat again to pop out the glass from the ir filter and use it to hold the new filter.


Yes! Glue can be remove easily with Heat Gun and a Sewing Needle. Smash to bits is never an option. (or the last one) I filed down I tiny screwdriver to loosen the set screw. filter.jpg


Using a heat gun allowed a clean removal of the filter, much easier than smashing. Would recommend just getting a $20 heat gun from home depot.


I'm getting crazy, there is massive amount of talk about mobius conversions but there is no single guidance for converting the mobius for ndvi. I've purchased DIY Filter Kit but now I have no idea about what to do. When we remove the IR-block filter, where do we place the new filter? Whether will it be glued or what? which glue? I need help from you guys, thanks in advance.

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The new filter can be put several different places:

  1. stick it on the front of the lens.
  2. Build a frame to hold it in front of the lens: https://publiclab.org/notes/cfastie/04-18-2014/mobius-filter-o-matic
  3. Stick it on the back of the lens where the IR cut filter was removed.
  4. Put it in the square chamber between the back of the lens and the sensor. You can cut a square piece and glue it in there like in the photos here: https://i.publiclab.org/system/images/photos/000/004/063/original/mobius_instructions.pdf

Chris


Thank you very much for your quick and informative answer, Mr. Chris. Among those choices, which one do you suggest? To me, #4 looks ideal (light cannot pass near the filter, if properly mounter it wont bend etc.) and easy to perform. Is there a special kind of glue? Do you have any kind of warnings, advises etc? (I've read the instructions like this one, but any feedback is welcomed) Thanks again,

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I like #4 too. I don't have a good answer for the type of glue. Something thick and quick drying. It does not have to be sticky, it just has to form a few globs in the corners which keep the filter in place. Let it dry before you close it up.


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