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That's a good point. I'm not sure how the removal of the IR filter will affect this work. I mentioned some prior work from the physics side [1] in another comme

That's a good point. I'm not sure how the removal of the IR filter will affect this work. I mentioned some prior work from the physics side [1] in another comment that explores the reflection characteristics in more detail. I don't think they explored the IR filter contribution as well, so this could be an interesting direction. [1] https://ieeexplore.ieee.org/document/8790792 https://ieeexplore.ieee.org/document/8790792

sturgl 5y ago
IR cut filters used in smartphone-style cameras are typically reflective. Installing an absorptive IR cut filter on top of the lens would decrease the amount of reflected light, and might hinder your approach. Those are pretty cheap to buy, so you could try it out pretty easily.
- fortran77 5y ago
  The key is they're _retroreflective_. From this paper: > Specifically, the hiddencamera embedded in the object reflects the incoming laser pulses at a higher intensity than its surroundings due to an effect called lens-sensor retro-reflection. This occurs when almost all light energy impacting an object is reflected directly back to the source (see Section 2.2). These unexpectedly high-intensity reflections from hidden cameras cause certain regions of the ToF sensor to be “saturated” and appear as black pixels. LAPD processes these saturated areas to automatically identify the hidden camera and its location and displays it on the user’s smartphone screen.
  - sturgl 5y ago
    Nevertheless, if you block all the ~850nm IR light from reaching the lens, then there cannot be any retro reflection at this wavelength, meaning the ToF cannot see anything.
    - frizensami 5y ago
      Yes, I agree. I believe military-grade optics detection systems use multiple wavelengths, and some even use short bursts of visible light, making it harder to block.
  - mnw21cam 5y ago
    The filter/sensor itself doesn't need to be retroreflective. The fact that it's sitting at the focus of a lens makes it retroreflective.
fortran77 5y ago
We were trying to detect audience members filming at live events and in movie theatres, esp. premieres and film festivals (and this was some time ago, when studios, etc, cared more about this). It worked pretty well, even in the early 2000s. I suspect you could do much better today with some "AI" behind it.

sturgl 5y ago

IR cut filters used in smartphone-style cameras are typically reflective. Installing an absorptive IR cut filter on top of the lens would decrease the amount of reflected light, and might hinder your approach. Those are pretty cheap to buy, so you could try it out pretty easily.

fortran77 5y ago
The key is they're _retroreflective_. From this paper: > Specifically, the hiddencamera embedded in the object reflects the incoming laser pulses at a higher intensity than its surroundings due to an effect called lens-sensor retro-reflection. This occurs when almost all light energy impacting an object is reflected directly back to the source (see Section 2.2). These unexpectedly high-intensity reflections from hidden cameras cause certain regions of the ToF sensor to be “saturated” and appear as black pixels. LAPD processes these saturated areas to automatically identify the hidden camera and its location and displays it on the user’s smartphone screen.
- sturgl 5y ago
  Nevertheless, if you block all the ~850nm IR light from reaching the lens, then there cannot be any retro reflection at this wavelength, meaning the ToF cannot see anything.
  - frizensami 5y ago
    Yes, I agree. I believe military-grade optics detection systems use multiple wavelengths, and some even use short bursts of visible light, making it harder to block.
- mnw21cam 5y ago
  The filter/sensor itself doesn't need to be retroreflective. The fact that it's sitting at the focus of a lens makes it retroreflective.

fortran77 5y ago

We were trying to detect audience members filming at live events and in movie theatres, esp. premieres and film festivals (and this was some time ago, when studios, etc, cared more about this). It worked pretty well, even in the early 2000s. I suspect you could do much better today with some "AI" behind it.