There is an Earthlike planet inhabited by Earthlike lifeforms including beings that are human for all practical purposes. The primary visible human civilisation is roughly equivalent in technology to Ancient Greece in the period 200 BC - 100 BC. However, the god-emperor-like semi-secret rulers of the world possess technology equivalent or greater than what we have today (2022 CE).

The secret rulers are aware that there are pesky human offworlders with comparable technology visiting their world and they wish to detect these intruders and restrict their intelligence gathering activities. While there are many measures being undertaken to achieve this detection / restriction strategy, this question is only concerned with detecting camera usage within population centres by offworlder spies. (Detecting the offworlders as they arrive, fumble their way through the language or exchange diseases with the locals are all out of scope.)


  1. Camera detection equipment is built into statues placed such that any person within a town will always be within 100 m of a statue.
  2. There is no cultural restriction on the materials that the statue can be made of - the populace know that their rulers have powers available, they just consider them magic rather than technology.
  3. Detection station technology is limited to that available in 2022.
  4. Power supply and maintenance for the detection stations are non issues, provided maintenance is not required more frequently than daily.
  5. Active scanning, if any, must not have adverse effects on the citizens or be detectable by the citizens (eg a solution that scans the area constantly with a multi-kilowatt laser/reflection detector combination that will send everyone blind within a few years is unacceptable, as is even a visible spectrum low-powered laser.
  6. Offworlder cameras may be concealable digital cameras or film cameras. However, they are equivalent to off-the-shelf consumer items that have been available in the period 1992-2022 on Earth, not custom creations of a spy agency.
  7. Offworlder cameras' electronics (if any) are shielded so RF detection is not an option. The offworlders are also smart enough to physically disable the flash (if any), to avoid any possibility of accidental activation.
  8. Offworlders will always deploy a camera from within their robes / under the fold of a cloak etc. They will not hold it up in the open to take pictures nor will they leave it unattended.
  9. Budget: Assume that each detection station may contain up to USD 100,000 worth of hardware with a separate and effectively unlimited software budget. Offworlder cameras are price limited to USD 1,000 each.
  10. Detection stations are only expected to be able to detect cameras that are pointing at them.
  11. Detection stations must recognise that eyes (human or other animals) are not cameras for this purpose and must not return a false positive every time a human, sparrow, moth or goat (or equivalent) looks in that direction.

Question: Given the above conditions, how long would the lens of a covertly employed camera 100 m away from a detection station need to be exposed in order for there to be a 50% (or greater) chance of the station detecting it?

Note 1: Every search along the lines of "detecting cameras" provided "how to find a hidden camera in my hotel room", which is how to slowly scan a single room for a camera rather than constantly monitoring a large area for the presence of every camera.

Note 2: While the rulers and the offworlders both have access to particular technologies that are more advanced than current real-world Earth technology, these do not impact this surveillance / counter-surveillance issue.

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    $\begingroup$ I'm pretty sure this is unanswerable. Nothing is going to be able to spot the difference between a camera in my bag with metalwork decorative items, stone/turtle-shell buttons and an empty bag with shiny stone/turtle-shell buttons and metalwork. Close examination/taking it apart would be needed. $\endgroup$ Commented Aug 31, 2022 at 12:01
  • $\begingroup$ Given that the vast majority of the inhabitants of the land have two built-in always-on photocameras and they depend on them for their daily lives, I am not sure that I understand what it is that the emperor-gods want to detect. $\endgroup$
    – AlexP
    Commented Aug 31, 2022 at 12:16
  • $\begingroup$ @AlexP there are story reasons why the emperor-gods are not concerned about biological eyeballs but are worried about cameras that can be used to make a permanent image of particular places. $\endgroup$ Commented Aug 31, 2022 at 22:05
  • $\begingroup$ I get that, but the difference is in the processing of the image after it was captured. The eye-cameras and the non-eye-cameras work in the exact same way. What happens to the image afterwards is afterwards. $\endgroup$
    – AlexP
    Commented Aug 31, 2022 at 22:37
  • $\begingroup$ @AngryMuppet I'm fine with an answer that says why this idea won't work, or won't work out to 100 m with something that can fit in a reasonably sized statue, or won't work for film cameras. I just lack the knowledge of optics to make that assessment myself. $\endgroup$ Commented Aug 31, 2022 at 22:38

3 Answers 3


Active digital cameras are easy to detect. CCD and CMOS cells are effectively retroreflectors, so a bright modulated infrared light source will be visible in them and detectable as long as there is line of sight. 100 meters might be probably optimistic, but many tens of meters would be possible. This is technology that is used in real life and is non-harmful to people.

This will detect close to 100% of cameras with open shutters pointed in direction of the detector that are close enough. Given level of society there should be no other retroreflective things in the open other than cats' eyes, so very few false positives. Use below to sort out the cats.

Film cameras are another matter, because filmstock is not reflective and lenses are usually coated in anti-reflection coatings, thus they are impossible to detect via reflected emissions. (In general case, they are basically impossible to remotely detect, since any box with a pinprick can be a camera obscura. Even CT-scan wouldn't be enough, you'd have to do chemical sample to detect if there is light sensitive chemicals present.)

If you can be sure that a film camera always looks like a camera, then best bet for "today's tech" would probably be to use AI image recognition to pick up lenses and camera looking things in surveillance feeds. This will also allow you to sort out any false positives from the CCD/CMOS sensor detector. Because of variability of AI tech, it is impossible to say how reliable this would be, but it could be anything from 50% to 95% for positives ("camera") with something around 80% likely, and high 90s for negatives ("not a camera").

  • $\begingroup$ Thanks for the response, good to know that film cameras are effectively undetectable without image recognition. Re the "bright modulated infrared source" - would this need to be a narrowly directional beam that needs to be moved around rapidly to repeatedly scan the area, or are you thinking that an omnidirectional IR light source would do the job? Focus of the question is how long to detect the camera - is it near-instantaneous or based on how long a laser/narrow beam can physically sweep 360 degrees horizontally and cover vertical variations? $\endgroup$ Commented Aug 31, 2022 at 22:20
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    $\begingroup$ @KerrAvon2055: Digital camera sensors are very sensitive to IR light, and therefore there is always an IR filter in front of them. This detection method works because the IR filter acts like a mirror: but it doesn't have to, it's just that we don't care. Surely the dastardly intruders do care. (And even with real-world unmodified cameras this doesn't work for all digital cameras. A digital SLR camera does not normally let light shine on the sensor except during the short fraction of a second when actually taking a picture.) $\endgroup$
    – AlexP
    Commented Aug 31, 2022 at 22:46

"Cameras". You leave open the type, the power source, the size, the connection. There is nothing specified about these hidden cameras except they are placed by the offworlders. These cameras might have nothing in common with each other except they stink of offworlders.

Sniff them out.

Magawa dies at 8: Heroic rat sniffed out land mines and helped save lives

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Animals can be trained to find devices based on the smell of those devices. Magawa here found 100+ land mines in his career as a bomb sniffing rat. It will be easier to find offworlder tech because offworld stuff has several different funky smells that are not present on this alien world. To a creature that perceives the world through smell these cameras are super obvious.

-- One could use teams of sniffer animals and search, like they do for mines. I assert though your secret rulers are not that obvious. They do not want the offworlders to know that anyone is searching for their tech. The animals could be trained to work solo and try to get into the cameras and open them up for a treat. Perhaps pest control civilians laconically patrol the city, looking for evidence of rat nests and rat damage and collect the cameras in a disinterested way along with rat nests and other trash, in the interest of civil cleanliness.

Or even stealthier - one could make mutant animals that are especially attracted to the offworld smell. Mutant mice become "super sniffers". We have this tech and so too your hidden rulers. I like wasps or bees for this. They choose sites that smell like offworlders to place their nests. The nests mess up the cameras. The wasps don't care if the cameras are high or low or hidden in a crack, or how they work: they smell like a good nest site. Even before the cameras are discovered the wasps mess them up. Insect control persons eventually remove the nests as a public hazard and the cameras too.

Added benefit - the wasps like other offworlder things for their nests. Nests are not just on cameras! The hidden rulers chuckle.

  • 1
    $\begingroup$ Jinx. I was about to post a comment along the lines of the OP might have better luck with pheromone-detection (of the aliens themselves, mind you). Sure, in the ancient world, the smell of plastics in particular would stand-out (volatile plasticisers and all). $\endgroup$ Commented Aug 31, 2022 at 13:47
  • $\begingroup$ In terms of detecting offworlders and their items, smell is certainly a factor, actually a planned plot point. However, as per point 8, cameras are not being left unattended, they are being briefly used by the person carrying them and then re-hidden in their clothing / bag / belt pouch, so this question is focused on detection in real time in populated areas where potentially many people are moving around. $\endgroup$ Commented Aug 31, 2022 at 22:55
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    $\begingroup$ When I saw "mine-detecting rat died," I assumed it was because it found a mine -- but no: he died of old age! In retirement!! $\endgroup$
    – Tom
    Commented Sep 1, 2022 at 0:46
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    $\begingroup$ @Tom They chose the species because they live sufficiently long, trainable, and too light to trigger mines. Unlike dogs. $\endgroup$
    – KC Wong
    Commented Sep 1, 2022 at 1:42

Maybe you could rephrase your problem statement and move detection to an earlier stage. For example, detect the intruders before they land on your planet. Camera is not the only spy tool. So yeah, think of a planet wide intruder detection system. If you can afford, send several satellites into orbit.

When you say world, you seem to assume a city. That's a fallacy I see in many Sci-Fi movies - when they say a planet they mean a city. Imagine having to monitor for camera use in thousands of cities around your world!

So better move it one level up.

  • $\begingroup$ This appears to be a comment (or two) on my question rather than an answer. Detection and interdiction of visitors before they arrive is a separate problem - or rather a series of problems which would result in a question so broad it would not be allowed on this site. As for the focus on population centres - with great but finite resources the god-emperor types are concentrating on the towns, they don't care if visitors take pictures of waterfalls or fields of wheat-like grain. A town solution can be applied to other vital assets such as mines if required. $\endgroup$ Commented Sep 1, 2022 at 22:52
  • $\begingroup$ The proposed solution was to install sattellites. $\endgroup$
    – ACV
    Commented Sep 2, 2022 at 15:07

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