Warning: This story contains some offensive language
So-called spy-cam porn has existed in China for at least a decade, despite the fact that producing and distributing porn is illegal in the country.
But in the past couple of years the issue has become a regular talking point on social media, with people - particularly women - swapping tips on how to spot cameras as small as a pencil eraser. Some have even resorted to pitching tents inside hotel rooms to avoid being filmed.
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The theory is to detect infrared light for the cam who utilise infrared night vision, but from the article it seems that they don’t use that at all, so when the light’s out no one can view anything. This might be the reason why it went undetected.
That’s one method, also camera sensors have a retro-reflective effect that makes using a camera flash highlight other cameras in its field.