You have to have special equipment, some of which has been taken off the market by certain camera makers when they found out people were using this equipment toward unsavory ends. Basically, certain frequencies of light that are not within the visible spectrum can penetrate certain types of fabrics that visible light cannot. This principle is used to make those bathing suits that allow you to get tan even whilst covering up your naughty bits. Visible light bounces off the material, making it appear opaque to the human eye, while UV light, which is what tans your skin, is able to pass through it because of its different wavelength. Some UV light passes through most bathing suit material, as a matter of fact.
Enter the 0 LUX camera... originally intended to make it so people could film in night-time conditions with no visible light (0 Lux) Even when there is no visible light, there is still oft times light that the human eye cannot see, making night-vision goggles and zero lux cameras possible. The trick came when people took these cameras out during the day. Normally, the amount of light during the day would make it impossible to use them on a 0 lux setting, but by attaching a special UV light filter to the lens to filter out the frequencies of visible light, you can "see" the UV light that is permeating the swimsuits and bouncing off these people's skin, rendering their bathing suits transparent.
When first introduced, this technology was billed as something akin to James Bond's xray glasses that would magically make certain materials disappear. In reality, it doesn't really work that well. Though some people still of course get off on it, the affect of looking through one of these cameras is similar to that night-time footage we always get from Iraq, everything is green and the picture is usually low quality. Further, while those special suntan bathing suits are nearly completely transparent, other materials are not completely, and you can still see the outline of them. The naked body beneath the clothes is usually ill-defined. Seems like it would be far easier just to visit a nude beach or something.
But, maybe it's a bigger problem in Japan, as I have noticed most of the pictures I've seen of the cameras actually in use in public settings come from public Japanese swimming pools. Still.... if someone is going to THAT much trouble just to see the outline of the crack of ass in shades of green... it seems almost cruel to foil their plans.