Dark flash
The dark flash (also: black flash ) is a technique of infrared photography .
Action
The dark flash method uses an infrared filter film in front of the flash instead of an infrared filter in front of the lens . This makes the flash invisible, only if you look directly into the reflector can you see a reddish glow. So while with ordinary infrared photography the visible light in front of the lens is filtered out and thus kept away from the film or sensor, with the dark flash method the subject is only illuminated with infrared from the start. In doing so, however, the film has to hold back a large part of the flash light, which puts an enormous strain on it and consequently causes it to fade very quickly.
application
With the dark flash method, people in the dark, such as cinema-goers, can be photographed unnoticed during the screening. However, their faces are displayed unnaturally, which is why this technique is of minimal importance in amateur photography. However, shy nocturnal animals can be documented so well.
In 2010, a speed trap was installed in the Britz tunnel in Berlin , which photographs motorists driving faster than allowed without them noticing ("black speed camera ").