In Wikipedia's high speed camera page, there was this thing 'High-speed film cameras can film up to a quarter of a million frames per second by running the film over a rotating prism or mirror instead of using a shutter...'
There was no other info about this. So, can someone explain me this prism or mirror thing... with a diagram?
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2 Answers
A normal film camera has a mechanism for stopping the film in the gate every frame while the shutter is open, then when the shutter closes pulling down the next frame, stopping it, exposing it, and so on. If the film isn't perfectly still while the shutter is open the image will be blurred.
To do this with high speed film is impractical, the forces involved in pulling down the next frame in 1/500,000th of a second and bringing it to a dead stop would simply destroy any film you could put in it. So in a high speed camera the film doesn't stop, instead there is a rotating prism, as stated in the article. The prism bends the incoming light as it rotates, and it does so in a way that the image follows the frame of film as it passes through the camera.
A good article explaining it here, this illustration in particular:
In this illustration the frame of film would be on the right side, moving downwards, following the spot of light.
The reverse of this can be seen in a Steenbeck editing machine that also uses a rotating prism to hold the image on the screen steady while the film moves continuously through the viewer.
There are also high speed cameras that use whats called a platter - think a large plate like wheel, with the film (a single layer) wrapped around the outside. The entire wheel is enclosed in darkness, sometimes so much as in a vacuum space to reduce both dust and friction of air/heat that would build up. The wheel is then rotated, using a high speed motor and perhaps a basic transmission, spinning up to a very very high RPM. And then the gate/shutter is opened. These cameras can achieve ridiculous speeds, the downside is they only record perhaps a few hundred frames a most. But that is suffice, for say capturing somthing like a bullet exiting a gun... triggering the timing of the camera to the event is the difficult part. Because in real time, the camera only records for a very short time, think 1/2000th of a second of "real time". But you have as a result, 300 frames which represent 1/2000 of a second. 300x2000 = 600,000 FPS The cameras that do 1 Million plus, which actually capture light traveling across a room as in, light turns on you can see the light actually move across the room. That is a whole different technology, where by they actually take millions of executions, separately, triggering the camera to record a frame in steps of 1/1M of a second later each time.
