Shutter Speeds (Integration Time)
The length of time that an image is exposed--that photons are collected--may be
varied in some cameras or may vary on the basis of video formats (see Table 3).
For reasons that have to do with the parameters of photography, this exposure
time is usually termed shutter speed although integration time
would be a more appropriate description.
Values of the shutter speed as low as 500 ns are available with commercially
available CCD video cameras although the more conventional speeds for
video are 33.37 ms (NTSC) and 40.0 ms (PAL, SECAM). Values as high as 30 s may
also be achieved with certain video cameras although this means sacrificing a
continuous stream of video images that contain signal in favor of a single
integrated image amongst a stream of otherwise empty images. Subsequent
digitizing hardware must be capable of handling this situation.
Again values as low as 500 ns are possible and, with cooling techniques based
on Peltier-cooling or liquid nitrogen cooling, integration times in excess of
one hour are readily achieved.