Specifications

Appendix G Interline CCD Cameras 87
SCAN
Mechanical Shutter
Acquire Readout
ClosedOpen
Exposure time
Shutter compensation
t
exp
t
c
Figure 44. Exposure of the CCD with shutter compensation (non-overlapped mode)
is low during readout, high during exposure, and high during shutter
compensation time.
Since most shutters behave like an iris, the opening and closing of the shutter will cause
the center of the CCD to be exposed slightly longer than the edges. It is important to
realize this physical limitation, particularly when using short exposures.
Continuous Exposure (no shuttering)
Cameras with interline capability may be used with or without a shutter. When operating
without a shutter, image smearing may occur due to a small amount of light leaking
through to the storage cells during the readout time. In the case of lens-coupled
intensified cameras (ICCDs), this effect can be eliminated by using a fast phosphor and
gating the intensifier at the same frame rate as the CCD.
The fraction of total signal due to smearing is the ratio of the readout time to the
exposure time divided by ~4000. Faster readout or longer exposure times will minimize
this effect. Note that while 1% smear is insignificant in an 8-bit camera (256 gray levels),
in a 12-bit camera (over 4,000 gray levels) 1% smearing is over 40 counts, enough to
obscure faint features in a high dynamic range image.