An
ICCD
high speed camera offers several kinds of exposure modi due to the fact
that the image intensifier may
principally be operated independently of
the CCD sensor. But, of course, the timing of these two independent
system components is always pretty good adjusted together. Though,
their coordination depends on the actual operation mode of the camera.
The two basic events are
- The gating, that opens the shutter
according to the length of the
gate pulse and thereby exposes the CCD chip to the incoming light. This
means, the gating builds up the image on the CCD sensor.
- The readout, that reads out the collected charges from the CCD chip
and transfers the frame, i.e. the image, to the PC. Thus, the readout
acquires the already existing image from the CCD sensor.
The timing diagram below shows a camera operating in single exposure
mode, where each gating event is followed by a readout. This operation
mode corresponds to a standard video camera, if the exposures are gated
continuously by e.g. the built-in trigger source.
A special case of the single exposure mode is the so
called double
frame mode. It allows you to place two gated exposures in two
consecutive frames each, with a time lag of only 500 ns between them.
In every case the image must first be built up by
gating the camera and
thereafter it can be acquired from the CCD sensor as a frame. But, the
image intensifier is an independent system component, thus it may be
gated several times prior to the readout of the CCD, i.e. prior to the
frame access. This kind of operation is called multiple-exposure mode
and is shown in the following timing diagram.
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Moreover,
the intensifier may be even gated periodically and
synchronous to, e.g., a high frequency pulsed laser for a certain
period. The gating frequency in this operation mode, i.e. the
multiple-exposure repetition rate, may be up to 10 MHz. The single
gating events are mostly individually triggered by a trigger source,
maybe the laser itself, to ensure synchronicity. In order to open the
shutter precisely at the desired point in time after the laser
excitation, i.e. after the arrival of the trigger pulse, all our
cameras provide a trigger delay unit which can be adjusted in steps of
10 picoseconds.
Some applications require a certain sequence of
single exposures which
cannot be triggered each by the experimental setup. In addition, the
single exposures may demand individual exposure times. Our cameras meet
this challenge by a RISC processor equipped CPU. You may program any
multiple-exposure sequence by means of our camera remote control
software in a few minutes. The complete exposure sequence may then be
started by a single trigger pulse. The diagram below gives a simple
example of such a programmed exposure sequence.
Furthermore, its possible to repeatedly acquire
whole exposure
sequences, independent of their complexity by our Dynamic Range Expansion system.
Amongst others, these so called scan sequences easily
allow for a serious enhancement of the image resolution and quality by
highly sophisticated frame adding techniques.
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