Exposure modi of high speed ICCD cameras
An high speed ICCD 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.
Independent gating and readout enables different exposure modi
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.
Single exposure mode
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.
Double frame mode
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.
Multiple exposure modes
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.
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 5 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.
Programmed exposure sequence (Burst mode)
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.