The light-sensitive photocathode of the ICCD converts incident photons into electrons, which are then multiplied within the micro-channel plate (which has approximately 6 million channels), before hitting a phosphor screen, which converts the electrons back into visible photons. The electrons, in turn, are relayed onto the CCD sensor by a fiber optic stub.
The ICCD sensor mechanism provides three benefits:
- Short Exposures - Exposures as short as 3ns are produced by rapidly turning on and off the photocathode, which then functions as a very fast shutter. The phosphor screen at the output of the intensifier has a relatively slow decay, which allows the CCD sensor enough time to capture the image.
- Increased Sensitivity - The MCP multiplies the electrons and can provide variable gain as the potential across the plate is varied. The light amplification is invaluable when taking very short exposures, but this gain control also allows the sensitivity of each of the ICCD units inside the camera to be matched, and provides the fine control necessary to vary the sensitivity from frame to frame when taking images of events that have rapid increase or decrease in intensity, including explosive events, plasma, or electrical discharges.
- Resolution - The combination of the SIM system’s fine-pitch MCP and a fine-grain, high-sensitivity P43 phosphor, carefully coupled to the CCD sensor using a low-distortion 4μm fiber optic stub, has resulted in a spatial resolution in excess of 50 lp/mm.
The analog video signal from each channel's CCD sensor is digitized to 12 bits using a dedicated video signal processor, which is located close to the ICCD to minimize any noise and interference, before being transmitted to the main control electronics. A large state-of-the-art field programmable gate array (FPGA) device then multiplexes the eight separate 12-bit data channels into a single block of high-speed memory where the image sequence is held until required for download to the controlling PC. Dealing with this amount of high-speed data requires very careful PCB design and layout to avoid the introduction of unnecessary noise.
Electronic Integration and Synchronization
Once a trigger is received, it is fed into a high-speed (1 GHz) timing generator, which closely controls the gating of all of the ICCD channels to give up to 1 billion frames per second, admittedly with overlapping shuttering times. A novel, RAM-based timing generator is implemented within an FPGA to allow leading-edge to leading-edge shuttering pulses from 1ns to 10ms. Complex algorithms offer independent control of each ICCD channel with exposure timings from 3ns to 10ms in 1ns steps. The timing signals are then converted into a train of high-voltage pulses to turn the image intensifiers on and off at the appropriate times.
The backend optics and electronics are important; however, the most important feature is simplicity, as these systems should be regarded as tools and not science projects. To this end, the majority of ultra-high-speed systems are USB 2.0, Camera Link, serial, or Ethernet controlled. The interfaces are built into the computer (although most modern laptops now do not come with a serial port as standard). Camera Link does require a specialist card, which can cause conflict during installation and subsequent operating system updates; it is ruled out with most laptops. As the computer industry adopts faster and faster speeds, Ethernet control is likely the best plug and play interface, particularly where an experiment, such as one that uses very fast or explosive subjects, is located at some distance from the controlling computer.
Capturing events that develop over a few seconds requires careful control, but imaging phenomena that last a millionth of a second or less require both extremely fine control of the timing, as well as a sophisticated triggering system. The triggering system has to remain simple yet flexible enough to accommodate the vast range of trigger sources available in the modern laboratory, including optical sensors, strain gauges, make-and-break circuits, or inductive pickup from high-voltage signals.
The triggering system of modern cameras must be capable of accepting any electrical signal in the range of -50V to +50V and should be sampled at a very fast rate so as to reduce the trigger jitter. For example, the SIM camera samples the trigger signal with a 1GHz clock, offering repeatability with a jitter of less than 1 nanosecond. The SIM camera’s triggering is further enhanced by the SURESHOT facility, which utilizes two separate trigger inputs so the exact moment to start recording can be pinpointed, even when test subject velocities are variable or unpredictable.