Gating is probably the most important single advantage of the PI·MAX camera system. It is the electronic shutter action produced by controlling the photocathode voltage of the image intensifier. Gating allows the detection of low light level signals in the presence of interfering light sources of much greater energy by temporal discrimination. For instance, in combustion research a pulsed laser probe is used to investigate the chemistry within a flame. Since the flame itself emits broadband light continuously, the total flame emission is much greater than the signal resulting from the laser probe (such as laser induced fluorescence or Raman). Fortunately, since the laser pulse is very short and the time at which it occurs is known, it is possible to gate for a few nanoseconds during the laser pulse, thus reducing the flame emission interference by a factor of 10 6 to 10 8 .Gate times as short as 2 nsec FWHM (full width at half maximum) optical gate times are possible Since the control is electronic, the shutter time can be made virtually as long as desired, so shutter times from 2 nsec up to seconds can be implemented in one instrument setup. The electronic shutter on/off ratio is very high, typically 5 × 10 6 or greater.
MCP bracket pulsing
Another feature that contributes to the PI·MAX extraordinary flexibility is provision for MCP bracket pulsing. Traditionally, intensified detectors discriminated against background signal by gating the photocathode. Although this technique yields very high peak Off/On ratios, on the order of 5 × 10 6 to 1 in the visible, background signal can still prove troublesome in low-duty factor measurements, particularly in the UV region where the rejection is only ~10 4 to 1. The PI·MAX allows bracket pulsing of the intensifier microchannel plate (MCP), in addition to the photocathode gating, to gain higher rejection (10 6 :1) in UV measurements. Note that bracket pulsing does not help in the visible region. Under extremely low duty-factor conditions, the only remedy is to install an external shutter ahead of the camera.
Princeton Instruments offers a selection of Gen II and Gen IV (also called Gen III enhanced) image intensifiers, covering the entire visible and NIR spectral region. Gen II intensifiers are available with red enhanced, blue enhanced, and compromise red-blue enhanced photocathodes as stock items. Quantum efficiencies above 1% are attainable from about 160 nm to about 850 nm with quartz or suprasil windows. Intensifiers with MgF 2 windows are also available with response in the 120-700 nm range. Gen IV intensifiers are available in the 400 - 900 nm region, and for the 800 - 1100 nm region. Some image intensifiers suffer from "iris effect" when gated. Iris effect is mostly a result of the distributed resistance and capacitance of the photocathode, causing the periphery of the photocathode to turn on before the center and vice versa on turn off. The usual solution to iris effect problems is to put a conductive underlay on the photocathode, but this reduces quantum efficiency. High QE intensifiers with low iris effect and without compromise are offered exclusively by PI. PI selects image intensifiers for minimum noise, negligible corona, highest gain, and longest operational and shelf life. In addition, because of our close relationships with most of the major intensifier manufacturers worldwide, we can usually provide state of the art custom photocathodes. For special requirements contact our office or your representative.
vs. Signal Measurement
Probably the most confusing aspect of ICCDs is the distinction between detection and measurement. Fiber coupled ICCDs like the PI·MAX, have the option of operating the intensifier at extremely high net gain, greatly intensifying the light reaching the CCD. Thus a single photoelectron from the photocathode can produce a signal of 80 to 100 A/D units ("counts") with a readout noise of only 1 A/D unit, making it very easy to determine if a photoelectron event occurred. This is detection, and ICCDs excel at it. In many experimental applications, the importance of detection is that it allows the experimental system to be set up and adjusted into operation. In the beginning, the various elements of the experiment are not optimized and the mere existence of an optical signal is a sign the experimenter is on the right track. Once the experiment is optimized, there is often sufficient optical signal so that existence is no longer important, but quantization is. As the magnitude of the signal increases, the photon shot noise of the signal itself becomes more significant, and soon becomes the dominant noise source in the experiment. The advantage of a high sensitivity ICCD is not that it provides a higher S/N ratio with a reasonably sized signal, but that it allows the experiment to be optimized to the condition where there is enough signal to measure. Of course, once you reach this state, you can adjust the gain downward in the high sensitivity ICCD and get higher dynamic range and even a slightly better S/N ratio than with a low gain ICCD. To maximize the signal to noise ratio of the data as the signal increases, the gain of the ICCD should be reduced, allowing the maximum number of photons to be captured within the dynamic range of the ICCD. Some ICCD vendors have used this as an argument against high gain ICCDs, but it is a false argument for three reasons:
For maximum S/N, the available signal should just fill the detector's dynamic range.
The experimental world often refuses to provide enough signal to fill the detector's dynamic range.
If you need extreme gain (detection mode) to set up and adjust the experiment, then it follows that with a "lowgain only" ICCD it will not be possible (or at least it will be much more tedious) to get the experiment optimized enough to use the low sensitivity settings.