Princeton Instruments PI·MAX is the first commercial ICCD, ( intensified CCD ) camera to provide MCP bracket pulsing in addition to photocathode gating for enhanced light transmission on/off ratio in UV measure-ments.
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. Applications which can benefit from this improved design include LIF of flames, and nanosecond pump-probe experiments. The glass used to make an MCP has the additional property of exhibiting photoelectric response to UV photons. Photons transmitted by the photocathode can excite the release of electrons from the MCP. 'These electrons can be drawn to and absorbed by the photocathode if it is more positive (i.e., off) but some are attracted by the electric field of the MCP and pass through the MCP's holes, getting multiplied as they go. This is the dominant response of a GEN Il image intensifier to photons when the photocathode is electrically "off", i.e., it is the main source of leakage and thus reduced on/off ratio.
Once electrons emerge from the MCP, they are accelerated to a metal coated phosphor screen by 5 to 6,000 volts. The phosphor screen is metalized both so that it can act as an electrode and to make it optically opaque. The goal however, is for the electrons from the MCP to penetrate the electrode and deposit their energy, creating light which can be coupled to the CCD. If the metal layer is too thick, electrons will not pass through it, if it is too thin, it will be too transparent optically ( like metal reflecting sunglasses). The optimum metal layer is a compromise between these, and this compromise contributes to the high but not infinite on/off ratio of intensifiers.
|Image Intensifier On/Off Ratio vs. Wavelength|
using laser induced fluorescence to probe combusting flows are
performed in the UV. Atomic emission from flames also has significant
UV content. If the flame is continuous, the UV background will
also be continuous. Even where a flame is transient (e.g., internal
combustion engine) its lifetime can be many seconds, compared
to the nanosecond time scale of the lasers used. This background
can last a million times as long. If the background is bright,
then a UV on/off ratio of 20,000: l will be inadequate. For high
dynamic range quantitative measurements, background must be kept
to an absolute um. MCP bracket pulse gating dramatically improves
the rejection of CW and even millisecond-time-scale background