Fluorescence microscopy benefits from advances in single-photon detectors

The benefits of technological advances in single-photon counting detectors to commercial microscope systems used for fluorescence-signal detection in live-cell imaging are discussed. Photon-counting PMT modules now have maximum count rates of tens of megahertz, such as the H7421-50 module from Hamamatsu, which has a pulse-pair resolution of 20 ns, for a maximum count rate of 50 MHz. Another improvement has been field-programmable gate arrays (FPGAs), which allow flexible logic to be programmed into a chip to provide the high-speed operations required to discriminate, count, and read out current pulses as the laser excitation is scanned over a single pixel of the sample. A group at Vanderbilt University has described the implementation of a simple photon-counting device to offer enhanced detector sensitivity in a two-photon microscope. Using a Hamamatsu H7421 series photon-counting PMT module, the group demonstrated that at low levels of fluorescence the home-built photon-counting detector offers a higher single-to-noise level.