Emitted fluorescence signal from the sample was detected with six optical fibers that were arranged around the ring holder as shown in Fig. 1. Six detector fibers were used so as to allow close to full-angle collection of the emitted fluorescent light. As we demonstrate, the signal from individual fluorescent microspheres and fluorescently labeled cells was detectable on each of the six detection channels, but in this work, these were summed to improve the overall signal-to-noise ratio (SNR). Specially cut filters centered at 700 nm with a 50-nm bandpass (ET700/50, Chroma Technology, Rockingham, VT) were placed in front of collection fibers; this blocked diffusely and specularly reflected light from directly entering the fiber and generating secondary autofluorescence. On the opposite end, the fibers were terminated on a custom-designed filter housing with collimating lenses and a second, 700-nm filter (Chroma) placed in front of each anode of an eight-channel PMT array (H9530-01, Hamamatsu Photonics, Japan). The use of two filters for each detector fiber was empirically determined to be necessary since the emitted fluorescence from individual cells was very small, and even modest amounts of autofluorescence or laser light leakage could obscure the signal. The output from each channel of the PMT was then amplified with a 1.6 GHz, eight-channel preamplifier with 26 dB gain (HFAM-26dB-10, Boston Electronics, Boston, MA) and passed into an eight-channel multichannel scalar (MCS) photon counting card (PMM-328, Boston Electronics) installed in a personal computer (NIXSYS Open Systems, Santa Ana, CA). This instrument design allowed high-sensitivity photon counting from each of the six detection optical fibers simultaneously (the two additional PMT channels were unused). The photon-counting threshold was set to for each channel, and the sampling rate was set to a rate of . The maximum number of photon counts per time sample was 65,535 (hardware limited) on each of the six detection channels. For each experiment, the MCS card was configured to acquire continuously for 7500 samples, which was equivalent to 75 s. The 75 s measurement “run” could be repeated an arbitrary number of times with approximately a 0.1-s time delay between cycles to allow for writing of the data to the hard drive. Experimental automation was performed using the analog outputs of a multifunction data acquisition card (DAQ; NI-USB-6251, National Instruments, Austin, TX) controlled with the same personal computer.