The selection of a specific wavelength of the white laser is achieved through a motor-controlled monochromator. The pulsed white light is directed via the built-in optical fiber into the optical system, where an H10 monochromator (ISA Instruments S.A., Longjumeau, France) sequentially selects the wavelengths from 680 to 850 nm. The monochromator has an exit slit of 1 mm with a dispersion of . From the monochromator, the light is coupled into an optical fiber bundle to be delivered to the sample. The measured power at the sample is about 0.3 mW at 750 nm, in an area of about . Diffusely reflected light is collected and processed at two detecting channels simultaneously. Each channel consists of a photomultiplier (PMT) detector (Models R7400U-20 and H10721-20; Hamamatsu Corporation, Bridgewater, NJ) that is included in a custom-made, handheld probe. Moreover two high-pass glass filters (Model NT66-045; Edmund Optics, Barrington, NJ) have been used for the detection to filter out photons below a certain wavelength (). The probe also allows multiple source detector distances (Fig. 3) by placing the source fiber in the different available positions. The PMT modules have a gain curve depending on the voltage control that goes from to . The modules have a sensitivity of at 630 nm, a sensing element with an 8-mm diameter, and very low dark noise. The detector sensitivity and the high brightness of the source allow us to obtain satisfactory signals for distances greater than those commonly seen in conventional FDPM instruments. The signals from the PMTs are converted in “photon pulses” using a constant fraction discriminator and directed to the FLIMbox (Laboratory for Fluorescence Dynamics, University of California Irvine, Irvine, CA) that is synchronized with the white laser frequency of 20 MHz. The FLIMbox performs the digital FD acquisition based on the heterodyning principle using a field programmable gate array (FPGA).8 It replaces the RF mixing of analog signals with a digital parallel mixer scheme in the FPGA (rather than using analog mixers, which are noisy and have a low conversion efficiency) by using multiple sampling windows within each excitation period, as shown in Fig. 4. Because the mixing is performed digitally, the signal can be processed by an arbitrary number of sampling windows without any loss of signal (photocurrent pulses) at a 100% duty cycle. The PMT can be operated at full power during the entire acquisition. The photon counting operation mode further improves the signal-to-noise (S/N) ratio of the detection system. Measurements have been obtained with an acquisition of about . We maintain the input laser power at the sample constant using a feedback circuit. The changes in intensity at the detector depend on the sample absorption and scattering. Since data collection terminates when a given number of photons is collected, the only consequence of the transmissions at different wavelengths is to modify the integration time in our acquisition protocol. This means that all data points are collected with the same statistics and the same input power. Figure 5(a) shows a picture of the actual A320 FLIMbox module (ISS Inc., Champaign IL). Figure 5(b) shows the schematics of the FLIMbox module, which includes the preamplifiers, constant fraction discriminators for the two independent input channels, the FPGA chip (Spartan 3E; Xilinx Inc., San Jose, CA), and the connections for synchronization with the laser frequency.