# Low-Light Photodetectors for Fluorescence Microscopy
## 3 Single Point Detectors
### High Resolution of Photon Counting  
HPDs exhibit better pulse height resolution than PMTs. Gain fluctuation of HPDs is significantly lower owing to much higher electron bombardment gain (about 1500 at a photocathode supply voltage of \(-8\) kV) than the first dynode gain of an ordinary photomultiplier tube (typically as low as 5-10). The first gain mostly determines the signal-to-noise ratio of the electron multiplication, which in turn represents the detector's capability to distinguish between one and multiple photons. As a result, HPDs offer high resolution of photon counting. As shown in Figure 8, signal peaks that correspond to 1, 2, 3, 4, and 5 photoelectrons can be identified in the output pulse height distribution.  
High Timing Resolution  
Time response characteristics of HPDs are largely determined by the junction capacitance of the internal avalanche diode, provided the diameter of the internal avalanche diode is around or larger than 1 mm. The internal avalanche diode with a diameter of 1 mm and a very low capacitance (4 pF), which is incorporated in the current HPD product, realizes a fast response. Figure 9a shows the time response waveforms of an HPD and a PMT. The FWHM for the HPD is 0.6 ns, which is smaller than that for PMT (1.6 ns).  
Figure 7: Afterpulse noise of PMT and HPD [32]. The data were obtained through detection of single photons from a continuous wave (CW) light source (wavelength = 470 nm). The cathode supply voltage and the reverse voltage of HPD were \(-8\) kV and 398 V, respectively.  
The TTS determines the instrument response function of HPD. The following three factors mainly affect the TTS for HPD: (i) the transit time within the photocathode; (ii) the variation in the time taken for the photoelectrons to move in the vacuum from the photocathode to the avalanche diode; and (iii) the electron transit time within the avalanche diode. The TTS for an alkali photocathode is about 50 ps [31]. The measured raw TTS for a GaAsP photocathode in an HPD is about 113 ps, which is larger than that for alkali because the GaAsP layer is thicker than the alkali layer. This raw TTS value included the laser pulse width (77 ps) and temporal resolution of the measurement system (30 ps), so the net TTS should be much smaller. Figure 9b shows the time response waveforms of an HPD and a PMT. The raw TTS for the GaAsP photocathode in a PMT is 300 ps, which is larger than that of an HPD. These characteristics are very important for time-resolved fluorescence detection because it contributes to accurate fluorescence lifetime measurement.  
Figure 8: Output height distribution of an HPD [32]. The incident pulsed lights (wavelength = 470 nm) were adjusted to make the photocathode emit three photoelectrons on average. The cathode supply voltage and the reverse voltage were \(-8\) kV and \(380\) V, respectively.  
Figure 9: Time response characteristics of an HPD and a PMT [32]: (**a**) time response waveforms of an HPD and a PMT; (**b**) transit time spread (TTS) of an HPD and a PMT. The data were obtained using a pulse laser (wavelength = 405 nm) with a pulse width of 77 ps. The cathode supply voltage and the reverse voltage of HPD were \(-8\) kV and \(390\) V, respectively.  
HPDs have a large effective area of more than several millimeters in diameter, enabling high photon collection efficiency. The active area of HPDs is comparable with that of PMTs, which is a marked contrast to SPADs with an effective diameter of only 10 micrometers.  
Table 2 lists the characteristics of PMTs, SPADs, and HPDs.