# Low-Light Photodetectors for Fluorescence Microscopy
## 5 Summary and Outlook  
This paper provides an overview of low-light point photodetectors used in fluorescence microscopy and introduces several point detectors and their operating principles, focusing on HPDs that exhibit high timing resolution and low afterpulse. In addition, we demonstrate application of HPD to wide-field single-molecule fluorescence detection.  
Fluorescence imaging with a point photodetector needs scanning to acquire an image and thus temporal resolution of the imaging is often limited. To overcome this limitation, excitation methods other than point excitation, such as multifocal excitation line excitation, have been proposed [45; 46]. Regarding photodetectors, imagers with a large number of pixels have been reported by many groups [47; 48; 49; 50]. For example, [PERSON] et al. reported scan-less wide-field FLIM using a camera consisting of a 500 \(\times\) 1024 SPAD array at a rate of 1 Hz [51]. [PERSON] et al. firstly reported the evaluation of a multi-pixel (8 \(\times\) 8) HPD developed by Hamamatsu Photonics [11]. [PERSON], an author of this paper, and his colleagues, reported another multichannel HPD that was composed of 32 channels (two lines of 16 pixels) on a chip, in which the size of each pixel was 0.8 \(\times\) 0.8 mm (Figure 19) [52]. It was confirmed that the timing resolution and afterpulse characteristics of the multichannel HPD are identical to the conventional single channel HPD. [PERSON] et al. reported an 1024-element SSPD array (a 32 \(\times\) 32 row-column multiplexing architecture) [53]. Fast acquisition methods for FLIM and multi-pixel photodetectors have been reviewed by [PERSON] et al. [22]. These point detector arrays can be widely applied to simultaneous multi-rameter observation, including simultaneous multi-wavelength fluorescence observation.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **SiPM** & **SSPD** \\ \hline Afterpulse & High & Low \\ Transit time spread (TTS) & \(\sim\)300 ps & \(\sim\)50 ps \\ Active area & \(\sim\)3 mm & \(\sim\)10 μm \\ Operating temperature & 250 K–room temperature & \(\leq\)4 K \\ \hline \hline \end{tabular}
\end{table}
Table 4: Characteristics of emerging photodetectors [24; 27].  
Figure 19: A photograph of the developed multichannel HPD. Adapted with permission from [52]. Copyright 2016, Elsevier.  
Low-light photodetectors find more applications in various fields not limited to biological fluorescence microscopy [23]. These applications include flow cytometry and polymerase chain reaction (PCR) in life science, positron emission tomography (PET) for medical diagnosis, elementary particle (neutrino etc.) detection and collision experiments in high energy physics, and semiconductor wafer inspection in industry. In the near future, combination of fluorescence microscopy and other modalities may make low-light photodetectors evolve further and may provide more detailed information on target biological specimens.  
Conceptualization, H.Y.; writing--original draft preparation, H.Y.; writing--review and editing, H.Y., A.F., M.H., and T.I.; investigation, H.Y. and A.F.; formal analysis, H.Y. and A.F.; supervision, H.Y.; visualization, H.Y. and A.F. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Science Research Promotion Fund of the Promotion and Mutual Aid Corporation for Private Schools of Japan (to H.Y., M.H., and T.I.) and the research grant of Tokai Foundation for Technology (to H.Y. and A.F.).  
The data that support the findings of this study are available from the corresponding author upon reasonable request.  
We would like to thank [PERSON] for his advice on pulse lasers.  
The author declares that there are no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.