Single Photon Detectors

  • Low Dark Counts: 25 Hz and 150 Hz
  • Detector Sizes of Ø20 µm and Ø50 µm
  • Active Quenching and Temperature Stabilization


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Complete with Cables, Power Supply, and Software

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Item # SPCM20A(/M) SPCM50A(/M)
Detector Type Si Avalanche Photodetector
Wavelength Range 350 - 900 nm
Active Detector Size Ø20 µm Ø50 µm
Typical Max Responsivity 35% @ 500 nm
Dark Count Rate
25 Hz (Typical)
60 Hz (Max)
150 Hz (Typical)
200 Hz (Max)
Max Count Ratea 28 MHz 22 MHz
  • For Pulsed Light


  • Low Dark Counts
    • SPCM20A(/M): 25 Hz (Typical)
    • SPCM50A(/M): 150 Hz (Typical)
  • Two Detector Sizes
    • SPCM20A(/M): Ø20 µm Active Area
    • SPCM50A(/M): Ø50 µm Active Area
  • Active Quenching
  • Temperature Stabilized
  • USB Interface
  • Pulse Output
  • TTL Gating/Trigger Input
  • Compact Size: 76.5 mm x 85 mm x 36.2 mm


  • Spectroscopy with Single Molecules
  • Spectro-Photometrical Measurements
  • Flow Cytometry
  • Photon Correlation Spectroscopy
Detection Probability vs. WavelengthClick to Enlarge
The Photon Detection Probability is shown here as a function of photon wavelength. The shaded region of this graph identifies the operating range of the detector.

Thorlabs' Single Photon Detectors, which use a silicon avalanche photodiode to detect single photons, are sensitive to photons emitted in the 350 to 900 nm range, with the maximum sensitivity around 500 nm (see the graph to the right). Incoming photons are counted by an internal 31-bit counter. In addition, an output SMA connection offers a signal that can be viewed on an oscilloscope or connected to an external counter. Please see the Tutorial tab for details about the functionality of this photon counter.

An integrated Peltier element stabilizes the diode's temperature below the ambient temperature to reduce the dark count rate. The two available models, SPCM20A and SPCM50A, have low typical dark count rates of 25 and 150 counts per second, respectively, which allows them to detect power levels down to 0.14 fW.

The active quenching circuit integrated into the diode of the single photon detector enables high count rates. Its high speed allows users to count a photon every 35 - 45 ns, depending on the model chosen. The SPCM20A provides an active area of Ø20 µm while the SPCM50A offers Ø50 µm.


The Single Photon Counter Module includes a software package with GUI for out-of-the-box operation. The following operating modes can be set by the software:

  • Manual Mode for manual operation
  • Free Running Timer Counter for counting incident photons for a certain number of "Time Bin Lengths"
  • Externally Triggered Timer Counter for triggering the timer start for counting incident photons for a certain time period
  • Externally Triggered Counter for starting and stopping the counter by an external trigger
  • External Gating for activating the counter and the APD externally

For more details about the software and its operation modes, please see the Software tab.

Item # SPCM20A(/M) SPCM50A(/M)
Detector Type Si Avalanche Photodetector
Wavelength Range 350 - 900 nm
Active Detector Size Ø20 µm Ø50 µm
Typical Max Responsivity 35% @ 500 nm
Dark Count Rate
25 Hz (Typical)
60 Hz (Max)
150 Hz (Typical)
200 Hz (Max)
Max Count Ratea 28 MHz 22 MHz
Dead Time 35 ns (Typical) 45 ns (Typical)
APD Gating Delayb 18 ns (Typical)
Gate / Trigger In
to Pulse Out Delayc
28 ns (Typical)
Afterpulse Probability 3%
APD Temperature Stability <0.1 K
Gating / Trigger Input TTL 50 Ω
Pulse Output TTL 50 Ω
Dimensions 76.5 mm x 85 mm x 36.2 mm
Power Supply 6 VDC / 1.5 A
  • For Pulsed Light
  • In external gating mode: delay between trigger signal (at Gate / Trigger In) and activating the APD gating.
  • In external gating mode: delay between trigger signal (at Gate / Trigger In) and Pulse Out.

Operating Principle of Single Photon Detectors

Avalanche photodiodes operated in the Geiger Mode have the ability to detect single photons. This single photon sensitivity can be achieved by biasing the APD above the breakdown voltage (Point A in Fig. 1). The APD will remain in a metastable state until a photon arrives and generates an avalanche (Point B). This avalanche is quenched by an active quenching circuit inside the APD (Point C), which lowers the bias voltage below the breakdown voltage (labeled VBR in Fig. 1).

Current Voltage Characteristics
Thorlabs Single Photon Counter SPCM in Geiger Mode
Figure 1: Current Voltage Characteristics of an Avalanche Photodiode Operated in Geiger Mode

Afterwards the excess bias voltage can be restored. During this time, which is known as the pulse dead time of the diode, the APD is insensitive to any other incoming photons. Spontaneously triggered avalanches are possible while the diode is in a metastable state. If these spontaneous avalanches occur randomly, they are called dark counts. If the spontaneously triggered avalanches are correlated in time with a pulse caused by a photon, it is called an afterpulse. To block such afterpulses in the measurement, an additional pulse dead time can be set in the software, which will cause the internal counter of the single photon detector to ignore all pulses occurring during this pulse dead time.


Geiger Mode:
In this mode, the diode is operated slightly above the breakdown threshold voltage. Hence, a single electron-hole pair (generated by absorption of a photon or by a thermal fluctuation) can trigger a strong avalanche.

Dark Count Rate:
This is the average rate of registered counts in the absence of any incident light and determines the minimum count rate at which the signal is dominantly caused by real photons. The false detection events are mostly of thermal origin and can therefore be strongly suppressed by using a cooled detector.

Active Quenching occurs when a fast discriminator senses the steep onset of the avalanche current and quickly reduces the bias voltage so that it is below breakdown momentarily. The bias is then returned to a value above the breakdown voltage in preparation for detection of the next photon.

Dead Time is the time interval the detector spends in its recovery state. During this time, it is effectively blind to incoming photons. The dead time fraction, which is an inherent feature of an active quenching circuit, may be defined as the ratio of missed to incident events.

During an avalanche, some charges can be trapped inside the high field region. When these charges are released, they can trigger an avalanche. These spurious events are called Afterpulses. The life of those trapped charges is on the order of a few tenths of a microsecond. Hence, it is likely that an afterpulse occurs directly after a signal pulse.


Version 1.1

Click the button below to visit the software page for these Single Photon Counters.

Software Download


The Single Photon Detector includes a software package with GUI for out-of-the-box operation. The following operating modes can be set by the software:

Manual Mode:
The counter is started and stopped manually by pressing the Start/Stop button (toggle function). The timer will be reset at each start.

Free Running Timer Counter:
Both the number of time bins (i.e, the number of measurements) as well as the minimum interval between two subsequent bins can be set.

Externally Triggered Timer Counter:
In this mode, the timer is started by an external trigger signal and counts incident photons during the set time bin length. The active trigger slope (rising or falling) can be selected.

Externally Triggered Counter:
In this mode, the external trigger signal will start and stop the counter.

External Gating:
The counter and the APD are activated externally.

Measurement Settings:
In the array mode, each data value is recorded to an array. In the continuous mode, the measurement is restarted after the preset number. Both modes can be saved as a .txt file. The measurement results can be represented as a bar (XY bar with counts vs. number of measurements), graph (curve), table (numeric) or alignment (numeric with additionally information) display. The number of measurements can be defined, and the measurements can be repeated.

Posted Comments:
MKiess  (posted 2021-08-24 07:56:07.0)
Dear, SOOHEYONG LEE, thank you for your inquiry. Negative and positive pulses can occur if you use a rather long cable and you have not terminated it with 50 ohms or have not set 50 ohms on the oscilloscope. This leads to ringing, which can show both positive and negative pulses. I have contacted you directly to discuss your application and further details on this with you.
Ben Sparkes  (posted 2021-03-10 21:56:12.767)
Dear Thor Labs, I was wondering what the timing jitter on your single photon detectors (SPCM20A/SPCM50A) is? Cheers, Ben
MKiess  (posted 2021-03-17 06:07:17.0)
Dear Ben, thank you for your inquiry. Unfortunately, we do not have data regarding the timing jitter for these two detectors.
Clara Spetebroodt  (posted 2020-09-30 13:08:44.427)
Dear Thorlabs team, Is it necessary to cover the whole sensor area of SPCM50A ? Best regards
MKiess  (posted 2020-10-01 09:47:33.0)
Dear Clara, thank you very much for your inquiry. It is not necessary to illuminate the entire active detector area to make a photon measurement.
Chul Woo Ahn  (posted 2020-08-18 00:35:12.52)
I would like to set up the Hanbury Brown and Twiss interferometer. I would like to know if it is possible to experiment with these 2 detectors and software. Thank you.
dpossin  (posted 2020-08-19 03:12:19.0)
Dear Chul Woo Ahn, Thank you for your feedback. We do have loan devices of our SPCM50A/M. I am reaching out to you in order to discuss the modalities.
Vygandas Jarutis  (posted 2019-11-15 07:27:48.693)
We have exactly the same question as was posted by Daniel Borrero, i.e. can we use 2 detectors simultaneously, and trigger one of the detector with the output of the other?
MKiess  (posted 2019-11-15 11:46:38.0)
This is a response from Michael at Thorlabs. Thank you very much for your inquiry! The SPCM50A has a trigger input. This means, you can send an external TTL trigger signal. Furthermore there are different external trigger modes. However, the SPCM20A cannot output a trigger signal. I contacted you directly to discuss the possibilities for your application and to provide further assistance.
Daniel Borrero  (posted 2019-10-28 12:15:12.453)
I would like to do coincidence counting, I would like to know if I can use 2 detectors simultaneously, and trigger one of the detector with the output of the other and somehow carry out the correlation measurement. I would like to set up a Hong Ou Mandel interferometer, and I would like to know if I can do the measurements using this detector. Thanks!
MKiess  (posted 2019-10-29 12:38:37.0)
This is a response from Michael at Thorlabs. Thank you very much for your inquiry! That sounds like an interesting setup. This Single Photon Counter does not have a special trigger output. However, a current pulse is generated by an incoming photon passes a pulse shaping circuit, which is shortening the APD's output TTL pulse duration from 35...45ns to ~14ns. These pulses are applied to the SMA "Pulse Out" connector. Maybe we can use that pulse for your application. I contacted you directly to discuss details and feasibility.
George Brown  (posted 2019-08-15 17:56:30.86)
I would like to run the SPCM50A in a simple free-running mode. Will I need to download the Thorlabs software to do so? If so, will I need to separately install the National Instruments software?
MKiess  (posted 2019-08-20 04:35:13.0)
This is a response from Michael at Thorlabs. Thank you very much for your inquiry! To run the SPCM50A in a Free Running mode, you can simply download the Software for Single Photon Counter Module Series from our website. A NI-VISA Runtime Engine® is required to operate the Single Photon Counter. If NI-VISA is not installed yet, the installation package will install version 5.0.3 automatically.
Firoz Khan  (posted 2019-07-22 10:32:19.76)
Hello Sir, I am Dr. Firoz. I have two SPCM50A/M detectors. I want to employ these two detector simultaneously by using your provided software but only one detector go only in running mode. Can we handle two detector with one software simultaneously?
dpossin  (posted 2019-07-25 11:05:46.0)
Hello Dr. Firoz, Unfortunately it is not possible to drive more than one detector with our software simultaneously. Alternatively you could trigger the SPCMs externally.
rickkreidler  (posted 2018-09-17 13:14:45.387)
I need to do fluorescence decay time measurements on inorganic phosphor powders. I have acquired an Edinburgh Instruments EPLED 250 pulsed laser for this task. The laser output consists of pulses having FWHM 10 nm at peak wavelength 250 nm. The pulse frequency is adjustable from 20 MHz to 2.5 kHz in15 steps. My expected decay times are in the range of 0.5 to 20 microseconds which means that I should operate the EPLED source at frequencies of 50 to 2000 kHz. The average power of the EPLED laser at 10 kHz is 1.2 microwatts. The EPLED produces an internal trigger pulse -380 mV into 50 Ohm impedance. The time duration of the trigger pulse is 3-5 ns. The EPLED can also be triggered by an external TTL trigger pulse. It is necessary to trigger Single photon counter at the same time as the EPLED. I have attempted to do the measurements using a Thorlabs avalanche photodiode (APD130A2) without success. Although I can detect the lase pulse, there is not enough signal from my phosphor samples to detect and measure their decay curves. All I see is noise. I have therefore concluded that my only option is to use a single photon counter. I have been displaying my signals on a Tektronix 50 MHz two channel digital oscilloscope. I have been able to record the spectrum of my samples using an Ocean Optics usb2000+ spectrometer. Sampling times of the order of 40 seconds were needed this merely confirms that my samples were excited by the pulsed laser. I will need your advice on whether a Thorlabs single photon counter will be able to produce measured decay curves from my samples and if so how it should be set up. Please note that the quantum efficiency of my samples is ~80% for excitation by 250 nm radiation. Sincerely, Eric R. Kreidler
wskopalik  (posted 2018-09-20 04:59:09.0)
This is a response from Wolfgang at Thorlabs. Thank you very much for your inquiry! In general the single photon counters would be an improvement for the detection of the fluorescence signals. There are however a few critical points which depend on the details of your setup and which we should discuss in greater depth. I guess that the fluorescence is emitted randomly in all directions. So one would need to make sure that as much light as possible is collected by the photodiode in the counter. The wavelength of the fluorescence would need to be in the spectral range of the single photon counters (i.e. 350 - 900 nm) as well. And also the resolution in time is limited by the dead time of the counter which is 35 ns. This means that after the detection of a photon, the detector cannot detect a new photon for 35 ns. So for a decay time of 0.5 µs you would get about 15 measurement samples. I will contact you directly regarding these points so we can find a suitable solution.
y5shi  (posted 2018-06-21 14:11:41.163)
I wonder how should I mount this module to a Nikon upright microscope with C-mount? We can't do free-space coupling so I'm not sure what is the best way to integrate this module to the microscope. Should I use a multimode fiber for the coupling?
YLohia  (posted 2018-06-21 05:33:44.0)
In order to mount to a C-mount, you will have to use the SM1A9 adapter. We really do not recommend multimode fibers for this since the SPCM20A and SPCM50A have active sensor diameters of 20um and 50um, respectively, which is significantly smaller than the core size of most standard multimode fibers. We do, however, sell 10um and 25um core size multimode fibers here: I will reach out to you directly to discuss this further.
parksj003  (posted 2018-05-14 19:38:04.24)
I would like to measure the tail of fl. lifetime which is quite small to be measured. To make it measurable, we can increase the fl. intensity and selectively measure the tail by TTL gating function. But I am worrying about the pile up problem. So would you let me know that it gates TTL output only or inactivates the photon detection? I think if it only gates TTL output, it is not possible to avoidable the pile-up problem which resulting in inaccurate life time curve in tail. Thank you.
mvonsivers  (posted 2018-05-17 07:01:04.0)
This is a response from Moritz at Thorlabs. Thank you for your inquiry. It is possible to operate the Single Photon Counter Modules in External Gating mode. In this case, the external gating signal directly controls the APD bias voltage. Therefore, the photon detection is only activated when a gating signal is applied.
tjwoehl  (posted 2017-06-29 10:49:30.357)
I am looking to possibly use this SPCM detector for a photon correlation spectroscopy application on an optical microscope, similar to fluorescence correlation spectroscopy. What is the minimum bin size and the minimum bin interval that this detector can achieve? In this mode, does the software directly record photons/bin as a function of time? What mode would the detector typically run in for photon correlation spectroscopy?
swick  (posted 2017-07-04 03:41:24.0)
This is a response from Sebastian at Thorlabs. Thank you for the inquiry. The Bin length and the Time Between Bins can be set from 0.000001 to 2147,483647 seconds. For example you can use the Free Running Timer Counter for counting incident photons for a certain number of "Time Bin Lengths". In order to recommend the correct operation mode for you application, I have contacted you directly for assistance.
cpepe  (posted 2016-03-21 10:53:52.76)
Hi, I was wondering what the timing resolution is for the SPCM detectors?
tschalk  (posted 2016-03-22 07:27:02.0)
This is a response from Thomas at Thorlabs. The specification of the max. count rate is 28MHz for SPCM20A and 22MHz for SPCM50A. I will contact you directly to discuss your application.
m.traulsen  (posted 2014-05-20 17:00:09.94)
Hello, my application is distance measurement and therefore the minimal time bin length of 1 microsecond is way too long. If I use the externally triggered counter mode can I achieve shorter bin length and therefore faster counting? And also If I use the external gating mode what is the shortest bin length then? And if I gate fairly fast (let's say 15 MHz so too fast for the software) am I still able to measure perfectly with my TSCPC or oscilloscope?
tschalk  (posted 2014-05-23 11:07:42.0)
This is a response from Thomas at Thorlabs. Unfortunately, it is not possible to achieve shorter bin length with external triggering. You can use the software and external gating mode at 10MHz. It is also possible to use 20MHz gating frequency using an oscilloscope for the readout. I will contact you directly for more detailed information.
esolarte  (posted 2013-03-03 06:06:38.3)
Can I detect 20ps photon pulses @ 780nm, with your SPCM20A? if yes, what is the time resolution of this detector? Need I some especial (additional) electronics?
cdaly  (posted 2013-03-06 14:39:00.0)
Response from Emily at Thorlabs: Thank you for your inquiry! The SPCM20A would detect a 20ps pulse. The detector itself has a timing resolution (FWHM) of 40ps. The SPCM20A does not perform time correlated single photon counting. Therefore to make time correlated single photon counting you would need additional electronics. You can connect your external electronics to the pulse out connector which delivers a TTL pulse. I will contact you directly to discuss your application.

Single Photon Counter Modules

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SPCM20A Support Documentation
SPCM20ASingle Photon Detector, 350 - 900 nm, Ø20 µm Active Area, 8-32 Tap
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SPCM50A Support Documentation
SPCM50ASingle Photon Counter Detector, 350 - 900 nm, Ø50 µm Active Area, 8-32 Tap
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SPCM20A/M Support Documentation
SPCM20A/MSingle Photon Detector, 350 - 900 nm, Ø20 µm Active Area, M4 Tap
SPCM50A/M Support Documentation
SPCM50A/MSingle Photon Detector, 350 - 900 nm, Ø50 µm Active Area, M4 Tap