FAQ
Answers to the most frequently asked questions on SPAD technology and our products.
Single-photon avalanche diodes, or SPADs, are detectors capable of detecting extremely faint light signals, down to individual photons. This allows for single-photon and photon-counting detection.
A SPAD array consists of a grid of individual SPAD pixels, each detecting light independently. When a SPAD array is large, it can also be referred to as a SPAD image sensor.
A SPAD array consists of a grid of individual SPAD pixels, each detecting light independently. When a SPAD array is large, it can also be referred to as a SPAD image sensor.
To read out the signals from SPAD pixels independently, digital electronics are built directly on the detector. These electronics enable to extract the number of photons the SPAD pixel detected during a defined integration time (termed photon counting) or to determine the exact pixel and time of individual photon arrivals (termed time tagging or time-correlated single photon counting [TCSPC]).
Photon counting and time tagging are complemented with time gating. Time gating enables to define a very short integration time (in the nanosecond range) and offers an alternative way to extract precise timing information in the picosecond range by shifting the gate.
Photon counting and time tagging are complemented with time gating. Time gating enables to define a very short integration time (in the nanosecond range) and offers an alternative way to extract precise timing information in the picosecond range by shifting the gate.
SPAD arrays exhibit an exceptionally high signal-to-noise ratio in low light conditions, making them ideal for detecting and measuring faint signals. This capability is of particular value when combined with the high-speed digital readout. The high sensitivity of SPADs allows them to distinguish individual photons, enabling accurate detection and quantification of low light sources.
SPAD arrays have incredibly precise timing capabilities, characterized by a low standard deviation. This feature enables them to measure the time of arrival of individual photons with remarkable accuracy.
SPAD arrays are thus best suited for the following measurement conditions:
* maximum number of photons heavily depends on the gate parameters being used
SPAD arrays have incredibly precise timing capabilities, characterized by a low standard deviation. This feature enables them to measure the time of arrival of individual photons with remarkable accuracy.
SPAD arrays are thus best suited for the following measurement conditions:
| Operating mode | Photon counting | Photon counting with time gating | Time tagging |
|---|---|---|---|
| Number of incident photons per integration per pixel | <300 | <1500* | <300 |
| Optimal integration time | <10 ms | ||
| Integration mode | Frame based: data sent regularly or synchronous to a clock | Frame based: data sent regularly or synchronous to a clock | Asynchronous readout: data sent as photons are detected |
| Timing precision | >1 µs | 100 ps | 100 ps |
Pi Imaging is offering three types of SPAD arrays: point, line and image sensor. Select one of the detectors based on application requirements. For example, SPAD23 for confocal/scanning applications, SPADλ for spectral detection/line scanning and SPAD512² for full image acquisition at high frame rates.
| Detector type | Point sensor | Line sensor | Image sensor |
|---|---|---|---|
| Detector name | SPAD23 | SPADλ | SPAD512² |
| Operating mode | Photon counting + time tagging | Photon counting + time tagging + time gating | Photon counting + time gating |
| Total photon throughput in photon counting mode | 180 Mphotons/s | 4 Gphotons/s | 25.6 Gphotons/s |
| Total photon throughput in time tagging mode | 70 Mphotons/s | 140 Mphotons/s | - |
| Integration mode | Frame based and asynchronous | Frame based and asynchronous | Frame based |
| Minimum integration time in time tagging mode | 10 μs | 10 μs | - |
| Maximum acquisition rate in photon counting mode | 2'000'000 dwells/s | 555'000 lines/s | 100'000 frames/s |
| Timing precision | 100 ps | ||
Smaller SPAD arrays compare best with photomultiplier tubes (PMTs) and hybrid detectors.
Larger SPAD arrays or image sensors compare best with EMCCD and sCMOS cameras.
| SPAD | PMT | |
|---|---|---|
| Scalability | High | Low |
| Detector area | μm² | mm² |
| Dynamic range | High | High |
| Timing precision | High | Low |
| Peak efficiency | Up to 55% | ~30% |
| Noise performance: | ||
| Dark count/current | <0.1 cps/μm2 | <100 cps, <1 nA |
| Read-out noise | No | Yes (in analog mode) |
| Excess noise | No | Yes (in analog mode) |
| Afterpulsing | <0.1% | >>1% (in counting mode) |
| SPAD | EMCCD | ICCD | sCMOS | Two-Tap CMOS | |
|---|---|---|---|---|---|
| Pixel size | μm² | μm² | μm² | μm² | μm² |
| Peak efficiency | Up to 55% | >80% | Up to 50% | >50% | >30% |
| Maximum frame rate 1-bit | 100'000 fps | - | - | - | - |
| Maximum frame rate 8-bit (dynamic range) | 1500 fps | <100 fps | <10 fps | <100 fps | <100 fps |
| Minimum exposure time | < 10 ns | > 10 ms | < 2 ns | > 10 µs | < 2 ns |
| Gating resolution | <20 ps | > 1 ns | <20 ps | > 10 ns | <20 ps |
| Dynamic range | > 100 dB | ~100 dB | > 80 dB | > 80 dB | > 60 dB |
| Read-out noise | No | Yes | Yes | Yes | Yes |
| Excess noise | No | Yes | Yes | No | No |
Time tagging detection mode is suitable for applications which require picosecond timing resolution and/or operate in conjunction with pulsed illumination.
Time tagging allows to precisely capture time-varying phenomena, such as FLIM or cross correlation spectroscopy. So whenever a time-varying signal is of interest, time tagging should be the preferred acquisition method.
On the other hand, time tagging leads to data expansion, where each photon typically generates a significant amount of data. Therefore, both acquisition bandwidth and data processing should be taken into account.
Time tagging allows to precisely capture time-varying phenomena, such as FLIM or cross correlation spectroscopy. So whenever a time-varying signal is of interest, time tagging should be the preferred acquisition method.
On the other hand, time tagging leads to data expansion, where each photon typically generates a significant amount of data. Therefore, both acquisition bandwidth and data processing should be taken into account.
Time tagging leads to the highest photon efficiency and accuracy.
However, when acquisition bandwidth is limited, or when the sensor is becoming too large, time gating is a viable alternative. It reduces the data rate compared to time tagging. Both time gating and time tagging allow us to extract the same timing information.
In our linear arrays, it is also possible to combine the use of the two methods. Time gating is used to select a time span of interest, whereas time tagging is used to tag each photon within this time span. This combination can be a powerful means to save bandwidth and still have the level of precision associated with time tagging.
However, when acquisition bandwidth is limited, or when the sensor is becoming too large, time gating is a viable alternative. It reduces the data rate compared to time tagging. Both time gating and time tagging allow us to extract the same timing information.
In our linear arrays, it is also possible to combine the use of the two methods. Time gating is used to select a time span of interest, whereas time tagging is used to tag each photon within this time span. This combination can be a powerful means to save bandwidth and still have the level of precision associated with time tagging.
