SPAD and conventional CMOS image sensors are different kinds of imagers, not interchangeable performance tiers. A single-photon avalanche diode (SPAD) can register individual photon events and, with timing electronics, record when they arrive. A conventional CMOS image sensor (CIS) integrates photodiode charge to produce an intensity image. SPAD is usually the better fit for photon counting, time-of-flight (ToF) ranging and precise photon timing; conventional CMOS is usually the better fit for high-resolution color photography, video and general machine vision.
There is no universal winner. “CMOS” can describe the manufacturing and circuit platform used to build a SPAD array, so the meaningful comparison is a SPAD imager versus a conventional integrating CMOS imager. Choose by the output, illumination and timing information your system actually needs.
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What the comparison actually means
In ordinary product language, “CMOS camera” usually means a conventional photodiode-based image sensor. Each pixel collects charge during an exposure; readout electronics convert the charge into a digital value representing light intensity. A SPAD (single-photon avalanche diode) is an avalanche photodiode biased above its breakdown voltage in Geiger mode. One detected photon can trigger an avalanche and generate a digital-like pulse. Quenching and recharge circuitry then return the pixel to a state in which it can detect another event.
Both devices may be fabricated with complementary metal-oxide-semiconductor (CMOS) processes. That shared manufacturing platform does not make their pixels operate alike. A SPAD pixel is an event detector with recovery behavior; a conventional CMOS pixel is an integrating detector that measures accumulated signal. SPAD arrays may add time-to-digital or time-to-amplitude converters, counters and reconstruction logic, while conventional CMOS arrays generally provide intensity samples.
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The technical review in Interface Focus describes the SPAD architecture and its trade-offs. For conventional global-shutter CMOS, Sony explains that all pixels sense light at the same time, convert it to digital data and transfer it to memory before processing in its Pregius implementation; that description applies to that architecture, not to every CMOS sensor.
How each sensor turns light into data
Conventional CMOS: integrate charge into an intensity value
A conventional CMOS pixel’s photodiode converts incoming photons into electrical charge during an exposure. The charge is read, amplified and digitized. Pixel design, photodiode quantum efficiency, full-well capacity, conversion gain, read noise, analog-to-digital resolution and image processing determine the resulting signal. A rolling-shutter device reads rows at different times; a global-shutter device captures the exposure for all pixels together and stores the result for readout.
This architecture naturally produces a dense two-dimensional intensity image. Color cameras add a color-filter array or other spectral separation, so each output pixel estimates red, green and blue information after demosaicing or related processing. Sensor size, pixel pitch, optics and exposure time all affect the useful image.
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SPAD: detect events, then count or time them
A SPAD’s avalanche is quenched after a detection and the diode must recharge. The resulting event can be counted over an interval, assigned to a pixel, or timestamped. An array can therefore produce a photon-count image, a histogram of arrival times, or a depth estimate derived from a round-trip ToF measurement. The pixel’s dead time, quenching method, timing circuitry and readout determine how quickly successive events can be recorded.
SPAD output is not automatically a conventional photograph. To make an intensity-like image, a system often accumulates counts and reconstructs an image. To make depth, it correlates timestamps with a light pulse or modulation pattern. Spatial resolution, fill factor, dark counts, optical filters and reconstruction quality are as important as the avalanche detector itself.
SPAD vs. conventional CMOS at a glance
| Axis | SPAD imager | Conventional CMOS imager | What a fair comparison requires |
|---|---|---|---|
| Sensitivity metric | Photon detection probability or photon detection efficiency (PDE), which varies with wavelength, excess bias, fill factor and design. | Quantum efficiency and signal-to-noise performance, affected by photodiode, pixel circuit, exposure, readout and processing. | Match wavelength, illumination, exposure, sensor area and whether a value is measured at pixel or system level. |
| Speed metric | Photon-arrival timing precision, event-count rate and array readout. Dead time and saturation limit the useful rate. | Frame rate, exposure time, readout mode and bit depth. Global shutter captures simultaneously; rolling shutter scans rows. | Compare the speed measure that the application needs. Timing resolution is not the same as frames per second. |
| Native output | Events, counts or timing histograms; depth may be reconstructed from timing. | Sampled light intensity, often with color information after filtering and processing. | Define whether the desired result is a photograph, count map, timestamp stream or depth image. |
| Noise and range | Dark counts, afterpulsing, optical/electrical crosstalk, dead time and saturation affect usable measurements. | Read noise, dark current, full-well capacity, clipping and fixed-pattern noise affect image quality and dynamic range. | Use measurements at the intended temperature, flux, wavelength and exposure. |
| Typical strengths | Single-photon detection, precise timing, ToF depth and selected scientific imaging. | Dense spatial detail, color photography, video, inspection and general intensity imaging. | Select the architecture by required information, not by the label alone. |
Which is more sensitive in low light?
SPADs can detect individual photon events, which is valuable when the signal contains very few photons or when the arrival time of each photon matters. That does not mean every SPAD camera produces a brighter or cleaner low-light photograph than every CMOS camera. SPAD sensitivity is commonly discussed using PDE or photon detection probability. The value depends on wavelength, excess bias, active area and pixel fill factor. Dark-count rate, afterpulsing and crosstalk can add false events that become significant when the real signal is weak.
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Conventional CMOS performance in dim scenes is described with quantum efficiency, read noise, dark current, conversion gain, full-well capacity and exposure behavior. A large pixel with low read noise and a long exposure may outperform a small, low-fill-factor SPAD array for an ordinary intensity image. Conversely, a SPAD system can recover useful timing or counts from extremely sparse light where an integrating camera would need a longer exposure or would bury timing information in noise.
Questions to ask before calling one “better”
- What wavelength reaches the sensor? PDE and quantum efficiency are wavelength-dependent.
- How many photons arrive per pixel during the measurement, and over what exposure or integration interval?
- What are the SPAD’s dark-count rate, afterpulsing probability, crosstalk and fill factor, or the CMOS camera’s read noise, dark current and full-well capacity?
- Do you need an intensity image, or is a timestamp for each detected event the useful signal?
- What temperature, aperture, optical filter and processing pipeline will be used?
Compare complete systems under identical optics and illumination. A detector-level PDE number cannot by itself predict the quality of a reconstructed image or depth map.
Which is faster: SPAD or CMOS?
“Speed” has at least three meanings here. SPAD timing resolution describes uncertainty in the measured arrival time of a photon. Conventional CMOS frame rate describes how often a complete image is output. A SPAD can offer very fine timing while producing a relatively slow reconstructed depth frame; a CMOS sensor can deliver hundreds of frames per second without measuring individual photon arrival times.
SPAD event throughput is constrained by pixel dead time, avalanche recharge, counter capacity, saturation and the architecture used to move events off the array. At high photon flux, a pixel may be unable to register photons arriving during its recovery interval. At low flux, timing precision can be the decisive advantage. Conventional CMOS speed is constrained by exposure, row or global readout, analog-to-digital conversion, interface bandwidth and bit depth.
Shutter behavior matters for moving scenes
A global-shutter CMOS sensor exposes all pixels together, which avoids the geometric skew associated with a rolling shutter. Sony’s Pregius explanation states: “The light simultaneously sensed by each photodiode of all pixels is converted to digital signal and transferred to their respective memory sections before processed.” This is a description of that Sony implementation, not a universal property of CMOS. A rolling-shutter sensor can still be fast, but rows are sampled at different times.
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What “image quality” means for each architecture
For a conventional camera, image quality commonly means spatial resolution, color fidelity, noise, dynamic range, sharpness and motion rendering. Pixel count, pixel pitch, optics, demosaicing, compression and exposure all contribute. A high frame rate or global shutter can improve motion capture, but neither guarantees low noise or accurate color.
For a SPAD system, quality may mean spatial detail in a count image, timing precision, depth accuracy, confidence at low photon counts, or the quality of a reconstructed scene. Fill factor and pixel pitch influence spatial sampling. Dark counts and crosstalk can create false structures. Dead time can make bright regions respond nonlinearly. The reconstruction algorithm may suppress noise, fill missing data or trade spatial detail for temporal precision.
These outputs are not interchangeable. A SPAD depth map can contain information a color CMOS photograph cannot provide, while a conventional CMOS camera can deliver dense color detail that a low-resolution SPAD array cannot reproduce. Define the output before comparing a specification called “resolution” or “quality.”
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SPAD-specific limits
- Dark counts: thermally or electrically generated avalanches that look like photons. Rates vary with temperature, bias and device process.
- Afterpulsing: a previous avalanche leaves trapped charge that can trigger a later false event.
- Crosstalk: an avalanche or its electronics can induce counts in neighboring pixels.
- Dead time: the interval after detection during which a pixel cannot accept another photon.
- Saturation and pile-up: at high flux, events are missed or timing histograms become biased toward earlier arrivals.
- Fill factor: support circuitry can occupy area that does not detect photons, reducing effective collection unless microlenses or other optics compensate.
Conventional CMOS limits
- Read noise: uncertainty introduced during charge conversion and readout, especially important for short exposures and faint signals.
- Dark current: thermally generated charge that increases with temperature and exposure time.
- Full-well capacity: the charge a pixel can hold before clipping; it contributes to highlight range.
- Fixed-pattern and column noise: spatially repeatable or readout-dependent variations that calibration and processing may reduce.
- Rolling-shutter distortion: moving objects can shift or skew when rows are exposed at different times.
Dynamic range is therefore not a single technology-wide number. Measure it in the light level, temperature, exposure and readout mode that your application will use.
Published figures, with the conditions attached
Specific published numbers illustrate why context is essential:
| Device or source | Reported figure | What it describes |
|---|---|---|
| 32 × 32 SPAD array in a 2016 Nature Communications paper | Approximately 20% photon-detection efficiency at 640 nm; approximately 100 Hz dark-count rate at room temperature; 390 ps array time resolution; 3.14% active-area fill factor. | A particular research array with independent silicon SPADs, CMOS electronics and a time-to-digital converter for each detector. These are not typical or universal SPAD specifications. See Photon-efficient imaging with a single-photon camera. |
| Sony IMX925 configuration announced November 19, 2024 | 394 frames per second at 10-bit output. | A manufacturer-reported operating mode for a conventional CMOS global-shutter image sensor. It is not a head-to-head speed test against the research SPAD array. See Sony’s announcement. |
The 390 ps figure is timing resolution, not a 390-picosecond video frame period. Likewise, 394 fps says how often that CMOS configuration outputs frames; it does not say how precisely it timestamps individual photons. There is no fair universal sensitivity or speed number that ranks all SPADs against all conventional CMOS sensors.
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Where a SPAD-based system is the better fit
Time-of-flight depth
In ToF ranging, a light source sends a pulse or modulation pattern and the detector measures the return time. SPAD timing electronics can identify sparse returned photons and build a depth estimate. Sony describes an industrial SPAD ToF depth-sensor product and its processing approach at its SPAD ToF overview. Check the current product specification, optical requirements and availability before selecting a system.
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Photon-starved scientific measurements
Experiments involving fluorescence lifetimes, quantum-optical signals, astronomical photons or other sparse events can benefit from counting and timestamping individual detections. The required wavelength range, timing jitter, dark-count rate and synchronization interface must be matched to the experiment.
Depth or timing rather than color
If the key output is a timestamp, histogram or depth map, a SPAD architecture can provide information a conventional integrating pixel does not preserve. It may still require a separate color or intensity camera when appearance and texture are also needed.
Where conventional CMOS is usually the better fit
Photography and video
Conventional CMOS sensors are designed to produce dense intensity images and can integrate color filters, high pixel counts, image stabilization and video-oriented readout. For scenes where the goal is a natural-looking photograph or conventional video stream, this output matches the workflow directly.
Industrial inspection and machine vision
Inspection systems often need predictable frame rates, global or rolling shutter choices, controlled exposure, calibrated grayscale or color values and established interfaces. A conventional CMOS sensor can meet those requirements without converting photon events into an image first.
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When fine detail across a large field of view is more important than photon timing, compare pixel count, pixel pitch, optical resolution, signal-to-noise ratio and readout bandwidth among conventional CMOS devices. A low-resolution SPAD array may not provide enough samples even if its timing is excellent.
How to choose: a practical decision procedure
- Define the required output. Choose “intensity image,” “color video,” “photon-count map,” “timestamp stream” or “depth map.” If you need more than one, state which is primary and whether two sensors are acceptable.
- Set the photon and wavelength conditions. Record the spectral band, expected irradiance, exposure or integration time, optical aperture and temperature. Request PDE curves for SPADs and quantum-efficiency/noise data for CMOS at that wavelength.
- Set the timing requirement. For ToF, specify depth range, distance precision, repetition rate and allowable latency. For video, specify frame rate, exposure time, shutter type and motion speed. Do not substitute one metric for the other.
- Calculate the usable signal range. For SPAD, check dark counts, dead time, afterpulsing, crosstalk, maximum count rate and fill factor. For CMOS, check read noise, dark current, full-well capacity, conversion gain, clipping and dynamic range.
- Check spatial and optical compatibility. Compare active area, pixel pitch, array dimensions, microlens or filter requirements, lens image circle and the spatial resolution delivered by the whole optical system.
- Evaluate data and reconstruction. Confirm whether the sensor outputs raw counts, histograms, timestamps or processed depth, and whether the interface can sustain the required data rate. Identify calibration, pile-up correction, demosaicing or denoising steps.
- Test at representative conditions. Use the same target, wavelength, illumination, temperature and optics for each candidate. Record false-event rate, temporal accuracy, spatial detail, dynamic range and latency rather than relying on a headline specification.
Specification checklist for a fair purchase or design review
- Sensor architecture: integrating CMOS, SPAD count array, or a hybrid system.
- Array dimensions, pixel pitch, active area and effective fill factor.
- Spectral response and the exact wavelength at which efficiency is quoted.
- SPAD PDE, dark-count rate, afterpulsing, crosstalk, dead time and maximum count rate; or CMOS quantum efficiency, read noise, dark current and full-well capacity.
- Timing jitter or time resolution for SPAD; frame rate, exposure, bit depth and shutter mode for CMOS.
- Raw versus processed output, histogram or timestamp access, synchronization and interface bandwidth.
- Operating temperature, cooling requirements, calibration stability and optical filtering.
- Whether the quoted number is a laboratory result, a named product mode, a typical value or a maximum.
Common comparison mistakes
“SPAD is CMOS, so they are the same”
CMOS can describe the fabrication and support electronics. The detector behavior remains different: SPADs produce avalanche events, while conventional CMOS pixels integrate charge.
“Single-photon detection means the best low-light photograph”
Photon detection is only one part of a camera. Dark counts, fill factor, array size, timing accumulation and reconstruction can outweigh the advantage for a conventional photograph.
“Picosecond timing beats hundreds of frames per second”
Those numbers answer different questions. Timing resolution concerns when an event is detected; frame rate concerns how often an image is delivered.
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Optics, pixel size, noise, dynamic range, shutter behavior and processing determine whether additional samples reveal more useful detail.
“One research prototype represents the whole SPAD market”
SPAD performance changes with wavelength, excess bias, temperature, fabrication process, active area and array architecture. Treat published prototype figures as device-specific evidence.
Bottom line: choose the information you need
Choose SPAD when your system must detect sparse photons, preserve their arrival times or derive ToF depth. Choose conventional CMOS when you need a dense intensity image, color, conventional video or predictable machine-vision capture. For low-light work, compare the specific SPAD’s PDE, dark counts, fill factor and reconstruction against the specific CMOS camera’s quantum efficiency, read noise, dark current, exposure and optics. A defensible decision compares complete systems under matching conditions rather than declaring one sensor class universally faster, more sensitive or higher quality.
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