SPAD Array Sensors: Driving Direct Time-of-Flight Technology Towards Excellence

In today’s 3D sensing and distance measurement field, time-of-flight technology is undoubtedly at the core. We generally divide it into two main categories: indirect time-of-flight and direct time-of-flight. The former is commonly found in Android phones, while the SPAD array sensor-based dToF is becoming the leader of next-generation 3D sensing technology due to its significant advantages in ranging distance and accuracy.

Core Concept Analysis

What is SPAD?

SPAD stands for Single-Photon Avalanche Diode. It is an extremely sensitive photodetector that operates in the “Geiger mode” above its breakdown voltage.
Its core characteristic is that when a photon hits its sensitive area, it triggers a chain reaction like an “avalanche,” producing a large and easily detectable electrical pulse.
In simple terms, a SPAD is an “ultra-sensitive switch” that can detect a single photon.

What is dToF?

The principle of dToF is very straightforward: it directly measures the flight time of photons.
The system emits a brief laser pulse towards the target and then receives the photons reflected back from the target. By accurately calculating the time difference Δt between the emission and return of the laser pulse, the distance d can be calculated using the speed of light c:

d = (c × Δt) / 2

How Does the SPAD Array Sensor Achieve dToF?

A single SPAD can only detect whether a photon is “present” or “absent,” but cannot perform precise timing. When tens of thousands or even millions of SPADs form an array and are combined with precise timing circuits, they create a powerful dToF system.

The workflow is as follows:

Pulse Emission: An integrated laser emits an extremely brief (usually in the nanosecond or even picosecond range) infrared laser pulse.
Photon Detection: After the laser pulse reflects off the target, some photons return to the sensor. Each pixel in the SPAD array acts like an independent “stopwatch.” When a SPAD detects a returning photon, it immediately generates an electrical signal.
Precise Timing: Each pixel integrates a precise timing circuit called a Time-to-Digital Converter (TDC). The TDC records the exact moment the laser pulse is emitted and the exact moment the SPAD receives the photon, calculating the time difference Δt.
Data Accumulation and Processing:

A single measurement may be inaccurate due to noise or weak signals. Therefore, the system repeatedly emits thousands or even millions of pulses at a very high frequency.
The SPAD array and TDC count the number of photons received in each very short time interval, ultimately forming a histogram.
The time point at which the peak photon count appears in the histogram corresponds to the main flight time of the photons, thus yielding the most reliable distance value.

Generating Depth Maps: Each pixel in the array independently calculates a distance value, and all this data combines to form a high-resolution 3D depth map.

Why is SPAD dToF Superior to the iToF Commonly Used in Android Phones?

Currently, Android phones widely adopt indirect time-of-flight technology. Its principle involves emitting continuously modulated light and indirectly calculating flight time by measuring the phase difference between the returning light and the emitted light.

It is this fundamental difference in principle that gives rise to the superior performance of SPAD dToF:

Feature	SPAD-based dToF	Indirect ToF
Principle	Directly measures the flight time of light pulses	Measures the phase shift of modulated light
Ranging Distance	Very long	Limited
Measurement Accuracy	Very high, almost unchanged with distance	Decreases with increasing distance
Power Consumption	Pulsed operation, lower average power consumption	Requires continuous emission of modulated light, higher power consumption
Resistance to Ambient Light Interference	Strong (through time gating, only focusing on the time window when the signal appears)	Weaker, easily affected by ambient light leading to decreased signal-to-noise ratio
Multipath Interference	Strong resistance (the histogram can distinguish between the main signal and reflected signals)	Weak resistance, easily causing measurement errors
System Complexity	Pixel circuits are complex (need to integrate TDC), high manufacturing process requirements	Pixel structure is relatively simple
Main Applications	High-end smartphones (such as Apple’s LiDAR Scanner), autonomous vehicles, robotics, AR/VR	Mid-range smartphones, facial recognition, gesture control

Advantages Explained:

Longer Ranging Distance: dToF emits pulsed light with instantaneous peak power, concentrating energy, making it easier to detect weak return signals at long distances. In contrast, iToF’s continuous light power is limited, and as distance increases, the signal attenuates rapidly, leading to a sharp deterioration in signal-to-noise ratio.
Higher Measurement Accuracy: The accuracy of dToF is primarily determined by the precision of the timing circuit (which can reach the picosecond level) and is essentially unchanged with distance. The accuracy of iToF, however, is related to the modulation frequency; high-frequency modulation can improve accuracy but is limited by system bandwidth, and in practical applications, measurement errors accumulate with increasing distance.
Better Power Consumption and Interference Resistance: The pulsed operation mode of dToF keeps it in a “sleep” state most of the time, resulting in low average power consumption. Additionally, by only detecting within the expected time window for signal return, it can effectively shield most ambient light interference outside of that time window, greatly enhancing reliability for outdoor use.

Conclusion

The combination of SPAD array sensors and direct time-of-flight technology represents a significant leap in the field of 3D sensing. By “capturing single photons” and “precisely timing their flight time,” it fundamentally addresses the bottleneck issues of iToF in long-distance, high-accuracy measurements.

Although its manufacturing process is complex and costs are higher, as technology matures and becomes more widespread, SPAD dToF is gradually moving from high-end devices (such as Apple’s LiDAR Scanner) to broader application scenarios, including future flagship Android phones, autonomous driving, industrial inspection, and metaverse interactions, bringing us more powerful and reliable 3D perception capabilities.