Indirect Time-of-Flight: Continuous-Wave or Pulsed – Which Suits Your Needs?

Time-of-Flight (ToF) imaging has become a driving force in depth sensing, powering applications that range from industrial automation to automotive safety and consumer electronics. Within ToF, indirect approaches dominate because of their ability to capture range information without requiring ultra-fast detectors.

However, two distinct modulation methods, namely Continuous-Wave (CW) and Pulsed, define the design trade-offs. Understanding how these approaches operate and where each excels is critical for building trusted embedded vision systems.

In this blog, you’ll get insights into the principles of Indirect ToF technology, the two modulation approaches, and how to pick the right one.

Principles of Indirect Time-of-Flight (ToF)

Indirect ToF relies on emitting modulated light and measuring the phase or temporal offset between emitted and received signals. Instead of detecting the exact travel time of a photon, the system observes how the returning waveform has shifted relative to the source.

This approach uses more complex hardware than direct ToF, while still enabling millimeter-level ranging at scale. The two modulation families, CW and Pulsed, achieve this correlation differently.

What is Continuous-Wave Modulation?

In CW-based ToF, the emitter projects a sinusoidal or square-wave modulated beam. The reflected signal returns with a phase delay proportional to distance. Multiple modulation frequencies are often used to address phase ambiguity and extend the measurable range.

Strengths of CW ToF

• Compact electronics: CW systems leverage relatively simple driver circuits since the source is continuously active.
• Flexibility in modulation frequency: Developers can tune for short or long distances by selecting frequency bands.
• High dynamic range: Amplitude and phase can be extracted simultaneously, providing information about both intensity and distance.

Trade-Offs with CW ToF

• CW systems require demodulation circuitry capable of handling multiple frequency bands with high accuracy.
• Ambient light interference and multipath reflections can introduce errors, especially in uncontrolled outdoor scenes.
• Thermal load on the emitter also rises, since the source operates continuously.

What is Pulsed Modulation?

In Pulsed ToF, the emitter releases discrete bursts of light, often with nanosecond-scale pulse widths. The returning waveform is correlated against the emission pattern to derive depth. Instead of phase delay, the measurement focuses on timing offset and correlation strength.

Strengths of Pulsed ToF

• Lower power consumption: Since light is emitted in short bursts, the source is idle between pulses, reducing thermal stress.
• Resistance to ambient light: The sharp timing structure of pulses helps isolate the signal from noise in high-background environments.
• Simplified long-range scaling: By adjusting pulse width and repetition rate, systems can extend coverage without large increases in modulation complexity.

Trade-offs with Pulsed ToF

• Pulsed approaches demand detectors and correlation circuits capable of capturing rapid transients.
• Synchronization between emitter and sensor must remain highly stable, which can raise integration costs.

Indirect ToF Side-by-Side Comparison: CW vs. Pulsed

Feature / Factor	Continuous-Wave (CW) ToF	Pulsed ToF
Modulation Method	Emits a sinusoidal or square-wave modulated beam	Emits discrete bursts of light
Measurement Principle	Distance derived from phase shift between emitted and received signals	Distance derived from timing offset between emitted and received pulses
Power Consumption	Higher, since emitter runs continuously	Lower average power, since light is emitted in bursts
Thermal Load	Increased, continuous operation heats emitter	Reduced, idle periods lower emitter stress
Ambient Light Tolerance	Susceptible to interference, especially outdoors	Strong resistance due to sharp pulse timing
Range Extension	Achieved by using multiple modulation frequencies	Achieved by adjusting pulse width and repetition rate
Hardware Complexity	Requires demodulation circuits for multiple frequencies	Requires detectors and circuits capable of capturing fast transients
Close-Range Accuracy	Strong at short distances with proper frequency selection	Strong at short distances, and actual proximity can improve accuracy further.
Dynamic Range	Amplitude and phase extracted simultaneously for richer scene data	Depth emphasized, intensity information secondary
Use Case Fit	Structured indoor/outdoor environments, automation, consumer electronics	Automotive safety, robotics in sunlight, outdoor monitoring

Indirect ToF Principle: Choosing Between CW and Pulsed

On the one hand, CW systems face a challenge when scenes shift away from controlled lighting and medium-range conditions. The ability to extract amplitude and phase makes them attractive for conveyor systems and robotic guidance.

On the other hand, Pulsed systems are favored in driver-assist and in-cabin monitoring due to resilience against sunlight and high dynamic range conditions. Their ability to extend range while conserving power is critical in vehicle deployments.

Recent research also explores hybrid modulation, where CW and Pulsed methods are combined to balance trade-offs. Dual-mode sensors can adapt to ambient conditions, operating in CW mode for fine indoor depth mapping and switching to Pulsed mode outdoors. Such adaptability ensures robustness across varying lighting scenarios.

e-con Systems’ High-Performance Time-of-Flight Cameras

Since 2003, e-con Systems has been designing, developing, and manufacturing customizable OEM camera solutions, including ToF cameras. These cameras use NIR wavelengths (940nm/850nm) to provide reliable 3D imaging, whether indoors or outdoors.

Some of the supported interfaces are USB, MIPI, and GMSL2. Moreover, we offer SDK compatibility for NVIDIA Jetson AGX Orin/AGX Xavier and X86-based systems.

Our ToF cameras include DepthVista_GMSL_IRD, an advanced GMSL camera module that supports the Continuous-Wave method and helps accurately measure 3D depth. It is based on onsemi’s AF0130 CMOS ToF sensor, offering high-resolution depth data at 1.2MP @ 60fps & VGA @ 100fps.

Know more about our ToF cameras

Use our Camera Selector to see our full portfolio.

Need help finding and deploying the perfect ToF camera solution?  Please write to camerasolutions@e-consystems.com.

Frequently Asked Questions

1. What is the main difference between Continuous-Wave (CW) and Pulsed Indirect ToF?
   CW ToF determines distance by analyzing the phase shift of a modulated beam that runs continuously, offering reliable results in structured indoor environments. Pulsed ToF instead works with short bursts of light, calculating distance based on timing offsets. The two approaches differ in modulation technique, circuit demands, and application suitability.

2. Which modulation method is better suited for outdoor environments?
   Pulsed ToF handles outdoor use more effectively because its sharp pulse timing resists interference from direct sunlight and high background illumination. This resilience makes it well-suited for automotive safety, robotics in open spaces, and monitoring systems exposed to natural light. CW systems face challenges with ambient light in these same scenarios.

3. Why does CW ToF consume more power than Pulsed ToF?
   CW emitters run with far less idle time compared to Pulsed ToF, which increases both energy usage and thermal stress on the source over time. Pulsed systems emit light intermittently, reducing average load on the emitter and extending system lifetime. For power-sensitive devices, the reduced demand of Pulsed ToF provides a clear advantage.

4. Can CW and Pulsed approaches be combined in a single system?
   Yes, hybrid solutions exist where CW is used indoors for fine depth mapping and Pulsed is used outdoors for long-range sensing. These systems switch modes depending on lighting conditions to optimize performance across environments. By combining both, designers can achieve adaptability without sacrificing reliability.

5. What does e-con Systems offer in the area of ToF cameras?
   e-con Systems develops high-performance ToF cameras that operate at 850nm and 940nm wavelengths for accurate 3D imaging indoors or outdoors. Our ToF lineup features the DepthVista_GMSL_IRD, a high-performance GMSL camera module for accurate 3D depth measurement. Powered by onsemi’s AF0130 CMOS ToF sensor, this camera supports the Continuous-Wave method and delivers 1.2MP depth output at 60 fps and VGA depth output at 50 fps.