Distance Image Sensors Questions & Answers

What is touchless interface?

Touchless interface is a human-machine interface technology that allows commanding the computer without physically touching a keyboard, mouse, or screen. Touchscreens are everywhere nowadays on laptops, kiosks, and smartphones. But the increasing hygiene concerns due to the COVID-19 pandemic could open a new era that touchless interface technologies have been waiting for. The three latest trends in touchless interface technologies are voice control, gesture control, and eye tracking. Voice control technology is working very well in certain circumstances, but it is not a good option in public spaces, where it can be noisy or you may not want to speak aloud. A touchless interface based on gesture control can be implemented using a distance image sensor, which can collect 2D imaging and depth information accurately in indoor and outdoor settings.

Figure 1. Touchless interface based on gesture control.



How does the ToF (time of flight) distance image sensor generate the depth information required for a touchless interface?

A 3D ToF camera typically consists of a pulsed IR light source, a distance image sensor, and the necessary optics and driver electronics (Fig. 2).

Figure 2. Basic components of a 3D ToF camera.



There are two different types of ToF cameras based on how the depth information of the object is calculated: direct ToF and indirect ToF.

The direct ToF measures the time delay Δ between when the light is emitted and when the reflected light is received by the sensor. The distance between the sensor and the object can be estimated as depth = cΔ/2, where c is the speed of light.

With indirect ToF, a higher distance accuracy can be achieved with easier electronic designs. Hamamatsu’s indirect ToF distance image sensor implements the dual-capacitor pixel structure (Fig. 3). The pulsed light reflects off an object, and strikes the pixel where the photo-generated charge will be recorded in capacitor Cfd1 or Cfd2 shown in Fig. 3. The capacitors are alternately connected to the pixel at the same frequency as the pulsed light source. The distance between the image sensor and the object can be calculated by the ratio of Q1 and Q2.

Figure 3. Operating principle of indirect ToF distance image sensors.



What are the advantages of the ToF distance image sensors?

ToF distance image sensors have many advantages compared to other 3D imaging technologies.

  • Eye-safe. ToF sensors can use a low-power infrared laser diode or LED as its light source and drives it by a modulated pulse, so you don’t need to worry about harming your eyes.
  • Fast and precise measurements. A ToF sensor is faster and more effective in ambient light. It is able to compose a 3D image of a scene in just one shot. Detection using ToF sensors is unaffected by humidity, air pressure, and temperature, making the sensors suitable for outdoor and indoor use.
  • Lower power consumption and less data processing. Compared to other 3D imaging technologies such as stereoscopic imaging and structured light, ToF sensors need much less calculation, so the power consumption is also much lower.

Visit Hamamatsu’s main website to learn more about distance image sensors.

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Meet the engineers

Lu Cheng is an applications engineer, specializing in all of our image sensors and driver circuits. Before joining Hamamatsu, she worked as an analog and mixed-signal ASIC designer. With more than 10 years’ experience in ASIC and circuit board design, she can support you to find the optimized product/solution not only from a user’s perspective but also from a designer’s. Traditional Chinese dancing is one of the things that make her learn about the beauty of the world from a different perspective.

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