Maria Montero

How do time of flight sensors work? A look at …

How do time-of-flight sensors determine distances? Learn more about ToF cameras using an example 3D camera.

There are three leading 3D imaging technologies vying for space in your cell phone and your car. Those technologies are stereoscopic imaging, structured light projection, and time-of-flight (or ToF) cameras. The data generated by these devices can provide pedestrian detection, authenticate users based on facial characteristics, detect hand movement, and power SLAM (simultaneous location and mapping) algorithms.

Two of the biggest players in this space are ams and Infineon. At the time of writing, ams carries one of each type of sensor and Infineon focuses only on time-of-flight sensors.

This article describes the technology behind time-of-flight sensors focusing on a “pico flexx” 3D camera development kit from pmdtechnologies. (Before you ask, both “pico flexx” and “pmdtechnologies” are presented in lower case on their website.)

The flexx pico is based on the REAL3 3D image sensor jointly developed by Infineon and pmd.

False color 3D image of a hand taken from pmdtec.com on April 29, 2019. Colors are mapped to individual pixels based on distance from the sensor to the hand (red is close, blue is far)

Please note that pmd provided us with a device for this article.

What is a time of flight sensor?

Time-of-flight sensors measure the time it takes for an object, often a particle, to travel a distance through a medium. Generally, this is the measure of the time elapsed between the emission of a particle, its reflection on an object and its return to the ToF sensor. A time-of-flight camera is a device that uses ToF measurement to determine the distances between the camera and objects or environments, creating images generated by individually measured points.

Applications for ToF cameras include laser-based scannerless LiDAR imaging systems, motion detection and tracking, object detection for machine vision and autonomous driving, topographic mapping, and more.

But how are these measures achieved?

Below I have represented three ways to use travel time to determine distance.

Methods of using travel time to determine distances.

At the top of the image, you can see the first method, which is to send pulses and measure the time interval until they return after reflection.

The middle part of the image shows the second method, in which you can modulate the amplitude of a light source and record the phase shift of the reflected wave.

The lower part of the image represents the third method, which transmits a 50% duty cycle square wave and records the amount of returned light that arrives in a specific interval.