ToF (Time-of-Flight) is a type of sensor capable of measuring the distance of an object, illuminating it with infrared and capturing the reflected light. Cell phones use this technology to perform facial recognition, adjust image focus, and blur the background in photos by changing the depth of field.
How the ToF sensor works
The ToF sensor is a depth sensor that calculates distances using light. It illuminates the object with a pulsed light source, such as a laser or LED; and measures how long it takes light to bounce back to the camera.
The technique is called “Time-of-Flight” measurement. The time interval in which the light travels is proportional to the distance between the camera and the object, as explains Texas Instruments.
Every ToF sensor is close to the main camera. It measures the distances to each pixel in the image, resulting in a depth map. Therefore, the ToF sensor is a 3D sensor, and its data can be used to adjust the depth of field in photos – which would normally require changing the focal length of the lens.
A Time-of-Flight system has 3 main parts:
- lighting module: contains a laser or LED, which generates NIR (near infrared) light to be emitted towards the object;
- ToF sensor module: collects light reflected from the object, converting it into depth data at each pixel;
- depth processor: converts the data received from the ToF sensor, combining it with autofocus information obtained by the image sensor, to generate the depth map.
The light pulsed (or modulated) by the ToF sensor is invisible to the human eye. It is usually in the near infrared (NIR) spectrum, with a wavelength of 850 nm or 940 nm; this means that the sensor can only analyze light rays with a specific frequency, reducing noise.
Simpler smartphones sometimes bring a depth camera: a 2 megapixel or 5 MP sensor that works alongside the main camera. The two cameras capture images from slightly different points, which are processed to generate a depth map. However, this stereo vision technique has lower accuracy than ToF.
ToF vs LiDAR: what’s the difference?
LiDAR is a type of ToF sensor: it has a scanner to deliver laser beams, used in Time-of-Flight measurement to measure distances. LiDAR has a longer range than other ToF 3D sensors, but tends to cost more.
ToF camera applications
- Facial recognition: this form of biometrics detects the smallest details of your face for greater security – according to Applethe chance of a random person unlocking your iPhone with Face ID is one in a million;
- autofocus: the ToF sensor accurately calculates the distance to objects, helping the camera to autofocus, especially in low-light scenes;
- portrait mode: the blurred background effect, to simulate the portrait mode of professional cameras, is more accurate thanks to the ToF sensor;
- Augmented reality: the 3D sensor is used to position virtual objects in the real world more accurately.
ToF sensors are also used in self-driving cars, robots and drones to detect objects, map the surrounding environment, and avoid obstacles. The technology is applied to detect people with greater privacy (by not capturing images of faces), in addition to automation in factories and, according to sonyeven in agriculture, to monitor productivity.
Advantages and limitations of Time-of-Flight sensors
ToF sensors offer the following benefits:
- Precision: ToF cameras measure accurate distance faster than ultrasonic or laser sensors; and are not affected by changes in air humidity, pressure or temperature;
- Long range: ToF sensors can measure long distances, with a field of view of up to 40 m in some cases;
- Security: this camera is safe for human eyes as it uses low power infrared light.
Despite the above advantages, there are two major “enemies” for Time-of-Flight sensors:
- Photos in bright light: bright light can prevent ToF cameras from detecting the infrared reflected from objects, so they may struggle outdoors or with very shiny surfaces;
- Irregularly shaped objects: measuring shapes that are too angular, or too curved, can be distorted if they reflect light several times before it reaches the ToF sensor.
Which cell phones use ToF sensor?
The first cell phone with ToF sensor is the LG G3, launched in 2014, according to TechInsights consultancy. The component came next to the rear camera to help with autofocus, according to university study TU Graz. The BlackBerry Passport and the LG G4 also have a ToF sensor, as explains the AnandTech.
Apple implemented a ToF sensor next to the front camera of the iPhone 7 to serve as a proximity sensor, according to TechInsights; the iPhone X brought a more advanced version of that technology to Face ID. The ToF sensor was used in the 2018 Huawei Honor View 20 as a 3D depth sensor for the rear camera.
Samsung, Xiaomi and Motorola also use ToF sensors in premium smartphones like the Galaxy S20 Ultra, Mi 11 Ultra and Motorola Edge+.
The ToF sensor sector is highly competitive, and there are no companies that dominate the market. These are the main manufacturers of this type of component:
- Texas Instruments
The ToF sensor market was valued at US$3.28 billion in 2021 by consultancy Straits Research; the sector is expected to grow to US$13.93 billion by 2030. Mordor Intelligencedemand is expected to come primarily from Asia Pacific, including China, to monitor and inspect electronics manufacturing.