A range of approaches exist to directly measure distance as the length of the reflected optical path from laser, to a target where it is reflected, and back to a sensor. These are commonly known by various names, including LIDAR and time of flight sensors, although there is actually overlap in principles between them.
The main system types are summarized in the table below. They are principally defined by the following key parameters:
- Optical distance measurement principle: This is the method by which optical depth in the z dimension is measured. Main approaches are indirect Time of Flight (iToF), direct Time of Flight (dToF), and Frequency Modulated Continuous Wave (FMCW).
- Scanning architecture: This is how the system measures multiple depth points across the x and y dimensions to create a 3D depth map. Main approaches here are either a single emitter with a sensor array, emitter array plus sensor array, and scanning mirror systems with only a single source/detector.
- Optical aperture/power: There is a trade-off between optical power and aperture size, and achievable range. There are two key categories: Compact, low power short range systems, for example integrated modules with wafer level optics for consumer electronics; and larger longer-range systems built using discrete components, more powerful sources and larger aperture bulk optics.
All typically operate in the infrared spectrum. This enables interference from ambient light to be minimized by using a matching infrared bandpass filter at the receiver, and for the system to appear largely invisible to users.