Wordology, Lidar

The word is an acronym for Light Detection and Ranging. The US military and NASA invented the Lidar technology during the early 1960s for measuring distance in space. Its first commercial usage did not occur until 1995.

It uses ultraviolet, visible, or near infrared light from lasers. Radar (Radio Detection and Ranging) uses radio or electromagnetic waves. Lidar used in cars is low powered and classed as 'eye-safe' allowing it to be used with few safety precautions.

Some refer to Lidar as laser radar, however it is not. It is more precise than radar, because the speed of light is a constant, so a laser can make extremely precise measurements of distance by computing the time between when the device emits a laser pulse and when it detects the reflection. Sound travels about 1,000 feet (300 meters) per second and light travels about 984,000,000 feet per second (300,000,000 meters). Also, radar wavelengths suffer from atmospheric conditions, such as humidity, fog, rain, snow, and temperature, but do perform better in smokey or dusty conditions.

A laser unit fires a short pulse of light. The pulse rebounds off a point, such as the rear of the car in front and is detected by a sensor in the laser unit. A computer connected to the unit measures the time between the initial pulse and the light return and, using the speed of light, calculates the distance the light has traveled. It creates a high-resolution 3D map of the surrounding environment. The best sensors can see details of a few centimeters at distances of more than 330 feet or 100 meters.

Currently most autonomous cars use some combination of Lidar, Radar, and camera. Lidar is precise, Radar is good at motion, and cameras are good for depiction. Each technology has strengths and weaknesses, so automakers and others are trying to find the best combination of strengths at the lowest cost.

Sound Mirrors

Mirrors can actually reflect sound as well as light. Mirrors that reflect sound waves are known as “acoustic mirrors,” and were used in Britain during World War I to detect certain sound waves coming from enemy aircraft from 8 to 15 miles away. This was before the development of radar.

Several were built around the coast of Britain, and are still standing today. They are located on both the north and south shores of England. They are also called listening stones.

Concrete acoustic mirrors were built on the south and northeast coasts of England between about 1916 and the 1930s. The ‘listening ears’ were intended to provide early warning of incoming enemy aircraft.

They did work, but the development of faster aircraft made them less useful, as an incoming aircraft would be within sight by the time it had been located. Increasing ambient noise made the mirrors harder to use successfully, and then radar rendered acoustic detection redundant.

There is also an example of one that is a parabolic sound mirror carved into boulders to dramatically magnify the sound of a nearby stream for listeners. It is inspired by satellite dishes, the seating in choir lofts where curved walls reflect sound and the antique hand-held sound magnifiers used in the days before hearing aids.