A centimetre is a thirtieth of a nanosecond
Light in vacuum travels 299,792,458 metres a second, so in one nanosecond it covers about thirty centimetres. A lidar measures distance by timing that trip.
Since 1983 the metre has been the distance light travels in vacuum in 1/299,792,458 of a second. The speed is exact by definition, and the centimetre is about a thirtieth of a nanosecond of it. A lidar sends a pulse, or a modulated beam, and times the return. Half the round trip, times the speed, is the distance. Automotive and surveying lidars are clocks with a lens, and their boast of centimetres is a boast about timing to a fraction of a nanosecond, in a box that also has to be cheap.
Air slows light by a few parts in ten thousand, which a careful survey corrects and a car often ignores, because a few parts in ten thousand of a hundred metres is less than the other errors. Rain, a dark target, and a bright Sun set the practical limit long before the definition of the metre does. The definition only says what the limit would be if the clock were perfect and the air were not there.
Caveats
- A real lidar’s accuracy is set by the pulse, the detector, and the target, not by this arithmetic alone. The arithmetic is the floor under the engineering.
- In glass, as another entry says, light is slower. A lidar in air is not a lidar in a fibre.
Sources
- 1Bureau International des Poids et Mesures. The International System of Units. SI Brochure, 9th edition, 2019.