The phase method laser rangefinder detects the distance by detecting the phase difference between the emitted light and the reflected light when they propagate in space. The laser rangefinder is light in weight, small in size, simple in operation, fast and accurate, and its error is only one-fifth to one-hundredth of that of other optical rangefinders. In the right figure, there are typical phase method rangefinders and pulse method rangefinders.

Laser is a new technology developed in the 1960s. It is a kind of light with pure color, highly concentrated energy and good directionality. Laser rangefinder is an instrument that uses laser to measure distance. Its working principle is simple: measure the distance by measuring the time from the beginning of the laser emission to the laser reflection from the target. For example, to measure the distance of the moon with a laser rangefinder, if the time from the beginning of the laser launch to the reflection from the moon is measured as 2.56 seconds, the one-way time from the laser launch to the moon is equal to 1.28 seconds, and the speed of the laser is the speed of light, equal to 300000 kilometers per second. Therefore, the measured distance from the moon to the earth is the product of one-way time and the speed of light, which is 384000 kilometers. In order to transmit and receive laser and time, the laser rangefinder is composed of laser transmitter, receiver, clock frequency oscillator and distance counter.