Laser distance meter is an instrument that accurately measures the distance of the target by using a certain parameter of the modulated laser. The ranging principle can be basically reduced to measuring the time required for light to and from the target, and then calculate the distance d through the light speed c = 299792458m / s and the atmospheric refractive index n. Because it is difficult to measure the time directly, it is usually to measure the phase of continuous wave, which is called phase measuring rangefinder. Of course, there are also pulse rangefinders. Usually, precision ranging requires the cooperation of total reflection prism, while the range finder used for house measurement is directly measured by the reflection of smooth wall, mainly because the distance is relatively close and the signal intensity reflected by light is large enough. From this, we can know that it must be vertical, otherwise the return signal is too weak to get the accurate distance.