First there is the most primitive way to measure the pull tape measure, pull measurement, four-quadrant component measurements, measure the photodiode array, and later developed to measure the PSD, CCD measurements, the latter four measurement in accordance with the nature and measurement of light sources can be divided into for the laser-and infrared-type. ... Laser has many advantages, such as highly directional, monochromatic, high brightness, therefore the source is generally difficult to interfere with the laser measurement, stability is also very good.