Laser beam welding is very much popular in Microsystems technology by its precise parameters. Generally it is characterized by transient nature and very short duration of the process. For real-time applications, the model should be computationally efficient and relate critical but unmeasurable variables such as weld bead penetration to easily measurable variables such as the surface temperature. So for accurate real time tracking and inspecting, the wide range of reliable sensor, such as CMOS, CCD, Infrared, Ultra violet and Photodiode based with high performance are extremely demanded in laser welding process. Penetration depth is most important parameter to fulfill the criteria of expected weld bead in laser beam welding. This article describes research review on online measurement techniques is use for identify weld quality (i.e. penetration depth) in real time and control system control the geometry of the weld pool and to prevent defects in laser beam welding