Abstract: For achieving a correct measurement of the extinction coefficient of visibility, parallelism importance of the probe beam was discussed, the detection method of probe beam divergence angle was proposed and the measuring device was designed. The device includes a fixed focus imaging system and a linear motion system. Divergence angle was derived by using the imaging geometry and spatial relationship. With ultraviolet (UV) LED as the light source, measurement systems and control software were completed. According to the imaging results, the horizontal and vertical divergence angles were calculated. The probe beam was analyzed with divergence angle. Experimental results show that the UV LED light horizontal divergence angle is 7.8 and the vertical-s is 6.9. By comparison, the visibility ratio of non-parallelism to parallelism is 0.51. Visibility by this method is closer to the actual value.