Parallelism detection of visibility meter's probe beam and the effect on extinction coefficient measurement

Extinction coefficient is the base parameter of visibility measurement, and the parallelism of visibility meter's probe beam decides the accuracy of extinction coefficient. In the paper, the detection method of probe beam divergence angle was proposed and the measuring device was designed. The device included a fixed focus imaging system and a linear motion system. Divergence angle expression was derived by the geometry and spatial relationship. Relative errors of horizontal and vertical divergence angles were 0.65% and 0.61%. Using this method, 15 probe beam divergence angles were measured. After divergence angles were amended, extinction coefficient of samples with deferent concentrations was measured. The results shows that the optical power decreases of different divergence angles are close in samples with different concentrations, the largest drop falls from 38% to 25%, and extinction coefficient is closer to the normal value. In high concentration sample, the maximum probe beam divergence angle can be increased by 50%.