Design of a free-form diffuser for the entrance optic to correct the cosine error in the photometer

摘要

The entrance optic is an important part of the photometer head. It can match the directional response of the photometer to the cosine function. In this paper, an entrance optic consisting of a free-form diffuser, a shadow ring, and an integrating cavity is introduced. An iterative optimization algorithm is presented to design a free-form diffuser that exhibits better cosine response characteristics. Diffusers of different materials and sizes are designed in a simulation experiment. After a finite number of iterations, in the absence of the shadow ring, the integral cosine error of the free-form diffuser is less than 1%. The directional cosine error is less than 3% for incidence angles between 0 degrees and 70 degrees. After adding a shadow ring to correct the directional response of the incident angle greater than 80 degrees, for incident angles between 0 degrees and 85 degrees, the cosine errors are typically less than 3%, except that the cosine errors of very few large incident angles are close to 5%. The experimental results show the effectiveness and robustness of the proposed method. In addition, the influence of an important percentage constant sigma on iterative optimization is studied. It is found that the larger the parameter sigma, the fewer the number of iterations, and the directional cosine error may be slightly larger but still acceptable. The wide range of values of sigma further embodies the versatility and flexibility of the proposed method. (C) 2019 Optical Society of America