An Approach To Measure The Field Of View Of Sun- Photometer

Field of view (FOV) is a basic parameter of sun-photometer and also the key parameter for transferring calibration and inter-comparison using an integrating sphere as light source in the laboratory of sun-photometers. High-precision FOV is important to improve the calibration accuracy of sun-photometer. According to the definition of solid angle and optical structure of CE318 sun-photometer, the relationship between the FOV and solid angle of CE318 sun-photometer has been determined, then we analyze the importance of FOV by deducing the theories of transferring calibration and inter-comparison using an integrating sphere as light source in the laboratory.Based on the matrix measurement method, FOV is measured through a multiband sun photometer in the field, and a system using laser as a light source to measure the FOV of CE318 sun-photometer in the lab is also developed, then we compare the FOV with transfer calibration method. The results indicate that the standard deviation of the matrix measurement is 0.3% or less, which shows a good repeatability. The matrix measurement with laser has a FOV uncertainty of 0.4%-1.1%. Because of the movement during the field measurement process and divergence angle of sun, the FOV’s uncertainty based on matrix measurement with sun is about 1.2%-1.5% which is larger than the laser result In addition, the more historical calibration records, the more accurate FOV obtained by using transfer calibration method to compare with matrix approach with laser, which suggests that the transfer calibration method can obtain accurate FOV when historical records are enough.