Cosine response characteristic and cosine error for spectral irradiance sensors

Accurate spectral irradiance measurement is very important in the field of Earth observation. In the ideal condition, the measurement of spectral irradiance is expected to follow the cosine law of angular response, but the cosine diffuser does not exhibit the ideal cosine response in the actual measurement, the cosine error will affect the measurement result of the instrument. In order to understand the effect of cosine diffuser on the performance of the irradiance senor, a spectral irradiance experimental measurement system was established. The cosine errors of three spectrometers were measured. Two different methods, the traditional and offsetting angle response normalization method, were used to analyze the experimental results, respectively. The experimental results showed that cosine response characteristic was affected by many factors such as diffuser material and size, the adaptability of different angle response normalization method was different. Three different sensors were involved in this experiment: HOC spectral irradiance sensor, EVO spectrometer, and Fieldspec Spectroradiometer. The results indicated that the traditional normalization method is more suitable for HOC and EVO spectral irradiance sensors, and the former presents good cosine characteristics, but the latter is obviously wavelength dependent by the influence of element reflection and diffraction. Compared with conventional normalization method, the Fieldspec Spectroradiometer showed good cosine response characteristics using the offset method. When θ is in the range of 0~60°, the cosine error of thissensor is reduced from 11% to 6%, and it is almost independent of wavelength change.