People's demand for light sources contains not only efficiency, energy conservation, and environmental protection, but is shifting towards healthy and comfortable lighting quality, which puts higher requirements on the measurement technology for luminescence characteristics of the light sources. At present, there are two main methods for measuring the luminescence characteristics of light sources, including far-field and near-field distributed photometric measurements. The far-field distributed photometric measurement based on a point light source model employs a single point photometric detector for spherical scanning to obtain the light intensity, which makes it difficult to accurately demonstrate the luminous information in the near-field distance. The near-field photometric measurement utilizes luminance images from different directions to build a near-field source model. By the conversion of photometric parameters and the processing method of luminance data, the luminescence characteristics of light sources can be characterized, and characteristic information such as the origin and propagation direction of light sources can be obtained. Although the near-field photometric measurement model is more complicated, it can more finely and completely characterize the luminescence characteristics of light sources. However, facing the application requirements for accurate luminescence characteristic measurement of light sources, the development of domestic near-field distributed photometric measurement systems is still in preliminary stages. Meanwhile, some systems have limitations in measurement size or measurement accuracy, which makes it difficult to characterize the luminescence characteristics of light sources. Therefore, by studying the near-field photometric measurement method and its measurement mechanism, we measure the luminous intensity distribution of a plane light source based on the self-developed near-field photometric measurement device.

The spatial distribution of luminous intensity information of a plane light source is obtained by a luminescence model of the plane light source. To build the model, firstly, the imaging luminance meter driven by the mechanical structure performs three-dimensional spherical scanning motion around the luminous body, while capturing luminance images of various directions on the scanning sphere. Therefore, the luminance spatial distribution information of the light source is obtained. Secondly, a luminescence model of the plane light source is built, which is composed of the point light source array. According to the transformation of the coordinate system and the conversion of photometric parameters, the light distribution in each direction of the luminous plane is obtained. Thirdly, the acquisition of the near-field luminous intensity distribution requires the luminous distribution calculation of the light source from multiple directions. Finally, the results of near-field-distributed photometric measurement are analyzed. The luminance measured value in our paper extracted from the center position of luminance images is compared with the luminance standard value traced to the National Institute of Metrology, China. Additionally, the calculated results of the near-field distributed photometric measurement are compared with far-field distributed photometric measurement by GO-R5000 photometer in far-field conditions.

The luminance images of the plane light source in different directions are collected by the imaging luminance meter driven by a mechanical turntable to complete 2π space swing scanning. Under the fixed rotation axis angle, the luminous area detected by the imaging luminance meter first increases and then decreases with the rotation of the pitch axis. The luminance distribution curves of measured and standard values are consistent. The absolute error of the measured luminance value is less than 1015.52 cd/m², and the relative error is better than 6.51%. The coincidence degrees of luminous intensity distributions obtained from near-field and far-field photometric measurements are relatively high. The absolute error of the near-field measurement is less than 14.74 cd, and the relative error is less than 8.38%. Meanwhile, the matching index between the luminous intensity distribution of near-field and far-field photometric measurements is calculated for overall evaluation. The matching index of the 0° photometric curve is as high as 98.33%. The results verify the effectiveness of the proposed method for near-field photometric measurement.

We collect the luminance data of the light source based on the self-developed near-field photometric measurement device and the light distribution of the luminous plane in all spatial directions is analyzed by the principle of photometry and geometric optics based on the luminescence model of a plane light source. Additionally, the luminous intensity distribution of the plane light source in near-field conditions is calculated. The results show that the luminous intensity distribution curves in the near- and far-field photometric measurements maintain good consistency. When the relative luminance error of the adopted imaging luminance meter is better than 6.51%, the relative error of luminous intensity from near- and far-field photometric measurements is less than 8.38%. The matching index of the 0° photometric curve is as high as 98.33%. The results show that the proposed method has yielded near-field distributed photometric measurement, and our study has high engineering application significance in lighting, displays, and other fields.