The traditional method based on passive light source for measuring vegetation indices (VIs) has poor adaptability to the light environment and is inconvenient to use. In this paper, utilizing the characteristics of constant proportion of solar radiation energy in each component band, the standard whiteboard response ratio C_W with constant value was introduced to construct a new method for calculating VIs. The measurement system composing of two narrow-band cameras was used to determine C_W for many times in 23 days under natural light conditions. The result of determination of C_W was applied to measure the Normalized Difference Vegetation Index (NDVI) of the wheat canopy in a box planting area. The measured values of both C_W and NDVI were stable and showed a strong adaptability to natural light intensity and measurement heights during tests. The coefficient of variation (CV) of C_W and NDVI were 3.72% and 1.95%, respectively. The measured NDVI data were compared with those by GreenSeekerRT200, and the maximum, average and standard deviation of the absolute deviation rate were 5.65%, 1.49% and 1.06%, respectively. This showed that the new measurement method was effective and provided a new idea for measuring VIs.

传统的基于被动光源测量植被指数（VIs）的方法对光环境适应性差，使用不便。本文利用各分量波段太阳辐射能量比例恒定的特点，引入恒定值的标准白板响应比C _W，构建了一种计算VI的新方法。由两台窄带相机组成的测量系统在自然光条件下，在23天内多次测定_CW。应用C _W测定结果测量了盒式种植区小麦冠层的归一化差异植被指数(NDVI)。C _W的测量值NDVI 稳定，在测试过程中对自然光强度和测量高度显示出很强的适应性。C _W和 NDVI的变异系数 (CV)分别为 3.72% 和 1.95%。实测的NDVI数据与GreenSeekerRT200进行比较，绝对偏差率的最大值、平均值和标准偏差分别为5.65%、1.49%和1.06%。这表明新的测量方法是有效的，并为测量 VI 提供了新的思路。