A new method for measuring vegetation indices based on passive light source

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 CW 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 CW for many times in 23 days under natural light conditions. The result of determination of CW was applied to measure the Normalized Difference Vegetation Index (NDVI) of the wheat canopy in a box planting area. The measured values of both CW and NDVI were stable and showed a strong adaptability to natural light intensity and measurement heights during tests. The coefficient of variation (CV) of CW 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.