Research on the Optimal Exposure Time of Digital Hemispheric Photography Method Based on Light Intensity

Abstract

Optimal exposure time is essential for accurate measurement of LAI (leaf area index) by DHP (digital hemispheric photography) method, The results showed that LAI was underestimated in different degrees under automatic exposure mode. To address this problem, in this paper we constructed a model of light intensity and optimal exposure time by studying the quantitative relationship between light intensity under the canopy and exposure time. The result shows that using the exposure based on this model rather than the automatic exposure, the comparison of LAI from the LAI-2200 and digital photographs is greatly improved, with R2 increasing from 0.398 to 0.845, and RMSE decreasing from 1.298 to 0.293. The method helps to solve the uncertainty of optimal exposure time in DHP method and improve the accuracy of DHP method in LAI measurement.