PROBLEM TO BE SOLVED: To determine a leaf area index highly accurately, even if each solar radiation in a forest and out of the forest is not measured simultaneously, or even if measurement itself of the solar radiation out of the forest is omitted. SOLUTION: When measuring indirectly the leaf area index, an image of a prescribed area is photographed by changing an exposure time in a plurality of kinds, relative to each of near-infrared light and red light by using a wide angle lens and an electronic imaging element, and an image whose brightness value is the nearest to the median of a gradation from insufficient exposure to excessive exposure is selected from among images having a plurality of kinds of exposure times, relative to each of the near-infrared light and the red light in each subdivided domain acquired by subdividing the prescribed area. A brightness value when the selected image is normalized into a prescribed exposure time is determined, and a brightness value ratio between the near-infrared light and the red light in each subdivided domain is determined from each brightness value of the near-infrared light and the red light determined in each subdivided domain, and a relative solar radiation is estimated by using the brightness value ratio, and the leaf area index is determined from the relative solar radiation. COPYRIGHT: (C)2007,JPO&INPIT
