The influence of time of day on differences in digital number (DN) between vegetation and background pixels in vertical upward-oriented photographs.

From A to H, the photographic times were 05:00, 07:00, 09:00, 11:00, 13:00, 15:00, 17:00, and 19:00 h, respectively. A and H were taken before sunrise and after sundown, respectively, when there was no direct sunlight.