Digital canopy photography: Exposed and in the raw

Abstract

Estimates of the canopy gap fraction, on which calculations of leaf area index (LAI) are based, are sensitive to photographic exposure in upward-facing images. In this article we describe a simple, automated method of image acquisition and processing that eliminates both subjectivity and the need for the operator to consider photographic exposure in the field. A key strength of our methodology was the use of a test apparatus (perforated aluminum screen) with a precisely known gap fraction; this allowed us to separate the confounding effects of gap size and gap fraction on the optimal photographic exposure for a canopy. We took photographs of the test apparatus at different photographic exposures; we varied the gap fraction by covering a proportion of the holes in the screen, and also varied gap size by varying the distance of the camera from the screen. We acquired both raw images and JPEG images. We found that the optimal exposure of JPEG images varied with both gap size and gap fraction, not just gap fraction as previously assumed. Underexposing by one stop yielded raw data that were never clipped resulting in data loss, but that used most of the 14-bit range of the raw file. We also found that it was easily possible to standardize photographic exposure during image processing by acquiring raw images in the field; thus eliminating the variation in estimated gap fraction and LAI associated with exposure variations. This result was replicated in both fisheye images and cover images that we acquired in real canopies. We recommend the following protocol for acquiring canopy images in future studies: 1. Shoot raw using one stop of underexposure. 2. Convert propriety-format raw files to DNG format using the free software Adobe DNG Converter. 3. Contrast stretch the blue channel of the image and save as JPG for further analysis. The authors can supply MATLAB script to perform this step. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved.