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ARS Home » Southeast Area » Tifton, Georgia » Southeast Watershed Research » Research » Publications at this Location » Publication #214697
Title: Preparation of a Low-Cost Digital Camera System for Remote Sensing

Author
item RITCHIE, GLEN - UGA
item Sullivan, Dana
item PERRY, C - UGA
item HOOK, J - UGA
item BEDNARZ, C - UGA

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/13/2008
Publication Date: N/A
Citation: N/A

Interpretive Summary: Off-the-shelf consumer digital cameras are convenient and user-friendly. However, the use of these cameras in remote sensing is limited because convenient methods for concurrently determining visible and near-infrared (NIR) radiation have not been developed. Two Nikon COOLPIX 4300 digital cameras were evaluated in tandem and a cross-camera calibration algorithm was developed. The accuracy of the calibrated system was evaluated by comparing uncalibrated and calibrated images to known reflectance targets (n=26). The relationship between camera channel brightness and target reflectance was nonlinear within each exposure, but sensitivity was linear between exposures. Thus an algorithm was developed to standardize all images to a common exposure level post-acquisition. The calibration algorithm was tested in the field over 36 cotton plots (Gossypium hirsutum) in an irrigation study in 2006. Images obtained on 8 dates during the season were corrected for exposure and converted to relative reflectance values. The normalized difference vegetation index (NDVI) values from the plots were then compared with ground-based spectrometer measurements of NDVI. Corrected camera-based NDVI values were closely correlated with the spectrometer NDVI values (r2 = 0.72), suggesting that the camera system can consistently estimate crop reflectance characteristics if exposure compensation is applied.

Technical Abstract: Off-the-shelf consumer digital cameras are convenient and user-friendly. However, the use of these cameras in remote sensing is limited because convenient methods for concurrently determining visible and near-infrared (NIR) radiation have not been developed. Two Nikon COOLPIX 4300 digital cameras were evaluated in tandem to determine the effectiveness of a cross-camera calibration procedure that would allow concurrent use of an unmodified digital camera and a NIR-sensitive digital camera without preset shutter speeds or aperture settings. The NIR-sensitive camera was modified to detect NIR by replacing the internal hot mirror with a Hoya R720 filter. Each camera was calibrated at 5 exposure levels using a Gretag-Macbeth ColorCheckerTM reflectance panel, and raw camera brightness values were converted to relative reflectance by exposure compensation equations. The method was tested on a series of 26 diffuse reflectance targets, which also yielded the same exposure compensation relationships. The relationship between camera channel brightness and target reflectance was nonlinear within each exposure, but sensitivity was linear between exposures. The procedure was tested on 36 cotton plots (Gossypium hirsutum) in an irrigation study in 2006. Images obtained on 8 dates during the season were corrected for exposure and converted to relative reflectance values. The normalized difference vegetation index (NDVI) values from the plots were then compared with ground-based spectrometer measurements of NDVI. Corrected camera-based NDVI values were closely correlated with the spectrometer NDVI values (r2 = 0.72), suggesting that the camera system can more consistently estimate crop reflectance characteristics if exposure compensation is applied.