Semi professional digital camera calibration techniques for Vis/NIR spectral data acquisition from an unmanned aerial vehicle

Abstract

ABSTRACTUnmanned aerial vehicles (UAVs) equipped with multispectral digital cameras are very effective in spectral information obtainment. In order to check the radiometric response of the multispectral digital camera and provide better target’s spectral dynamics investigation, the used camera must be carefully calibrated in laboratory and field conditions. This article aimed to achieve the radiometric calibration of a multispectral Vis/NIR camera attached to an UAV, using as reference a hyperspectral sensor. First, a laboratory calibration, under controlled conditions, was performed. Then, a field calibration with no controlled conditions was accomplished. Finally, a cross calibration between laboratory and field data was developed. Multispectral Fujifilm S200-EXR digital camera, sensible to infrared spectrum radiation and 8 bits of radiometric resolution (256 digital numbers (DNs)) and Fieldspec 3 Jr spectroradiometer (ASD Inc.), with spectral resolution of 3 nm between 350 and 1400 nm and 30 nm between 1400 and 2500 nm were used for spectral data acquisition. Six tarpaulins were used as reference targets. In laboratory, reference values were collected by Fieldspec 3 Jr and photographs of the reference targets and Spectralon panel were taken with no optical filter and using seven optical filters, which block the visible (Vis) and near-infrared (NIR) radiation in different intensities. In field, reflectance values were collected by the hyperespectral sensor on ground level and Vis/NIR images were taken using two identical Fujifilm S200-EXR cameras coupled to a Tarot Iron Man 1000 octocopter (UAV) at 200 and 600 m flight altitude on 22 January 2016 in clear weather conditions. All models and calibrations equations obtained by the correlation of DNs and reflectance values were significant to the Student’s t-test (p ≤ 0.05). As in laboratory as well as in field, for red, green, blue, and NIR bands were obtained great correlations (r > 0.90) except for red band in laboratory (r = 0.88). The cross calibration (laboratory vs field) obtained models for red, green, blue, and NIR bands presented high Pearson coefficients (r > 0.90). Under these circumstances, the calibrations models for red, green, blue, and NIR bands, point out the potential of cross calibration such way that the reference target’s reflectance values can be acquired in laboratory and, from digital images obtained by cameras attached to UAVs, DNs in field conditions can be collected, making able the easy obtainment of trusty spectral information.