Comparison of spectral indices obtained using multiple spectroradiometers

To determine the degree of comparability between three spectrometers (Analytical Spectral Devices FieldSpec Pro FR (FR), Analytical Spectral Devices HandHeld (HH), and UniSpec Spectral Analysis System (UN)), leaf spectra of three species (Cafea arabica, Lantana camara, Eriobotrya japonica), recorded from each instrument, were compared using two illumination, viewing, and field of view (FOV) scenarios. Scenario 1 eliminated differences due to illumination, viewing, and FOV conditions. Scenario 2 represented a ‘typical’ illumination and viewing set-up for each instrument. Six vegetation indices were computed from the raw spectra as well as spectra (1) interpolated to 1-nm intervals (the sampling interval of the FR) and (2) interpolated to 3.3 nm (the sampling interval of the UN). The spectra measured from the three instruments differed in both shape and amplitude, more so for scenario 2 than scenario 1. In many cases, indices obtained using one instrument differed significantly from the same indices obtained using the other two instruments (but the same leaves), regardless of scenario. The severity of these differences varied between indices. Interpolation was generally ineffective in ‘matching’ the spectra from the various instruments. Care should be exercised when comparing indices generated from spectra measured from different instruments.