I got for analysis data obtained of a machine for remote sensing detecting of leaf reflectance. Data within the green and blue spectral area are negative. Can someone to say me if this is possible or the measurement with that machine was incorrect.
Negative values of NDVI usually correspond to urban areas.
http://www.eumetrain.org/data/3/36/print.htm
http://www.sciencedirect.com/science/article/pii/S092427161300107X
Prof. Maneva, if I understand your question correctly, you are trying to understand whether it is possible to have negative reflectance values in the blue and green wavelength region while measuring the reflectance of a leaf? Can you please clarify the name/model of this 'machine' that you mentioned in your question and the specific leaf type? Is the machine an FTIR or a Spectrometer or something else? What processing steps did you carry out to derive the reflectance values? How did you calibrate the instrument/machine?
Generally negative reflectance values in remote sensing do not have any physical meaning and are avoided. However, I would need more information in order to say something about this specific case of leaf reflectance.
https://www.researchgate.net/post/Hyperspectral_Image-Pixel_value-Reflectance-Negative_Values
Dear James Varghese, spectral reflectance measurement were performed by means of a portable fibre-optics mictospectrometer USB400 ocean optic Inc., USA with spectral resolution of 0.6 nm.. Reflectaaaance measurements were performed in the VIS and NIR spectral ranges under controlled illumination conditions. The light source was a halogen lampe400-900 nm. You cam see attached the graphic of obtained spectral reflectance characteristics. The leaf samples were fresh apple leaves treated and untreated with chemicals for plant protection
I will say in addition that during last 6 years I , together with an other scientist, had several investigations on possibility to use remote sensing for early diagnostic of plant diseases. We obtained very good results and we have several publications on the topic. We have never obtained negative values of the reflectance. I am the mathematician of the group who process the obtained data - first derivation, statistical significance of the values within different wave lengths, calculation of several vegetation coefficients and discussion of the results.
I hope that addet explanation will help you to answer my question
Thank you Prof. Maneva for clarifying your question. I suspect there may be an issue with the instrument calibration. I found some links to similar discussions.
https://www.researchgate.net/post/What_does_it_mean_when_the_IR_absorbance_is_positive_or_negative
https://www.researchgate.net/post/How_to_interpret_negative_absorptions_in_ATR-FTIR_spectra
https://www.researchgate.net/post/Why_does_negative_absorption_occur_in_ATR-IR_measurements
https://www.researchgate.net/post/What_is_the_significance_of_Negative_reflectance
https://www.researchgate.net/post/What_does_Negative_values_in_Remote_sensing_reflectance_mean
https://www.researchgate.net/post/What_is_the_meaning_of_negative_transmittance
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