Comparative study on estimation of chlorophyll content in spinach leaves using various red edge position extraction techniques

The position of the inflection point in the red edge region (680～780 nm) of the spectral reflectance signature, termed the red edge position (REP), has been used as a means to estimate foliar chlorophyll or nitrogen content and leaf area and also as an indicator of vegetation stress. At present, extraction techniques for REP can be divided into two groups. The one is based on derivative and the other uses curve fitting. There was no conclusion about which technique can adequately track variations in spectral reflectance near the chlorophyll sensitive peaks, and precisely estimate the chlorophyll content. Maximum first derivative, Lagrangian interpolation, four-point interpolation, inverted–Gaussian modeling, polynomial fitting and linear extrapolation techniques were used to extract the REP from spinach leaf hyper reflectance spectra, and their performance for prediction of leaf chlorophyll content were also compared. Results show that the results of REP calculations are dependent upon the choice of method, and the relationship between REPs extracted by different techniques and leaf chlorophyll content were all good. REPs extracted by four-point interpolation technique are biased towards the longer wavelength and most insensitive to leaf chlorophyll content. The maximum first derivative technique is not appropriate for estimating leaf chlorophyll content because of the discontinuity it creates in the REP data and the REP/chlorophyll relationship. Lagrangian interpolation and inverted-Gaussian modeling to some extent are less sensitive to higher chlorophyll content due to the REPs saturate when leaf SPAD higher than 60. REPs extracted by polynomial fitting technique were unstable due to its high sensitivity to the spectral resolution, waveband range and the highest order of polynomial. The linear extrapolation technique was the most practical and suitable method because of its relative high correlation with a wide range of chlorophyll content, relative simple algorithm, relative high sensitivity to chlorophyll at high or low content and application to more wider bandwidth spectra. However, appropriate REP extraction technique should be chosen according to the practical situation.