Measuring spectral differences in vegetation canopies by a reflectance ratio meter

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Abstract
Summary: A simple, robust and inexpensive hand‐held meter is described which senses the presence of green vegetation against brown soil by measuring the ratio of near infra‐red (0·75 μm) to red (0·65 μm) radiances: ratios are higher for green canopies than for soil, due to strong absorption by chlorophyll of red wavelength and the high near‐infrared reflectance of healthy vegetation. Field tests showed that radiance data were correlated closely with plant density and growth stage. Such spectral assessments provide a rapid, nondestructive method of monitoring changes in green‐leaf biomass. Although only small differences in radiance values were recorded from monocultures of various non‐flowering species there was a positive correlation with content of white clover in five grass/clover swards. In addition to being a research aid, these findings confirm the potential of remote sensing of agricultural targets, e.g. detection of weed patches for herbicide spraying, or gaps in swards for sowing seed.