A SIMPLE RED-FAR-RED SENSOR USING GALLIUM-ARSENIDE PHOSPHIDE DETECTORS

1. A low-cost portable instrument was designed to measure the red:far- red ratio (660:730 nm) and photosynthetically active radiation (PAR, 4 00-700 nm). The sensor head comprises three gallium arsenide phosphide photodiodes associated with tinted-glass filters. 2. Such detection l akes advantage of the spectral response curves of these diodes, which provide a narrow bandpass response when combined with an appropriate f ilter. No interference filtering and no amplification is required. The output is displayed on a battery-powered unit enclosing two digital v oltmeters for both PAR and red:far-red ratio readings. 3. The unit is easy to produce and requires no knowledge of electronics. The instrume nt was tested under field conditions against a spectroradiometer. 4. T his approach may provide a basis to assess other spectral vegetation i ndices such as NDVI.