Developing a Light Emitting Diode Sensor to Measure Plant Canopy Phenology

Light emitting diodes (LEDs) are commonly found in lighting sources due to their minimal cost and lower energy consumption compared to incandescent light sources. From previous research it has been found that LEDs can be used in reverse mode to measure light at wavelengths the LED emits essentially functioning as a photodiode. The use of an LED as a light sensor has proven to be useful for detecting the reflectance of plant canopies. For this experiment red and near infrared (NIR, 0.7 – 1 mm) LEDs were used because the red and NIR reflectances are used to calculate the normalized difference vegetation index (NDVI). The NDVI is a common index used to monitor changes in plant canopy structure by sensors on satellites.  The sensor in this study is constructed of 4 LEDs to measure the incoming and outgoing red and NIR.

Using the data collected for the year 2010 at Sherman Island, a field site with complex canopy phenology, it was seen that the LED sensor data does indeed follow the plant phenology expected. This means that the LED absorbance compares well with the spectrometry measurements made over the plant canopy every 2-3 weeks during 2010. Due to the statistical relationship between the LED and spectrometry measurements a calibration function was derived. Developing a rigorously tested LED sensor is important because of its low cost and ability to measure absorbance throughout the day continuously. With an efficient sensor it will be possible to determine changes in plant canopy structure accurately.