High pressure sodium lamps generally only have two positions: on or off. The light spectrum provided by high pressure sodium lamps corresponds very poorly to the spectrum plants use during photosynthesis. Plants do not receive very much of the blue and red light that they need the most. They do, however, receive a great deal of infrared light, which is harmful to some crops, and yellow light, which the plants cannot utilize to any great extent.
"Everything in modern greenhouses is very high tech except for lighting," said Anna-Maria Carstensen, a PhD student in automatic control at Chalmers. "Temperature and nutrition are meticulously controlled. Lighting regulation, however, lags far behind."
The Chalmers research project aims to ascertain how much and what type of light different plants require at specific times. A spectrometer is used to measure which wavelengths are sent back by the plants. The plants send back light in two different ways:
- Direct reflection, where the light bounces back without being absorbed by the leaf.
- Fluorescence, which is light emitted by plants. This light is created by photosynthesis and consists of wavelengths other than those from the supplied light. Researchers can analyze these signals to determine which light plants require.
"This is uncharted territory," said Torsten Wik, associate professor of control engineering and head of the research project. "How plants react to light is generally determined by taking manual samples on or close to leaves using special equipment. We perform the analysis remotely, however, using the lamp's control options. This means that an entire plant population can be measured, which automatically enables a representative average for the kind of light they need."
Plants' response will control lighting
The project's aim is to produce a system that employs the plants' response to automatically regulate the lights in the greenhouse. Natural sunlight can then be supplemented with light from lamps to ensure the plants get all the lighting they need, both in terms of brightness and light spectrum.
This can be achieved using advanced LED lamps, which consist of several groups of dimmable light emitting diodes with different colour spectra. This kind of lamp can also be programmed to provide lighting that is adjusted to the needs of the plants.
"The technology has enormous potential for energy savings," said Wik. "We are counting on being able to save about 30 per cent by switching from sodium lamps to LED. Furthermore, it is possible to save 20 per cent by regulating the light's intensity and spectrum using our method. This means that greenhouses in Europe alone would be able to save as much electricity as half of Sweden's electricity consumption."