Greenhouse Lighting
The Electromagnetic Spectrum
Energy from sunlight is called the electromagnetic spectrum. In the electromagnetic spectrum (EM) there are many different types of waves with varying frequencies and wavelengths as shown in Figure 8.1. The continuous spectrum is subdivided into some familiar types of electromagnetic energy like x-rays, ultraviolet rays, visible, infrared, microwaves, and radio waves. These different types of electromagnetic energy are categorized by their positions, or wavelengths, in the electromagnetic spectrum. The wavelengths of light are typically expressed nanometer (nm).
Photosynthetically Active Radiation
Visible light corresponds roughly to the photosynthetically active radiation (PAR) from about 400 to 700 nm and can be used by the plant to drive photosynthesis (Figure 8.1). Photosynthetically active radiation measures the intensity of light that directly affects photosynthesis; that is, the portion of the light spectrum that makes up the colors of the rainbow red, orange, yellow, green, blue, indigo, and violet. Photosynthetically active radiation is a much more accurate way to measure light intensity since it deals with the portion of the light spectrum that drives photosynthesis. Approximately half of the energy that comes from the sun falls within the photosynthetic waveband. The remaining amount of energy has shorter wavelengths (such as UV light) or longer wavelengths (such as infra-red radiation).
Recommended PPFD for Plants
Understanding the different growth stages is crucial for optimizing conditions like PPFD. These stages include the seedling, vegetative, and flowering phases, each with its own unique light requirements. For example, the PPFD levels for flowering or fruiting plants are often higher than PPFD levels for leafy green plants. Microgreen crops are considered to be low-light intensity (100–200 µmol∙m–²∙s–1) crops while tomatoes, for instance, are considered as high-light intensity (600 µmol∙m–²∙s–1) crops.
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