Light is a type of electromagnetic radiation emitted by the sun. This radiation reaches the earth in the form of waves. The waves are filtered by the atmosphere and thus reach the earth as global radiation. The global radiation can be roughly divided into ultraviolet (UV), the part used by plants (PAR), infrared (NIR) and long wave thermal radiation (FIR). Ultraviolet radiation has a short to very short wavelength, while infrared radiation has a much longer wavelength.

It is generally believed that light that has a spectrum similar to sunlight guarantees normal plant development. Sunlight contains, expressed as a percentage of all photons between 400 and 800 nanometers (nm), approximately 21% blue (400-500nm), 26% green (500-600nm), 27% red (600-700nm) and 26% far red light . The different wavelengths affect different plant processes.

The list below shows the influences of the different wavelengths on the plant processes.

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The process of photomorphogenesis leads to the final shape, color and flowering of the plant. This is largely genetically determined, but is controlled by light.

Some of the light is used by plants for photosynthesis. This part of 400-700nm is called Photosynthetic Active Radiation (PAR).

In chlorophyll synthesis (chlorophyll synthesis), chlorophyll (chlorophyll), the green pigment in leaves and stems, is produced. The sunlight is collected and processed in the chloroplasts (chloroplasts).

For many plants, the day length (light period) is a source of information that determines the moment to form shoots or start flowering. The behavior and development of the plants are thus influenced by the photoperiod.

The color of certain flowers and fruits can be improved by UV radiation. The radiation ensures a more compact growth, shorter internodes and smaller and thicker leaves. UV radiation at higher intensities is harmful to crops, such as a negative effect on photosynthesis. On the other hand, there is a positive effect due to the formation of flavors and antioxidants.

The blue radiation has an effect on the photosynthesis process. For the photosynthesis process, the blue and red radiations are equivalent. Blue light is especially important for the formation of chlorophyll, the development of chloroplasts, stomata opening, the production of enzymes and the 24-hour cycle of photosynthesis and photomorphogenesis. An increased proportion of blue light in natural light has an inhibitory effect on cell elongation, causing stems to become shorter and leaves thicker. Conversely, a decrease in the amount of blue light results in an increase in leaf area and stem elongation. The amount of blue light cannot be reduced indefinitely for all plant species. Too little blue light can have negative effects on plant development. (Source: Dieleman A. et al., 2006, Is lighting appropriate for energy-efficient pepper cultivation?) Many plant species require a minimal amount of blue light for normal plant development. This requirement differs per species and varies from 5-30 mol m²/s for lettuce and peppers to 30 μmol m²/s for soybean. In the Netherlands there is naturally enough blue in the natural light for plants (also in the greenhouse). The plant shape can be controlled by influencing the amount of blue light. (Source: Hemming S., 2004, Optimal use of natural light in greenhouse horticulture)

The red radiation is most efficient for the photosynthesis of plants. The energy content of a red photon (600-700nm) is 1.75 times lower than that of a blue photon (400-500nm). This means that more energy is needed to make a blue photon than for a red photon, while the photons for photosynthesis are equivalent. For example: at 400nm 1 Watt is 3.4 mol and at 700nm 1 Watt is 5.8 mol. (Source: Hemming S., 2004, Feasibility study fluorescent energy screen)

Red radiation contributes to the production of chlorophyll (leaf green) and plays a role in the processes of photoperiodism and photomorphogenesis. Selectively shading red light, which reduces the ratio between red and far-red light, can reduce the formation of side shoots and lint.

The near infrared (NIR), with a wavelength of 700-3000nm, is the part of the solar spectrum that is hardly used by the plants; it is mainly converted into heat (tangible and latent) in the greenhouse. Depending on the location and the season, this can have a beneficial effect on the greenhouse climate or it can introduce the problem of overheating.

The radiation portion of 700-800nm ​​is called far red. This contributes to photomorphogenesis, especially stem elongation and photoperiodism of plants.

Far infrared radiation

Far infrared radiation (FIR) with wavelengths of 3000 - 10000 nm does not result from direct solar radiation, but is heat radiation emitted by any warm 'body'. This radiation is of great importance in greenhouses, because it causes part of the greenhouse effect. (Source: Hemming S., 2004, Optimal use of natural light in greenhouse horticulture)