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Light and Plants

Light and plants

Plants have a different sensitivity response than humans to light of various wavelengths. For green plants, the sensitivity to red light is highest and it is lowest for green light. For this reason, the ‘lux’ unit is not the right measure for the lighting of green plants. Light sources may be regarded as sources of energy particles; these are called light quanta or photons. Most lighting products are developed for applications with human beings at center stage. The units ‘lumen’ and ‘lux’ are relevant for these. The unit lux has its origins in photometry and is based on the average sensitivity of the human eye. The maximum sensitivity is around 555 nm (yellow-green light) and reduces in case of longer (red) and shorter (blue) wavelengths.

All photons, from blue to red (400 to 700 nm), are used for photosynthesis, but this does not happen for every photon with the same output. This is termed the ‘plant sensitivity graph’.


Studies carried out by universities and testing stations show that the speed of growth of plants is mainly determined by the number of photons between 400 and 700 nm that they absorb. The growth light is indicated by the abbreviation PAR, and is the only reliable measure to indicate whether a lamp is suitable for photosynthesis. The higher the PAR value per Watt, the more efficient the lamp. For this reason, Philips specifies the PAR value for all its ‘growth lamps’, expressed in micromol photons per second (µmol/s).

The Double-ended Lamp

HPS lamps have long been the favored light sources for plant growth, owing to the high proportion of radiation in the spectral range of greatest value to photosynthesis. Philips was an early pioneer in the development of lamps having a further optimized spectrum, for instance the original SON-T Agro 400W. This features enhanced output in the blue as well as red regions to stimulate photosynthesis with maximum efficiency.

In order to increase growing yields, there has been a continual trend to the use of higher irradiation levels, which in turn calls for higher output lamps. This resulted in the GreenPower range of HPS lamps having only the increased red component, which raises performance because growth is stimulated much more by red light than blue.

To fuel the desire for still higher light levels, Philips launched the first 1000W plant growth lamp in 2005, in the form of this GreenPower TD. The double ended construction was chosen so as to maximise optical efficiency in the luminaire, because unlike for single ended lamps there is no frame support wire running alongside the arc tube, which may cast a shadow in the beam. The position of the arc tube with respect to the pinch-seals is also tightly controlled, which permits optimal positioning within the luminaire’s optics. The Philips GreenPower 1000W lamp is a gas-filled lamp rather than vacuum. This permits a reduction of arc tube length for a given wattage, further improving the degree of optical control. The high thermal conductivity of the gas filling does however make the lamp extremely sensitive to its environment in the luminaire, especially the reflector design.

In 2017 DLI launched the DLI 1000W DE Horticulture Plus Lamp to increase the photosynthetic efficiency. By enhancing the gas chemistry a high life can be maintained. ALso the enhanced red and blue spectrum encourages growth of all types of horticulture, therefore creating a unique 1000W DE lamp.


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