Although incadient sources are the cheapest, it has been shown that they can overheat the plant, consume energy, and provide a limited range of red and blue, which is necessary for the breeding and health of the plant.
Halogen sources have a light color very close to the sun (about 6000K) and a longer lifespan than incadescent (up to 15,000h). They are characterized by poor efficiency, as well as overheating. They can provide light in one narrow area of the spectrum, eventually damaging and burning plants by giving too much light to certain zones under the plants, and not enough to other zones.
Fluorescent sources (picture 1) are very widespread, but they lack the red spectrum, which is important for blooming. Luminaire systems with a more complete spectrum of light have been developed, especially those with increased blue and red spectrum, in order to achieve sustainable PPF (photosynthetic photon fluence) required for high yield. However, the spectrum and intensity of fluorescence sources is not stable over a longer period of time, and such systems are expensive and inefficient.
Picture 1: Fluorescent sources
Discharge lamps, such as MH and HPS sources (picture 2), have so far been the most widely used in horticulture. They have relatively high efficiency up to 200 lm / W and high PA (Photosynthetic Active Radiation) up to 40% and are used in greenhouses and other plants for growing plants. HPS sources give light in the warm, red part of the spectrum and are dominant in
relative to MH or fluorescent sources for growing flowers. However, due to the poor blue color reproduction index, the plants look faded, so it is necessary to combine two types of HPS sources to get the red and blue colors of the light.
Picture 2: HPS and MH sources
The use of these light sources, which do not have the full range of spectrum and have high electrical power consumption, as a serious disadvantage also has the effect that it is not possible to place lamps near the plant due to high temperatures that can damage the plant (picture 3).
The development of LED technology in recent decades has introduced a new light source in the field of plant cultivation, with many advantages.
Picture 3: Led lamps
First of all, LED sources can provide the full range of wavelengths (picture 4) required for growth and development of plants or achieving only a certain color of light in certain stages of development. The plant absorption spectrum can be adjusted using tunable LEDs. LED sources are far more convenient than conventional HPS, whose maximums vary greatly from the greenhouse plant absorption spectrum.
Picture 4: Wavelengths of LED sources
Many plants do not need all the colors from the visible part of the spectrum. For example, for flowering plants an entire visible spectrum of light is needed, but some wavelengths are more important than others. Red light is of crucial importance for stimulating flowering and the process of fruiting, while the blue light stimulates the compact "greenish" look of a healthy plant. Therefore, breeders who rely on artificial lighting such as LEDs concentrate on the blue and red part of the spectrum.
The exact amount of light must be provided artificially, where one should bear in mind that too much light can damage the plant, be inefficient and cost too much.
Of all artificial light sources, LED sources have the highest PAR efficiency, in the range of 80-100%. Since the LED emits a blue, green, yellow, orange, red light, these can be used as basic colors, their combinations, or special wavelengths to increase efficiency and yield, thanks to their narrow wavelength range.
High efficiency, low operating temperature and small LEDs allow the sources to be placed very close to the plant. A long lifetime and easy control make them ideal for greenhouses throughout the year. LED technology is intended to replace conventional sources and introduce revolutionary changes in controlled conditions of plant growth.