Light and plant

Of the visible light only a small part is being used (photosynthesis) of plants, with wave length between 400 and 700 nm contributes one part. This one is called, however, Par-field (PAR = Photosynthetic Active Radiation). At daylight approximately 45% of the overall radiation lie between 400 and 700 nm. Therefore approximately 45% of overall radiations are Par-light. An efficient lamp for plant growth must convert as much as possible electric energy into Par-radiation energy.

Photosynthesis is a photochemical process where light energy is absorbed by chlorophyll and carotenoid of the leaves. This energy is used for the shaping of sugars carbon dioxide (CO 2) that is taken by the leaves. This process can be reflected as follows:

6 CO2 + 6 H2O + light energy -> C6H 12O6 + 6 O2
carbon dioxide + water + light energy gives oxygen + sugar

A light source can be considered as a source which transmits energy parts. These particles are called light quants or photons. Energy -content of a photon coincide with the wave length. Total to transmitted photons between 400 and 700 nm per second, are called PPF (Photosynthetic Photon flux) with unit of µmol/s. PPF are with the term compare lumen, however then on the basis of plant sensitivity. Energy -content of 400 nm-photon (blue) are for example 1.75 time higher than those of 700 nm-photon (red), but for the photosynthesis process both photons are equivalent. The surplus to energy of a blue photon is for the greater part converted into warmth.

The speed of photosynthesis is stipulated by the number of photons between 400 and 700 nm that by the plant are absorbed and not by the sum of energy -content of these photons. For this reason the number of photons is between 400 and 700 nm that per second on a certain area fall, indicated with the abbreviations PPFD (Photosynthetic Photon flux Density), the only suitable half-measure in which the quantity must be expressed slightly for the photosynthesis process. PPFD are with the term compare lux, however then on the basis of plant sensitivity. THE PPFD are measured with quantum sensor and are expressed in µmol photons by m² per second (µmol. m -2 s -1). When one speaks of PAR (Photosynthetic Active Radiation) then it concerns energy -content of light between 400 and 700 nm in W/m².

Also within Par-field are not plants for all wave length just as sensitive. This is among others caused by the specific absorption of all kinds of pigments in the leaf, of which chlorophyll is confessed most. As a result of a relatively strong reflection and transmission greenness slightly it is less effectively used by the leaf. This explains why leaves for the human eye are observed as greenness.

The impact of radiation of several wave lengths on the plant increase can be made visible with a plant sensitivity curve. Because photosynthesis is the most important process for increase we use an action spectrum in which the photosynthesis speed has been stipulated at several wave lengths. This action spectrum for photosynthesis has been based on the number of absorbed photons (light quants) by wave length. Such an action spectrum also are called ' spectral quantum efficient '.

From research (McCree 1972) becomes clear that the average deviation by plant variety is with respect to the average no longer than 5%. Also the quantum efficient prove to be the highest in the orange red area, therefore the orange red slightly offer the highest efficiency on photosynthesis. This does not mean moreover that to plant can become exclusively under slightly of this colour being cultivated. For a good development it is very important that the plants get a balanced spectrum. Thus the share of blue is very important for a good plant development. A shortage to blue gives exuberant stem drawing and sometimes yellow leaves. Also the proportion red/ver-rood is important for plant development. A low proportion of ver-rood prevents stem drawing. These sensitivities are by plant variety differing.