GREENHOUSES

Greenhouse lighting for horticulture:

Light is the energy source for photosynthesis and the most important factor in plant growth. The unit of light for plant growth is expressed in µmol/m²/s (number of light photons in micromoles per square metre of leaf area per second). The rate of photosynthesis, or assimilation rate, is largely determined by the availability of light for growth.

Under light-limited conditions, supplementary lighting is necessary to increase production and enable year-round cultivation of plants. Factors such as intensity, duration of illumination and light colour (or light spectrum) are of great importance here, although it should be noted that the amount of light is not the only factor determining the rate of assimilation or photosynthesis.

Other factors, such as plant species, temperature, water (including nutrients), humidity and CO₂ concentration in the air, also determine how efficiently the plant can utilise the light.

All light consists of electromagnetic waves within a spectrum ranging from ultraviolet (UV) to infrared (IR). The wavelength of light is measured in nanometres (nm), for both visible and invisible light.

Photons (particles of light) are a quantum, which is the smallest, indivisible unit of a quantity. As individual photons possess only an extremely small amount of energy, this is expressed in moles (mol); one mole is equal to 6.02 × 10²³ photons. A micromole (µmol) is one millionth of a mole.
Photons possess different amounts of energy, depending on their wavelengths.

The quality of light depends on the relative number of photons at the various wavelengths. The quality of light therefore depends on the spectral distribution – that is, the relative number of photons in each part of the spectrum, both visible and invisible, emitted by the light source.

When a plant converts light into energy during photosynthesis, it requires light of specific wavelengths. We refer to this as photosynthetically active radiation (PAR); this spectrum ranges from 400 to 700 nm. The part of the light spectrum that is most visible to humans, around 555 nm (green, yellow and orange light), has the least effect on plants.

Light meters designed to measure light visible to humans (in lumens, lux or candelas) are therefore not particularly effective for determining light levels for plants. To measure plant lighting more effectively, quantum meters can be used; these measure the entire PAR spectrum.

Quantum meters differ from visible-spectrum meters in that they measure the number of photons falling on one square metre per second. This provides a snapshot of the light received by the plant, expressed as the number of photons (in micromoles) per square metre, i.e. µmol/m².

High-pressure sodium gas discharge lamps:

High-pressure sodium lamps, such as SON-T, are the most commonly used light source in greenhouse horticulture. The main reasons for this are their high growth light efficiency (µmol/Watt), their availability in high-wattage versions (600 / 1000 Watt) and the relatively small size of the lamps. Although the light colour or spectral composition differs from natural daylight, plant development in greenhouse horticulture is generally good under this type of light.

LED lamps (Light Emitting Diodes):

The development of Light Emitting Diodes (LEDs) with a very high growth light output offers good prospects for their use in horticulture. LED lamps have a number of advantages over other types of lamps and are increasingly being used in practice as supplementary lighting or assimilation lighting in multi-tier cultivation systems and for inter-row lighting.

However, the efficiency of LEDs compared to SON–T is constantly increasing and also offers prospects for the use of LEDs as top lighting. LED lamps emit virtually no heat from the front and are available in many light colours. This offers advantages over standard SON-T; however, before SON-T can be replaced by LEDs, more knowledge is needed, amongst other things, regarding which light colour is optimal for the growth and development of the crop in question, as well as what the optimal light/heat balance is.

LED lamps have potential applications across a range of horticultural uses but, for the time being, cannot serve as a complete replacement for photosynthetic lighting.