Light is measured differently depending on what part of the light spectrum is being measured. The total light spectrum coming from the sun (400 to 1100 nanometer wavelengths) is measured in units of watts/m2. On a clear sunny summer day, there may be 1000 Watts/m2.
Light may be measured in terms of its intensity (lux) or the number of photons reaching a surface (photon flux density). The part of the spectrum that humans can see, called visible light (380 to 770 nanometer wavelengths) is measured in lumens. The lumen is the metric unit of light intensity and the term lux refers to the number of lumens per square metre of surface area.
In horticulture, the number of photons reaching a surface is more important. Photons are basically packets of energy which make up a stream of light. The number of photons trapped by a leaf determines the level of photosynthesis and therefore the amount of plant growth.
The part of the spectrum that plants use is called photosynthetically active radiation (PAR) and relates to light in the 400 to 700 nanometer wavelength which is almost the same as visible light but not quite. It is measured in units of µmol/m2/s and describes the photon flux density, that is, the number of packets of energy which reach a surface.
The amount of light entering a greenhouse is influenced by;
The greenhouse should be positioned north-south to provide more uniform light and reduce the shading effect of the support structure. The support structure must also be minimised to avoid shading. Metals make good structural material because of their strength which means narrower trusses and purlins can be used. A typical greenhouse frame can reduce light transmission by more than 10%. The type of covering material will also influence the level of light in the greenhouse.
Finally the shape of the roof will impact on how much light enters the greenhouse. For example, a flat roof will limit the amount of light due to reflection while a curved roof provides the greatest annual light transmission.
A balance of light across the PAR range is considered to be preferable, however there is increasing research being conducted in the area of light spectrum modification for improved plant growth.
Diffuse light is better than direct light because it is able to reach the lower parts of the canopy (less shadowing) and it will not cause sunburn. Irrespective of whether the light is direct or diffuse, it must be of sufficient intensity (lux).
The selected covering material may also be used to increase the amount of diffuse light. A textured surface on glass, for example, can increase the proportion of diffuse light without significantly reducing the total level of light transmitted.
The colour of plastic films affects the total level of light that enters the greenhouse.
A clear film will transmit the most amount of light. Blue and green coloured plastics will transmit a lot of the light in the blue to blue-green wavelengths, but cut out much of the light in the red wavelengths. From the diagram above looking at PAR, it can be seen that red light is the most efficient waveband for plant growth. A blue plastic is likely to produce a slower growing, shorter, tougher plant.
Also of interest is that plants have been shown to use far-red light as a way of determining how much competition there is for light. This is because green surfaces, such as leaves from other plants, reflect a lot of far-red light. If the plant perceives that there is a lot of competition, it will put less energy into growing roots and more into growing tall, quickly.
A green plastic is likely to produce a stretched, slow growing, poor performing plant.
A white film will reduce the total amount of light transmitted by as much as 20%, but the light spectrum entering the greenhouse will remain similar to the natural light spectrum.
Plants have an optimal intensity of light. This is the point at which the process of photosynthesis is maximised and plant growth is greatest. If the level of light is less, growth is reduced. The point where an increase in light intensity will not increase photosynthesis any more is called light saturation.
Cucumbers for example require at least 250 µmol/m2/s of photosynthetically active radiation (PAR). Below this level, productivity will decline. The amount of light in the greenhouse should be maximised wherever possible. The management of heat is the only reason for reducing incoming light levels.
Low light slows growth and increases the cost of production but excessive light intensity can damage some plants and/or fruit. Light is increased by minimising objects above the plants including frames, pipes, lights and other equipment. The level of radiation entering a greenhouse can be reduced with screening materials.
Supplementary lighting is not considered to be economical for producing most crops in Australia, however, supplementary light may be used to improve seedling uniformity during propagation.