Photoselective Filtering Films
Photosynthesis and Plant Growth
It is commonly believed that the only light spectrum required by plants was in the 400 to 700NM. This is known as the P.A.R. or photosynthetically active radiation. However, plant scientists and horticulturists know that plants have evolved and have adapted to the natural light found in their habitat. The quality, intensity and duration of the light in a plant's habitat provide a baseline for the type of spectral filter film that they should be grown under. For instance, if one takes a plant that normally grows in full sun it does not grow well in the shade, or a plant which grows well in the shade does not grow well in full sun.
Plant scientists also know that the ratios (UV- Blue- Near Red- Far Red) are different depending upon where one is in the world. It is also known that the spectrum of light that floor growing plants get in a woodland shaded area is also different. As plants have evolved to grow in these different conditions we know that not all plants want the same spectrum of light.
Most Glass and polythene will block out the UV below 350nms. However research has shown that lower levels of UV will help in the flower and leaf colour, particularly of red fruited subjects like Fragaria, Lycopersicon and leafy plants like Photinia. Research in Spain has shown that Fragaria had a better red colour right through the fruit and arguably more flavoursome. Recent research in the U.K. also showed enhanced root growth and better blues on plants such as Gentian. This is hardly surprising as Gentians are alpines and so would normally grow with a higher UV level. High levels of UV will also produce bushier plants with shorter intermodal growth.
There are films that remove all UV light because just as you see in Yellow light then flying insects (Whitefly for instance) see in UV light. It is known that if all UV is blocked below 380NMs bumblebees are working in 60% of their visible light. At 400NMs they are only seeing 5% of their visible light. So films which block to 380NMs are a useful compromise between full insect control and still allowing more robust pollinators to work. Dramatic reductions in infestation have been recorded, cutting down infestation by as much as 90% using this type of pest management control. Fungal diseases such as Botrytis are also controlled if you block UV. They need UV light to be able to create a viable type.
If you put floras under a UV blocking film which already have Botrytis on them the Botrytis will still grow, but its reproductive capacity is very much reduced. Many growers have commented on how, within a short space of time, the disease will have died out and plants will become clean again. The best control is achieved if you can block the UV light up to 400NMs, but this again compromises the ability of pollinators. Incidentally this UV blocking effect has no effect on natural predators.
The peak of blue light occurs at around 475NMs. Blue light is required for the opening of the stomata and there is relevance between the ratio of blue to red light in the photosynthesis effect. Blue light also affects the phototropism of plants. In general plants which are grown under films which transmit good levels of blue light will be stronger and there is some evidence to show that Blue light affects cell wall thickening and therefore the sturdiness of the plant.
The peak of green light occurs at around 510NMs. Plants mostly do not use green light. For instance, if one looks at grass wavelengths such as red and blue are absorbed however green light is reflected away as it is not required, therefore grass appears green.
The peak of Yellow light occurs at around 570NMs. Yellow light is important in the production of chlorophyll and in conjunction with Red is important for photosynthesis.
The peak of red light occurs at around 660NMs. However, red is very important to plants and is split into Near Red (Pr form) and Far Red (Pfr). Much of the photosynthesis is triggered by the red part of the spectrum and since plants are producers of protein (phytochromes) through photosynthesis their autonomic response is to search out red light for growth. They will naturally try to keep their apical bud in the sun. They also have a switch which modifies their production of growth and plants which have not adapted to grow in shade conditions (open prairie or savannah type) will switch on a trigger that makes them grow more elongated between the distances of the internodes. However, floras which have adapted and evolved to grow under a forest canopy or woodland floor have evolved to grow with a lower level of the near red.
This has been absorbed by the canopy before the surplus has been reflected back for use by the plants on the floor. This is not just a natural lowering of the light level but an altering of the natural spectrum as well. Plants that have adapted to grow in these conditions will thrive.
Similarly scientists can use this natural switch that plants have by modifying the ratio of Pr to Pfr. If we lower the Pfr more than we do the Pr many plants switch from creating etiolating growth and will become bushier with shorter internodes, more breaks closer to the ground, better colour and excellent rooting. This is being more and more widely used as a natural Growth regulator without using chemicals.
Near Infra-Red is light closest to the P.A.R. but it is not used by the plant. It is generally in the 730 to 1350NMs and is in the heat wavelengths. The clearer the sky and the more direct the sunlight the higher the levels of NIR and IR are. That is why in summer your tunnel gets hotter. Many plants have reflectance to NIR and IR rays and in general the older a leaf the better its NIR reflectance is. However, high temperature also lead to water stress and in extreme situations if a plant cannot take-up enough water its cells will rupture. That is why filters have been developed that are capable of reflecting part of the NIR and IR so that temperatures within the structure are reduced.
Filtering films and the make up
Why Co-extruded is better than Mono-extruded? All quality greenhouse films these days are produced using a co extruded system of at least three layers, even though it appears to be only one. This enables us to put together chemicals which would normally not mix well together by putting complementary compounds in the three different layers. A cross section of a co-extruded film also shows the different molecular structure of the three layers so that it is built-up just like a sheet of plywood. Mono-extrusion films have to be relatively simple compounds and do not have the same strength as the co-extruded films.
Diffusion films scatter the light so that as they enter the greenhouse there are no shadows so that all plants get equal amounts of light. It also overcomes the plants natural tendency to keep its apical bud in full light, because no plant is being shaded from its source of photosynthesis by its neighbour. Diffusion films also transmit a greater percentage of the available light into the structure when the sun’s angle is low. This is because the light is deflected, rather than reflected which is what happens with both glass and clear polythene. This is dramatically better in winter and can make as much as 50% better light transmission under diffused polythene than under glass.
Thermal heat barrier (T.H.B.)
The diffusion additive is normally calcium carbonate which not only diffuses the light but also acts as a reflector to the NIR spectrum of light. Great claims are made about the effects of THB’s but in reality it will make around 10-15% difference to the temperature of the greenhouse on a hot day. Instead of being 400c it will be 350C. Still too hot. However, the air temperature is not the only factor as the leaves and fruit parts of the plant can also be substantially lower than leaves and fruits under direct light films.
Until recently it was not possible to manufacture a UV open film. This allows the plant to receive the same amount of UV radiation as it would if it were growing outside. This is because older UV stabilisers used this blocking effect to create a longer life to the film. New UV stabilisers and antioxidants have overcome this problem and so long life films are now able to be made with this UV open benefit.
Pr/Pfr or Near Red/Far Red filters
One lowers the Near Red (Pr) so that it mimics the light spectrum on a forest floor. The second lowers the Far Red Pfr more than it does the Near Red. This is becoming widely used by growers of bedding plants where it will give, not only a natural dwarfing effect on many subjects without resorting to the use of PGR’s, but because it has turned off the natural switch in the plant to produce massive growth it prolongs the saleable life of a crop. It must also be noted that it will approximately take two weeks longer to have the plant ready for market. Some of the Pr/Pfr filter films used are tinted and therefore change the colour of the film to Green or Blue for example. There is however, a white film available from some manufacturers that merely lowers the overall light level across the whole spectrum. In our experience its light transmission level is down to about 35% within six months of being exposed to sunlight.
Filtering films without any UV stabiliser lasts only a few months, even in the U.K. In the UK the average number of units of Ultra Violet (Kilo Langley’s) is 76Kl. In the Middle East this could be as much as 300Kl’s. UV stabilisers are used because the low level radiation destroys the molecular structure of the filtering molecule, so that it becomes brittle and breaks. So a five year film for Northern Europe will need resistance against 380Kls of UV (76Kl X 5 years).
When films had a natural yellow tint they used a UV stabiliser which was a Class One carcinogenic. If anybody offers you a Nickel Quench stabilised film this uses the carcinogenic stabiliser. A good stabiliser but harmful to the environment. Most films today use a H.A.L.S. (Hindered Amine Light Stabiliser) which work by scavenging the free radicals created by UV light so that they do not harm the film. There are two forms of H.A.L.S. stabilisers with either the Monomeric or Oligomeric molecules. The difference is that the Monomeric molecules are small molecules and migrate out of the film quickly but the Oligomeric with larger molecules migrate slowly. A good filtering film therefore has a complex mixture of the two types so that the film will last for the design life of the film. When thin films are manufactured they need greater amounts of UV stabilisers than a thick film, but no longer do you need a thick film to get a long life.
M.L.L. and Linear Low Density Copolymers
Modern polymer technology has allowed us to manufacture films which are not only thinner, last just as long as thicker films, but are also stronger. Thinner films means that every ton of polymer covers a greater surface area, costs less to manufacture, less to transport, and there is less to reprocess. M.L.L. co polymers are however expensive ingredients so they do not lower the cost of the film, per square metre proportionally with their thickness. How do they work and how common are they? They use a modified form of polymer molecule which has a long, rather than a short, molecule chain. It is this long chain that gives the greater strength. Thinner greenhouse films are gaining favour in Europe and today most greenhouse films in America are now 150 Microns.
Thermal films were the first of the spectral filters. They work by allowing shortwave Infra-Red radiation from the sun to enter the structure. This then heats up all of the items inside the structure, the floor, pots and plants. They radiate this heat back again as long wave infra-red radiation. The film has a filter in it which reflects much of this heat back into the structure again. Many films which use a diffusion additive will be around 55-65% thermal but a real thermal film will give you 85% thermal retention. Films with this level of thermicity will typically give you around 4ºC frost protections or around a 15% energy saving, heating the greenhouse.