Switchable optical filters: the future of greenhouse covering materials?

Switchable optical filters: the future of greenhouse covering materials?

One of the main tasks of the Smart Materials project was completed last year. Greenhouse simulation models were used to explore the potential effect of smart covering materials with switchable optical filters on microclimate, use of resources and crop performance.

Until recently, the only way to modify the amount and quality of light reaching the crop in a greenhouse was to use temporary coatings or screens. Three main groups of filters were identified and simulated: filters reflecting both PAR and NIR radiation, filters selectively reflecting only the NIR part, and filters reflecting the FIR spectrum.

Different climate regions and types of coverings were analysed to factor in a range of scenarios. The results highlight a significant potential improvement in microclimate and yield associated with the use of switchable optical filters for all the analysed climates, even if the optical properties are less than ideal.

For some of the simulated filters there are other alternatives available which perform equally well (thermal/energy saving screens). Further research is needed to analyse the technical and economic feasibility of these theoretical filters.

New LED lamp rates highly in hybrid lighting trial

New LED lamp rates highly in hybrid lighting trial

A trial with hybrid lighting (SON-T + LED) at Dutch tomato nursery Gebroeders Koot has yielded good results. The LED lamp used in the trial, which was developed on British soil with Dutch input, offers several advantages. One stand-out benefit is its clever design which makes it easy to integrate into existing SON-T installations.

Yields up by more than nine percent after seven months (weeks 48-26). That was the auspicious outcome of a greenhouse trial at Prominent growers Gebroeders Koot in Poeldijk, the Netherlands, where a tomato crop grown under 150 μmol/m2/sec SON-T grow light was compared with an identical crop supplemented with 58 μmol deep red with a little blue LED light. Geert Koot, who had had no previous experience in growing under grow light, was very impressed. “I hadn’t expected the higher light level to make such a difference,” he says. “That will appeal to a lot of growers. The same goes for the lamp itself, which has a surprisingly simple design. It’s fully interchangeable with SON-T, so it fits seamlessly into an existing system.”
“A lot of thought has gone into the functional design,” cultivation specialist Maarten Klein adds. He and his assistant, Tim Valstar, oversaw the trial, which was run on behalf of the British LED manufacturer Plessey. Klein, who has had a lot of experience with grow light, developed this lamp in collaboration with the technology company.

Smarter design

“Most LED systems are difficult if not impossible to integrate into existing lighting installations,” Klein continues. “Growers looking to switch to hybrid lighting currently have to install a whole new system alongside their existing one, often with extra C profiles. That pushes up the cost and results in more light interception, which causes problems all year round. Plessey Semiconductors in Plymouth wanted to eliminate these problems.”
To test the practical value of the lamp in the greenhouse setting, Klein approached several Dutch nurseries. In addition to Gebroeders Koot, trial setups were installed at nearby alstroemeria and gerbera growers and a pot plant nursery.

Trial setup

Although Gebroeders Koot were not growing tomatoes under artificial lighting, they did have a SON-T system in place in a section that had previously been let to another grower. These 1000W lamps supplied 151 μmol/m2/s extra grow light and, of course, the usual radiated heat. LED lamps were added in one bay, ramping up the artificial light level to 209 μmol.
Tim Valstar assisted with the trial and, together with Geert Koot, took measurements in the trial and reference sections. All the relevant crop and fruit features of the variety grown, Brioso, were recorded, varying from growth rate and stem thickness to leaf size, leaf colour, fruit weight and Brix value.


The plants arrived in the greenhouse in week 46. “That’s later than the usual for an artificially lit Brioso crop – they would usually go in in mid-October – but the lighting period was long enough to get a reliable impression of any differences,” Koot says. “The plants developed well in both light environments. But the plants under the higher light level were that little bit stronger with slightly thicker stems and more dark green leaves.”
Due to the extra vigour, the plants under the hybrid lighting regime held the first trusses for longer and they were harvested a few days later than those in the reference sections. The higher yield potential quickly expressed itself in a higher average fruit weight. To maintain the desired fineness, one fruit more was kept on the truss (11 instead of 10) from the tenth truss onwards, without the plants forfeiting vigour.
Valstar: “After week 26 we stopped taking measurements and were able to take stock.” The harvest under the hybrid lighting regime was 38.32 kg per m2 compared with 35.04 kg under SON-T. That represents an increase in yield of 9.35%. The average fruit weight was also slightly higher than under SON-T, at 39.2 grams compared with 38.8 grams.

Flexible use

The attractive increase in yield can’t be ascribed solely to the higher light levels in the periods when both systems were in use. The SON-T system was switched off and the CHP unit shut down for maintenance at the beginning of week 19, whereas the LED system was used from 4 am to 7 am for a further three weeks.
“The option to only use the LED lamps either end of the lighting season would be an extra benefit,” Klein says. “Those are often the times when you don’t need the radiated heat produced by the SON-T lamps. LEDs have virtually no impact on the climate. You can always switch them on if you need more grow light. And because they are much more energy-efficient than SON-T lamps, you also have more flexibility when it comes to deciding whether to generate the energy yourself with CHP.”

375 and 600W

Klein is keen to point out that the prototype trialled at Gebroeders Koot was developed exclusively for research purposes. But the lamp has since undergone further development and a commercial 375W version was launched at IPM 2017. All the LEDs are now in one bay and the fitting, which has integrated cooling ribs, can be attached directly to the trellis.
The lamp is called Hyperion 1000 because it has a photon flux of 1000 μmol/s. “Because of the higher uptake of deep red light, it’s the equivalent of a 600W SON-T lamp but it uses 40 percent less electricity,” the cultivation specialist says. “The producer has also recently brought out a more powerful 600W version which is the equivalent of a 1000W SON-T lamp.”

Ten years ago

There is a lot of added value in the new lamp, Koot believes. “It’s efficient, it has a broad spectrum, and its clever design makes it easy to incorporate into an existing system. That will appeal to a lot of growers. I’m also quite impressed. But because of my age and the fact that I have no successor in place, I have decided not to invest in any more grow lights now. If this trial had taken place ten years ago, I would almost certainly have gone for them. But we very much enjoyed taking part in the trial.”


A new type of LED lamp produced in the UK is achieving interesting results. The clever design makes the lamp particularly attractive. It can be attached to the trellis without the use of C profiles and can be integrated into existing 600W SON-T systems with standard connectors. A more powerful version equivalent to a 1000W SON-T lamp was brought out earlier this year.

Text and images: Jan van Staalduinen.


Dynamic simulator of solar and outdoor light conditions

Dynamic simulator of solar and outdoor light conditions

Light plays an important role in demanding crop science research applications. A lot of research requires accurate replication of real-time outdoor light conditions to achieve different goals.

But it is impossible to replicate those conditions to the extent to which data collected indoors would be relevant. LightDNA was created to solve this problem. The system consists of Valoya’s latest technology: the 8-Channel Light, a high-power LED fixture with eight channels of light, an internet connected microcomputer and both local and cloud-based software for processing the light data.

Optimised LED configuration

The configuration of LEDs is optimised to meet outdoor light conditions with 90% or higher accuracy (380-780 nm range).
Other advantages are: light-weight, compact design suitable for various applications; strong light intensity up to 2000 µmol/m2/s with uniform light output; low heat emission with active cooling system and energy saving.
Stand number: 12.424


Right light spectrum on bottom layer with half the energy

Right light spectrum on bottom layer with half the energy

The LED lamps in the light fittings underneath the top growing layer shine brightly on the plants in the cultivation greenhouse at phalaenopsis growers De Vreede in Bleiswijk in the west of the Netherlands. The light may look white but actually it’s the right combination of colours. It’s one of the innovations that brothers Herman and John de Vreede are working on as part of their drive to supply large volumes of uniform quality orchids more sustainably. They did most of the preliminary research into the right light spectrum themselves.

The phalaenopsis nursery moved to Bleiswijk in 1995. The brothers soon bought the nursery next door and then another two sites 300 metres and 2 kilometres away, making a total of 12.5 hectares of growing space. Each of the sites is equipped for a specific purpose.
The cultivation greenhouse, where the plants spend their first 35 weeks, is heated to a temperature of 28ºC. Then they move to the spike induction site, where they stay until about week 55. Here the plants start off warm and after a few weeks the temperature is reduced to 19ºC to induce flowering. In this phase, the plants are spaced wider apart, staked and sorted by flower size, colour and number of buds. Finally, they are transferred to the finishing site for three to four weeks. Orders are packed and shipped from there.

Large volumes

De Vreede produces 12 million plants per year. Even Herman de Vreede finds it hard to get his head around those numbers. A massive 200,000 young tissue culture plants arrive from various locations every week and leave the nursery again as adult plants more than a year later.
De Vreede specialises in eight outstanding orchids – exclusive varieties with a long life span and offering great value for money. They come from two breeders, with most of their stock supplied by Anthura. “We test about 30 varieties a year, including from other breeders. We want to keep up with the latest innovations.”
The brothers work with large volumes. “We are equipped to fulfil orders of 500,000 units at a time. The biggest challenge for us is getting all the plants to the same stage at the right time. Much of what we do is automated now. Soon we plan to install industrial Fanuc robots which will enable us to respond even more efficiently to market demand.”

Sustainable lighting solution

Orders arrive in peaks. “We supply more than half of our annual production in the first five months of the year,” de Vreede says. “There are a lot of special occasions like Women’s Day and Mother’s Day at that time of year. To accommodate peak production we decided to install a second growing layer above part of the cultivation greenhouse. We now have four hectares of growing space there instead of three. That helps make the crop more sustainable to grow because we’re maximising our space.”
It wasn’t practical to install a second growing layer directly above the original one, either in terms of climate or air circulation. So the brothers decided to put in a second layer along the sides of the three cultivation areas. It is relatively low, just 1.5 metres above the bottom layer. Lighting is needed to make up for the lack of daylight. The standard lighting with SON-T lamps used elsewhere in the nursery can’t be used here.
“There are SON-T lights above this part, but with 600W output, slightly less than the 1000W from the other lamps we use,” Herman de Vreede says. “We went with LED grow lights for the bottom layer. Not only because they generate less heat, but also because they are a sustainable solution. They use less energy and you can choose a particular combination of light colours.”

Three years of tests

At the time there was no such thing as a standard solution. So before they started building in October 2016, they ran tests over a three-year period to see which light spectrum produced the best results. “We tested the effect of different light spectra on properties such as development rate, root development and the hardiness of the plant, both inside and outside the nursery. A lot of knowledge is needed for that, as you have to see what the best result is for each situation. The light spectrum that is most suitable for the vegetative phase of phalaenopsis is not necessarily the right one for the spike induction phase, for example.”
The tests in the nursery were overseen by Simone de Vreede, who had gained a lot of experience in this area and carried out research at her parents’ nursery while still at university. Once they had decided on the light spectrum they wanted, the next step was to find out where to source the lights from. Ultimately they chose Philips GreenPower LED top lighting, which fitted the bill nicely. The lights give out light that looks white. The advantage of this is that it makes it easier to visually inspect the plants being grown in the greenhouse.

More stable climate

“Installing a second growing layer blocked out the daylight from the bottom layer,” says Stefan Hendriks of Philips. “They couldn’t use SON-T because of the short distance between the crop and the lamps: they would generate too much heat. With LED you can create a controllable climate in which phalaenopsis can be grown very efficiently with relatively little light.”
Since the second growing layer was installed in October 2016, the plant specialist has been visiting the nursery every two weeks to carry out analyses and take crop measurements, including length, leaf splitting and dry matter concentrations. In addition, the climate is intensively monitored by means of PAR, temperature and humidity sensors. These observations are linked to the climate data from the computer. “Based on this data, we want to fine-tune the use of the lamps and optimise our cultivation even further. Experience and knowledge are essential when using LEDs. That’s why we carry out a lot of in-depth analyses here,” says Hendriks.


The phalaenopsis grower is also considering buying in LED lights for the other sections when the time comes to replace the SON-T lamps there. Hendriks adds: “Besides being more energy-efficient, LEDs last longer. The life span of the models we use is given as L90. That means that after 25,000 hours of operation, the light output is still 90% of the original level. But the module will still go on working fine after that and will have many burning hours left in it.”
At De Vreede the lamps will probably wear out sooner than that, due to the number of hours they operate. With 14 hours of lighting a day, they are in use for 5,110 hours a year. But that also means that the LED lighting in the new no-daylight situation will pay for itself more quickly.


Dutch phalaenopsis growers De Vreede have 12.5 hectares divided into cultivation, spike induction and finishing sites. In order to have enough growing space available at peak times, they invested in a second growing layer above part of their cultivation area. To light the bottom layer, now in shade, they installed LED lighting with the right light spectrum for the vegetative phase, having first done their own in-situ research into which spectrum to use.

Text and images: Marleen Arkesteijn.


Radiation monitor improves understanding of plant processes

Radiation monitor improves understanding of plant processes

A new online app quantifies the effect of radiated heat loss on crop temperature and energy loss from the greenhouse in a simple, user-friendly way. The radiation monitor is a handy tool for growers who want to get a better understanding of aspects such as the use of screens and greenhouse cover materials. More knowledge of physical and phytophysiological processes in the greenhouse and the crop can help the grower produce even better results.

Anyone with a PC can use the radiation monitor. The app was launched recently, and Aat Dijkshoorn, Next Generation Growing (NGG) project manager in the Netherlands, is very happy with the result. “This program makes it easy to calculate the effects of screening on energy consumption and vertical temperature distribution. It helps growers take concrete decisions – such as whether or not to close the screens tonight – and supports the trend towards energy-efficient growing.”

Know your temperatures

Knowing the plant temperature helps the grower grow more efficiently and accurately. As crop adviser Peter Klapwijk recently put it on HortiNext: “When I talk to growers about their climate strategy, I often realise that they still see the greenhouse air temperature as the most important reference variable. Many of them understand the importance of plant temperature but dismiss it because it’s ‘so difficult to measure reliably’. So people don’t tend to pay much attention to it. But this is a misconception because it is essentially the temperature of the plant that determines the crop’s growth rate and how it is steered.”
Measuring plant temperature is by no means easy, Dijkshoorn admits. Temperature is a result of all the energy flows that occur inside and outside the greenhouse. A simple sensor unit or thermal camera will only capture part of all that data. Then there’s the problem that the equipment needs to be incredibly accurate to register the differences, which are often only a matter of decimal places. I’m convinced that the radiation monitor makes that a thing of the past as well. The simulation model gives an excellent picture of cause and effect, which makes the plant temperature much easier to steer accurately.”

Relative humidity and transpiration

It’s a well-known fact that there is a link between screening and temperature, and the use of screens has risen substantially in recent years as NGG gains in popularity. So it’s no surprise that all kinds of initiatives are being launched to attempt to shed more light on this relationship. The radiation monitor does that very well.
The program was devised by Wageningen University & Research in the Netherlands. Researcher/developer Feije de Zwart understands exactly what lay behind this assignment. “The fact is that many growers are still reluctant to use screens intensively and will only close their screens if the difference between the indoor and outdoor temperature is more than 10°, for example. They understand straight away that a screen saves energy but what they often don’t realise is that it can also bring about more homogeneous vertical temperature distribution in the greenhouse. Many growers mainly see screening as a way of increasing relative humidity. And that can be risky. After all, the more humid the air in the greenhouse is, the lower the difference in vapour pressure between the crop and the greenhouse air will be and the less the crop will transpire.”

Screening is good

Time and again, practical experience shows that intensive use of screening is not necessarily detrimental to crop quality and production. The same conclusion was reached in the “Transpiration at the head” study. This study revealed that reducing radiated heat loss by screening substantially increases the temperature at the head of the crop, leading in turn to higher levels of transpiration at the head. Intensive screening can limit transpiration from the crop as a whole but, conversely, stimulates it from the head of the crop. Increasing understanding of this phenomenon by explaining the theory, demonstrating measurements in practice and producing a software tool that quantifies the various effects could help raise awareness of the importance of screening even further.
“That’s exactly what the radiation monitor does,” Dijkshoorn points out. “The application produces the numbers to back up what we have been seeing in practice for some time, namely that the use of screens not only helps save energy but also benefits the crop. It tells you exactly when you can expect to save energy. As I said before, with this model at their fingertips, growers can optimise their use of screens even further.”

Input screen

The radiation monitor calculates the energy balance (the sum of the incoming and outgoing energy flows) based on a large number of relevant parameters. De Zwart: “As everyone knows, the most important parameters are the outdoor and greenhouse air conditions, the greenhouse envelope, the number and type of screens, the crop and any lighting used. The physical properties of the greenhouse envelope, screens and lighting then determine exactly what the energy balance will be. When we wrote the program we decided to show these parameters in the extended help document but without making them editable.”
Other properties can be configured by selecting different greenhouse roofs, screens, crops or lighting systems, but not by changing the parameters at user level. “This way we can guarantee that only realistic parameters are used. The model then calculates the temperatures at various crop heights and, where applicable, at projecting parts of the plant, such as the flowers on gerbera, for example. The app also displays the energy consumption and light intensity at crop height. That is relevant when lighting or transparent screens are used during the day.”

Comparing scenarios

According to Dijkshoorn, the application really comes into its own when different scenarios are compared. “Then you can select a scenario with the screens open and compare it with a scenario with one or two screens closed, for example. But it is important to choose realistic values for the greenhouse climate, especially as the RH in the greenhouse can change,” he says. Kas als Energiebron, one of the initiators of the project, envisages even more uses for the model and would like to see it extended to include more selection options. “For example, at the moment you can only choose between a closed screen or an open one,” Dijkshoorn explains. “But in practice some growers work with screens closed 80% of the way. How does leaving these gaps impact on the temperature? These are aspects that the model can fine tune even further.”
The user-friendly radiation monitor app is already available to growers. It uses a minimal number of input fields: just enough to produce useful calculations of the effect of screening while offering the user plenty of scope for selecting starting points they can recognise. Detailed instructions for use are available on the website and an instruction video is currently being produced. The people behind the app also hope to encourage growers to use the app in workshops, information sessions and through the Next Generation Growing course.


The radiation monitor displays the realistic effects of greenhouse roofs, screens, lighting and other user settings on temperature distribution in the crop and light intensity at the head of the crop. The monitor also shows temperatures on the surface of the screen and the greenhouse roof. All this gives the grower a better understanding of the effect of screening. The program is an internet application that can be operated on the PC.

Text: Jojanneke Rodenburg. Images: Leo Duijvestijn and Jan van Staalduinen.