TECHNICAL FIELD

The present invention relates to a light pollution screening arrangement or a greenhouse with artificial lightening which reduces the amount of light escaping from a greenhouse, while at the same time providing ventilation of heat and moisture from said greenhouse.

BACKGROUND OF THE INVENTION

The aim of protected cultivation in greenhouses is to modify the natural environment to increase yield, to improve product quality, to conserve resources, to extend production areas and crop cycles among others.

Greenhouse screens are used frequently for energy saving, shading and temperature control. One known type of greenhouse screens comprises a plurality of flexible strips of film material extending in parallel and which by means of a knitting, warp-knitting or weaving process and a yarn system are interconnected to form a continuous product, where the strips form a major part of the surface area of the product. Such a greenhouse screen is known for example through EP 0 109 951 . Other examples of screens of this type are shown in FR 2 071 064, EP 1 342 824 and in WO 2008/091 192.

The strips of flexible material can be of selected materials providing desired properties with respect to reflection and light and heat transmission.

Supplementary lighting during the dark hours is used to increase the production of common greenhouse crops. Light pollution from greenhouses using supplementary lighting is a growing issue around the world. In the Netherlands, a country with an advanced cluster of greenhouses, this has become such a big problem for the society that legislation is in place to limit the amount of light that is allowed to escape the greenhouse during dark hours.

The generally accepted method to prevent the light from escaping from or into a greenhouse, is to use a greenhouse screen. The screen limits the light transmissibility to <1 %. However, greenhouse screens having such low limits for light transmissibility generally also have very limited air permeability and heat will build up causing

temperatures to soar during lighting hours which will create problems for crops grown underneath such screens. Thus, while greenhouse growers are legally obliged to take measures to prevent emission of light from the artificial lighting system used during dark hours of the day, they at the same time have to create an agreeable climate for the crops grown inside the greenhouse with regards to ventilation, humidity and light.

Temperature and humidity can normally be regulated by letting heated air escape through openings in the roof or walls of the greenhouse. However, this is of no use if the greenhouse is provided with a screen with low light transmissibility as gaps or openings created in the screen to allow heated air to escape will inevitably also let light escape through the same openings.

EP1 17401 15 discloses a black out and/or light assimilation screen reducing the amount of light escaping from a greenhouse or preventing the light from entering the greenhouse. This greenhouse screen transmits air while efficiently blocking any light from dissipating through the screen. However, the ventilation through the screen is not sufficient and humidity and heat has a tendency to build up inside the greenhouse resulting in condensation on the screen when the rising hot and humid air reaches the cold screen or walls.

Temperature and moisture control can also be obtained by fans and air passages equipped with light traps. However, fans and fan control systems bring substantial investments to greenhouse owners. Also the forced ventilation requires costly use of energy for operating the fans.

EP2702864 describes a solution for measures regarding light pollution and aeration. However, an advanced structure that requires large amounts of material, special manufacturing techniques and complex structures that is difficult to clean is disclosed. Also the proposed air permeable means are installed close to ground preventing air exchange introduced by wind, or air movements induced by hot lamps. Most importantly it only focuses on the side walls which are a small part of the total surface, lacking windows to ventilate the growing area (at least in Dutch circumstances).

EP0715054A2 also proposes a structure allowing air flow while preventing light passage. This structure provides the same drawbacks as mentioned for the air permeable means of EP2702864.

It is therefore the object to provide a screening arrangement for a greenhouse wherein the light emission is kept within bounds of the regulations and wherein the ventilation of the greenhouse can take place as required. SUMMARY OF THE INVENTION

A first object is to provide a screening arrangement for preventing emission of artificial light from a greenhouse. In practice the term "greenhouse" is intended to mean an agricultural or horticultural greenhouse suitable for cultivating vegetables, fruits, flowers or the like. The screening arrangement as described herein comprises a first retractable screen and a second retractable screen. The retractable screens are provided with retracting means which enables opening and closing of the screens inside the

greenhouse. If artificial lights are required after daylight hours the first and second retractable screens may be installed to prevent emission of artificial light from the greenhouse during dark hours. When the artificial lights have been turned off or in the mornings when the natural daylight is strong enough the screens may be retracted in order to take advantage of the natural daylight. The installment and retraction of the screens may be performed manually or automatically.

The first and second retractable screens each have a longitudinal extension in a direction y and a transverse extension in a direction x. The x- and y-directions are generally perpendicular to each other. Each screen has a longitudinal extension y _s , which during manufacture generally extends in the machine direction and an extension x _s (width) which extends in the transverse (x) direction. Two or more narrow screens may be arranged and connected together side-by-side along their longitudinal extending sides to form a screen with the required width. The screening arrangement has a direction z perpendicular to said x- and y-directions. In the screening arrangement as described herein the first retractable screen is arranged facing the crop growing area and the second retractable screen is arranged facing the ceiling of the greenhouse.

The first and second retractable screens each have one or more light assimilation sections. The light assimilation sections are areas of the screen which are substantially impermeable to light. This means that preferably less than 2 %, and more preferably less than 1 % light will pass through the light assimilation section.

Each light assimilation section has a longitudinal extension y _LA in the y-direction and a transverse extension x _LA in the x-direction analogous with the x- and y-directions of the first and second retractable screens.

The first retractable screen is arranged at a distance d in said z-direction from said second retractable screen. The screening arrangement disclosed herein is

advantageously installed in the upper part of a greenhouse and the first retractable screen is arranged superposed to the second retractable screen with a distance d there between such that a space is formed between the first and second retractable screens. Thus the first and second retractable screens form a double layer of screens separated by a distance d.

In the first and second retractable screens, the light assimilation sections are arranged side by side along their longitudinally extending sides with gaps there between. Thus, in each screen a first light assimilation section is located next to a first gap which in turn is located next to a second light assimilation section, which is located next to a second gap and so on throughout the transverse extension _s of the screen. The number of light assimilation sections and gaps in each screen will depend on the width of each section as discussed below.

The one or more light assimilation sections in said first retractable screen are arranged to partially overlap with one or more light assimilation sections in said second retractable screen. With the term "partially overlap "is meant that parts of the light assimilation section in the first retractable screen overlaps parts of one or more light assimilation sections in said second retractable screen in the x-direction. Thus, a light assimilation section located in a first retractable screen will partially overlap a first light assimilation section located in the second retractable screen. The same light assimilation section located in a first retractable screen will also completely overlap a gap located next to said first light assimilation section, and also partially overlap a second light assimilation section located on the other side of the gap in the second retractable screen.

Heated air from the artificial lights and moisture from the growing crops will rise towards the upper part of the greenhouse and reach the screening arrangement. The screening arrangement as disclosed will allow air and moisture to pass through the gaps between the light assimilation sections in the first retractable screen and enter into the space between the first and second retractable screens. After having entered said space the hot and moist air will flow in the x-direction within said space and thereafter rise and exit through the gaps located in the second retractable screen.

In one alternative screening arrangement the first and second retractable screens consist of only one single light assimilation section each. This means that both of the first and second retractable screens consist of continuous light assimilation sections without any gaps. In this alternative the first retractable screen is pulled from a first side of the greenhouse towards a second side opposite the first side and past the middle of the greenhouse. The second retractable screen is pulled from the second side of the greenhouse and past the middle such that the first retractable screen overlaps the second retractable screen, creating a space with a distance d between said retractable screens.

Heated and moist air will rise towards the screening arrangement, move in the x- or y- direction and escape through the space between the first and second retractable screens. However, the extent of overlap between the first and second retractable screens is important to prevent light from escaping through the screening arrangement. The dissipation of light may be prevented by decreasing the distance d between the two screens to < 40 cm, preferably < 30 cm, more preferably < 20 cm, more preferably < 15 cm. The distance d may in some embodiments equal zero. Due to the "light trapping arrangement" provided by the partially overlapping light assimilation sections in the first and second retractable screens, light will not be able to escape through the screening arrangement. While effectively preventing light from escaping to the surroundings, the screening arrangement as disclosed herein at the same time provides an effective ventilation of the greenhouse allowing hot and moist air to escape therefrom.

In an alternative embodiment, each of said first and second retractable screens may comprise one or more air permeable sections and one or more light assimilation sections, wherein said one or more light assimilation sections in said first retractable screen are arranged to completely overlap with one or more air permeable sections in said second retractable screen.

The air permeable sections are areas of the screen which will allow air as well as moisture to pass freely through the screen in these sections. Each air permeable section has a longitudinal extension y _AP in the y-direction and a transverse extension x _AP in the x- direction analogous with the x- and y-directions of the first and second retractable screens.

The air permeable sections and light assimilation sections are arranged side by side along their longitudinally extending sides in the first and second retractable screens. Thus, in each screen a first light assimilation section is located next to a first air permeable section which in turn is located next to a second light assimilation section, which is located next to a second air permeable section and so on throughout the transverse extension X _s of the screen. The number of light assimilation sections and air permeable sections in each screen will depend on the width of each section as discussed below.

The one or more light assimilation sections in said first retractable screen are arranged to completely overlap with one or more air permeable sections in said second retractable screen. The first retractable screen in the screening arrangement is located in relation to the second retractable screen such that a light assimilation section in the first retractable screen completely overlaps an air permeable section in the second retractable screen. With the term "completely overlap" as used herein is meant that the width (i.e. the transverse extension X _LA of the light assimilation sections located in the first retractable screen is at least as wide or wider than the width (transverse extension X _AP ) of the air permeable sections located in the second retractable screen. The light assimilation sections always extend beyond both longitudinally extending sides of the air permeable sections in the x-direction.

The light assimilation sections of the first retractable screen partially overlap with one or more light assimilation sections in said second retractable screen. With the term "partially overlap" is meant that parts of the light assimilation section in the first retractable screen overlaps parts of one or more light assimilation sections in said second retractable screen in the x-direction. Thus a light assimilation section located in a first retractable screen will partially overlap a first light assimilation section located in the second retractable screen, completely overlap a first air permeable section arranged next to said first light assimilation section, and also partially overlap a second light assimilation section arranged next to said first air permeable section. The partial overlap will be in the x- direction.

Heated air produced from the artificial lights and moisture from the growing crops will rise towards the upper part of the greenhouse and reach the screening arrangement. The screening arrangement as disclosed will allow air and moisture to pass in the z-direction through the air permeable sections in the first retractable screen and enter into the space between the first and second retractable screens. After having entered said space between the first and second retractable screens the hot and moist air will flow in the x- direction in the space and thereafter rise in the z-direction and exit through the air permeable sections located in the second retractable screen. Due to the "light trapping arrangement" provided by the partially overlapping light assimilation sections in the first and second retractable screens, light from the artificial lighting arrangement inside the greenhouse will not be able to escape through the screening arrangement. While effectively preventing light from escaping to the surroundings, the screening arrangement as disclosed herein at the same time provides an effective ventilation of the greenhouse allowing hot and moist air to escape therefrom.

The screening arrangement as described herein will not prevent outside light from entering the greenhouse. For example low set or nearly horizontal solar incident radiation, or lights from surrounding infrastructure such as e.g. cars or houses may enter the greenhouse. However, the screening arrangement works efficiently in a greenhouse environment wherein the artificial light is directed substantially vertical downwards towards the plants and away from the screening arrangement and the artificial lights located in the greenhouse are advantageously provided with downwardly directed reflectors. The extent of overlap between the light assimilation sections and air permeable sections as well as the distance d between the first and second screens will prevent deviant directed artificial light that may arise from reflections onto leafs, floors and other structures present inside the greenhouse.

A further advantage of the screening arrangement as disclosed is that an air exchange is provided in the hottest area of the greenhouse, i.e. right below the second screen in the screening arrangement. By cooling the air in the top section of the greenhouse closest to the screen, the temperature gradient in this section is lowered and thereby the propensity for developing unwanted condensation on the screen is also reduced.

Additionally, the area above the first retractable screen in the screening arrangement, i.e. the space between the first and the second screens is warmed up when hot and moist air from the greenhouse enters this space which results in less condensation on the underside (i.e. the surface facing the growing crops) of the first screen. Also the area above the screening arrangement, i.e. the space between the greenhouse ceiling and the top of the second screen is heated which results in less condensation on the screens. Any moist air which escapes through the screening arrangement may be cooled off against inside of the greenhouse ceiling, recirculated down to the greenhouse base and back into the greenhouse. The moisture from the ventilated air may also be recovered through condensation and reused in irrigation of the plants. In all, the increased airflow around the screens in the upper part of the greenhouse contributes to the elimination of moisture and reduces unwanted condensation on the screens.

In some situations, e.g. on cold evenings or nights, it may instead be advantageous to retain the heat inside the greenhouse. In such situations it is possible to decrease the distance d between the first and second retractable screens to a minimum or to zero. This way the screening arrangement may provide an efficient insulation, protecting the cultivation area inside the greenhouse from the cold outside environment.

Each of said first and second retractable screens comprises strips of film material that are interconnected by a yarn system by means of hosiery, knitting, warp-knitting or weaving as is well known to the person skilled in the art. The strips of film material may be made from polyester, polyethylene or polypropylene, or a combination thereof. The light assimilation sections in said first and second retractable screens comprise strips of film material that are interconnected by said yarn system by means of hosiery, knitting, warp-knitting or weaving and are arranged closely side-by-side and edge to edge to form substantially continuous sections wherein light is prevented from passing through. This means that preferably less than 2% and more preferably less than 1 % light will pass through the screening arrangement.

The yarn system comprises warp threads forming loops or stitches and primarily extending in the longitudinal direction y. The warp threads are connected to one another by weft threads extending across the strips. The light assimilation sections may comprise strips of film material which extend both in the longitudinal and transverse directions in order to reinforce the light assimilation ability.

The air permeable sections in said first and second retractable screens are formed by portions of said yarn system without strips of film material to provide sections through which air as well as moisture is permitted to pass freely. The yarn system comprises warp threads forming loops or stitches and primarily extending in the longitudinal direction, y. The warp threads are connected to one another by weft threads which extend across and between said warp threads.

The air permeable sections may in an alternative screening arrangement contain a few strips of film material to provide stability to the yarn system in the air permeable sections. A few strips of film material in the yarn system also prevents the screens from tangling when bundled up during storage or when they are not in use. However, it is important to limit the number of strips in the air permeable sections to a minimum to not obstruct the free passage of heat and moisture there through.

Alternatively the threads making up the yarn framework may advantageously comprise at least two different components, wherein at least one component may be a thermoplastic polymer yarn component having a softening temperature of between 5-200 ^ lower than the other component, such that individual yarn threads in the yarn system may be thermally bonded to other yarn threads in the yarn system. This thermal bonding of yarn threads will create a stiffer yarn framework which is less prone to tangling.

The thermoplastic polymer yarn component may have a softening temperature of between 7-185°C, preferably of between 10-175 °C lower than the other yarn component. The thermoplastic polymer yarn component may be chosen from polymer materials of the group consisting of polyethylene or copolymers thereof, polypropylenes or copolymers thereof, polyamides, polyesters or copolymers thereof. The thermoplastic polymer yarn component may be incorporated into the yarn framework by intertwining one or more fibers together, wherein at least one of the fibers may comprise said thermoplastic polymer yarn component.

The thermoplastic polymer yarn component may form a coat covering a fiber core material and said fiber core material may have a higher melting temperature than the thermoplastic polymer yarn component. Alternatively the thermoplastic polymer yarn component may form a coat covering at least part of the yarn framework.

Both transverse threads and longitudinal threads may comprise the thermoplastic polymer yarn component. Alternatively only the transverse threads may comprise the

thermoplastic polymer yarn component

Advantageously at least some of the strips in the light assimilation sections comprise a film material in the form of a multilayer film having at least two layers, wherein at least one layer has a light color and at least one layer has a dark color, thereby providing strips having one light shaded surface and one dark shaded surface. Thus, when strips having one light shaded surface and one dark shaded surface are weaved together in sections, light assimilation sections having one light shaded surface and one dark shaded surface are formed.

Advantageously the strips having one light shaded surface and one dark shaded surface are oriented in said light assimilation sections of the first and second retractable screens such that the strips with dark shaded surfaces in the first retractable screen are oriented towards the strips with a dark shaded surface in the second retractable screen. The first and second retractable screens are arranged such that the dark shaded surfaces of the light assimilation sections in both screens are oriented towards each other and face the space in between the first and second retractable screens. The advantage with this arrangement is that artificial light inside the greenhouse which is emitted through the gaps/air permeable sections in the second retractable screen, i.e. the lowest screen in the screening arrangement which is closest to the artificial light and the growing crops, will be absorbed by the dark shaded surfaces of the light assimilation sections facing the space between the first and second retractable screens.

The light shaded surfaces of the strips are advantageously oriented such that in the first retractable screen the light shaded surfaces face the greenhouse ceiling and in the second retractable screen the light shaded surfaces face the growing crops. This way artificial light is reflected back into the greenhouse area from the screening arrangement. By reflecting the light from the screening arrangement the yield will increase by up to 10%, which makes it possible to use fewer light fixtures.

In order to increase the light reflecting capacity of the screening arrangement inside the greenhouse, areas of said one or more light assimilation sections in said first retractable screen which is completely overlapping with one or more air permeable sections in said second retractable screen may advantageously be provided with strips of film material having two light shaded surfaces. If areas of the light assimilation sections in the first retractable screen which overlap air permeable sections in the second retractable screen is provided with strips having two light shaded surfaces, light which passes through the air permeable sections of the second retractable screen may reflect back into the

greenhouse, thereby increasing the lighting capacity of the artificial lights further.

Advantageously said light shaded surface is a reflective surface which reflects light well, such as a white or metallic colored surface. Advantageously the light shaded surface may also comprise between 0.05 and 5 weight-% of a UV-stabilizer, preferably triazine based, such as Tinuvin ^® 1577 from BASF. The film strips having a white coloured surface advantageously comprise the white pigment Ti0 ₂ preferably of rutile type. The white layer comprises polyester and a white pigment in an amount between 5 and 50 weight-% based on the total weight of said white layer. Preferably the amount of white pigment is at least 15 weight-%. Ti0 ₂ gives the best whiteness and UV protection relative to particle loading. At least some of the strips at the reflective surface may advantageously consist of light and/or heat reflecting foil strips, e.g. a low emitting metal foil, preferably an aluminum foil. The foil strips are supported by plastic film strips which advantageously consist of a halogen or phosphorus containing polymer, such as polyvinylchloride (PVC),

polyvinylidenechloride (PVDC), polychlortrifluorethylene (PCTFE), polyvinylfluoride (PVF), polyvinylidenedifluoride (PVDF), fluorinated ethylenepropylene (FEP),

polytetrafluorethylene (PTFE), ethylenetetrafluorethylene (E TFE) or Polyethylene terephthalate (PETP).

The halogen-containing polymeric strips may comprise up 10 wt.%, preferably up to 5 wt.%, of commonly known additives, such as for example processing aids or UV- resistant additives.

At least a portion of the halogen-containing polymeric strips may comprise a light- reflective film or foil laminated to the halogen-containing polymeric strips. The light- reflective film or foil may be a low emitting metal foil, preferably an aluminum foil. Preferably, all of the halogen-containing polymeric strips comprising a light reflective film or foil laminated to the halogen-containing polymeric strip are arranged at one side of the screen such that all the light-reflective films or foils are located on the same surface of the screen. In particular in the first screen the reflective surface will be facing the growth area and the in the second screen the reflective surface will be facing the greenhouse ceiling.

The light reflective film or foil may be laminated to the halogen-containing polymeric strips by any known suitable means, including co-extrusion, extrusion of the halogen-containing polymer onto the light reflective film or foil, or by application of a glue at the interface of the light reflective film or foil and the halogen-containing polymeric strips. Preferably, the light reflective film or foil is laminated to the halogen-containing polymeric strips by application of a glue.

Advantageously said strips having a dark shaded surface are black and may comprise the opacifying agent carbon black. The amount of carbon black is preferably between 0.2 and 10 weight-% based on the total weight of said black layer. Preferably the amount of carbon black is at least 1 weight-%. The black layer may also contain an amount of white pigment, for example Ti0 ₂ . Preferably the amount of white pigment in the black layer is at least 1 weight-%, and not more than 25 weight-%. By adding a white pigment to the black layer the optical density of the layer is increased. The thickness of the black layer is at least 2 μηι, preferably at least 4 μηι.

Screens in greenhouses can be a potential fire hazard, since a fire starting by for example an electrical failure in a lighting arrangement can be spread to the entire greenhouse by the screen causing huge economic damages. The multilayer polyester film may advantageously comprise a flame retardant agent. The flame retardant agent may be a phosphorous-containing flame retardant and the multilayer film may comprise between 1500 and 3500 ppm phosphorous. Advantageously the flame retardant is a co- polymerisable phosphorus-containing flame retardant compound and is selected from compounds of formula (I):

wherein: R is an ester-forming group selected from -COOR ⁴ , -OR ⁵ and -OCOR ⁶ ;

R ² and R ³ are independently selected from halogen atoms, hydrocarbon groups having 1 - 10 carbon atoms and R ;

R ⁴ is a hydrogen atom, a carbonyl group or a hydrocarbon group having 1 -10 carbon atoms which may contain a hydroxyl group or a carboxyl group;

R ⁵ is a hydrogen atom or a hydrocarbon group having 1 -10 carbon atoms which may contain a hydroxyl group or a carboxyl group;

R ⁶ is a hydrocarbon group having 1 -10 carbon atoms which may contain a hydroxyl group or a carboxyl group;

A is a divalent or trivalent hydrocarbon group having 1 -8 carbon atoms;

n1 is 1 or 2; and

n2 and n3 are each 0, 1 , 2, 3 or 4, particularly wherein said compound contains two ester- forming functional groups.

In order to obtain an optimal ventilation of the greenhouse while at the same time preventing light from escaping through the screening arrangement, the size of gaps/air permeable sections as compared to the total area of the screen, the distance between the two retractable screens as well as the extent of partial overlap between the light assimilation sections in the first and second retractable screens have to be carefully tuned.

One factor which influences the ventilation in the greenhouse as well as the light dissipation through the screening arrangement is the total size of the gaps/air permeable sections in relation to the total area of the screen. A greater proportion of gaps/air permeable sections in the screen provides better air flow through the screening arrangement. However, the greater proportion of gaps/air permeable sections in relation to the total screen area, which also means little or no partial overlap between the light assimilation sections in the first and second retractable screens, the greater risk of having light escape through the screening arrangement.

The size of the gaps/air permeable sections may advantageously be expressed as the fraction (in %) of gaps/air permeable area in relation to the total area of the screen. For obvious reasons this fraction must always be less than 50 %, as the total area of light assimilation sections must always be greater than the total area of the gaps/air permeable sections for the light assimilation sections to completely overlap the gaps/air permeable sections. With little or no partial overlap (i.e. a large proportion of air permeable sections in the screen) there is good ventilation but a great risk that light dissipates through the screening arrangement. With a large partial overlap (i.e. a small proportion of air permeable sections in the screen) there is little or no light dissipation, but also decreased ventilation.

The proportion of air permeable sections in the screen is advantageously 5-45%, more preferably 7.5-40% and most preferably 10-30%.

Another factor which greatly influences the air flow and light diffusion through the screening arrangement is the distance between the first and second retractable screens. The more the distance between the two retractable screens is increased, the better the air flow through the screening arrangement becomes. However, with an increasing distance between the screens, the greater risk is for light escaping through the screening arrangement.

This may in turn be compensated by increasing the partial overlap between the light assimilation sections of the first and second retractable screens, i.e. decreasing the area proportion of gaps/air permeable sections in the screen. Large partial overlaps will also decrease the air flow capacity through the screening arrangement. Thus, the ventilation and light dissipation through the screening arrangement is also highly dependent on the relationship between the distance (d) from the first to the second retractable screens and the proportion of gaps/air permeable sections in the screens. Therefore depending on the area proportion of gaps/air permeable sections in the screen the distance (d) between the two screens must be adjusted to enable optimal ventilation and minimal light dissipation through the screening arrangement.

The relationship between the distance d from the first to second retractable screen and the area proportion of air permeable sections in the screen may be expressed as the relative distance (r _d ) as expressed in percent (%). The relative distance r _d equals the distance (d ) divided with the sum of the widths (X _LA and X _P ) of one light assimilation section and one gap/air permeable section x 100. For example in a screening

arrangement with a relative distance r _d of 25%, wherein the width of the light assimilation sections is 200 cm, and the width of the air permeable sections is 40 cm, the distance between the first and second retractable screens is 60 cm. As illustrated in the examples below the distance d is very much dependent on the extensions {X _LA and X _A p) of the light assimilation and air permeable sections. The screening arrangement may advantageously further comprise one or more third retractable screens extending in the z-direction perpendicular to said first and second retractable screens. The side walls of the greenhouse may also need to be covered to avoid light dissipation into the surroundings. One or more third retractable screens may also be required to close off areas of the greenhouse from plants which do not require artificial light after dark.

A further aspect relates to a greenhouse comprising wall surfaces, at least one ceiling surface, at least one crop growing area, at least one artificial light element and the screening arrangement as described herein. The screening arrangement is installed in an upper part of said greenhouse wherein the first retractable screen of the screening arrangement is oriented towards said ceiling surface and said second retractable screen is oriented towards the crop growing area.

One or more artificial light sources are installed at a distance of at least 20 cm from the screening arrangements and facing the crop growing area, or at a distance prescribed by the providers of the greenhouse lights.

The greenhouse may advantageously comprise at least one or more air inlets in a lower part of the greenhouse. In order to improve ventilation inside the greenhouse area parts of the outside walls may be furnished with air permeable devices. Said air permeable devices must be provided light-blocking means in order to block the emission of light through the greenhouse into the surroundings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic view of a greenhouse environment comprising a screening arrangement

Figure 2 is a schematic view of a greenhouse environment comprising an alternative screening arrangement

Figure 3 is a cross sectional view of a screening arrangement

Figure 4 is a schematic view of a screening arrangement comprising light and dark shaded surfaces

Figure 5 is a view disclosing the light assimilation and air permeable sections of a first or second retractable screen in detail Figure 6 is a view of a light assimilation comprising strips extending in both longitudinal and transverse directions

Figure 7 is a schematic view of an alternative screening arrangement comprising light and dark shaded surfaces

Figure 8 is a graph showing possible configurations of the screening arrangement

Figure 9 is detailed view of the graph in figure 8.

DETAILED DESCRIPTION

Figure 1 shows a perspective view of a screening arrangement 1 when installed in a greenhouse furnished with artificial lightening. The screening arrangement 1 according to a first embodiment comprises a first retractable screen 2 and a second retractable screen 3. The retractable means used for installing and retracting the screens inside the greenhouse are not shown in the figures. The first and second retractable screens 2, 3 each have an extension y _s in the longitudinal direction y (which during manufacture generally extends in the machine direction), and an extension x _s in a transverse direction x (see figure 1 ). Two or more screens may be arranged together side-by-side along their longitudinal extending sides to form a screen with the required width. The screening arrangement 1 has a z-direction perpendicular to said x- and y-directions (figure 1 ).

The first and second retractable screens 2, 3 each have one or more light assimilation sections 4a, 4b, which are areas of the screen which are substantially impermeable to light. With the term "substantially impermeable to light" is meant that preferably less than 2 % and more preferably less than 1 % light will pass through the screening arrangement to the outside. Each light assimilation section 4a,b has a longitudinal extension y _LA in the y-direction and a transverse extension x _LA in the x-direction, analogous with the x- and y- directions of the first and second retractable screens 2, 3.

In one embodiment the light assimilation sections 4 are arranged side by side along their longitudinally extending sides with gaps 6 there between as seen in figure 1 . Thus, in each screen 2,3 a first light assimilation section 4a, 4b is located next to a first gap 6a, 6b which in turn is located next to a second light assimilation section 4a, 4b, which is located next to a second gap 6a, 6b and so on throughout the transverse extension X _s of the screen 2, 3.

The one or more light assimilation sections 4a in said first retractable screen 2 are arranged to partially overlap with one or more light assimilation sections 4b in said second retractable screen 3. This means that the light assimilation section 4a located in a first retractable screen 2 will also completely overlap a gap 6b located next to said first light assimilation section 4b, and also partially overlap a second light assimilation section 4b located on the other side of the gap 6b in the second retractable screen 3 (see Figure 1 ). In an alternative embodiment, each of said first and second retractable screens 2, 3 comprise one or more air permeable sections 7 and one or more light assimilation sections 4 (see figure 2). In this embodiment the one or more light assimilation sections 4a in said first retractable screen 2 are arranged to completely overlap with one or more air permeable sections 7b in said second retractable screen 3. Each air permeable section 7a, 7b has a longitudinal extension y _AP in the y-direction and a transverse extension x _AP in the x-direction analogous with the x- and y-directions of the first and second retractable screens 2, 3. The air permeable sections 7a, 7b and light assimilation sections 4a,b are arranged side by side along their longitudinally extending sides in the first and second retractable screens 2, 3.

The first retractable screen 2 is arranged at a distance d in said z-direction from said second retractable screen 3. The screening arrangement 1 is installed in the upper part of the greenhouse and the second retractable screen 3 is arranged superposed a distance d from the first retractable screen 3 such that a space 5 is formed between the first and second retractable screens 2, 3. Thus the first and second retractable screens 2, 3 form a double layer of screens separated by a distance d (see figure 3).

The first retractable screen 2 in the screening arrangement 1 is located in relation to the second retractable screen 3 such that each one of the light assimilation sections 4a in the first retractable screen 2 completely overlaps at least one air permeable section 7b in the second retractable screen 3. The width of the light assimilation section 4a extends past the width of the air permeable section 7b on both longitudinally extending sides in the x- direction. The light assimilation sections 4a of the first retractable screen 2 partially overlap with one or more light assimilation sections 4b in said second retractable screen 3 (see figures 2 and 3).

Heated air produced from the artificial lights and moisture from the growing crops will rise towards the upper part of the greenhouse and reach the screening arrangement 1 (see wavy arrows in figure 4). Air and moisture pass through the gaps/air permeable sections 6a/7a in the first retractable screen and enter into the space 5 between the first and second retractable screens 2, 3. After having entered said space 5 between the first and second retractable screens 2, 3 the hot and moist air will flow in the x-direction in the space 5 and thereafter rise and exit through the gaps/air permeable sections 6b/7b located in the second retractable screen 3.

The light assimilation sections 4a, 4b in said first and second retractable screens 2, 3 comprise strips of film material 8 that are interconnected by a yarn system 9, 10a, 10b by 5 means of hosiery, knitting, warp-knitting or weaving and are arranged closely side-by-side and edge to edge to form a substantially continuous sections wherein light is prevented from passing through (see figure 5).

The light assimilation sections 4a, 4b in said first and second retractable screens 2, 3 comprise a plurality of narrow film strips 8 held together by a yarn framework 9, 10a, 10b.

10 The strips 8 are preferably arranged closely edge to edge, so that they form a

substantially continuous surface. In the figure the distance between the strips 8 has been exaggerated for the sake of clarity to make the yarn framework 9, 10a, 10b visible. The first and second retractable screens 2, 3 have a longitudinal direction, y, and a transverse direction, x, wherein the strips 8 extend in the longitudinal direction y. In the embodiment

15 shown in Fig. 6 there are strips 8 ^' extending also in the transverse direction x. A typical width of the strips is between 2 mm and 10 mm.

In Fig. 5 the strips 8 are interconnected by a warp knitting procedure as described in EP 0 109 951 . The yarn framework 9, 10a, 10b comprises warp threads 9 forming loops or stitches and is primarily extending in the longitudinal direction, y. The warp threads 9 20 are connected to one another by weft threads 10a and 10 b extending across the strips 8.

Fig. 5 shows an example of a mesh pattern for a fabric manufactured through a warp knitting process in which four guide bars are used, one for the strips 8, two for the connection threads 10a and 10b extending transversely to these and one for the longitudinal warp threads 9.

25 In the light assimilation sections the strips 8 are located closely edge to edge. The

longitudinal warp threads 9 are arranged on one side of the screen, the underside, while the transverse connection threads 10a and 10b are located on both sides of the fabric, the upper and the underside. The term "transverse" in this respect is not restricted to a direction perpendicular to the longitudinal direction, but means that the connection threads

30 10a and 10b extend across the strips 9 as illustrated in the drawings. The connections between the longitudinal weft threads and the transverse threads are preferably made on the underside of the fabric. The strips can by that be arranged closely edge to edge without being prevented by the longitudinal weft threads. The longitudinal warp threads 9 in Fig. 5 extend continuously in unbroken fashion along opposite edges of adjacent strips 8, in a series of knitted stitches, in a so called open pillar stitch formation.

The transverse threads 10a and 10b pass above and below the strips 8 at the same place, i.e. opposed to each other, to fixedly trap the strips 8. Each knitted stitch in the longitudinal warp threads 9 has two such transverse threads 10a and 10b engaging with it.

Fig. 6 shows another embodiment of a woven screen as described in US 5,288,545 comprising film strips 8 (warp strips) extending in longitudinal direction, y, and film strips 8 ^' (weft strips) extending in transverse direction x. The strips 8 ^' in the transverse direction may as shown in Fig. 4 always be on the same side of the strips 8 in longitudinal direction or may alternate on the upper and underside of the longitudinal strips 8. The warp and weft strips 8 and 8 ^' are held together by a yarn framework comprising longitudinal and transverse threads.

The air permeable sections 7a,b in said first and second retractable screens 2, 3 are formed by portions of said yarn system 9, 10a, 10b without strips of film material to form sections through which air as well as moisture is permitted to pass freely. The yarn system 9, 10a, 10b comprises warp threads 9 forming loops or stitches and primarily extending in the longitudinal direction, y. The warp threads 9 are connected to one another by weft threads 10a and 10b which extend across and between said warp threads 9 generally in the x-direction (see in figure 5).

The strips 8 may be comprised of a multilayered film material comprising a laminate of two films having different properties or shades, or an aluminum foil + at least one film layer. Advantageously the strips comprise at least two layers wherein one layer may have a light shaded surface 1 1 and the second layer has a dark shaded surface 12. Strips 8 having one light shaded surface 1 1 and one dark shaded surface 12 are weaved together in the light assimilation sections 4a, 4b producing screens 2, 3 having surfaces with two different shades, one light shaded surface 1 1 and one dark shaded surface 12.

To optimize the utilization of artificial light inside the greenhouse the first and second retractable screens 2, 3 are arranged such that the dark shaded surfaces 12 of the light assimilation sections 4a, 4b in both screens 2, 3 are oriented towards each other and face the space 5 in between the first and second retractable screens 2, 3 (see figure 4). The light shaded surfaces 1 1 of the strips 8 are oriented such that in the first retractable screen 2 the light shaded surfaces 1 1 face the growing crops and in the second retractable screen 3 the light shaded surfaces 1 1 face the greenhouse ceiling. This way light is reflected back into the greenhouse area from the screening arrangement 1 (see dotted arrow in figure 4).

In a further embodiment of the screening arrangement 1 (see figure 7), areas of the one or more light assimilation sections 4a in the first retractable screen 2 which is completely overlapping with one or more air permeable sections 7b in said second retractable screen 3 may be provided with strips 8 of film material having two light shaded surfaces 1 1 . In this embodiment the light (see dotted arrow) which passes through the gaps/air permeable sections 6a/7a of the first retractable screen 2 may reflect back into the greenhouse from the light assimilation area 4b in the second retractable screen 3, thereby increasing the lighting capacity of the artificial lights further (see figure 7).

As discussed above the optimal flow of heated and moist air, while at the same time preventing the dissipation of light, is dependent on the fraction of gaps/air permeable sections 6a, 6b/7a, 7b in relation to the total area of the screens 2, 3, as well as the distance d between the first and second retractable screens 2, 3 in the screening arrangement 1 .

The proportion of gaps/air permeable sections (in %) may be calculated by Formula I I :

∑ extension x _AP of all ^ ^a P ^S permeable sections

gaps/airpermeable fraction F _AP =— — : — x 100%

∑ extension x _s of first and second retractable screens

Depending on the fraction of gaps/air permeable sections F _AP in the screening

arrangement 1 the distance d between the first and second retractable screens 2, 3 must be adjusted to achieve optimal ventilation through the screening arrangement 1 .

The optimal distance d from the first to second retractable screens 2, 3 and how it relates to a given fraction of gaps/air permeable sections F _AP in the screen may be found in the following mathematical relationship:

A relative distance (r _d ) as expressed in percent (%) = (the distance d) divided with (the sum of the extension x _LA ) for one light assimilation section 4a, 4b and the extension x _AP for one gap/air permeable section 7a,b) x 1 00. The relative distance r _d may be calculated by Formula I II :

relative distance (r _d ) =— -— x 100%

^X LA ^+X AP

Figure 8 discloses a graph wherein the fraction gaps/air permeable sections F _AP is plotted against the relative distance (r _d ). The same graph also discloses how much light escapes through the screening arrangement 1 for the given values. Thus when the fraction of gaps/air permeable sections F _AP in the screening arrangement is known, the optimal distance d between the first and second retractable screens 2, 3 can be found in the graph.

The screening arrangement 1 may advantageously further comprise one or more third retractable screens 13 extending in the z-direction perpendicular to said first and second retractable screens 2, 3 (see figures 1 and 2) as the side walls of the greenhouse may also need to be covered to avoid light dissipation into the surroundings. One or more third retractable screens 13 may also be required to close off areas of the greenhouse from plants which do not require artificial light after dark (see figures 2 and 3).

Example 1

A screening arrangement 1 comprises a first and a second retractable screen 2, 3. Each screen 2, 3 is approximately 500 cm wide. The first retractable screen 2 comprises fourteen light assimilation sections 4a having an extension X _LA of 29 cm in the x-direction and thirteen gaps/air permeable sections 6a/7a having an extension X _P of 7 cm in the x- direction. The second retractable screen 3 comprises thirteen light assimilation sections 4b having an extension X _LA of 29 cm in the x-direction, and fourteen gaps/air permeable sections 6b/7b having an extension X _AP of 7 cm in the x-direction. The second screen also comprises two light assimilation sections 4b which are located along the two outer edges of the second screen 3. Each edge light assimilation section has an extension X _LA of 1 1 cm in the x-direction (see Table 1 ).

The fraction of gaps/air permeable sections is calculated using formula II, wherein

∑ extensions X _AP of all gaps/air permeable sections = 14 x 7 cm (first retractable screen 2) + 13 x 7 cm (second retractable screen 3) = 189 cm

∑ extensions X _s of first and second retractable screens = 14 x 29 cm + 13 x 7 cm (first retractable screen 2) + 13 x 29 cm + 2 x 1 1 cm + 14 x 7 cm (second retractable screen 3) = 994 cm

Gaps/air permeable fraction F _AP = (189 cm/994 cm) x 100% = 19.0 %

If we want to attain a maximum light transmission of about 1 % we can find from the graph in figure 9 that in a screening arrangement with a gap/air permeable fraction of 19.0% we require a relative distance r _d of about 27%.

From formula II I we calculate the distance d required between the first and second retractable screens 2 and 3 to attain a maximum light transmission of about 1 %. d = r _d (X _LA + X _A p) 1 100% = 27% (29 cm + 7 cm) / 1 00% = 9,7 cm

Thus, in order to attain a light transmission of about 1 % in the screening arrangement having light assembling sections with a width of 29 cm, gaps/air permeable sections with a width of 7 cm and a total air permeable fraction of about 19.04% as described above, the distance between the first and second retractable screen should not be longer than 9.7 cm (see Table 1 ).

Table 1

Example 2

In a different embodiment of the screening arrangement the extensions of the light assimilation sections and gaps/air permeable sections are about twice the width compared to the sections in Example 1 . The total width of the screening arrangement is about the same (see table 1 ). It can be concluded that although the air permeable fraction of the screening arrangement in this embodiment is nearly the same as for the screening arrangement in Example 1 , the maximum distance between he first and second retractable screens can be approximately doubled to 20.8 cm to retain the same maximum light emission of 1 %. Here it is obvious that the widths of the light assimilation sections and gaps/air permeable sections have a great impact on the distance between the two screens. Wider sections allow for a greater distance, while narrow sections require a closer distance between the screens.

Example 3

This screening arrangement is the same as in Example 1 but the maximum light emission permitted is set to 1 .5 %. In table 1 it is seen that the distance between the two screens is increased from 9.7 cm to 13.7 cm when more light is allowed to escape through the screening arrangement.

Example 4

This screening arrangement is the same as in Examples 1 and 3 but the maximum light emission permitted is set to 0.5 %. In table 1 it is seen that the maximum distance allowed between the two screens is 8.6 cm when only 0.5% light emission is permitted.