A plant lighting device arranged to be positioned above the plant tray so as to illuminate the plants in the plant tray.

Abstract

A plant lighting device (2) arranged to be positioned above a plant tray (4) to illuminate a plant (5) therein, the device comprising a reflector (12) and a light source (6), the light source being arranged to emit light directed towards the reflector, the light source and the reflector being positioned such that light emitted by the light source is reflected by the reflector towards the plant tray, both the reflector and the light source being elongated along a length direction (I), the reflector having a shape in a cross-sectional plane perpendicular to the length direction, the shape being such that the reflector is elongated along a central portion (12) and the light source is elongated along a length direction (I) and the reflector has ...). a central portion having a reflector section (13) and two peripheral portions (14) adjacent to the central portion, the shape of which is symmetrical with respect to an axis of symmetry extending through the central portion along a height direction (h) perpendicular to the length direction, the central portion having a protrusion (17) on the axis of symmetry, the protrusion protruding downward along the height direction toward the light source, the central portion of the reflector being positioned to receive light from the light source and reflect the received light toward the peripheral portions of the reflector, and the peripheral portions of the reflector being positioned to reflect the light received from the central portion toward the plant tray.

Description

[Technical field]

[0001] The present invention relates to a plant lighting device arranged to be positioned above a plant tray so as to illuminate a plant in the plant tray, as well as a plant growing system comprising such a plant lighting device. The present invention also relates to a method for making such a plant growing system. [Background technology]

[0002] Plant lighting devices for illuminating plant trays have been developed in the prior art. The plant tray comprises a planting surface for receiving plants. For example, plant lighting devices for illuminating plant trays are known from. The planting surface is a surface with a length and width direction perpendicular to each other, i.e. the planting surface is perpendicular to the height direction. The plant lighting device comprises a light source that is long in the length direction. In FIG. 4 of EP 3772897, four parallel light sources are provided, each long along the length direction. In the known device, the light source directly illuminates the plants in the plant tray. This had the disadvantage that the irradiance of the light reaching the plants in the plant tray depends on the position of the plant in the plant tray, i.e. the plant directly below the light source receives light with a higher irradiance than the plants at a different position in the width direction. However, it has been found that the irradiance of the light affects the growth of the plants, and thus conventional plant lighting devices have the problem that the plants in the plant tray are not illuminated with a uniform irradiance, resulting in non-uniform plant growth. A known solution in the state of the art is to increase the number of parallel long light sources per unit length along the width direction to improve the uniformity of the irradiance, however this leads to high capital and costs. Summary of the Invention

[0003] The present invention provides a solution to a problem encountered in the state of the art. To that end, the present invention provides a plant lighting device according to the first claim, which is arranged to be positioned above a plant tray so as to illuminate the plants in the plant tray. In use, the plant tray comprises a planting surface for receiving a plant. The planting surface is a surface with length and width directions perpendicular to each other, i.e. the planting surface is perpendicular to the height direction. Usually, the plant tray is positioned horizontally on the floor or hung horizontally from the ceiling, and the height direction is along the gravitational acceleration vector. However, sometimes the plant tray is mounted on a wall, and the height direction is perpendicular to the gravitational acceleration vector. The term "top" or "upper" in the present invention corresponds to a position or element farther from the plant tray along the height direction relative to the position or element represented by the term "lower" or "lower". "Up" and "down" mean the directions "from lower position to upper position" and "from upper position to lower position", respectively. The plant lighting device comprises a light source that is long in the length direction. In this patent application, the long direction of the light source determines the length direction of the lighting device. The plant lighting device further comprises a reflector in addition to the light source mentioned above. The reflector is positioned above the light source. The light source is arranged to emit light directed at least to the reflector, and preferably only to the reflector, i.e. the light source does not directly illuminate the plant tray. The light source and the reflector are positioned such that the light emitted by the light source is reflected by the reflector towards the plant tray. In other words, in use, the light source is positioned between the reflector and the plant tray. Both the reflector and the light source are long along the length direction, and the reflector has a shape in a cross-sectional plane perpendicular to the length direction, the shape comprising a central portion and two peripheral portions adjacent to the central portion. The shape is symmetrical with respect to an axis of symmetry extending through the central portion along the height direction. Assuming that the reflector is long in the length direction, the reflector is symmetrical along a plane of symmetry that includes the axis of symmetry and extends along the height and length directions. The central portion comprises a protrusion at the axis of symmetry, which protrudes downward along the height direction towards the light source. Preferably, the light source is provided adjacent the protrusion along the width direction, ie directly below or in close proximity to the protrusion.The central portion of the reflector is arranged to receive light from the light source and to reflect the received light towards the peripheral portion of the reflector. The peripheral portion of the reflector is arranged to reflect the light received from the central portion towards the plant tray. The reflector has the advantage that a uniform light irradiance can be obtained across the planting surface (i.e. the uniformity of the irradiance can be improved). In fact, the light is spread across a wide width by the reflector, so that it is no longer concentrated at a position directly below the light source, as is the case in the prior art. The present invention has the additional advantage that the power supplied to the light source can be reduced. In fact, in the prior art, the light source is overpowered so that the minimum amount of light required reaches the plants at the periphery of the plant tray. The present invention has the additional advantage that a smaller number of light sources need to be provided per unit length along the width direction. The distance between the light sources is, for example, between 140 cm and 240 cm. The reduction in the number of lighting devices is accompanied by a reduction in the amount of water cooling required, a reduction in installation costs, a reduction in the amount of material required to build the plant cultivation system, etc.

[0004] According to one embodiment of the invention, the peripheral portion is arranged to reflect light in a partially or fully diffuse manner, i.e. as opposed to a fully specular manner. This has the advantage of improving the uniformity of the light irradiance at the planting surface (i.e. improving the uniformity of the irradiance), e.g. providing the plants with diffuse light which promotes plant growth by reducing the shadowing effect between plants or plant parts. Preferred implementation details regarding the amount of diffuse reflectance of the peripheral portion are given further below.

[0005] According to one embodiment of the present invention, the protrusion divides the reflector into two adjacent concave elements.

[0006] According to one embodiment of the invention, the central portion on one side of the axis of symmetry includes an approximated parabola that terminates on the axis of symmetry to form a protrusion of the central portion of the reflector. For completeness, due to symmetry, the central portion on the other side of the axis of symmetry also includes an approximated parabola that terminates on the axis of symmetry to form a protrusion of the central portion of the reflector. This creates a parabolic reflector that has the advantage of improving the uniformity of the light irradiance across the planting surface (i.e. improving irradiance uniformity). According to one embodiment of the invention, the approximated parabola is a piecewise linear approximation of a parabola. This embodiment facilitates the manufacture of the reflector. Preferably, the piecewise linear approximation includes at least three linear sections. Alternatively, the approximated parabola is a parabola, i.e. forms a smooth parabolic curve.

[0007] According to one embodiment of the invention, the reflector has a width direction perpendicular to the height direction and the length direction, and the central portion of the reflector is a portion of the reflector shaped such that the height along the height direction increases with increasing distance from the apex of the protrusion along the width direction. According to one embodiment of the invention, the reflector has a width direction perpendicular to the height direction and the length direction, and the peripheral portion of the reflector is a substantially flat portion of the reflector or the peripheral portion of the reflector is a portion of the reflector shaped such that the height along the height direction increases with increasing distance from the apex of the protrusion along the width direction.

[0008] According to one embodiment of the invention, the reflector comprises separate parts, namely a central part provided with a central portion and a ceiling part provided with two peripheral parts, preferably made of different materials.

[0009] It has been found that, depending on the type of plant, certain wavelengths of light are optimally absorbed by the plant, while other wavelengths are simply reflected by the plant. Furthermore, it has been found that a certain combination of wavelengths of light, i.e. a certain spectral mixture of light, a certain mixture depending on the type of plant, can optimally grow the plant. Thus, the inventors have found that the light source of the plant lighting device can be adapted to optimally grow a specific plant type. One implementation method involves implementing the light source as a single long light strip with lighting elements, such as LEDs, at least two of which are arranged to emit light in different wavelength ranges, for example, one LED arranged to emit mainly blue light and another LED arranged to emit mainly red light. To optimally grow a specific plant type, it is only necessary to provide a long light strip that includes the correct amount of all kinds of lighting elements, i.e. arranged to emit a specific wavelength range. However, the inventors have also found that providing a single long light strip with different types of lighting elements has the disadvantage that providing such a single strip significantly complicates the design of the light strip PCB ("printed circuit board"). After all, due to the very large surface that can be illuminated with one plant lighting device and the large amount of power desired from the light source, a large number of lighting elements, such as LED diodes, must be arranged on a small PCB strip. Thus, according to a second preferred implementation, the light source comprises a plurality of parallel long light strips, i.e. long light strips along the length direction, on each side of the axis of symmetry. The plurality of long light strips are arranged adjacent to each other along a width direction perpendicular to the height direction and the length direction. At least two of the plurality of long light strips on each side of the axis of symmetry are arranged to emit light in different wavelength ranges, i.e. with different spectral distributions, for example centered around different dominant wavelengths. Thus, each light strip comprises the same lighting elements, e.g. the same type of LEDs, all arranged to emit light in the same wavelength range. This greatly facilitates the design of the PCB driving the light strips. Preferably, the light source is arranged below the protrusion, adjacent to the protrusion along the width direction, i.e. directly below or close to the protrusion.If the light source comprises a single light strip, the light strip is preferably located directly below the protrusion. If the light source comprises multiple light strips, the light strip is preferably located below and close to the protrusion along the width direction. The multiple light strips are, for example, concentrated around the protrusion on a plane located below the protrusion. However, the closer the lighting element is located to the axis of symmetry, the more sensitive the light distribution is to the exact positioning of the lighting element relative to the axis of symmetry. Therefore, the light strip is preferably spaced apart from the axis of symmetry along the width direction by a distance between 1 and 10 centimeters, for example between 1 and 5 centimeters. Preferably, the multiple light strips are located directly below the central portion and not directly below the peripheral portion.

[0010] According to one embodiment of the invention, the light sources are arranged to emit PAR light. According to one embodiment of the invention, at least one of the long light strips on each side of the axis of symmetry is arranged to emit mainly blue light, and at least one of the long light strips on each side of the axis of symmetry is arranged to emit mainly red light.

[0011] According to one embodiment of the invention, the long light strip is a strip of LEDs. Preferably, the LEDs are cooled by a cooling system such as that described in EP 3772897, which is incorporated herein by reference.

[0012] The inventors have found that it is important not only to have a substantially uniform irradiance across the planting surface (i.e. high irradiance uniformity) so that all plants receive the same stimulus for growth, but also to have a substantially uniform spectral mix of light on the planting surface (i.e. high spectral uniformity), i.e. it is beneficial if all plants in the plant tray receive light with the same spectral mix. If a single long light strip with different types of lighting elements as described above is positioned on the axis of symmetry, the light reflected by the reflector has a substantially uniform spectral mix in the planting surface below the reflector, i.e. independent of the position along the width direction. However, according to a preferred form of the invention as described above, the different light strips are spaced apart from each other along the width direction. The result of this is that the light coming from the different light strips is reflected differently by the reflector towards the plant tray, i.e. depending on the position of the light strip along the width direction, the light rays of the light strip are reflected by the central part of the reflector towards different positions of the peripheral part of the reflector and thus towards different positions along the width direction of the plant tray. This results in a reduced irradiance uniformity of the light at the planting surface and a reduced spectral uniformity. To solve this problem, both the central and peripheral portions of the reflector are arranged to diffusely reflect the light from the light source, i.e. they are both not perfectly specular. Furthermore, the diffuse reflectance, i.e. the degree of diffuse reflection, of the peripheral portion is selected to be higher than the diffuse reflectance of the central portion. Preferably, the central portion is arranged to reflect the light in a partially diffuse manner. Preferably, the peripheral portion is arranged to reflect the light in a substantially completely diffuse manner.

[0013] The optical properties of a material determine how light behaves when it strikes it. The most important optical properties are light transmission, absorption, and reflection. Preferably, the total integrated scattering (i.e., total reflection) of the central part of the reflector and / or the peripheral part in the case of the reflector is greater than 90%, preferably greater than 95%. This ensures that losses are minimized. Reflection determines at what angle light is reflected when it strikes the material from a given angle. A perfectly specular material reflects light to the normal of the material's surface at the same angle of incidence to the normal of the material's surface (i.e., the light is mirrored around the normal of the surface of the object it strikes). A perfectly diffuse material scatters the incident light equally in all directions, regardless of the angle of incidence to the normal of the material's surface. The degree of scattering can also be anywhere between these two extremes (i.e., between perfect diffusion and perfect mirror). In this case, after interaction with the reflective material, the light remains somewhat bunched around the reflection direction obtained with perfect specular reflection. This phenomenon is illustrated in Figures 6a, 6b and 6c, which show perfectly specular, perfectly diffuse and partially diffuse reflection respectively. The figures show a plane containing the incident ray and the surface normal.

[0014] In a first optional implementation, the degree of light scattering in partial diffuse reflection can be expressed by the full width at half maximum (bidirectional reflectance distribution function) of the BRDF around the reflection direction obtained in perfect specular reflection (also called the "specular reflection direction"). The maximum BRDF value is obtained in the specular reflection direction. Preferably, the BRDF model is symmetric. Preferably, the full width at half maximum of the three-dimensional BRDF is evaluated in a plane containing the incident ray and the surface normal, preferably perpendicular to the length of the reflector. Preferably, the material in the central part of the reflector exhibits an optimal diffusivity that can be expressed as having a full width at half maximum in the range of 20° to 120°, preferably in the range of 30° to 90°, more preferably in the range of 40° to 60°. It should be noted that these values ​​are the full width at half maximum scattering angles obtained at half the maximum BRDF value, and in case of symmetry of partial diffuse reflection around the specular reflection direction (e.g. when considering a symmetric BRDF model) it may be clear to describe the half width at half maximum, which is simply half the full width at half maximum. The half width is therefore preferably in the range of 10° to 60°. Preferably, the material of the peripheral part of the reflector exhibits a diffusivity that can be expressed as having a full width at half maximum greater than 120°, preferably greater than 160°. Preferably, the material of the peripheral part is such that there is no full width at half maximum, i.e. the radiant intensity does not fall below half maximum at any scattering angle in the range of -90° to +90° for a flat surface.

[0015] In a second preferred embodiment (i.e. instead of or in addition to using the full width at half maximum values ​​mentioned above), the degree of light scattering in partial diffuse reflection around the reflection direction obtained in perfect specular reflection (also called the "specular reflection direction") can be expressed as a "cos^n" BRDF (bidirectional reflectance distribution function) model, where "n" is a variable parameter. Preferably, the BRDF model is symmetric. Preferably, the three-dimensional BRDF model is evaluated in a plane containing the incident ray and the surface normal, preferably perpendicular to the length of the reflector. Figure 7 shows such cos^n functions for n = 0, 1, 10 and 30. The horizontal axis represents the scattering angle, i.e. the deviation from the specular reflection direction. The vertical axis represents the amount of scattered light per steradian. The curves shown are normalized so that the integrals of the BRDF model in spherical polar coordinates are equal.

[0016] The higher the diffuse reflectance of the surface, the lower the n value of the cos^n function, and the wider the light scattered upon reflection. For a perfect diffuser, n is equal to 0. The higher n, the more the light remains bundled around the specular reflection direction. The discovery in this patent application is that the material of the central portion of the reflector exhibits an optimal diffusivity that can be expressed as a cos^n function, with n being between 10 and 200, preferably between 25 and 50, and preferably approximately 30. This discovery can be rephrased, i.e., that the diffuse reflectance of the central portion is between cos^10 and cos^200, preferably between cos^25 and cos^50, and preferably approximately cos^30. This allows the reflected radiation to be well "directed", but still well scattered. The more one deviates from this optimal material property, the less good the uniformity of the total irradiance on the planting surface and its spectral composition. Preferably, the central portion is made of aluminum. After all, aluminum can be easily processed to obtain the above-mentioned diffuse reflectance. Similarly, the diffuse reflectance of the peripheral portion is preferably less than cos^20, preferably less than cos^10, more preferably less than cos^5. Preferably, the peripheral portion is made of MCPET or l-reflect or coated with MCPET or l-reflect. These materials make it possible to obtain the above-mentioned diffuse reflectance.

[0017] It is a further object of the present invention to provide a plant lighting unit comprising a plurality of plant lighting devices as described above, preferably arranged parallel to one another by arranging the elongated light sources of each plant lighting device parallel to one another.

[0018] A further object of the present invention is to provide a plant cultivation system comprising a plant tray extending in length and width direction and a plant lighting device as described above. Preferably, the plant cultivation system comprises a plurality of plant lighting devices, i.e. a plant lighting unit as described above. The plant lighting device (or the plant lighting unit, if applicable) is positioned in height direction above the plant tray, such that the light source is positioned between the reflector and the plant tray. According to an embodiment of the present invention, the plant tray is covered with a reflective material, preferably MCPET or l-reflect. This ensures that when the plant tray is not yet completely covered by the leaves, no light is absorbed by the plant tray and the plant is also illuminated from below. According to an embodiment of the present invention, the plant tray is a hydroponic based plant tray. Preferably, the hydroponic based plant tray is a plant tray in which the plant is positioned in a groove, e.g. a movable groove. Preferably, the groove is covered with a reflective material as described above.

[0019] It is a further object of the present invention to provide a method for providing a plant cultivation system as described above, the method comprising the steps of: Obtaining boundary conditions including: a) the distance between the light source and the planting surface of the plant tray; b) the dimensions of the planting surface of the plant tray; and c) the distance between the light source and the protrusion of the reflector; determining the shape of the reflector and the diffuse reflectance of the central and peripheral portions to optimize the spectral uniformity and irradiance uniformity of the light reflected by the reflector on the planting surface below the reflector.

[0020] According to one embodiment of the present invention, the above-described method is a computer-implemented method, ie, a method implemented by a computer.

[0021] According to one embodiment of the present invention, the determining step is carried out using a light simulation computer program such as "TracePro". Preferably, the determining step is carried out by applying an optimization process that varies the shape and diffuse reflectance values ​​of the reflector and calculates the degree of spectral uniformity and irradiance uniformity of the light on the planting surface for given boundary conditions. Preferably, the shape of the central part of the reflector is set as a piecewise linear approximation of a parabola, with the end points at the axis of symmetry fixed and the shape optimized by varying the positions of the remaining connection points between the piecewise linear parts. [Brief description of the drawings]

[0022] [Figure 1a] FIG. 1 is a diagram of a plant cultivation system including a plant lighting unit having a plurality of parallel plant lighting devices as known in the prior art; FIG. 2 is a cross-sectional view of the plant cultivation system cut along its length. [Figure 1b] 1a is a diagram of a plant cultivation system comprising a plant lighting unit having a plurality of parallel plant lighting devices as known in the prior art, the cross section of the plant cultivation system being cut along the width direction and along the cross section AA shown in FIG. [Figure 1c] FIG. 1 is a diagram of a plant cultivation system comprising a plant lighting unit having a plurality of parallel plant lighting devices as known in the prior art; FIG. 2 is a top view of the plant lighting unit of the plant cultivation system; [Figure 2a] FIG. 2 is a diagram of a plant cultivation system including a plant lighting unit having a plurality of parallel plant lighting devices according to an embodiment of the present invention; FIG. 3 is a cross-sectional view of the plant cultivation system cut along its length; [Figure 2b] 2a is a diagram of a plant cultivation system including a plant lighting unit having a plurality of parallel plant lighting devices according to one embodiment of the present invention, and FIG. 2b is a cross-sectional view of the plant cultivation system, the cross-section being cut along the width direction and along cross-section BB shown in FIG. 2a. [Figure 2c]FIG. 2 is a diagram of a plant cultivation system including a plant lighting unit having a plurality of parallel plant lighting devices according to an embodiment of the present invention, and is a bottom view of the lighting unit of the plant cultivation system, shown in dotted lines, with the light source positioned therein. [Diagram 3] FIG. 3 is a diagram of a variation of the plant cultivation system of FIG. 2 in which the light source of each plant lighting device comprises multiple parallel light strips on each side of the axis of symmetry, each light strip arranged to emit light in a different frequency range. [Figure 4a] FIG. 4 shows a portion of the plant cultivation system of FIG. 3 having several paths of light rays emanating from two light strips emitting light in different frequency ranges, as shown by dotted and dashed lines, where the plant lighting device is of a suboptimal type. [Figure 4b] FIG. 4b is a plot of light irradiance as a function of distance from a reflector protrusion in the width direction when the plant lighting device of FIG. 4a is used. [Figure 5a] FIG. 4 shows a portion of the plant growing system of FIG. 3 with several paths of light rays emanating from two light strips emitting light in different frequency ranges, as shown by dotted and dashed lines, where the plant lighting device is of an improved type. [Figure 5b] FIG. 5b is a plot of light irradiance as a function of distance from a reflector protrusion in the width direction when the plant lighting device of FIG. 5a is used. [Figure 6a] FIG. 1 is a diagram of a perfect specular reflection situation. [Figure 6b] FIG. 1 is a diagram of a perfect diffuse reflection situation. [Figure 6c] FIG. 1 illustrates the partially diffuse reflection situation. [Figure 7] A diagram of several cos^n functions, where n is 0, 1, 10, and 30. [Figure 8] FIG. 4 shows a variation of the plant growing system of FIG. 3 in which the central portion of the reflector on each side of the axis of symmetry is a piecewise linear approximation of a parabola. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] [Drawing Description] Plant lighting devices 2 have been developed in the prior art for illuminating a plant tray 4 in a plant cultivation system 1. One such prior art plant lighting device 2 is shown in FIG. 1. The plant tray 4 comprises a planting surface 3 for receiving a plant 5. For example, a plant lighting device for illuminating a plant tray is known from EP 3772897. The planting surface 3 is a surface with mutually perpendicular length (I) and width (w) directions, i.e. the planting surface is perpendicular to the height (h) direction. The plants 5 are watered by a water supply duct 11 as described in EP 3772897. In particular, a plant lighting unit is shown which is a group of several parallel plant lighting devices 2, each plant lighting device having a light source 6 which is elongated in the length direction (I). The plant lighting unit shown in FIG. 1 comprises four plant lighting devices 2 (shown in FIG. 1c, but only two plant lighting devices are depicted in FIG. 1a and FIG. 1b). As shown in Fig. 1a and Fig. 1c, the distance along the width direction by which the light sources of adjacent plant lighting devices 2 are separated is W1. Each light source 6 comprises a single light strip 7 comprising a plurality of lighting elements 8. In particular, the light strip 7 is an LED strip comprising a plurality of LEDs 8. The light sources are cooled by heat conducting ducts 9 conveying a cooling fluid 10 in the direction indicated by the arrows, as described in EP 3772897. As shown by the light beams emitted from the lighting elements 8 in Fig. 1, the light sources 6 directly illuminate only the plants 5 in the plant tray 4. This had the disadvantage that the irradiance of the light reaching the plants in the plant tray depends on the position of the plants 5 in the plant tray 4, i.e. the plants directly below the light source receive light with a higher irradiance than the plants at different positions in the width direction (w). However, the irradiance of the light has an effect on the growth of the plants, and therefore it has been found that the conventional plant lighting device shown in Fig. 1 has the problem that the plants in the plant tray are not illuminated with a uniform irradiance, resulting in uneven plant growth.

[0024] In order to solve the above-mentioned problems, a plant lighting device 2 and a plant cultivation system 1 of the present invention are provided. One embodiment of such a lighting device 2 and a plant cultivation system 1 is shown in FIG. 2. The plant lighting device 2 comprises a reflector 12 in addition to a light source 6. The light source 6 comprises a single light strip 7 with a plurality of lighting elements 8 as described above. In particular, a plant lighting unit is shown which is a group of a plurality of parallel plant lighting devices 2, each of which comprises an elongated light source 6 as described above and an associated reflector 12. The plant lighting unit shown in FIG. 2 comprises in particular two parallel plant lighting devices 2. In each plant lighting device 2, the reflector 12 is positioned above the light source 6. The light source 6 is arranged to emit light only towards the reflector 12, i.e. not directly towards the plant tray 4 as in the case of the prior art described above. The light source 6 and the reflector 12 are positioned such that the light emitted by the light source is reflected by the reflector 12 towards the plant tray 4. In other words, in use, the light source 6 is positioned between the reflector 12 and the plant tray 4. Both the reflector 12 and the light source 6 extend long along a length direction (I). As best shown in FIG. 2a, the reflector 12 has a shape in a cross-sectional plane perpendicular to the length direction, which shape comprises a central portion 13 and two peripheral portions 14 adjacent to the central portion 13. The illustrated reflector 12 comprises two separate portions: a central portion 15 where the central portion 13 is provided, and a ceiling portion 16 where the two peripheral portions 14 are provided. The ceiling portion 16 of one plant lighting device 2 is shared with an adjacent plant lighting device 2. This can be seen in FIG. 2a, where the same ceiling portion 16 extends above the left and right light sources 6. For each plant lighting device 2, the shape of the reflector described above is symmetrical with respect to an axis of symmetry extending through the central portion 13 along a height direction (h). Assuming that the reflector 12 is long in the length direction (I), the reflector 12 is symmetrical along a plane of symmetry that includes the axis of symmetry and extends along the height (h) and length (I) directions. The central portion 13 is provided with a protrusion 17 at the axis of symmetry that protrudes downwards along a height direction (h) towards the light source 6. The central portion 13 of the reflector 12 is positioned to receive light from the light source 6 and reflect the received light towards a peripheral portion 14 of the reflector 12.The peripheral portion 14 of the reflector 12 is arranged to reflect the light received from the central portion 13 towards the plant tray 4. This reflector 12 has the advantage that a uniform light irradiance is obtained across the planting surface (i.e. it improves the uniformity of the irradiance). It has the additional advantage that fewer light sources have to be provided per unit length along the width direction. Indeed, as shown in Figures 2a and 2c, the separation distance along the width direction (w) between the light sources 6 of adjacent plant lighting devices 2 is W2. The distance W2 shown in Figures 2a, 2c is greater than the distance W1 shown in Figures 1a, 1c.

[0025] According to a preferred embodiment of the invention, as shown in FIG. 3, the light source 6 comprises two parallel light strips 7a, 7b spaced apart from each other along the width direction (w) on either side of an axis of symmetry through the protrusion 17. The light strip 7a holds LEDs arranged to emit blue light, and the strip 7b holds LEDs arranged to emit red light. The result of this is that the light coming from the different light strips is reflected differently by the reflector 12 towards the plant tray 4, i.e. depending on the position of the light strip along the width direction, the light rays of that light strip are reflected by the central part of the reflector towards different positions of the peripheral part of the reflector and thus to different positions along the width direction of the plant tray 4. This is illustrated in FIG. 4a, where the light rays emitted by the light strip 7a are shown in dashed lines and the light rays emitted by the light strip 7b are shown in dashed and dot-dash lines. It should be noted that only one light ray leaving each light strip is shown. This is, of course, for illustrative purposes only. In reality, multiple light rays leave each light strip, and the direction in which the light rays are emitted is not along the height direction (h) itself, i.e. the light strip emits light rays in multiple directions centered on the height direction (h). This leads to a lower irradiance uniformity of the light at the planting surface 3 and a lower spectral uniformity, as exemplarily shown in FIG. 4b, where the irradiance of the blue light emitted by the light strip 7a is shown in dashed lines and the irradiance of the red light emitted by the light strip 7b is shown in dashed and dot-dash lines. To solve this problem, the reflector 12 is adapted as shown in FIG. 5a, where the light rays emitted by the light strip 7a are shown in dashed lines and the light rays emitted by the light strip 7b are shown in dashed and dot-dash lines. In particular, both the central part 13 and the peripheral part 14 of the reflector 12 are arranged to diffusely reflect the light from the light source 6, and the diffuse reflectance of the peripheral part 14 is selected to be higher than the diffuse reflectance of the central part 16. For example, the central portion is made from aluminum that has been treated to have a diffuse reflectance of cos^30. The peripheral portion 14 is coated with, for example, MCPET or I-reflect to have a diffuse reflectance of less than cos^10.The high irradiance uniformity and spectral uniformity are exemplarily shown in FIG. 5b, where the irradiance of blue light emitted by light strip 7a is shown in dashed lines and the irradiance of red light emitted by light strip 7b is shown in dashed and dotted lines.

[0026] Figures 6a, 6b and 6c respectively show the situations of perfect specular reflection, perfect diffuse reflection and partial diffuse reflection. In all three situations, an incident ray is shown as indicated by reference number 18. The incident ray reaches the surface 19 of the material at an angle of incidence a with respect to the normal 23 to the surface. The incident ray is reflected differently in Figures 6a, 6b and 6c as indicated by the reflected ray 21. In the case of perfect specular reflection as shown in Figure 6a, the reflected ray is reflected at an angle of reflection equal to a. In the case of partial or perfect diffuse reflection, the reflected ray 21 is distributed over multiple reflection angles as indicated by the envelope 24 that bundles the multiple possible reflected rays 21.

[0027] Figure 7 shows several cos^n functions with n = 0, 1, 10, and 30. The horizontal axis represents the scattering angle, i.e. the deviation from the specular direction. The vertical axis represents the amount of scattered light per steradian. The curves shown have been normalized so that the integrals of the BRDF model in spherical polar coordinates are equal.

[0028] Figure 8 shows a variation of the plant growing system of Figure 3, in which the central portion of the reflector 12 on each side of the axis of symmetry is a piecewise linear approximation of a parabola, each piecewise linear approximation of a parabola including, inter alia, three linear sections 22a, 22b, 22c.

Claims

1. A plant lighting device (2) arranged to be positioned above a plant tray (4) to illuminate a plant (5) therein, the device comprising a reflector (12) and a light source (6), the light source being arranged to emit light directed towards the reflector, the light source and the reflector being positioned such that the light emitted by the light source is reflected by the reflector towards the plant tray, both the reflector and the light source being elongated along a length direction (I), the reflector having a shape in a cross-sectional plane perpendicular to the length direction, the shape being a central portion (13). and two peripheral portions (14) adjacent to the central portion, wherein the shape is symmetrical with respect to an axis of symmetry extending through the central portion along a height direction (h) perpendicular to the length direction, the central portion having a protrusion (17) on the axis of symmetry, the protrusion protruding downward along the height direction toward the light source, the central portion of the reflector being arranged to receive light from the light source and reflect the received light toward the peripheral portion of the reflector, and the peripheral portion of the reflector being arranged to reflect the light received from the central portion toward the plant tray.

2. 10. The plant lighting device of claim 1, wherein the protrusion divides the reflector into two adjacent concave elements.

3. 3. The plant lighting device of claim 2, wherein the central portion on one side of the axis of symmetry comprises an approximate parabola that terminates on the axis of symmetry to form the protrusion of the central portion of the reflector.

4. 4. The plant lighting device of claim 3, wherein the approximating parabola is a piecewise linear approximation of a parabola, the piecewise linear approximation including at least three linear sections.

5. 2. The plant lighting device of claim 1, wherein the reflector includes a width direction (w) perpendicular to the height direction and the length direction, and the central portion of the reflector is shaped such that its height along the height direction increases as its distance from the apex of the protrusion along the width direction increases.

6. 6. The plant lighting device of claim 5, wherein the peripheral portion of the reflector is a substantially flat portion of the reflector, or the peripheral portion of the reflector is a portion of the reflector shaped such that its height along the height direction decreases as the distance from the apex of the protrusion along the width direction increases.

7. 10. The plant lighting device of claim 1, wherein the light source is arranged to emit PAR light.

8. 2. The plant lighting device of claim 1, wherein the light source (6) comprises a plurality of parallel long light strips (7a, 7b) arranged adjacent to each other along a width direction (w) perpendicular to the height direction and the length direction on each side of the axis of symmetry, and at least two of the plurality of long light strips on each side of the axis of symmetry are arranged to emit light in different wavelength ranges.

9. 9. The plant lighting device of claim 8, wherein at least one of the plurality of long light strips on each side of the axis of symmetry is arranged to emit predominantly blue light and at least one of the plurality of long light strips on each side of the axis of symmetry is arranged to emit predominantly red light.

10. 9. The plant lighting device of claim 8, wherein the long light strip is a strip of LEDs.

11. 9. The plant lighting device of claim 1 in combination with claim 8, wherein both the central portion and the peripheral portion of the reflector diffusely reflect the light from the light source, the diffuse reflectance of the peripheral portion being higher than the diffuse reflectance of the central portion.

12. 12. The plant lighting device of claim 11, wherein the diffuse reflectance of the central portion is between cos^10 and cos^200.

13. The plant lighting device of claim 11 , wherein the diffuse reflectance of the peripheral portion is less than cos^10.

14. 14. A plant cultivation system (1) comprising: a plant tray (4) extending in the length and width directions; and a plant lighting device according to any one of claims 1 to 13, wherein the plant lighting device is positioned in the height direction above the plant tray so that the light source is positioned between the reflector and the plant tray.

15. 15. The plant growing system of claim 14, wherein the plant tray is covered with a completely diffusely reflective material.

16. 15. The plant growing system of claim 14, wherein the plant tray is a hydroponic-based plant tray.

17. 15. A method for providing a plant cultivation system according to claim 14, said method comprising: Obtaining boundary conditions including: a) the distance between the light source and the planting surface of the plant tray; b) the dimensions of the planting surface of the plant tray; and c) the distance between the light source and the protrusion of the reflector; determining the shape of the reflector and the diffuse reflectance of the central and peripheral portions to optimize the spectral uniformity and irradiance uniformity of the light reflected by the reflector on a planting surface below the reflector; A method comprising:

18. 20. A computer-implemented method comprising the method steps of claim 17, wherein the method is performed by a computer.

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