WO2012175809A2

WO2012175809A2 - Illumination for activating photosensitive biological processes

Info

Publication number: WO2012175809A2
Application number: PCT/FI2012/050646
Authority: WO
Inventors: Jukka Lindfors; Juha Viljanen; Ilkka Tiainen
Original assignee: Laser Growth Ltd.
Priority date: 2011-06-23
Filing date: 2012-06-21
Publication date: 2012-12-27
Also published as: WO2012175809A3; FI20115654A0; FI20115654L

Abstract

An illumination device (1) for producing an illuminating light beam (7) to activate a photosensitive biological process comprises a light source element (2) configured to emit light at a wavelength range overlapping with the absorption wavelength band of the photosensitive biological process. According to the present invention, the light source element comprises a plurality of laser diodes(2) configured to emit primary light beams(3)at different wavelengths; and the illumination device (1) further comprises at least one diffusor surface (6a, 6b) configured and placed to diffusively redistribute the energy of the primary light beams(3) into the illuminating light beam (7) so that the energy density of the illuminating light beam leaving the illuminating device does not exceed the maximum permissible exposure of a human eye.

Description

ILLUMINATION FOR ACTIVATING PHOTOSENSITIVE BIOLOGICAL PROCESSES

FIELD OF THE INVENTION

The present invention relates to activation of differ- ent photosensitive biological processes like photosyn^¬ thesis. The present invention is focused on a lighting device for producing an illuminating light beam to activate such processes, a use of such lighting device, and a method for producing an illuminating light beam to activate such processes.

BACKGROUND OF THE INVENTION

A great variety of various biological photosensitive processes exists, where the activation energy of the process originates from light absorption in biological molecules or systems.

One known example of those processes is photosynthe^¬ sis, which is vital for all aerobic life on Earth. As well known, photosynthesis occurs e.g. in plants, where the process consumes carbon dioxide and water to produce glucose and release oxygen. Photosynthesis is the basic process required for growth of a plant. Thus, conditions enabling efficient photosynthesis are vital as well in agriculture as in plant growth in greenhouses and the like.

The biochemical process of photosynthesis is initiated by energy absorbed in chlorophyll and other photosen- sitive molecules in the plants. In the nature and in agriculture, this light energy is originated from the sun. This is also often the case in greenhouses. How^¬ ever, due to the insufficient lighting conditions e.g. during the wintertime, also artificial lighting condi- tions are often necessitated. For example, high pres- sure sodium lamps have been used traditionally in the greenhouses to produce sufficient light energy.

Another example of photosensitive biological processes is the production of vitamin D in the skin when 7- dehydrocholesterol reacts with ultraviolet light (UVB) . Also in this process, the sunlight is the pri^¬ mary energy source to initiate the process. As well known, e.g. in the northern countries with relatively cool climate and less solar energy available, the ex^¬ posure of the skin to the sunlight is often insuffi^¬ cient to cover the appropriate need of vitamin D.

In the case of artificial lighting conditions used to activate a photosensitive biological process, some fundamental facts must be taken into account. First, absorption of light only occurs at some particular wavelengths defined by the properties of the absorbing molecules or systems. Moreover, the width of the ab- sorption wavelength range, i.e. the absorption band, is typically narrow. For example, in photosynthesis, the absorption peaks of chlorophyll are concentrated at wavelengths 450 - 500 nm and 650 - 700 nm, the highest peak widths being very narrow. Respectively, in vitamin D production, the useful wavelengths are between 270 and 300 nm, with peak synthesis occurring between 295 and 297 nm.

The narrow absorption bands of the biological process- es mean that when using artificial illumination, the light energy should be concentrated in the absorption wavelengths only. Unfortunately, this is not always the case. For example, the spectrum of the high pres^¬ sure sodium lamps is far from an ideal for photosyn- thesis with emission peaks between 550 and 600 nm. This means that a large portion of the emitted light energy lies outside the most efficient wavelength ranges and is wasted in heating the plants and the greenhouse (which of course sometimes can be also use^¬ ful) . Moreover, the efficiency of the primary conver^¬ sion of electrical energy into light is rather low in the conventional lamps. The same problem of non- optimized emission spectrum and poor energy efficiency is shared also e.g. by the UV lamps used in solariums to facilitate the production of vitamin D. Recently, the intense development in the field of light emitting diodes LEDs have opened new possibili^¬ ties in different lighting and illumination applica^¬ tions, and also LED based plant illumination methods and apparatuses have been introduces and launched on the market. LEDs possess some definite advantages over the conventional technologies. For example, the effi^¬ ciency of energy conversion is better in LEDs than e.g. in the high pressure sodium lamps. In addition, the emission band can be narrower. LEDs also can pro- vide lower device costs and longer lifetime than the conventional technologies.

Though providing some advantages, also LED-based acti^¬ vation of biological photosensitive processes is far from ideal. Particularly the overall energy efficiency of plant illumination would be highly desirable to be further enhanced.

PURPOSE OF THE INVENTION

The purpose of the present invention is to provide further enhanced solutions for activating photosensi^¬ tive biological processes like photosynthesis or pro^¬ duction of D-vitamin via absorption of light in the skin .

SUMMARY OF THE INVENTION The present invention is characterized by what is pre^¬ sented in claims 1, 14, and 15.

The first aspect of the present invention is focused on an illumination device for producing an illuminating light beam to activate a photosensitive biological process, the illumination device comprising a light source element configured to emit light at a wave^¬ length range overlapping with the absorption wave- length band of the photosensitive biological process.

By a biological process is meant here any biochemical or other type of process occurring in any biological organism like a plant or an animal or a human being. Photosensitive means here a process which can be acti^¬ vated, i.e. initiated and/or supported by energy orig^¬ inating from light absorbed in the organism.

The illuminating light beam can be of any size and form. Also the intensity distribution within the beam can be of any type suitable for the actual applica^¬ tion. By overlapping is meant here that at least a portion of the energy of the illuminating light beam lies at the absorption wavelength band in which said absorption of light can take place. Preferably, as big portion of the overall energy of the illuminating light beam as possible is concentrated in such absorp^¬ tion wavelength band. According to the present invention, the light source element comprises a plurality of laser diodes config^¬ ured to emit primary light beams at different wave^¬ lengths. Emitting primary light beams at different wavelengths means in this document that the peak emis- sion wavelengths of the laser diodes are different, i.e. separated from each other. As well known for those skilled on the art, a laser diode is an optoelectronic semiconductor device con^¬ verting electrical current into light by means of stimulated emission. The details of the laser diode are not in the core of the present invention and are thus not discussed further in this document. A laser diode of the illumination device according to the pre^¬ sent invention can be configured according to the well-known principles in the art. For example, the emission wavelength, by which is meant here the wavelength of the peak emission of the typically very narrow emission spectrum of a laser diode, depends on the actual material compositions in the active region, i.e. the light generating region of the laser diode. Moreover, various technologies exist for producing and controlling tunable laser diodes, where the emission wavelength band is adjustable.

When compared to LEDs, high pressure sodium lamps, and fluorescence lamps, a laser diode has several signifi^¬ cant advantages as a light source element of an illu^¬ mination device for activating a photosensitive bio^¬ logical process. First, the attainable light power is high, at least when compared to the LEDs. A light power of several watts is possible from a single semiconductor compo^¬ nent, thereby providing savings in both the costs and the size of the illumination device.

In addition, the emission spectrum of a laser diode is typically very narrow, which means that when the emis^¬ sion band is matched to the absorption band of the photosensitive biological process, waste energy re- mains low, resulting in high efficiency. A narrow and accurately adjusted wavelength range of the illuminat^¬ ing light beam also enables accurate control of the activation, and e.g. simultaneous monitoring of luminescence from the process without undesired interfer^¬ ence . Further, the energy efficiency of a laser diode is typically very high. In semiconductor lasers, in converting electrical energy into light, conversion effi^¬ ciencies of as high as 50 % have been measured. Moreover, the optical intensity of the output of a la^¬ ser diode is easily adjustable by controlling e.g. the current through and the temperature of the diode. Also pulsed operation of the diode can be used, which can further increase the power efficiency of the illumina- tion in some applications, or facilitate e.g. studying the luminescence taking place during the photosensi^¬ tive biological process.

Plurality of, i.e. several laser diodes in a single device increases the total output light power of the device in comparison to a single laser diode. But it is even more important advantage in the present inven^¬ tion that the plurality of laser diodes configured to emit primary light beams at different wavelengths al- lows the different wavelengths, at which the plurality of laser diodes is configured to emit the primary light beams, to be matched to a plurality of ab^¬ sorption peak wavelengths of the photosensitive bio^¬ logical process. In a preferred embodiment of the pre- sent invention, the plurality of laser diodes is con^¬ figured to emit the primary light beams so as to be so matched. This has the great advantage in that a possi^¬ bly broad absorption wavelength band of the photosensitive biological process formed by several absorption peak wavelengths can be covered efficiently with nar^¬ row emission spectra of the plurality of laser diodes adjusted to those particular absorption peaks. Moreo- ver, also the intensities of the laser diodes can be adjusted separately from each other to match with the possibly different absorptances of the different ab^¬ sorption peaks. In other words, the emission intensity of a laser diode emitting at a wavelength correspond^¬ ing to a high absorption peak can be adjusted higher than that of a laser diode emitting at a wavelength corresponding to a lower absorption peak. By the absorption peak wavelengths is meant in this document the local maxima in the absorption spectrum of the photosensitive biological process at issue. These absorption peaks can be absorption peaks of the initial mechanism triggering the biological process, e.g. the absorption peaks of chlorophyll molecules in the case of photosynthesis. On the other hand, an ab^¬ sorption peak of the photosensitive biological process can also result from absorption at an intermediate stage of the already initiated biological process. For example, in the case of photosynthesis, there is such an intermediate absorption peak at 495 nm, which is different from the initial absorption peaks of the chlorophyll a molecule. Further, according to the present invention, the illumination device also comprises at least one diffusor surface configured and placed to diffusively redis^¬ tribute the energy of the primary light beams into the illuminating light beam so that the energy density of the illuminating light beam leaving the illumination device does not exceed the maximum permissible expo^¬ sure of a human eye.

By the illuminating light beam leaving the illumina- tion device is meant the situation when the illuminat^¬ ing light beam exits the illumination device so that it is accessible from the exterior of the device. In other words, the illuminating light beam having left the device can be looked by a person or guided by some external structures. The illumination device can be configured so that the illuminating light beam exits the device at the diffusor surface. Alternatively, there can be a housing arranged to prevent access to the illuminating light beam before it has propagated a certain distance from the diffusor surface. In diffusive redistribution, a point or sub-area of the diffusor surface does not redirect the light inci^¬ dent thereon according to the specular reflection or refraction of light, where light with a given angle of incidence is reflected or refracted into one single direction defined by the geometrical law of reflection or by the Snell's law, respectively. Instead, in dif^¬ fusive redistribution, the incident light energy with a given direction of incidence is distributed into several different directions. Diffusive redistribution of light energy at a diffusor surface can take place so that even in the case of a ideally flat surface of a piece of a suitable material, a light ray incident at a point of the surface is divided into several sub- rays with different directions. In that case, diffu- sion is due to the interaction of light with the in^¬ ternal structure of the material. However, more often diffusive redistribution of light occurs due to the random or controlled roughness of the surface. In that case, light rays with the same direction of incidence with respect to the macroscopic direction of the dif^¬ fusor surface at the respective points thereof are di^¬ rected differently.

Diffusive redistribution means redistribution of light energy so that in the illuminating light beam, the light rays are not collimated and they do not share a common virtual point of origin. Hence, an essential consequence of diffusive redistribution is that diffu^¬ sively redistributed light energy cannot be anymore collimated or focused by means of reflecting and/or refracting optical elements to increase the intensity in the illuminated light beam over the intensity thereof at the diffusor surface. Thus, the maximum in^¬ tensity of the illuminating light beam exiting the il^¬ lumination device according to the present invention is limited to the maximum intensity at the diffusor surface.

Moreover, because the diffusor surface is configured and located so that the energy density of the illumi^¬ nating light beam leaving the illumination device does not exceed the maximum permissible exposure of a human eye, the illumination device is an eye-safe device. This enables safe use of the laser-based illumination device in any application and enables implementation of the illumination device according to the present invention even as a consumer product.

There are different well known standards determining the maximal permissible exposure of the human eye for a laser beam. The maximum permissible exposure is typ- ically determined either as energy density or power density of the incident light beam. In both cases, the maximum exposure depends on the exposure time and the wavelength. Thus, no accurate quantities can be deter^¬ mined here for the maximum permissible exposure, but those must be determined in each case according to given conditions of the actual application, the stand^¬ ard used, and also the possible local safety regula^¬ tions. Anyway, it is standard engineering for a person skilled in the art, having been given the basic prin- ciple of the present invention, to configure and lo^¬ cate the diffusor surface, together with the laser diodes, so as to meet the above conditions. There are a great variety of different types of diffu^¬ sor elements commercially available. The details of the diffusor surface are thus not in the core of the present invention. In one embodiment of the present invention, the at least one diffusor surfaces comprise a surface of a transmission type diffusor element. In one embodiment, the at least diffusor surfaces com^¬ prise a surface of a reflection type diffusor element. If there are more than one diffusor surfaces, they may all be based on the same type of surfaces. Alterna^¬ tively, they may comprise the both types of surfaces. Selection of the most appropriate types of the diffu^¬ sor elements can be made in each application according to the special need thereof.

The illumination "device" according to the present invention can be an entire, operable, stand-alone unit having, in addition to the light source element and the diffusor surface, also all housings, electronics, power sources, and other equipment necessitated in the operation of the device. Such complete construction can be implemented e.g. as a plant lamp. On the other hand, the illumination device of the present invention can be also implemented as an intermediate module, comprising the core components only, to be incorpo^¬ rated into a larger device assembly.

In one embodiment of the present invention, at least one of the laser diodes is configured to operate as pulsed. This can provide advantages in some applica^¬ tions .

In one preferred embodiment, the illumination device further comprises a detector for monitoring lumines- cence from a biological organism where the photosensi^¬ tive biological process takes place. The detector can be of any known type suitable for detecting light at the wavelength range of the actual luminescence. Moni- toring the luminescence provide information about the operation of the photosensitive biological process. This information can be used to control the operation of the illumination device. For this purpose, in one preferred embodiment, the illumination device further comprises control means for controlling the operation of the laser diode based on the monitored lumines^¬ cence. Control means can comprise any suitable elec^¬ tronic, electric, hardware, and software means, which are commonly known.

In a preferred embodiment, the photosensitive biologi^¬ cal process is photosynthesis. There are readily available laser diodes emitting at suitable wave^¬ lengths to activate the photosynthesis processes in different organisms. For example, the absorption band at 650 - 700 nm of the chlorophyll can be covered by a laser diode where the active layer is formed of galli^¬ um arsenide GaAs or one or more of its alloys, e.g. aluminum gallium arsenide AlGaAs.

In the case of photosynthesis as the photosensitive biological process, said monitoring of luminescence from the biological organism can be used to monitor the fluorescence of chlorophyll, the intensity of which is proportional to the state of activation of the photosynthesis. Thus, for example in a greenhouse, said monitoring can be used to control and adjust the growth of the plants to be illuminated by the illumi^¬ nation device.

In one preferred embodiment where the photosensitive biological process is photosynthesis, the plurality of laser diodes is configured to emit the primary light beams substantially at wavelengths of 495 nm, 660 nm, and 735 nm. In other words, the peak emission wavelengths of the laser diodes emitting at different wavelengths lie substantially at those wavelengths. This is very advantageous because it has been surpris^¬ ingly found by the inventors that the intensity of the photosynthesis process in plants is highly enhanced if absorption of incident light takes place also at the side absorption peaks of 495 nm and 735 nm in addition to the highest absorptance of chlorophyll a at about 660 nm.

In another preferred embodiment, the plurality of la- ser diodes is configured to emit the primary light beams substantially at wavelengths of 430 nm, 495 nm, 660 nm, and 735 nm. The additional wavelength of 430 nm further increases the intensity of the photosynthe^¬ sis in comparison to the three wavelength case above.

In the above, the peak emission wavelengths are de^¬ fined with an accuracy of 5 nm, which means that that the laser peak emission wavelengths are within +/- 5 nm from the specified values.

The first aspect of the present invention can also be implemented as an illumination system for activating a photosensitive biological process. According to the present invention, the illumination system comprises at least one illumination device as defined in the above, and at least one site for receiving a biologi^¬ cal organism to be illuminated by the at least one il^¬ lumination device. The site can comprise any means, possibly equipped with some special structures, for receiving any type of biological organism at the site so that the illuminating light beam can illuminate the organism. The illumination system can be, for example, a greenhouse system. In one specific embodiment, the illumination system comprises a chamber configured to produce controlled environmental conditions within the chamber, the at least one site for a biological organism being located within the chamber. The chamber can be e.g. a climate chamber or an incubator. In one preferred embodiment, the chamber is a refrigerator. This embodiment makes it possible e.g. to enhance preservation of or grow vegetables and fruits in a refrigerator. According to a second aspect, the present invention comprises use of an illumination device as defined in the above to activate a photosensitive biological pro^¬ cess. Such use can be performed to activate photosyn^¬ thesis of plants in a greenhouse, at florist's, or in any other application where activation of a photosensitive biological process by illumination is needed.

According to a third aspect, the present invention is focused on a method for producing an illuminating light beam to activate a photosensitive biological process, the method comprising generating light at a wavelength range overlapping with the absorption wavelength band of the photosensitive biological process. According to the present invention, said generating light comprises generating primary light beams at dif^¬ ferent wavelengths by means of a plurality of laser diodes, and the method further comprises diffusively redistributing the energy of the primary light beams into the illuminating light beam so that the energy density of the accessible illuminating light beam does not exceed the maximum permissible exposure of a human eye .

In a preferred embodiment, the method further compris- es monitoring luminescence from a biological organism where the photosensitive biological process takes place. In the case of such monitoring, the method preferably further comprises controlling the operation of the laser diode based on the monitored lumines- cence . Said controlling can comprise adjustment of e.g. the intensity, pulsing, or the possibly adjusta^¬ ble emission wavelength of the laser diode.

Regarding to the detailed definitions of the expres- sions above as well as the principles and advantages of the method according to present invention, a reference is made to the above discussion on the first as^¬ pect of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is discussed in more detail in the following by reference to the accompanying figures which show schematic illustrations of some preferred embodiments according to the present invention. Fig- ures la and lb show illumination devices and Figures 2 and 3 illustrate illumination systems according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Figures la and lb each show an illumination module 1 comprising three laser diodes 2 configured to emit primary light beams 3 at predetermined wavelengths ac^¬ cording to an absorption wavelength of a photosensitive biological process, e.g. photosynthesis. (In Fig- ure lb, only one laser diode 2 and primary beam 3, re^¬ spectively, is visible in the drawing.) Though not il- lustrated in the schematic drawings, each of the laser diodes can be packaged in an appropriate housing, if needed. The laser diode is mounted, directly or via the possible housing thereof, on a cooling unit 4 for preventing the laser diode from overheating. The cooling unit can be either passive, when cooling is based on convention or conduction, or active, when cooling is performed by means of a thermoelectric component like a Peltier element.

The laser diodes may be connected in series or in par^¬ allel to each other. The different laser diodes emit at different wavelengths. In the case of photosynthe^¬ sis as the biological process to be activated, the three laser diodes emit light preferably at, i.e. the emission peak wavelengths are adjusted at the wave^¬ lengths of 495 nm, 660 nm, and 735 nm. In another al^¬ ternative embodiment, there are four laser diodes in a single illumination module, the fourth one emitting at 430 nm. Those wavelengths above are defined with an accuracy of 5 nm, which means that that the laser peak emission wavelength are within +/- 5 nm from those specific values. The laser diodes 2 are electrically connected to a control unit 5 arranged to control the operation of the laser diodes by supplying pulsed current through it. Alternatively, the laser diodes could also be operated in a continuous mode.

As an important feature of the present invention, the illumination modules 1 of Figures la and lb are eye- safe components. This is achieved by two essential features of the modules. First, there is a diffusor surface 6a, 6b diffusively redistributing the light power of he primary light beams 3 into a secondary light beam 7 which is the actual illuminating beam produced by the illumination module. In Figure la, the diffusor surface 6a is a surface of a transmission type optical element. Figure lb illustrates an alter^¬ native where the diffuser surface 6b is a reflecting surface. As clear for those skilled in the art, the diffusively redistributed light energy can no more be collimated, focused, or in any other way collected such that the intensity of the secondary beam would exceed the maximum value it had at the diffuser sur^¬ face. Moreover, the diffuser surface 6a, 6b is so con^¬ figured and located with respect to the laser diodes 2 that intensity of the secondary, i.e. the illuminating light beam 7 leaving the module is below the maximum permissible exposure of the human eye. In the module of Figure la, the diffusor surface 6a is a part of the outer surface of the housing 8 of the module. Thus, the diffusor surface 6a is located at a distance from the laser diode where said maximum intensity condition is met. In the module of Figure lb, the diffusor sur^¬ face 6b lies within the module and the illuminating light beam exits the module through a window 9. In this case, in addition to the distance of the diffusor surface 6b from the laser diode, also the light path between the diffusion surface 6b and the outer surface of the window 9 affects the intensity of the illumi^¬ nating beam 7 exiting the illumination module 1. Thus, also the configuration of the diffusor surface 6b, particularly its effect on broadening the illuminating light beam 7 is to be taken into account when deter^¬ mining the maximum intensity of the illuminating beam 7.

As known, there are different standards defining val^¬ ues for maximum permissible exposure of laser light for a human eye. For example, IEC 60285 standard de^¬ fines, for wavelength of 660 nm suitable for activat- ing photosynthesis, a maximum eye-safe beam intensity I_max of 2 mW/cm². As illustrated in Figure la, a laser diode typically produces a primary light beam 3 with specific beam di^¬ vergence determined by the half power angles 6_t for the fast axis and the slow axis. These values depend on the used semiconductor system and the detailed la^¬ ser diode structure. As an example, typical AlGalnP semiconductor laser operating at 660nm wavelength has a fast axis divergence angle of 6^ = 17° and a slow axis divergence of 6_S = 10° . Thus, the beam shape is ellipti- cal and the beam intensity degreases approximately ac^¬ cording to the Gaussian function from the intensity maximum at the beam center.

Because of the beam divergence, the intensity of one single primary beam 3 decreases with the distance d from the laser output facet. The elliptical beam has a half value intensity area A = n - a - b , where a and b are the half axis lengths in the ellipse, a = d - t&nO_f and b = d - t&n6_s . With these definitions, we get for the beam area at the intensity half value point the following equation: A = π - d² - tan0_f - tan^ . With a laser power P, the power density at beam center is then given approximately by / = 2 - Ρ/(π - d² - tan0_f - tan^) , and obviously de^¬ creases proportional to the squared distance from the emitting surface of the laser diode. The closest eye- safe viewing distance d_m±_n for a predetermined maximum intensity I_max is then given as

. As an example, one can cal^¬ culate for a laser diode emitting a primary light beam of 100 mw at 660 nm with the above given beam divergence parameters that the minimum eye safe distance is about 24.3 cm. This result assumes there are no re^¬ fracting, reflecting, or diffracting elements between the laser and the spectator's eye. Such elements may significantly change the safe viewing distance either by collimating or diverging the beam. Especially col- limating and/or focusing the beam by any surface should be prevented. Focusing the beam can make it dangerous for an eye even at a very large distance from the emitting laser diode.

With high power lasers, the eye-safe minimum distance min can become impractically long for implementation of an eye-safe illumination module. In such case, the minimum distance at which the maximum intensity de- creases below the maximum permissible exposure can be shortened by an optical component expanding the prima^¬ ry beam. Figure lb shows an example of this. A diver^¬ gent lens 10 diverging the primary beam 3 is placed between the laser diode 2 and the diffusor surface 6b.

The calculations above are performed, for simplicity, for a single primary beam emitted by a single laser diode. To convert them into the case of two, three, or more laser diodes and primary light beams, respective- ly, it of course has to be taken into account that each of the multiple primary light beams contribute to the total power density of the illuminating light beam. Figure 2 shows a greenhouse system 11 wherein an illu^¬ mination panel 12 has been placed above a table 13. Plants 14 have been placed on the table 13 to be illu^¬ minated by the illumination panel 12. The illumination panel 12 comprises a plurality of illumination modules 1 as illustrated in Figures la and lb, each producing an illuminating light beam 7, integrated to form a large array. The modules 1 emit light at the activa^¬ tion wavelengths of the photosynthesis. The illumination panel 12 is controlled by a central unit 15 supplying the required voltages and currents to the illumination modules 1. The greenhouse system 11 further comprises a photode- tector 16 for monitoring the chlorophyll fluorescence occurring during photosynthesis. The amount of light emitted in the chlorophyll fluorescence is a measure of the intensity of the photosynthesis taking place in the plants 14. The detector 16 is connected to the control unit 15 which controls the operation of the light panel based on the monitored level of chloro- phyll fluorescence.

Figure 3 shows a home system 17 where an illumination model according to the present invention is used to activate photosynthesis. The system 17 comprises an illumination module 1 as shown in Figures la and lb integrated in a refrigerator 18 to illuminate a plant 14, e.g lettuce, placed on a shelf 19 in the refriger^¬ ator . The above examples have to be understood as illustra^¬ tive examples of the present invention only, no way limiting the possibilities to implement the basic principles of the present invention. Instead, the em^¬ bodiments of the present invention can freely vary within the scope of the claims.

Claims

1. An illumination device (1) for producing an illuminating light beam (7) to activate a photosensitive bi^¬ ological process, the illuminating device comprising a light source element (2) configured to emit light at a wavelength range overlapping with the absorption wavelength band of the photosensitive biological process, characteri zed in that

the light source element comprises a plurali- ty of laser diodes (2) configured to emit primary light beams (3) at different wave^¬ lengths; and that

the illumination device (1) further comprises at least one diffusor surface (6a, 6b) con- figured and placed to diffusively redistrib^¬ ute the energy of the primary light beams (3) into the illuminating light beam (7) so that the energy density of the illuminating light beam leaving the illumination device does not exceed the maximum permissible exposure of a human eye.

2. An illumination device (1) as defined in claim 1, wherein the different wavelengths at which the plural- ity of laser diodes (2) is configured to emit the pri^¬ mary light beams (3) are matched to a plurality of ab^¬ sorption peak wavelengths of the photosensitive bio^¬ logical process.

3. An illumination device (1) as defined in claim 1 or 2, wherein the at least one diffusor surfaces (6a, 6b) comprise a surface (6a) of a transmission type diffu^¬ sor element.

4. An illumination device (1) as defined in any of claims 1 to 3, wherein the at least one diffusor sur- faces (6a, 6b) comprise a surface (6b) of a reflection type diffusor element.

5. An illumination device (1) as defined in any of claims 1 to 4, wherein at least one of the laser di^¬ odes (2) is configured to operate as pulsed.

6. An illumination device (1) as defined in any of claims 1 to 5, wherein the illumination device (1) further comprises a detector (16) for monitoring luminescence from a biological organism (14) where the photosensitive biological process takes place.

7. An illumination device (1) as defined in claim 6, wherein the illumination device (1) further comprises control means (15) for controlling the operation of at least one of the laser diodes based on the monitored luminescence .

8. An illumination device (1) as defined in any of claims 1 to 7, wherein the photosensitive biological process is photosynthesis.

9. An illumination device (1) as defined in claims 2 and 8, wherein the plurality of laser diodes (2) is configured to emit the primary light beams (3) sub^¬ stantially at wavelengths of 495 nm, 660 nm, and 735 nm.

10. An illumination system (11, 17) for activating a photosensitive biological process, the illumination system comprising an illumination device (1) as defined in any of claims 1 to 9, and a site (13, 19) for receiving a biological organism (14) to be illuminated by the illumination device (1) .

11. An illumination system (11) as defined in claim 10, wherein the illumination system is a greenhouse system (11).

12. An illumination system (17) as defined in claim 10 or 11, wherein the illumination system comprises a chamber (18) configured to produce controlled environ^¬ mental conditions within the chamber, the site (19) for receiving a biological organism (14) being located within the chamber (18) .

13. An illumination system (17) as defined in claim

12, wherein the chamber is a refrigerator (18) .

14. Use of an illumination device (1) as defined in any of claims 1 to 9 to activate a photosensitive bio^¬ logical process.

15. A method for producing an illuminating light beam (7) to activate a photosensitive biological process, the method comprising generating light at a wavelength range overlapping with the absorption wavelength band of the photosensitive biological process, charac^¬ teri zed in that

said generating light comprises generating primary light beams (3) at different wave^¬ lengths by means of a plurality of laser di^¬ odes (2); and that

the method further comprises diffusively re- distributing the energy of the primary light beams (3) into the illuminating light beam (7) so that the energy density of the acces^¬ sible illuminating light beam (7) does not exceed the maximum permissible exposure of a human eye.

16. A method as defined in claim 15, wherein the meth^¬ od further comprises monitoring luminescence from a biological organism (14) where the photosensitive bio^¬ logical process takes place.

17. A method as defined in claim 16, wherein the method further comprises controlling the operation of the laser diode (2) based on the monitored luminescence.