Plant growing 4 phosphor fluorescent lamp having a photon flux ratio of from 0.8 to 1.0 for light in the 600 NM-700 NM and 700 NM-800 NM bands
This application is a continuation of application Ser. No. 08/013,067, filed Feb. 3, 1993 (abandoned). 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     In recent years, research and development, as well as application to practical use, have been vigorously conducted in regard to bio-nursery for growing elite seeds and new spices by using bio-technologies such as tissue culture, cell fusion and gene recombination, as well as on plant factories intended for stably producing various pants with high product quality under an artificial environment which is free from natural conditions. A bio-nursery and a plant factory essentially require that light necessary for growing plants supplied from an artificial light source. Hitherto, lamps such as fluorescent lamps and BID lamps, e.g., metal halide lamps, high-pressure sodium lamps, have been used independently or in combination as the artificial light source. Among these lamps, fluorescent lamps are easy to handle and capable of being placed in close proximity to the object plants to satisfactorily illuminate the lamp. In addition, although the fluorescent lamps have been considered as being unsuitable as light sources for plant growth due to small light output per unit lamp as compared with BID lamps, compact fluorescent lamps having high output power have been developed. For these reasons, fluorescent lamps are nowadays expected to be one of the prominent artificial light sources for growing plants. 
     A fluorescent lamp for plant growth has been known which has two components: namely, a red fluorescent component and blue fluorescent component. The light emitting spectrum of this type of fluorescent lamp resembles that of the chlorophyll bio-synthetic action spectrum of plants. Typically, magnesium fluorogermanate activated with manganese is used as the red fluorescent component, while calcium tungstate is used as the blue fluorescent component. 
     Morphology of plants is a very important factor, particularly when the plants of high quality and high commercial value are to be produced. The elongation growth which largely affects the morphology of a plant is closely related to ratio of photon fluxes contained in two wavelength bands which are centered at 660 nm and 730 nm, i.e., the ratio between the photon flux included in the wavelength band of 600 to 700 nm and that in the wavelength band of 700 to 800 nm, represented by PF 600-700  /PF 700-800 . When this value is large, the internode elongation of the plant is suppressed to cause a dwarf tendency of the plant, whereas, when this ratio is large, the elongation is promoted. Actually, however, the conventional fluorescent lamps for plant growth substantially lack the light in the wavelength band of 700 to 800 nm, as shown in FIG. 2. In addition, since the fluorescent lamp for plant growth is designed to have such characteristics that the light is abundant in the wavelength band between 600 and 700 nm (see Table 1) so as to approximate the bio-synthetic curve of chlorophyll, the aforesaid ratio PF 600-700  /PF 700-800  of such lamp is 16.9, which is much greater than that, 1.1, of the natural light (standard daylight D 65 ). 
     
                       TABLE 1                                                     
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              PPF per lamp                                                
              input                                                       
Light source  (relative value)                                            
                          PF.sub.600-700 /PF.sub.700-800                  
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Standard daylight                                                         
              556          1.1                                            
(D.sub.65)                                                                
Incandescent lamp                                                         
              100          0.7                                            
Fluorescent lamp for                                                      
              243         16.9                                            
plant growth                                                              
(PG type)                                                                 
Three band    422         10.8                                            
fluorescent lamp                                                          
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     Therefore, the plants grown under the light of conventional fluorescent lamps for plant growth suffer from suppression in the internode elongation and growth of the veins, thus causing dwarfing of the plants. Furthermore, as shown in FIG. 3, other ordinary illuminating fluorescent lamps, e.g., three band fluorescent lamp, substantially lack light in the wavelength band of 700 to 800 nm, so that the aforesaid ratio PF 600-700  /PF 700-800  is as large as 10.8, thus enhancing the dwarf tendency of the plant. It is also to be pointed out that, as shown in FIG. 2, the conventional fluorescent lamp for plant growth has a smaller amount of photosynthetic photon flux than three band fluorescent lamps in the wavelength band of 400 to 700 nm which is effective in photosynthesis. 
     An object of the present invention is to provide a fluorescent lamp for plant growth which is effective in growing plants similar to those grown under natural daylight in terms of morphology, i.e., height, leaf shape and size and so forth, and which effectively promotes photosynthesis. 
     To this end, according to the present invention, there is provided a fluorescent lamp, comprising: a glass tube; and a fluorescent layer composed of three fluorescent components having light emission peaks in the bands of 440 to 460 nm, 540 to 560 nm and 600 to 620 nm, respectively, and a far-red emission fluorescent component having peak of light emission in the wavelength band of 700 to 800 nm, the fluorescent layer having the PF ratio ranging from 0.8 to 1.2 in terms of the ratio between the photon flux PF contained in the wavelength band of 600 to 700 nm to that in the wavelength zone of 700 to 800 nm, covering the PF ratio of natural daylight (D 65 ) which is 1.1. 
     According to the invention, a high photosynthesis promotion effect can be attained by using, as the fluorescent component which emits light of wavelength band of 400 to 700 nm which is effective in photosynthesis, by virtue of the use of a three band fluorescent lamp which has three types of fluorescent components activated by rare earth metals and which exhibits the highest emission efficiency among the fluorescent lamps. Furthermore, according to the present invention, a fluorescent component is used which emits far-red rays of a wavelength band of 700 to 800 nm effective in the control of plant morphology, and the ratio of the PF between the PF contained in the wavelength band of 600 to 700 nm and that contained in the wavelength band of 700 to 800 nm is determined to range between 0.8 and 1.2. Owing to these features, the present invention makes it possible to efficiently produce plants of high quality, while attaining plant morphology similar to that obtained under natural light, in terms of the height, shape and size of leaves, and so forth. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a spectral distribution chart showing the characteristics of an embodiment of the fluorescent lamp in accordance with the present invention; 
     FIG. 2 is a spectral distribution chart illustrative of the characteristic of a known fluorescent lamp for plant growth; 
     FIG. 3 is a spectral distribution chart illustrative of the characteristic of a three band fluorescent lamp; 
     FIG. 4 is a view of a fluorescent lamp having a fluorescent layer in accordance with the present invention; and 
     FIG. 5 is a cut-away view of a portion of the fluorescent lamp shown in FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A description will now be given of a preferred embodiment of the present invention. Barium aluminate magnesium (BAM) activated with bivalent europium is used as the fluorescent component which emits in the wavelength region of 440 to 460 nm. Lanthanum phosphate (LAP) activated with cerium and terbium is used as the fluorescent component which emits light in the wavelength band of 540 to 560 nm. Yttrium oxide (YOX) activated with trivalent europium is used as the fluorescent component which emits light in the wavelength band of 600-620 nm. Finally, lithium aluminate (ALF) activated with iron is used as a fluorescent component which emits light in the wavelength band of 700 to 800 nm. With these fluorescent components, a fluorescent suspension liquid is formed at a weight ratio of BAM 16%, LAP 32%, YOX 32% and ALF 20%. A fluorescent layer is formed on the inner wall surface of a glass tube by an ordinary method using this fluorescent suspension liquid, thus obtaining a compact 55 Watt fluorescent lamp. The spectral distribution characteristic of the thus obtained fluorescent lamp is shown in FIG. 1. 
     In a walk-in type growth cabinet which is air-conditioned to maintain air of 25° C. and relative humidity of 70% with CO 2  concentration of 300 ppm, four plant cultivation zones were defined which employ, respectively, a fluorescent lamp embodying the present invention, a standard daylight light source, a conventional fluorescent lamp for plant growth and three band fluorescent lamp, as the light sources. In each such cultivate zones, the light intensity on the culture surface was set to a level of 200 μmol.m -2 .S -1  in terms of photosynthetic photon flux density (photon flux density in wavelength band of 400 to 700 nm, PPFD), and seedlings of sunflowers were water-cultured in a rock wool culture. The state of growth in each zone was examined and studied. The photosynthetically effective photon flux density in the photon environment of each plant cultivation zone, together with the ratio of the photon flux density PF 600-700  contained in the wavelength band of 600 to 700 nm to that PF 700-800  contained in the wavelength band of 700 to 800 nm, is shown in Table 2. In the case of the illustrated embodiment, the above-mentioned ratio was 1.2. 
     
                       TABLE 2                                                     
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              Photosynthetically                                          
                            Photon flux                                   
              effective photon                                            
                            density ratio                                 
Types of light                                                            
              flux density PPFD                                           
                            PPF.sub.600-700 /                             
source        (μ · m.sup.-2 · S.sup.-1)              
                            PPF.sub.600-700                               
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Fluorescent lamp of                                                       
              200            1.2                                          
embodiment                                                                
Standard daylight                                                         
              200            1.1                                          
lightsource                                                               
Fluorescent lamp for                                                      
              200           16.9                                          
plant growth                                                              
(PG type)                                                                 
Three band    200           10.8                                          
fluorescent lamp                                                          
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     Seeds of sunflowers, homogeneity of which has been genetically assured, were used in the experiment. The seeds were dipped in water for 24 hours so as to bud and to sufficiently develop the cotyledon. Homogeneous seedlings, obtained 8 days after the inoculation, with third and fourth leaves being developed, were extracted so as to be used as test plants. The sunflower seedlings, as the test plants as selected as above, were grafted to the aforesaid four culture zones which employ different types of light sources under the same air conditions in terms of the air temperature, relative humidity, CO 2  density and photosynthetic photon flux density, and were cultured in the rock wool cultures of the respective culture zones. Table 3 shows the states of growth of the test plants as observed 8 days after the grafting to these culture zones. 
     
                       TABLE 3                                                     
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             Fresh   Dry       Grass Grass                                
Types of light                                                            
             weight  weight    height                                     
                                     area                                 
source       (g)     (g)       (cm)  (cm.sup.2)                           
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Fluorescent lamp                                                          
             14.0    1.5       19.1  312.3                                
of embodiment                                                             
Standard daylight                                                         
             14.8    1.6       19.3  308.0                                
light source                                                              
Fluorescent lamp                                                          
              9.0    1.0        8.3  205.2                                
(PG type)                                                                 
Three band    8.8    0.9        8.1  199.7                                
fluorescent lamp                                                          
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     The test plants grown in the culture zone employing the fluorescent lamp of this embodiment, as well as the culture zone under the natural light, showed greater weight those of the other culture zones in terms of both fresh and dry weights. As to the plant morphology, the test plants in the culture zone using the conventional fluorescent lamp for plant growth and the test plant in the culture zone using the three band fluorescent lamp, showed grass height and leaf area which are respectively about 40% and about 65% smaller than those obtained in the culture zone employing the standard daylight light source. In contrast, the test plant grown in the culture zone employing the fluorescent lamp in accordance with this embodiment showed results substantially equivalent to those obtained in the culture zone illuminated by the natural daylight light source. 
     As will be understood from the foregoing description, the fluorescent layer has a composition of BAM 16%, LAP 32%, YOX 32% and ALF 20% in terms of weight ratio, and exhibits the PF ratio of 1.2. This, however, is only illustrative, and the fluorescent layer can have a composition of BAM 13%,LAP 26%, YOX 26% and ALF 35% in terms of weight ratio, with the PF ratio value of 1.0, or a composition of BAM 10%, LAP 20%, YOX 20% and ALF 50% in terms of weight ratio, with the PF ratio of 0.8, as shown in Table 4. It is therefore possible to obtain a plant growing fluorescent lamp in accordance with the present invention, by selecting the contents of the respective fluorescent components so as to make the PF ratio value fall within the range of from 0.8 to 1.2. 
     
                       TABLE 4                                                     
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       BAM   LAP    YOX    ALF  PPF.sub.600-700 /PPF.sub.700-800          
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Embodiment                                                                
         16%     32%    32%  20%  1.2                                     
Embodiment                                                                
         13%     26%    26%  35%  1.0                                     
2                                                                         
Embodiment                                                                
         10%     20%    20%  50%  0.8                                     
3                                                                         
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     The physical structure of an embodiment of a fluorescent lamp in accordance with the present invention will now be described with reference to FIGS. 4 and 5. As shown in FIG. 4, fluorescent lamp 1 includes tube 2, which is preferably a glass tube. FIG. 5 shows a cut-away view of a portion of fluorescent lamp 1. As shown in FIG. 5, tube 2 has inner surface 2a, on which is formed fluorescent layer 3 in accordance with the present invention, whose composition has been described above. 
     As will be understood from the foregoing description, according to the present invention, there is provided a fluorescent lamp, comprising: a glass tube; and a fluorescent layer composed of three fluorescent components having light emission peaks in the bands of 440 to 460 nm, 540 to 560 nm and 600 to 620 nm, respectively; and a far-red emission fluorescent component having peak of light emission in the wavelength band of 700 to 800 nm, the fluorescent layer having the PF ratio ranging from 0.8 to 1.2 in terms of the ratio between the photon flux PF contained in the wavelength band of 600 to 700 nm to that in the wavelength zone of 700 to 800 nm. By virtue of these features, the present invention makes it possible to promote photosynthesis and to attain a plant morphology similar to that obtained under natural daylight in terms of grass height, leaf shape and size, thus enabling efficient growth of plants with high quality.