Abstract:
A fluorescent lamp having a non-fluorescent substance layer and a phosphor layer laminated and coated on, or having a fluorescent composition composed of a mixture of a non-fluorescent substance and a phosphor, coated on the inner wall of a glass bulb, wherein the non-fluorescent substance is an oxide containing at least one rare earth element selected from the group consisting of yttrium, gadolinium and lanthanum.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fluorescent lamp having the optical output controlled. 
     2. Description of the Background 
     Japanese Unexamined Utility Model Publication No. 117558/1985 discloses a cold cathode fluorescent lamp having the optical output of a fluorescent lamp controlled without changing the emission wavelength by coating on the inner wall of a glass bulb, a phosphor containing a non-fluorescent substance, for example, a manganese-activated zinc orthosilicate phosphor powder or a cerium.terbium-activated lanthanum silicophosphate phosphor powder and a calcium pyrophosphate powder or an aluminum powder. 
     Further, U.S. Pat. No. 4,069,441 discloses a gas discharge lamp having at least two layers of phosphor particles laminated on the surface of a support, whereby a high level of optical output has been made possible by converting the radiant energy generated by gas discharge to a radiation with a long wavelength, by incorporating a non-fluorescent white substance which absorbs no more 20% of ultraviolet rays with a wavelength of 240 μm or longer, for example, an ultraviolet reflective white substance such as barium sulfate or calcined calcium phosphate, to the phosphor layers except for the layer on the discharge side. 
     Furthermore, Japanese Unexamined Patent Publication No. 40854/1990 discloses a fluorescent lamp whereby high luminance and high color rendering are attained by coating a second layer comprising one or more phosphors on a first layer comprising a non-fluorescent substance of the formula LnPO 4  (Ln=La, Gd, Y or Al), or by coating a mixture of such a non-fluorescent substance and phosphors. 
     Still further, European Patent No. 228010 discloses an inexpensive fluorescent composition with high luminance having a phosphor and an alkaline earth metal sulfate fused to each other. 
     SUMMARY OF THE INVENTION 
     The above fluorescent lamps have a drawback that as compared with fluorescent lamps having no non-fluorescent substance incorporated, a decrease with time of the optical outputs (luminous flux-maintaining ratios) is substantial. 
     It is an object of the present invention to overcome such a drawback and to provide a fluorescent lamp whereby the optical output can be controlled without lowering the luminous flux-maintaining ratio of the fluorescent lamp. 
     The present invention provides a fluorescent lamp having a non-fluorescent substance layer and a phosphor layer laminated and coated on, or having a fluorescent composition composed of a mixture of a non-fluorescent substance and a phosphor, coated on the inner wall of a glass bulb, wherein the non-fluorescent substance is an oxide containing at least one rare earth element selected from the group consisting of yttrium, gadolinium and lanthanum. 
     Now, the present invention will be described in further detail with reference to the preferred embodiments. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the present invention, as the rare earth element for the above non-fluorescent substance, yttrium and gadolinium are excellent in the luminous flux-maintaining ratio, and yttrium is particularly preferred from the viewpoint of various properties including the initial luminous flux and the luminous flux-maintaining ratio. 
     The oxide of such rare earth element may, for example, be represented by the formula (Y x  Gd y  La z ) 2  O 3  where 0≦x≦1, 0≦y≦1, 0≦z&lt;1, and x+y+z=1. 
     In the present invention, it is preferred to incorporate at least one element selected from the group consisting of an alkaline earth metal and boron in addition to the above oxide of such rare earth element in order to avoid a decrease of the luminous flux-maintaining ratio. The content of such a substance is usually very small, and it is advisable to incorporate such an element in an amount of not higher than 5000 ppm, preferably from 10 to 3000 ppm. It is particularly preferred to incorporate boron in an amount of from 10 to 1000 ppm, more preferably from 50 to 500 ppm, or an alkaline earth metal in an amount of from 0 to 1500 ppm, more preferably from 10 to 1000 ppm. The alkaline earth metal is preferably selected from the group consisting of Ba, Sr, Ca and Mg, and it is particularly preferred to use Ba as an essential component. 
     As the above non-fluorescent material, it is particularly effective to use particles preliminarily fired at a temperature of at least 1000° C., preferably from 1100° C. to 1600° C. It is believed that by the firing, the crystallizability of the powder is improved, whereby the deterioration of the useful life can be suppressed. 
     Further, it is preferred to incorporate the above-mentioned at least one element selected form the group consisting of an alkaline earth metal and boron by mixing or coprecipitation prior to the firing. 
     Further, the particle size of the non-fluorescent substance is preferably relatively close to the particle size of the phosphor to be coated from the viewpoint of the uniformity of the coated surface. It is advisable that the average particle size of the non-fluorescent substance is smaller than the average particle size of the phosphor to be used, and it is particularly recommended that the average particle size of the non-fluorescent substance is smaller by from 1 to 3 μm than the average particle size of the phosphor to be used. The particle size is preferably within a range of from 0.5 μm to 10 μm, more preferably from 1 μm to 5 μm, by FSSS (Fisher Sub-Sieve Sizer). 
     In the present invention, in addition to the oxide containing the above-mentioned rare earth element, the non-fluorescent substance may contain a conventional ultraviolet reflective white or transparent inorganic substance (such as silica, alumina or barium sulfate). 
     The phosphor to be used in the present invention may be any phosphor commonly used for conventional fluorescent lamps or any phosphor which emits a light under excitation by ultraviolet rays. In the following Examples, a tri-color lamp has been described, for which a long useful life is particularly desired at present, but the present invention is by no means restricted to such specific Examples. 
     The present invention resides in suppressing as far as possible the change with time of the properties in the switched-on state of a fluorescent lamp having the optical output controlled, by incorporating the above-mentioned oxide of a rare earth element as a non-fluorescent substance to the phosphor. The amount of the rare earth oxide added, is determined depending upon the required optical output and is not particularly limited. 
    
    
     Now, the present invention will be described with reference to Examples. However, it should be understood that the present invention is by no means restricted by such specific Examples. 
     EXAMPLES 1 to 10 
     A phosphor mixture comprising 38% by weight of Y 2  O 3  Eu (red-emitting phosphor), 28% by weight of LaPO 4  :Ce,Tb (green-emitting phosphor) and 34% by weight of (Sr,Ca,Ba,Mg) 20  (PO 4 ) 2  Cl 2  :Eu (blue-emitting phosphor) and, as a non-fluorescent substance, a powder obtained by firing Y 2  O 3  at 1400° C. (containing 160 ppm of B, 320 ppm of Ba and 20 ppm of Sr i.e. the total amount of B, Ba and Sr being 500 ppm) were mixed in the various proportions as identified in Table 1, and fluorescent lamps FCL30EX-N (as prescribed in JIS) were prepared. With respect to each lamp, the initial luminous flux (lm) and the luminous flux-maintaining ratio upon expiration of 2000 hours were examined, and the results are shown in Table 1. 
     The &#34;initial luminous flux&#34; is the luminous flux (lm) upon expiration of 100 hours after switching on, and the &#34;luminous flux-maintaining ratio&#34; is the percentage of the luminous flux upon expiration of 2000 hours/the luminous flux upon expiration of 100 hours (the same will apply hereinafter). As is evident from Table 1, all the fluorescent lamps are capable of controlling the optical outputs (luminous flux) without reducing the luminous 
     
                       TABLE 1______________________________________                              Luminous                       Initial                              flux-Example   Phosphor          luminous                              maintainingNo.       mixture  Y.sub.2 O.sub.3                       flux (lm)                              ratio (%)______________________________________1         100       0       2080   88.02         90       10       2000   88.23         80       20       1980   88.24         70       30       1900   88.55         60       40       1795   88.46         50       50       1715   88.67         40       60       1575   88.68         30       70       1490   89.29         20       80       1270   89.510        10       90        950   91.3______________________________________ 
    
     EXAMPLE 11 
     A fluorescent lamp was prepared in the same manner as in Example 4 except that instead of Y 2  O 3  as the non-fluorescent substance, La 2  O 3  (containing the same amounts of B, Ba and Sr) was used as a non-fluorescent substance, and the initial luminous flux and the luminous flux-maintaining ratio were measured. This fluorescent lamp had an initial luminous flux of 1950 lm and a luminous flux-maintaining ratio of 88.0%. 
     EXAMPLE 12 
     A fluorescent lamp was prepared in the same manner as in Example 4 except that Y 2  O 3  as the non-fluorescent substance, Gd 2  O 3  (containing the same amounts of B, Ba and Sr) was used as a non-fluorescent substance. This fluorescent lamp had an initial luminous flux of 1920 lm, and a luminous flux-maintaining ratio of 88.3%. 
     EXAMPLE 13 
     A fluorescent lamp was prepared in the same manner as in Example 4 except that Y 2  O 3  containing no B, Ba or Sr was used. This fluorescent lamp had an initial luminous flux of 1765 lm and a luminous flux-maintaining ratio of 88.0%. 
     COMPARATIVE EXAMPLES 1 to 4 
     Fluorescent lamps were prepared in the same manner as in Examples 1 to 10 except that instead of Y 2  O 3  as the non-fluorescent substance, Ca 2  P 2  O 7 , alumina, BaSO 4  and LaPO 4  were, respectively, incorporated in an amount of 30%, evaluated in the same manner. The results are shown in Table 2. As is evident from Table 2, a decrease in the luminous flux-maintaining ratio is substantial with the fluorescent lamps having such non-fluorescent substances incorporated. 
     
                       TABLE 2______________________________________                           Luminous       Non-       Initial  flux-Comparative fluorescent                  luminous maintainingExample No. substance  flux (lm)                           ratio (%)______________________________________1           Ca.sub.2 P.sub.2 O.sub.7                  1855     78.72           Al.sub.2 O.sub.3                  1700     86.23           BaSO.sub.4 1935     86.64           LaPO.sub.4 1880     82.3______________________________________ 
    
     The present invention has made it possible to present a fluorescent lamp which is capable of controlling the optical output without reducing the luminous flux-maintaining ratio of the fluorescent lamp by adopting the above construction.