Abstract:
A coating system for a fluorescent lamp, and fluorescent lamps provided therewith. The coating system includes a phosphor-containing coating containing a mixture of phosphors that contain less than 10% weight rare earth phosphors. The phosphor-containing coating emits visible light having a color rendering index of at least 87 when excited by UV radiation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part patent application of co-pending U.S. patent application Ser. No. 13/247,350, filed Sep. 28, 2011. The contents of this prior application are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to lighting systems and related technologies. More particularly, this invention relates to fluorescent lamps and coating systems utilized by fluorescent lamps to generate visible light. 
     Fluorescent lamps have been in use and commercialization since the 1930s. More recently, both consumers and producers have voiced increased concerns for energy efficiency and environmental impact of products, spanning all industries, including the lighting industry. As such, fluorescent lights have seen an increase in usage due to their increased energy efficiency when compared to conventional incandescent lights. Fluorescent lights see a great deal of competition from light-emitting diode (LED) lights, due to a potential for greater efficiency and luminosity of LEDs. Significant effort and research have been made in the interest of improving fluorescent light lumen output without increasing power requirements or significantly increasing material costs. 
     A nonlimiting example of a fluorescent lamp  10  is schematically represented in  FIG. 1 . The lamp  10  is represented as having a sealed glass tube comprising of a glass envelope or shell  12  enclosing an interior chamber  14 . The chamber  14  is preferably at very low pressure, for example, around 0.3% atmospheric pressure, and contains a gas mixture having at least one constituent that can be ionized to generate radiation that includes ultraviolet (UV) wavelengths. According to the current state of the art, such a gas mixture includes one or more inert gases (for example, argon) or a mixture of one or more inert gases and other gases at a low pressure, along with a small quantity of mercury vapor. Electrodes  16  inside the chamber  14  are electrically connected to electrical contact pins  18  that extend from oppositely-disposed bases  20  of the lamp  10 . When the contact pins  18  are connected to a power source, the applied voltage causes current to flow through the electrodes  16  and electrons to migrate from one electrode  16  to the other electrode  16  at the other end of the chamber  14 . In the process, this energy converts a small amount of the liquid mercury from the liquid state to a charged (ionized) gaseous (vapor) state. The electrons and charged gas molecules move through the chamber  14 , occasionally colliding with and exciting the gaseous mercury molecules, raising the energy level of the electrons in the mercury atoms. In order to return to their original energy level, the electrons release photons. 
     Due to the arrangement of electrons in mercury atoms, most of the photons released by these electrons are in the ultraviolet (UV) wavelengths. This is not visible light, and as such for the lamp  10  to emit visible light these photons must be converted to a visible light wavelength. Such a conversion can be performed by a coating  22  represented in  FIG. 1  as disposed at the interior surface of the glass shell  12 . The coating  22  contains phosphor powders and is often separated from the glass shell  12  by a UV-reflecting barrier layer  24  of, for example, alumina (Al 2 O 3 ). The UV radiation emitted by the ionized mercury vapor is absorbed by the phosphor composition within the coating  22 , resulting in excitation of the phosphor composition to produce visible light that is emitted through the glass shell  12 . More particularly, when electrons of the phosphor atoms are struck by photons, the electrons become excited to a higher energy level and emit a photon to return to their original energy level. The emitted photon has less energy than the impinging photon and is in the visible light spectrum to provide the lighting function of the lamp  10 . The color and luminosity of the lamp  10  are largely the result of the phosphor or phosphors used in the coating  22 . 
     The apparent, or perceived, color of a light source can be described in terms of color temperature, which is the temperature of a black body that emits radiation of about the same chromaticity as the radiation considered. A light source having a color temperature of 3000K has a larger red component than a light source having a color temperature of 4100K. As additional examples, a fluorescent lamp having a perceived “warm white” color may have a CCT of approximately 3000K, whereas a fluorescent lamp having a perceived “cool white” color may have a CCT of approximately 4000K. Another measure of fluorescent lamp performance is the color rendering index (CRI). The CRI of a light source does not indicate the apparent color of the light source, but instead is a quantitative measure of the ability of a light source to reproduce the colors of objects faithfully in comparison with an ideal or natural light source. CRIs can only be accurately compared among two light sources having the same CCT. The highest possible numeric CRI value is 100. Incandescent lamps, which are essentially blackbodies, have CRIs of 100. Typical LEDs have CRIs of 80 or more, with CRIs of up to 98 being claimed, whereas fluorescent lamps typically have CRIs in a range of about 50 to about 90. In this regard, a high CRI for fluorescent lamps can be considered to be about 80 and higher, particularly at least 87. 
     Because fluorescent lamps utilize phosphors to produce visible light, the CRI and CCT of a fluorescent lamp is strongly influenced by the particular amounts of phosphors used and their compositions. Significant improvements in the CRIs of fluorescent lamps have been achieved with rare earth phosphors, in particular, phosphors containing one or more rare earth elements generally considered to include the fifteen lanthanides, scandium, and yttrium. Though fairly abundant, rare earth elements are becoming increasingly prohibitive to procure due to the majority of known rare earth reserves being found in limited locations, and the need to separate individual rare earth elements or compounds from relatively low concentrations found in various mined compounds. In addition, demand for rare earth elements has increased in a wide variety of technologies, for example, medical technologies, wind turbines, hybrid automobiles, televisions, smart phones, and computers. As such, the use of rare earth elements in phosphor coatings of fluorescent lamps has or may become increasingly cost-prohibitive, and effective alternative solutions are being sought. However, suitable alternatives preferably should not detrimentally affect the CRI or CCT of a fluorescent lamp. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides phosphor compositions suitable for use in fluorescent lamps to produce visible light, and to lamps utilizing such phosphor compositions. 
     According to a first aspect of the invention, a coating system comprises a phosphor-containing coating containing a mixture of phosphors. The mixture of phosphors contains less than 10% weight rare earth phosphors, and the phosphor-containing coating emits visible light having a color rendering index of at least 87 when excited by UV radiation. 
     Other aspects of the invention include fluorescent lamps having coating systems comprising the composition and characteristics described above. 
     A technical effect of the invention is the ability of a fluorescent lamp equipped with the phosphor-containing coating to exhibit a high color rendering index (CRI) with very limited use of rare earth phosphors. The phosphor-containing coating is particularly well-suited for use in fluorescent lamps that are desired to have CCTs at a value within a range of about 3000K to about 4100K. Such fluorescent lamps have the capability of exhibiting desirable lighting characteristics while minimizing certain disadvantageous aspects relating to the use of rare earth elements. 
     Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents a fluorescent lamp, including a fragmentary cross-sectional view of a glass tube of the lamp and an inner surface of the tube provided with a UV-reflecting coating and phosphor-containing coating. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will be described hereinafter in reference to the lamp  10  shown in  FIG. 1 , though it will be appreciated that the teachings of the invention are not limited to the lamp  10  and instead are more generally applicable to various lighting applications in which visible light is generated with the use of phosphor compounds, such as non-linear fluorescent lamps, or compact fluorescent lamps (CFLs), or other types of lamps. 
     The invention relates to coating systems that include a phosphor-containing coating, such as the single phosphor-containing coating  22  of  FIG. 1 , applied to a transparent or translucent substrate, such as the glass shell or envelope  12  of the fluorescent lamp  10 . In the nonlimiting example of  FIG. 1 , a discrete UV-reflecting layer  24  is represented as a constituent of a coating system that includes the phosphor-containing coating  22 . The UV-reflecting layer  24  may contain a scattering agent selected on the basis of its ability to scatter incoming UV rays emitted from an ionized constituent (for example, mercury) within the chamber  14  of the lamp  10 . The scattered UV rays are then absorbed by the adjacent phosphor-containing coating  22 , which as a result emits visible light. 
     According to an aspect of the invention, the phosphor-containing coating  22  has a composition that enables the lamp  10  to have a CRI of at least 87, and preferably to emit visible light emitted having a CCT at a value in a range of about 3000K to about 4100K, for example, at 3000K, 3500K, or 4100K. In addition, rare earth-containing phosphors constitute not more than about 10 weight percent of the phosphor content of the coating  22 , with the balance of the phosphor content being non-rare earth phosphors, of which at least one is at least one white halophosphor. 
     A preferred aspect of the invention is the capability of improving fluorescent lamp lumen output without significantly lowering CRI through the use of a white halophosphor. According to a preferred but nonlimiting embodiment of the invention, the entire phosphor content of the phosphor-containing coating  22  is a mixture of phosphors consisting of at least one white halophosphor, at least one strontium red phosphor, at least one blue halophosphor, and at least one green-blue emitting rare earth phosphor. The strontium red phosphor and the blue and white halophosphors are “non-RE” phosphors having broadband emitting distribution, which are believed to significantly improve lamp CRI. The green-blue emitting rare earth (RE) phosphor represents a relatively small fraction (not more than 10 weight percent) of the phosphor mixture, yet is preferably present in an amount capable of promoting the CRI of the coating  22  while also obtaining a color temperature consistent with the aforementioned CCT range. 
     Various phosphors that can be employed by the invention are commercially manufactured and available. The processes for manufacturing and introducing these phosphors into a phosphor mix are known to those skilled in the art, and therefore do not require further discussion here. A nonlimiting list of notable phosphors that are suitable for use in this invention is found in Table 1. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Type 
                 Name 
                 Formula 
               
               
                   
               
             
             
               
                 Non-RE strontium red 
                 SR 
                 Sr 3 (PO 4 ) 2 :Sn 2+   
               
               
                 phosphor 
               
               
                 Non-RE blue halophosphor 
                 Blue Halo 
                 Ca 5 F(PO 4 ) 3 :Sb 
               
               
                 Non-RE white halophosphor 
                 White Halo 
                 Ca 5 (PO 4 ) 3 (F,Cl):Sb 3+ ,Mn 2+   
               
               
                 RE green-blue phosphor 
                 BAMn 
                 BaMgAl 10 O 17 :Eu 2+ ,Mn 2+   
               
               
                 RE green-blue phosphor 
                 SAE 
                 Sr 4 Al 14 O 25 :Eu 2+   
               
               
                   
               
             
          
         
       
     
     In investigations leading to the present invention, evaluations performed on phosphor mixtures confirmed that appropriate additions of a white halophosphor to phosphor mixtures containing a strontium red phosphor, a blue halophosphor, and a green-blue emitting rare earth phosphor were capable of improving lamp lumen output while maintaining nearly the same performance in CRI. In Table 2 below, relative amounts (by weight percent) of Ca 5 (PO 4 ) 3 (F,Cl):Sb 3+ ,Mn 2+  as the white halophosphor (White Halo) are indicated in relation to a phosphor mixture containing, respectively, about 72%, about 22%, and about 6% (by weight) of Sr 3 (PO 4 ) 2 :Sn 2+ , Ca 5 F(PO 4 ) 3 :Sb, and BaMgAl 10 O 17 :Eu 2+ ,Mn 2+  as the strontium red (SR) phosphor, blue halophosphor (Blue Halo), and green-blue emitting rare earth phosphor (BAMn) (respectively). 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Phosphor Mixtures 
                   
                 Lamp Performance 
               
             
          
           
               
                 Sample 
                 SR-BAMn-BlueHalo 
                 White Halo 
                 CRI 
                 Lumen % 
               
               
                   
               
               
                 A 
                 100%  
                  0% 
                 90.6 
                 100 
               
               
                 B 
                 90% 
                 10% 
                 88.9 
                 103 
               
               
                 C 
                 85% 
                 15% 
                 87.6 
                 106 
               
               
                   
               
             
          
         
       
     
     The test data indicate that increasing the proportion of white halophosphor within the phosphor mixture improved lumen output without significantly reducing lamp CRI. Such a capability enables a lamp to achieve improved lumen performance while reducing the amount of rare earth phosphor(s) within the mixture and without negatively affecting the apparent or perceived color (described in terms of color temperature) of the light. In the particular examples evaluated during the investigation, an addition of 15 weight percent white halophosphor increased lumen output by about 6% while CRI was reduced by only 3 points, while still meeting standards for assessing light sources for suitability of use in a wide variety of applications, including retail, residential, hospitality, etc. 
     In addition to Samples B and C of Table 2, based on the results of the investigation it was concluded that a lamp utilizing a phosphor mixture of strontium red, blue halophosphor, and green-blue rare earth phosphor and modified to contain a white halophosphor should be capable of exhibiting improved CRI as compared to lamps that only utilize the same or similar tri-phosphor mixture of strontium red, blue halophosphor, and green-blue rare earth phosphor. The relative amounts of each phosphor component in phosphor mixtures of this invention include (but are not limited to), by weight, about 55% to about 90% and more preferably about 65% to about 80% of a strontium red (non-RE) phosphor, from about 10% to about 30% and more preferably about 15% to about 25% of a blue (non-RE) halophosphor, above 0% to about 20% and more preferably about 5% to about 15% of a white (non-RE) halophosphor, and about 0% to about 10% and more preferably about 4% to about 8% of a green-blue rare earth phosphor. 
     While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of a lamp could differ from that shown, and materials and processes other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.