WO2008129489A2

WO2008129489A2 - Fluorescent mercury vapor discharge lamp comprising trichromatic phosphor blend

Info

Publication number: WO2008129489A2
Application number: PCT/IB2008/051496
Authority: WO
Inventors: Thomas JÜSTEL; Cornelis Reinder Ronda; Jörg Meyer
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N. V.
Priority date: 2007-04-23
Filing date: 2008-04-18
Publication date: 2008-10-30
Also published as: WO2008129489A3

Abstract

A fluorescent mercury vapor discharge lamp for generating white light with good color rendering of illuminated objects is achieved by blending three different phosphors, namely a blue green-emitting phosphor Ba1-a-bMgA110O17:EuaMnb, wherein 0.01 ≤ a ≤ 0.5 and 0 ≤ b ≤ 0.5, a yellow-emitting phosphor selected from the group of (Sr1-x-aCax)2Si04:Eua, wherein 0.01 ≤ a ≤ 0.5 and 0 ≤ x ≤ 1; Sr1-aLi2Si04:Eua, wherein 0.01 ≤ a ≤ 0.5; (Ca1-c-aSrc)Si2N2O2:Eua, wherein 0.01 ≤ a ≤ 0.5 and 0 ≤ c ≤ 1; and (Ca1-c-aSrc)SiAlN3:Cea, wherein 0.01 ≤ a ≤ 0.5 and 0 ≤ c ≤ 1; and a red-emitting phosphor selected from the group of (Y1-y-z-aGdyLuz)2O2S:Eua, wherein 0.01 ≤ a ≤ 0.5, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1; and (Y1-y-z-aGdyLuz)2VO4:Eua, wherein 0.01 ≤ a ≤ 0.5, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1; and (Y1-y-z-aGdyLuz)2(V1-wPw)O4:Eua, wherein 0.01 ≤ a ≤ 0.5, 0 ≤ y ≤ l, 0 ≤ z ≤ l; and (Y1-y-z-aGdyLuz)2(V1-wNbw)O4:Eua, wherein 0.01 ≤ a ≤ 0.5, 0 ≤ y ≤ l, 0 ≤ z ≤ l and 0 ≤ w ≤ l; and (Y1-y-z-aGdyLuz)2(V1-w-xPwNbx)O4:Eua, wherein 0.01 ≤ a ≤ 0.5, 0 ≤ x ≤ 10 ≤ y ≤ l, 0 ≤ z ≤ 1 and 0 ≤ w ≤ 1.The phosphor particles may be coated with a protective coating. The lamp can be operated under saturated or unsaturated mercury vapor pressure conditions.

Description

FLUORESCENT MERCURY VAPOR DISCHARGE LAMP COMPRISING TRICHROMATIC PHOSPHOR BLEND

TECHNICAL FIELD OF THE INVENTION

The invention relates to a fluorescent mercury vapor discharge lamp for general illumination and display applications, in particular to a fluorescent mercury low- pressure gas discharge lamp that operates under reduced or unsaturated mercury vapor pressure conditions.

BACKGROUND OF THE INVENTION

In a fluorescent mercury vapor discharge lamp, mercury constitutes a primary component for the generation of ultraviolet radiation ("primary radiation"). To convert the emitted ultraviolet radiation into other wavelengths in the visible range

("secondary radiation") for general illumination and display applications, a luminescent layer comprising a luminescent material is provided on an inner wall of the discharge vessel of the lamp. It is known that in a conventional fluorescent mercury vapor discharge lamp, during operation, mercury is consumed by interaction with different lamp components, such as the glass envelope, electrodes and in particular also by the luminescent material.

Due to the consumption of mercury during operation, a significant excess in the initial dosing of mercury is necessary to achieve a sufficiently long lifetime of the lamp.

Such excessive use of mercury is not desirable and is detrimental to the environment. This is in particular the case if the fluorescent mercury vapor discharge lamps are injudiciously processed after the end of their lifetime. There is therefore a need for the development of technologies that suppress the amount of mercury consumption in fluorescent mercury vapor discharge lamps, but without causing a reduction of the lamp life. From WO2004/032180 an "unsatured" mercury vapor discharge lamp is known, i.e. a mercury vapor discharge lamp that operates under unsaturated mercury vapor conditions. Such a lamp combines a minimum mercury content with an improved lumen performance at elevated temperatures.

It is not an easy task to operate a low-pressure mercury vapor discharge lamp under unsaturated mercury conditions, while simultaneously realizing a long lifetime of the discharge lamp. As mentioned above, the luminescent layer is known to add substantially to the mercury consumption in a fluorescent mercury vapor discharge lamp. The prior art document therefore suggests to apply the luminescent layer to the outside of the discharge vessel. Such a measure will certainly lead to a reduction of the mercury consumption. But luminescent layers are known to lack durability and to be susceptible to scratches and wear, to which an outside coating will be prone.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved lamp of the type described in the opening paragraph, providing white light with pleasing characteristics and a long lifetime.

The present invention relates to a fluorescent mercury vapor discharge lamp provided with a discharge vessel, the discharge vessel enclosing, in a gastight manner, a discharge space provided with a gas filling of a mixture of inert gases with mercury, and a luminescent layer comprising a luminescent material comprising a trichromatic phosphor particle blend, the lamp also comprising means for igniting and maintaining a gas discharge in the discharge space, said trichromatic phosphor particle blend comprising blue green-emitting phosphor Bai__a_bMgAli₀Oi₇:EuaMnb, wherein 0.01 < a < 0.5 and 0 < b < 0.5, a yellow-emitting phosphor selected from the group of (Sri__x_ _aCa_x)₂SiO₄:Eu_a, wherein 0.01 < a < 0.5 and 0 < x < 1; Sr^Li₂SiO₄ :Eu_a, wherein 0.01 < a < 0.5; (Cai__c__aSr_c)Si₂N₂O₂:Eu_a, wherein 0.01 < a < 0.5 and 0 < c < 1; and (Cai__c_ _aSr_c)SiADSf3:Ce_a, wherein 0.01 < a < 0.5 and 0 < c < 1; and a red-emitting phosphor selected from the group of (Yi_y-_z-_aGd_yLu_z)₂O₂S:Eu_a, wherein 0.01 < a < 0.5, 0 < y < 1, 0 < z < 1; and (Yi__y__z__aGd_yLu_z)₂VO₄:Eu_a, wherein 0.01 < a < 0.5, 0<y≤l,0<z≤l;and (Yi__y__z__aGd_yLu_z)₂(Vi__wP_w)O₄:Eu_a, wherein 0.01 < a < 0.5, 0<y≤l,0<z≤l;and (Yi__y__z_ _aGd_yLu_z)₂(Vi__wNb_w)O₄:Eu_a, wherein 0.01 < a < 0.5, O≤y≤l.O≤z≤landO≤w≤l; and (Yi__y__z__aGd_yLu_z)₂(Vi__w__xP_wNb_x)O₄:Eu_a, wherein 0.01 < a < 0.5, O≤x≤lO≤y≤l.O≤ z≤landO≤w≤l.

Such a trichromatic blend fulfils a set of requirements for phosphor particle blends to be used in mercury vapor discharge lamps, viz.:

High efficiency at high lamp operating temperatures,

Chemical stability at said high temperature and under intense UV radiation,

Sensitivity to both short and long wave UV from the discharge,

Overall light generation should be white, not too far from the blackbody locus.

Stability throughout very long lamp lives. By the application of a three-phosphor particle blend according to the invention a white lamp with a very high color rendering index (> 90) in the color temperature range from 2700 to 8600 K can be achieved.

By limiting the components of the blend to three phosphors, blends of an intended color point and color rendering index can be realized with improved reliability, in comparison with the multi-component blends according to the state in the art.

Preferably, the phosphor particle blend comprises 55 to 65 % of the blue green -emitting phosphor, 12.5 to 17.5 % of the yellow-emitting phosphor and 18 to 32 % of the red-emitting phosphor.

According to a preferred embodiment the luminescent material comprises a protective layer formed on the phosphor particles.

Preferably, the protective layer is formed by an inorganic compound selected from the group consisting of a layer of Me₂θ3, MePO₄, or MeBθ3, wherein Me is selected from Al, Sc, Y, La and Lu or mixtures thereof. Such inorganic compounds will provide a dense continuous coating on each phosphor particle separately or on a continuous layer of phosphor particles.

It has been found that a continuous coating of an inorganic compound selected from the group consisting of a layer of Me₂θ3, MePO₄, or MeBθ3, wherein Me is selected from Al, Sc, Y, La and Lu or mixtures thereof, overlying the phosphor particles substantially increases lamp brightness, reduces maintenance loss and reduces the color shift of the fluorescent lamp during operation. The protective coating suppresses the consumption of mercury due to adsorption to the phosphor particles. In this manner it is also precluded that in operation a recombination of mercury ions and electrons on the phosphor surface, or the incidence of excited mercury atoms and electrons on the phosphor layer, causes the emissive power of the phosphor to be reduced in the course of time. Excitation of the phosphors due to the action of UV radiation, however, is not influenced thereby. The protective layer is preferably formed by homogeneous precipitation or in a Chemical Vapor Deposition process.

According to one embodiment the total mass of mercury in the filling is greater than that of mercury in a saturated mercury vapor phase at nominal operation.

According to an alternative embodiment the total mass of mercury in the filling is less than that of mercury in a saturated vapor phase at nominal operation.

In particular, it has now been found that a luminescent material composed of a trichromatic blend of a blue green -emitting component consisting of BaMgAli₀Oi₇:Eu,Mn, a yellow-emitting component consisting of (Sr₅Ca)₂SiO₄IEu and a red-emitting component consisting of Y₂O₂S :Eu.has the desired properties with regard to light quality and longevity.

According to an alternative embodiment the mercury vapor discharge lamp comprises a trichromatic phosphor particle blend comprising BaMgAli₀Oi₇:Eu,Mn, (Sr₅Ca)₂SiO₄ :Eu, and YV0₄:Eu.

Although lamps providing a color rendering index of 80 or less are sufficient in terms of color rendering and color point stability for most of the standard illumination purposes, in some applications better color rendition and color point stability is required. Higher color rendering and/or color point stability is required e.g. for LCD backlighting and for the illumination of objects in museums and galleries.

The fluorescent mercury vapor discharge lamp according to the invention is formulated to provide a color rendering index of at least 90 and separate special color rendering indices, Rl to R8, greater than 80 at a color temperature of up to 8600 K. It is therefore especially useful, not only for general illumination, but also for display applications, such as LCD backlighting, lighting in museums and galleries and special applications such as desk illumination, aquaria illumination and plant illumination.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a cross-sectional view showing a schematic structure of a low pressure mercury vapor discharge lamp according to the invention.

Fig. 2 shows the emission spectra of seven samples of fluorescent mercury vapor discharge lamps comprising a first trichromatic phosphor particle blend according to a first specific embodiment.

Fig. 3 shows the emission spectra of seven samples of fluorescent mercury vapor discharge lamps comprising a second trichromatic phosphor particle blend according to a second specific embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions are used in the description and claims of the present application:

The term "nominal operation" is used to indicate operational conditions in which the vapor pressure of the discharge-maintaining composition is such that the radiant output of the lamp is at least 80% of the maximum radiant output for that lamp, i.e. operating conditions in which the pressure of the radiating species is optimal.

Although a fluorescent mercury vapor discharge lamp according to the invention usually is designed to emit white light, it may be lightly colored, such as bluish white or yellowish white, if designed for general illumination applications. Such slightly colored, white-like light is also referred to as "white" light in this disclosure.

White-like light can be described by a "correlated color temperature" (CCT). The correlated color temperature of a light source is defined as the temperature at which a blackbody radiator produces the chromaticity having the closest color match to the light source in question. Color temperature and CCT are expressed in Kelvin. The color match is typically represented and compared on a conventional CIE (Commission International de l'Eclairage) chromaticity diagram. The color of a white- like radiation can then be described in the CIE chromaticity chart referring to the Black Body Line (BBL).

Generally, as the color temperature increases, the light becomes more blue. As the color temperature decreases, the light appears more red.

White-like colors can also be described by a "color rendering index" (CRI). The color rendering index (CRI) is a measure of the degree of distortion in the apparent colors of a set of standard pigments when measured with the light source in question as opposed to a standard light source. It is established by a visual experiment. To this end the correlated color temperature of a light source to be evaluated is determined. Then eight standard colored pigment samples are illuminated first by the light source and then by a light from a blackbody having the same color temperature. If a standard color sample does not change color, then the light source has a perfect CRI value of 100. A general color rendering index is termed "Ra", which is an average of the color rendering indices Rl to R8 of all eight standard color samples.

Light sources having a relatively continuous output spectrum, such as incandescent lamps, typically have a high CRI, e.g. equal to or near 100. Light sources having a multi-line output spectrum, such as conventional fluorescent mercury vapor discharge lamps, typically have a CRI ranging from about 60 to 80, while the fluorescent mercury vapor discharge lamp according to the invention can provide a CRI > 90.

Preferably the fluorescent mercury vapor discharge lamp is of the low pressure type. Here, in this specification the term "low pressure discharge" relates to discharge wherein the pressure of the fill during nominal operation of the lamp stays below atmospheric pressure. Usually, the total pressure of the gas fill in the lamp in operation will be well below 1000 hPa, e.g. 200 hPa.

The present invention relates in general to a fluorescent mercury vapor discharge lamp provided with a discharge vessel, the discharge vessel enclosing, in a gastight manner, a discharge space provided with a gas filling of a mixture of inert gases with mercury, and a luminescent layer comprising a luminescent material comprising a trichromatic phosphor particle blend. The lamp also comprises means for igniting and maintaining a gas discharge in the discharge space, The design of the fluorescent mercury vapor discharge lamp according to the invention may comprise electrodes as means for igniting and maintaining the mercury vapor discharge. The electrode-comprising design is either of the typical "tube lamp"- type (TL) as known in the art, with the main electrodes being arranged inside the discharge vessel. Or the lamp design is of the "dielectric barrier discharge"-type with at least one main electrode being arranged outside the vessel or - for capacitive operation - both main electrodes being arranged outside the discharge vessel.

Alternatively the discharge means for igniting and maintaining a discharge in the discharge space are selected from the means for electrodeless operation, such as inductive operation, microwave or radiofrequency-driven operation.

Irrespective of the mode of igniting and maintaining the mercury vapor discharge, the discharge vessel encloses a discharge space containing a gas filling that includes an appropriate amount of mercury and one or more types of inert gases. The inert gases may be, for example, argon (Ar) and /or neon (Ne) gas. Suitably, a mixture ratio of these gases is, for example, 90 to 95 vol% of Ne gas and 5 to 10 vol% of Ar gas.

Those skilled in the art know that a mercury vapor discharge lamp can be designed so as to be either unsaturated (dose limited) or saturated (vapor pressure limited) with regard to the mercury content.

In the description and claims of the current invention, the designations "unsaturated" or "unsaturated mercury conditions" are used to refer to a low-pressure mercury vapor discharge lamp in which the amount of mercury dosed into the discharge vessel during manufacture of the low-pressure mercury vapor discharge lamp is equal to or lower than the amount of mercury needed for a saturated mercury vapor pressure during nominal operation of the discharge lamp.

A saturated (vapor pressure limited) design requires a portion of mercury to be present as condensate during operation of the arc. During operation, a non-uniform temperature distribution is formed in the discharge vessel due to internal convection. Typically, at least one hot region and at least one cold region are formed, resulting in thermal gradients across the discharge vessel. Typically, the mercury in the discharge vessel migrates to the coldest part of the discharge vessel ("Cold Spot") and condenses on the wall. The value of this cold spot temperature depends on the physical characteristics of the discharge vessel itself as well as on the variations in characteristics ofthe discharge-maintaining means ofthe lamp. Thus, in a vapor limited lamp design, the total mass of the mercury filling in the lamps is greater than that of mercury in the vapor phase at nominal operation. As a result of this, the vapor phase is in equilibrium with the condensed phase located on the cold spot of the discharge vessel. The design of the fluorescent mercury vapor discharge lamp according to this invention is preferably of the unsaturated type. Accordingly the dosing of mercury is limited to a wall load below 0.01 mg/mm², as known from the prior art. The wall load is defined as the ratio of the weight, expressed in mg of mercury, and the product of the internal diameter, expressed in mm, and the length, expressed in mm, of the discharge vessel.

Operating a mercury vapor discharge lamp under unsaturated mercury conditions has a number of advantages. Generally speaking, the performance of unsaturated mercury discharge lamps (light output, efficacy, power consumption, etc.) is independent of the ambient temperature as long as the mercury pressure is unsaturated. This results in a constant light output which is independent of the way of burning the discharge lamp (base up versus base down, horizontally versus vertically). In practice, a higher light output and an improved efficacy of the unsaturated mercury vapor discharge lamp is obtained in the application. Thus, an unsaturated mercury discharge lamp gives a relatively high system efficacy in combination with a relatively low Hg content. FIG. 1 shows a fluorescent low-pressure mercury vapor discharge lamp

100 with an elongated outer envelope 105 which encloses a discharge space 107 in a gastight manner. The lamp 100 shown in the illustrative example of FIG. 1 is a tubular lamp, preferably having a length of approximately 15 to 220 cm, operating on a current from approximately 0.160 to 1.500 Amps, and a lamp power from approximately 4.0 to 215 Watts, for example. However, the lamp may alternatively be a compact fluorescent lamp, and the lamp may have other operating parameters and other shapes, like curved shapes, such as a U-shape or a circular shape, or any other desired shape.

The lamp 100 has a conventional electrode structure 110 at each end which includes a filament 115 made of tungsten, for example. Alternatively, the electrode structure 110 may be provided at only a single end, particularly for compact fluorescent lamps. The filament 115 of the electrode structure 110 is supported on conductive lead wires 120 which extend through a glass press seal 125 located at one end of a mount stem 130 near the base 135 of the lamp 100. The leads 120 are connected to pin- shaped contacts 140 of their respective bases 135 fixed at opposite ends of the lamp 100 through conductive feeds 150. A center lead wire 160 extends from each mount 130 through each press seal 125 to support a cathode ring 170 positioned around the filament 115. A glass capsule 180 with which mercury was dosed is clamped on the cathode ring 170 of only one of the mounts 130. The other mount does not contain a mercury capsule, however a cathode guard 170 may be provided around its filament 115, which has been omitted in FIG. 1 in order to show the filament 115.

A metal wire 190 is tensioned over the mercury glass capsule 180. The metal wire 190 is inductively heated in a high frequency electromagnetic field to cut open the capsule 180 for releasing mercury into the discharge space 107 inside the envelope 105.

The discharge space 107 enclosed by the envelope 105 is filled with a discharge- sustaining filling which includes an inert gas such as argon, or a mixture of argon and other inert gases, at a low pressure. The inert gas and a small quantity of mercury sustain an arc discharge during lamp operation. Mercury is dosed in a quantity such that the entire dose is vaporized to provide an unsaturated mercury vapor discharge lamp.

During operation of the lamp 100, when the electrodes 110 are electrically connected to a source of predetermined energizing potential via the contact pins 150, a gas discharge is sustained between the electrodes 110 inside the envelope

105. The gas discharge generates ultraviolet (UV) radiation which is converted to visible light by the phosphor blend in the phosphor luminescent layer shown as numeral 210 in FIG. 1.

In particular, the inner surface of the outer envelope 105 may be pre- coated with a single layer of a metal oxide, such as aluminum oxide AI2O3 200, over which a luminescent layer 210 is formed. The alumina pre-coat 200 reflects the UV radiation back into the luminescent layer 210, through which it has already passed, for further conversion of the UV radiation to visible light. This improves the phosphor utilization and enhances the light output. The alumina pre-coat 200 also reduces mercury consumption by reducing mercury diffusion into the glass lamp envelope 105. To further reduce mercury consumption, the glass mount stems 130 and press seals 125 may also be coated with an alumina pre-coat layer 215, to reduce mercury bound to the glass mount stems 130 and press seals 125.

An electric ballast is integrated in known manner in the lamp holder, which ballast is used to control the ignition and the operation of the gas discharge lamp.

The chemical composition of the phosphor blend determines the spectrum of the light or its tone. The materials that can suitably be used as phosphors must absorb the radiation generated by the mercury vapor discharge and emit said radiation in a suitable wavelength range and enable a high fluorescence quantum yield to be achieved.

The mercury vapor discharge lamp according to the invention includes a phosphor particle blend that is a mixture of three phosphors which emit light in blue- green, yellow, and red wavelength ranges when exposed to the ultraviolet radiation emitted by the mercury discharge.

In particular, the phosphor particle blend comprises as a first component a blue green-emitting phosphor Bai__a_bMgAli₀Oi7:Eu_aMnb, wherein 0.01 < a < 0.5 and 0 < b < 0.5. The first component of the phosphor blend, when excited by the ultraviolet radiation generated by the mercury discharge, exhibits an emission spectrum centered generally in the blue green region of the visible spectrum.

This phosphor has a broad excitation band around 254 nm and its emission spectrum exhibits two emission bands, viz. at 453nm and at 515nm, the intensity ratio of both bands being a sensitive function of the Eu/Mn ratio.

The phosphor particle blend also comprises at least one yellow-emitting phosphor as the second component. The yellow-emitting phosphor is selected from the group of (Sri__x__aCa_x)₂Siθ4:Eua, wherein 0.01 < a < 0.5 and 0 < x < 1; Sri__aLi₂Si0₄ :Eu_a, wherein 0.01 < a < 0.5; (Cai__c__aSr_c)Si₂N₂O₂:Eu_a, wherein 0.01 < a < 0.5 and 0 < c < 1; and (Cai__c__aSr_c)SiAlN₃:Eu_a, wherein 0.01 < a < 0.5 and 0 < c < 1.

These phosphors are well excitable by 254nm radiation and emit in the spectral range from 520nm and also show a broad emission band due to the activators Eu(II) and Ce(III), relying on 4f5d transitions.

The phosphor particle blend comprises also at least one red-emitting phosphor selected from the group of (Yi_y-_z-_aGd_yLu_z)₂O₂S:Eu_a, wherein 0.01 < a < 0.5, 0

< y < 1, 0 < z < 1; and (Yi__y__z__aGd_yLu_z)₂VO₄:Eu_a, wherein 0.01 < a < 0.5, 0 < y < 1, 0 < z

< 1; and (Yi__y__z__aGd_yLu_z)₂(Vi__wP_w)O₄:Eu_a, wherein 0.01 < a < 0.5, 0 < y < l, 0 ≤ z < l; and (Yi__y__z__aGd_yLu_z)₂(Vi__wNb_w)O₄:Eu_a, wherein 0.01 < a < 0.5, O ≤ y ≤ l. O ≤ z ≤ l and O < w < 1; and (Yi__y__z__aGd_yLu_z)₂(Vi__w__xP_wNb_x)O₄:Eu_a, wherein 0.01 < a < 0.5, O < x < 10 < y < l, O ≤ z < 1 and O ≤ w ≤ 1.

The remaining component of the mixture exhibits a multiple line emission or a very narrow band emission which is located primarily in the red region between 590 and 700 nm.

The manufacture of the phosphor particles themselves is customarily carried out by means of a solid-state reaction of the starting compounds in the form of fine-grained powders of precursors such as oxides, carbonates or nitrates and possibly various fluoride fluxes having a fine grain size distribution and thermally decomposing the mixture including a source of each metal in an open or inert atmosphere at elevated temperatures.

The optimum particle size range for all three phosphors is between 0.5 and 7.0 microns. The grain size is determined by the properties of the phosphor to absorb

UV radiation and absorb as well as scatter visible radiation, but also by the necessity to form a phosphor coating that bonds well to the glass wall. The latter requirement is met only by very small grains, the light output of which is smaller, however, than that of slightly larger grains. The phosphor particles may be coated with a protective coating. This makes it possible to prevent degradation of the phosphor particles caused by a chemical reaction with mercury, and the consumption of the mercury in the discharge space caused by adsorption to the phosphor particles.

The dense coatings also restrict the contact of oxygen with the phosphor, so that thermal oxidation of the activator cannot occur. Plasma sputtering of the phosphor by the discharge species is also less harmful, since the removal of the coating layer does not reduce the phosphor efficiency until the coating layer is completely sputtered away.

The inorganic compound constituting the protective coating layer may be at least one member selected from Me₂Os, MePO₄, or MeBθ3, wherein Me is selected from Al, Sc, Y, La, Lu or a combination of these materials. The metal Me constituting the inorganic compound may be the same metal as that included in the phosphor, or a different metal, but it is particularly preferable to use AI₂O₃ and Y₂O₃.

Suitable coating materials must have a high band gap to be sufficiently transparent to the incident UV radiation.

Y₂O₃ has a transmissivity of approximately 85% for 254-nm radiation and a low transmissivity for light with a wavelength of 200 nm or less. For this reason, Y₂O₃ has a blocking effect to 185-nm light that degrades phosphors, which is preferable.

While there are no particular restrictions on the amount of the inorganic compound to be added, it is preferable for the inorganic compound to be added such that, for example, the inorganic compound makes up approximately 0.1 to 0.6 parts per weight of the phosphor layer for 100 parts per weight of phosphor particles and having a thickness of about 10 nm to about 500 nm. The protective coating layer will have insufficient strength if not enough coating material is applied and luminance will be insufficient if there is too much of the coating material.

It should be noted here that the above protective layer can be applied to one phosphor species but also to all phosphors. Preferably it is provided as a particle coating, forming a dense continuous vitreous shell on each phosphor particle separately. Such a coating is achieved e.g. if the protective coating is deposited in a homogeneous precipitation process or in a Chemical Vapor Deposition process. Particularly preferred is a fluidized bed Chemical Vapor Deposition process. In the example given, the sample phosphor material was prepared by the following method of homogeneous precipitation. First, the phosphor was dispersed in a solvent such as 500 ml of ethanol. A metal alkoxide solution containing one metal Me selected from Al, Sc, Y, La, Lu or a combination of these materials was added to the phosphor-dispersed solution to provide slurries. If necessary, a soluble phosphate or borate is added. A predetermined amount of the metal was used to form the protective layer of the phosphor material, e.g. an amount in the range of 0.005 to 3.0 wt%, such as 1 wt% relative to the phosphor weight. The slurries were kept in a stirred vessel for a predetermined period at 70 ⁰C to 90 ⁰C to cause hydrolysis of the metal alkoxide solution. Polymerization of the metal oxide occurred on the surface of the phosphor particles. An excess of the alcohol solvent was volatilized and the polymerized metal oxide was dried and heated so as to obtain the continuous metal oxide layer on the phosphor particles. Alternatively the protective coating may be applied by Fluidized Bed Chemical Vapor Deposition.

Preferably, the phosphor particles are coated by means of Chemical Vapor Deposition (CVD). Conventional means for performing CVD are provided, including a reaction chamber and the appropriate reactant materials that, when combined and heated, will deposit a layer of the desired material on all exposed surfaces, including phosphor particles.

Once the particles have been placed inside the reaction chamber the deposition process is initiated. Throughout the deposition process the phosphor particles are continuously tumbled to ensure that the coating that each particle receives is uniform and has a controllable thickness. Once a layer of a protective coating of the desired thickness has been deposited on the particles, the reaction is terminated.

If an alumina (AI2O3) film is to be coated by fluidized bed CVD, trimethylaluminum as an alumina precursor material may be vaporized in an inert gas and introduced into a fluidized bed CVD system in which water vapor and phosphor particles are accommodated. The alumina film may be formed by reacting water vapor and the alumina precursor on the surfaces of phosphors. The reaction temperature may be about 150⁰C to about 250⁰C.

If an yttria (Y2O3) film is to be coated by fluidized bed CVD, it is preferable to use yttrium caprylate, yttrium 2-ethylhexanoate, or yttrium octylate as the precursor material.

Yttrium caprylate also reacts with a part of a surface of the phosphor particles to which moisture tends to adhere, thereby forming a dense continuous vitreous coating layer of yttria on this part. Next, the coated phosphor blend is applied as the luminescent layer to the inner wall of the discharge vessel.

In general, a luminescent layer is formed in four steps: (A) adjusting a phosphor suspension; (B) applying the phosphor suspension to a lamp; (C) drying; and (D) baking. The suspension may include a binding agent, thickening agent, or the like, as necessary. The binding agent is, for example, a phosphorous or boron binding agent, and the thickening agent is nitrocellulose or the like. In this case, it is suitable for the amount of the added binding agent to be approximately 0.1 to 2 parts per weight based on 100 parts per weight of phosphor particles, and for the amount of added thickening agent to be approximately 0.3 to 2.5 parts per weight for 100 parts per weight of phosphor particles. To apply the phosphors to the walls of the gas discharge vessel use is customarily made of a flooding process. The coating suspensions for the flooding process contain water or an organic compound such as butylacetate as the solvent. The suspension is stabilized by adding auxiliary agents, for example cellulose derivatives, polymethacrylic acid or polypropylene oxide, and influenced in its rheological properties. Customarily, use is made of further additives such as dispersing agents, defoaming agents and powder conditioning agents, such as aluminum oxide, aluminum oxynitride or boric acid. The phosphor suspension is provided as a thin layer on the inside of the gas discharge vessel by pouring, flushing or spraying. The coating is subsequently dried by means of hot air and burnt in at approximately 600⁰C. The optimum thickness of the luminescent layer on the inner face of the discharge vessel lies in the range from approximately 15 to 50 μm as, on the one hand, the layer must only be so thin that still sufficient UV radiation is absorbed while, on the other hand, it must only be so thick that not too much visible radiation, formed in the innermost grains of the phosphor layer, is absorbed. To obtain the desired white chromaticity, the relative amounts of a red- emitting phosphor, a blue-green-emitting phosphor and a yellow-emitting phosphor are selected and blended in different weight ratios to obtain the trichromatic blend.

Note that a mixture of phosphors of different compounds may be used for one color. One example is to use BaMgAli₀Oi₇:Eu,Mn for blue-green, (Sr₅Ca)₂SiO₄IEu for yellow, and Y₂O₂S :Eu and YVO₄ :Eu for red.

Preferably, the phosphor particle blend comprises 55 to 65 % of the blue- green-emitting phosphor, 12.5 to 17.5 % of the yellow-emitting phosphor and 18 to 32 % of the red- emitting phosphor

The relative proportions of the individual components of the light- generating medium are such that when the radiations are blended there is produced white light of predetermined CIE coordinates or color temperatures, and because of the selected spectrum of the radiations generated, the color rendition of objects illuminated thereby is excellent. In accordance with the present method, there is blended together a first light component which is yellow to yellow-green in color and a second light component which is red in color and a third light component which is greenish-blue in color. These blended light components have at most only a limited amount of radiation of a wavelength shorter than 430 nm and at most only a limited amount of radiation of a wavelength longer than 650 nm, as well as at most only a limited amount of radiation of a wavelength of about 500 nm to 575 nm. The relative light intensities of the three light components are controlled with respect to one another to produce a white light of predetermined CIE coordinates. Upon application of a current, the mercury discharge produces UV radiation, which is converted by the trichromatic phosphor blend to red light, yellow light and blue-green light. The combination of blue green, red, and yellow light produces a pleasing white light having a preferred combination of parameters such as a color temperature of between 2700 K and 8600 K, for example 2700 K, 2900 K, 4000 K, 5000 K, 6300 K, 8000 K or 8600 K, a CRI of typically greater than 90, e.g. between about 85 and 96, and a device luminous efficacy of 50 to 120 lumens per watt of input electric power.

Figs. 2 and 3 each show the emission spectra and photometry results, the correlated color temperature (CCT) and the general color rendering index Ra, for seven samples of fluorescent lamps according to the present invention.

The CRI values meet the color acceptance criteria of both the ANSI and ISO standards, which require the color rendering CRI or Ra to be over 90, and the separate special color rendering indices Rl to R8 to be over 85.

Referring to FIGs.2 and 3, it is interesting to note that in accordance with the present invention, most yellow-appearing radiations are minimized as much as possible in order to provide a good color rendition of illuminated objects. To express this in another way, the available energy is concentrated in other regions of the visible spectrum in order to achieve the best possible efficiency of light generation (i.e., lumens per watt) commensurate with good color rendition of illuminated objects. This would appear to be contrary to the relatively high degree of sensitivity of the human eye for such yellow-appearing radiations.

The following embodiments are examples of applicable trichromatic phosphor blends that have the property of providing a white-like light with a CCT between 85 and 96. Note that there are no limitations on the combination of phosphors from the list given above.

First Specific Embodiment

A series of fluorescent lamps having the structure shown in Fig .1 was made in the following way: First, there were provided a series of trichromatic blends of BaMgAli₀Oi7:Eu,Mn, (Sr₅Ca)₂SiO₄IEu, and Y₂O₂SiEu. All phosphor particles were coated by Al₂θ3. The mixture ratio of these three phosphors was adjusted such that a color temperature thereof ranged from 2700 K to 8600 K.

Each trichromatic phosphor particle blend was dispersed in a mixed solvent composed of butyl acetate and turpentine to obtain a suspension. Before dispersal of the phosphors, nitrocellulose and a boric acid binding agent were dissolved in the mixed solvent. Next, the coating material was applied to an inner side of a standard soda- lime glass tube. Application of the coating material to the glass tube was performed using a conventional up-flush process.

Next, dry air was supplied into the glass tube to dry a layer composed of the applied coating material. This drying of the layer was performed while rotating the upright glass tube. Then a baking process was performed using an electric furnace set to 580⁰C.

Electrodes were attached to the discharge tube and a mercury-comprising pill was added. Next, the interior of the glass tube was evacuated, inert gases (Ne: Ar = 95:5) were enclosed therein, and the glass tube was sealed. The lamp was connected to a TL lamp driver and driven under standard conditions. As shown in Fig.2, the lamps of this embodiment had a color point range from 2700 K to 8600 K and a color rendering index RA ranging from 90 to 96.

Second Specific Embodiment

A second series of fluorescent lamps having the structure shown in Fig.1 was made by the same process as outlined in the first embodiment, using a series of trichromatic blends of BaMgAli₀Oi₇:Eu,Mn, (Sr₅Ca)₂SiO₄ :Eu, and YV0₄:Eu. The mixture ratio of these three phosphors was adjusted such that a color temperature thereof ranged from 2700 K to 8600 K.

As shown in Fig.3, the lamps of this embodiment had a color point range from 2700 K to 8600 K and a color rendering index Ra ranging from 85 to 92.

While the present invention has been described in particular detail, it should also be appreciated that numerous modifications are possible within the intended spirit and scope of the invention. In interpreting the appended claims it should be understood that: a) the word "comprising" does not exclude the presence of elements other than those listed in a claim; b) the word "consisting" excludes the presence of elements other than those listed in a claim; c) the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements, d) any reference signs in the claims do not limit their scope; and e) several "means" may be represented by the same item of hardware or software implemented structure or function.

Claims

CLAIMS:

1. A fluorescent mercury vapor discharge lamp provided with a discharge vessel, the discharge vessel enclosing, in a gastight manner, a discharge space provided with a gas filling of a mixture of inert gases with mercury, and a luminescent layer comprising a luminescent material comprising a trichromatic phosphor particle blend, the lamp also comprising means for igniting and maintaining a gas discharge in the discharge space, wherein the trichromatic phosphor particle blend comprises blue green- emitting phosphor Bai__a__bMgAlioOi7:EuaMn_b, wherein 0.01 < a < 0.5 and 0 < b < 0.5, a yellow-emitting phosphor selected from the group of (Sri__x_ aCa_x)₂Si0₄:Eu_a, wherein 0.01 < a < 0.5 and 0 < x < 1; Sr^Li₂SiO₄ :Eu_a, wherein 0.01 < a

< 0.5; (Cai__c__aSr_c)Si2N₂O₂:Eu_a, wherein 0.01 < a < 0.5 and 0 < c < 1; and (Cai__c_ _aSr_c)SiAlN₃:Ce_a, wherein 0.01 < a < 0.5 and 0 < c < 1; and a red-emitting phosphor selected from the group of (Yi_y-_z-_aGd_yLu_z)₂O₂S:Eu_a, wherein 0.01 < a < 0.5, 0 < y < 1, 0

< z < 1; and (Yi-_y-_z-_aGd_yLu_z)₂VO₄:Eu_a, wherein 0.01 < a < 0.5, 0<y≤l,0<z≤l;and (Yi__y__z__aGd_yLu_z)₂(Vi__wP_w)O₄:Eu_a, wherein 0.01 < a < 0.5, 0<y≤l,0<z≤l;and (Y_1-y-z- _aGd_yLu_z)₂(Vi__wNb_w)O₄:Eu_a, wherein 0.01 < a < 0.5, 0<y<l,0<z≤land0<w<l; and (Yi__y__z__aGd_yLu_z)₂(Vi__w__xP_wNb_x)O₄:Eu_a, wherein 0.01 < a < 0.5, 0≤x≤10<y<l,0< z< 1 andO≤w≤ 1.

2. A fluorescent mercury vapor discharge lamp according to claim 1, wherein the phosphor particle blend comprises 55 to 65% of the blue green-emitting phosphor, 12.5 to 17.5% of the yellow-emitting phosphor and 18 to 32% of the red emitting phosphor.

3. A fluorescent mercury vapor discharge lamp according to claim 1, wherein the luminescent material comprises a protective layer formed on the phosphor particles.

4. A fluorescent mercury vapor discharge lamp according to claim 3, wherein the protective layer is formed by an oxide selected from the group consisting of a layer of Me2θ3, MePO₄, or MeBC>3 , wherein Me is selected from the group ofAl, Sc, Y, La and Lu.

5. A fluorescent mercury vapor discharge lamp according to claim 3, wherein the protective layer is formed by homogeneous precipitation or in a Chemical Vapor Deposition process.

6. A fluorescent mercury vapor discharge lamp according to claim 1, wherein the gas filling is saturated with regard to mercury at nominal operation.

7. A fluorescent mercury vapor discharge lamp according to claim 1, wherein the gas filling is unsaturated with regard to mercury at nominal operation.

8. A fluorescent mercury vapor discharge lamp according to claim 1, wherein the trichromatic phosphor particle blend is formed by BaMgAli₀Oi7:Eu,Mn, (Sr₅Ca)₂SiO₄ :Eu, and Y₂O₂SiEu.

9. A fluorescent mercury vapor discharge lamp according to claim 1, wherein the trichromatic phosphor particle blend is formed by BaMgAli₀Oi7:Eu,Mn, (Sr₅Ca)₂SiO₄ :Eu, and YV0₄:Eu.

10. Use of the lamp according to claim 1 for general illumination and display applications.