Gas discharge lamp comprising a mercury-free gas fill

TECHNICAL FIELD OF THE INVENTION

The invention relates to a gas discharge lamp comprising a light-transmitting discharge vessel enclosing, in a gas-tight manner, a discharge space comprising a mercury- free gas fill, the mercury-free gas fill comprising a molecular radiator compound, the gas discharge lamp further comprising discharge means for maintaining a discharge in the discharge space.

BACKGROUND OF THE INVENTION

Light generation in gas discharge lamps is based on the principle that charge carriers, particularly electrons, are accelerated so strongly by an electric field applied to the discharge lamp that collisions with the gas atom or molecule species in the gas fill of the discharge lamp cause these species to be excited or ionized. When the excited species in the gas fill return to the ground state, a more or less substantial part of the excitation energy is converted into radiation. Conventional gas discharge lamps comprise mercury as a primary component for generation of radiation. It is a disadvantage of mercury-containing discharge lamps that mercury emits considerable radiation in the high energy but non- visible UVC range of the electromagnetic spectrum, e.g., there exists a radiation line centered at 254 nm. To use such radiation for lighting purposes, it has to be converted into visible radiation e.g. by means of phosphors. Though quantum efficiency of most phosphors is near unity, there is always a waste of energy ("Stokes' loss") if one UV-photon is converted to one visible photon and heat.

Moreover, mercury is a highly toxic and environmentally hazardous substance. Thus, nowadays, mercury in discharge lamps is widely objected to and wherever possible is no longer used at all or at least reduced.

Some new designs of discharge lamps are dosed with easily vaporizable metal compounds, known as "molecular radiators". These lamps comprise no mercury at all and so cause no pollution. But while their emission is at least partially in the visible range of the electromagnetic spectrum their white light efficiency is insufficient.

It is therefore an object of the present invention to provide a discharge lamp of the type mentioned above with improved cnvjrømricπlal lnaidhucbs and high white light efficiency.

SUMMARY OF THE INVENTION

This object is achieved according to the invention by providing a discharge lamp comprising a light-transmitting discharge vessel enclosing, in a gas-tight manner, a discharge space comprising a mercury-free fill, the mercury-free fill comprising a molecular radiator compound, selected from the group of metal halides of ruthenium(III), osmium(III,IV), rhodium(III) and rhenium(III), wherein the halide is selected from the group of chloride, bromide and iodide and combinations thereof. The fill also comprises a buffer gas for aiding in starting and buffering the discharge, the discharge lamp further comprising discharge means for igniting and maintaining a discharge in the discharge space.

In the lamp according to the invention a molecular discharge takes place, emitting radiation in the visible and near UV range of the electromagnetic spectrum. The lamp produces visible radiation of the cold- white type and reduced emission in the UV range. The principal advantage of this lamp technology is that it does not require the use of mercury vapor, which is an environmentally hazardous material with known human toxicity. It is therefore environmentally safe. The second advantage is its potential to become an efficient white light illumination source not requiring the use of phosphors. Thereby, e.g., losses by Stokes' shift are reduced and less energy is wasted by radiative output in the ultraviolet. Thus, the invention provides a lamp with a high white light efficiency.

The preferred molecular radiator compound is ruthenium (III) halide. However other metallic species such as osmium(III,IV), rhodium(III) and rhenium(III) may be used as well, alone or in combination with ruthenium(III).

In this context, it might be appropriate noticing that the use of the metals ruthenium, osmium, rhodium, and rhenium as additives in discharge lamps for improving their radiation characteristics is known since a long time. Thus, for example, JP 61-208742 A proposed a "Discharge lamp using microwave discharge has translucent container filled with rare gas, mercury and chloride of osmium, ruthenium, rhodium, or rhenium". But it has to be noted that all these prior art documents employ discharges based on mercury as the main filling compound using these further metals only as minor constituents for influencing their spectrum. Thus, e.g., JP 61-208742 A adds these further metals to increase the luminous output of an electrodeless electric-discharge lamp for the near ultraviolet region, i.e., the

luminous output of such a lamp for the near ultraviolet region can be greatly increased by properly selecting the amount of the chloride of osmium or a similar element charged in the lamp.

This is in sharp contrast to the invention at hand, which aims for the complete disuse of mercury, i.e., is not looking for additives to mercury improving the mercury spectrum but for a mercury replacement. And, thus, has to deal with the expectation that its candidate mercury- free discharges will be of a completely different physical nature than the prior known mercury discharges. Accordingly, the use of the metals ruthenium, osmium, rhodium, and rhenium as additives in the prior art mercury discharges does not prejudice the inventive effort of recognizing that using these metals in themselves as the main discharge medium yield an environmentally friendly and highly efficient white light source.

In another embodiment, the fill of an inventive lamp may further comprise a regenerative additive for regenerating ruthenium(III), osmium(III,IV), rhodium(III) and rhenium(III). Such regenerative additive may improve the lamp's lifetime by the following envisaged mechanism. The radiative species selected from the group of the halides of ruthenium(III), osmium(III,IV), rhodium(III) and rhenium(III) are redox-amphoter. Redox- amphoter behavior means that the radiative species can be degraded either by oxidative or by reductive chemical reactions. To protect the radiative species from the attack of the omnipresent oxidative species, such as oxygen and water, it might be advantageous using a mild reducing regenerative additive that will keep the radiative species at their respective oxidation level, but will not reduce them to the metallic phase. Such improved composition of the gas fill may reduce the rate of loss of the radiative metallic species from the gas fill and, thus, inhibit loss of luminosity due to discoloration of the discharge vessel walls by deposited metals. However, while being of advantage in some situations, the use of such a regenerative additive is not required in every embodiment of the invention.

Regenerative additives may be used either in a volatile or in a solid compound form.

In a preferred embodiment the regenerative additive is a volatile compound selected from the group of sulfur, selenium, sulfur dioxide, and carbon monoxide. A substance, which may be preferably used, is sulfur. Adding a sulfur containing substance to the lamp fill may also have the effect of providing spectral emphasis to the spectrum.

In another embodiment, the additive may be supplied as a solid, selected from the group of metallic titanium, silicon, and aluminum, especially if the operating temperature within the discharge vessel is high enough.

It may also be possible to use a combination of the additives disclosed herein together in a single lamp fill.

Preferably, the molar ratio of said molecular radiator compound and said regenerative additive is between 1 : 1 and 5:1 and the concentration of the molecular radiator compound is between 10 ^~10 and 10 ^~5 mole/cm ³ .

The gas fill comprises also a buffer gas, preferably an inert gas selected from the group formed by helium, neon, argon, krypton, and xenon or mixtures thereof. Such substances cause the lamp to start more reliably and they also buffer the discharge.

Advantageously, the gas discharge lamp is constructed as a High Intensity Discharge (HID) lamp .

Typically the discharge means for maintaining a discharge in the discharge space are selected from the means for inductive operation, capacitive operation, or microwave operation, electrode-emissive operation being an alternative.

In an embodiment of the lamp, wherein no phosphors are used, the lamp is advantageously used as a lamp for diagnostic, therapeutic, cosmetic, or germicidal lighting applications, or as a lacquer-curing lamp.

For applications, where a lower color temperature is desired, the lamp may be combined with appropriate phosphors, e.g. in a luminescent layer coating. In these embodiments the gas discharge vessel may comprise a luminescent layer on the inside or outside surface of the wall of the discharge vessel or a luminescent layer on an additional outer bulb.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates an electroded embodiment of the lamp operated at 50Hz. Fig. 2 displays the optical spectrum of an electrodeless RUCI ₃ discharge at

600W microwave power.

DETAILED DESCRIPTION OF THE INVENTION

The present invention focuses on an improved fill comprising a molecular radiator compound, selected from the group of metal halides of ruthenium(III), osmium(III,IV), rhodium(III) and rhenium(III), for any design of a discharge lamp.

Such designs comprise lamps equipped with means for electrode operation (electrode emissive or capacitive), or electrodeless inductive or microwave operation for

igniting and maintaining the molecular radiator gas discharge, thereby generating a radiating plasma.

The design with electrodes is either of a typical "tubular lamp"-type as known in the art, with the main electrodes inside the discharge vessel. Otherwise the electrode- comprising lamp design is of the "dielectric barrier discharge"-type (DBD) with - for capacitive operation - at least one main electrode outside the vessel or both main electrodes arranged outside the vessel.

In further embodiments the lamp according to the invention does not necessarily rely on electrodes but rather produce light by inductively coupling the lamp gas fill to intense radio wave radiation of several MHz, thereby inducing an electrical discharge operating at radio frequencies of several MHz. According to further embodiments a microwave induced plasma operating at the frequency of 916 MHz and higher is used.

All of these electrodeless lamp designs have in common that the power is supplied to the lamps not through electrodes but rather by being subject to an externally produced electromagnetic oscillation. The variation of the pattern of the electromagnetic field depends on the structure and operation of the external source of the electromagnetic field.

As used herein, the phrase "radio wave radiation", as well as the acronym "RF", is intended to encompass electromagnetic radiation frequencies in either the conventional radio frequency range or in the conventional microwave frequency range. The RF source is an RF antenna, a probe, or the like for introducing RF energy into the waveguide.

A preferred electrodeless embodiment features a cylindrically-shaped arc tube of a height less than its outside diameter, a light transmissive outer envelope disposed around the arc tube and defining a space there-between, and excitation means for coupling radio- frequency energy to the arc tube fill. Such a construction was e.g. described in

US 4,810,938 Al and has the advantage that these lamps can be operated as relatively isothermal devices not experiencing various thermal losses found in electroded lamps, particularly at the walls and ends, as well as found in other electrodeless lamp constructions of the type having a relatively long and narrow arc tube. Furthermore, such a construction allows raising the cold spot wall temperatures from usually below 750° C up to around

900° C allowing an efficacy gain attributable to increased vapor pressure of the lamp fill. In this construction, the arc tube can e.g. be formed of a high temperature glass, such as fused quartz, or an optically transparent ceramic, such as polycrystalline alumina.

The filled arc tube generates a plasma arc during lamp operation by excitation from a solenoidal electric field employed in the lamp, all in known manner. The excitation is created by a magnetic field, changing with time, to establish within the tube an electric field which closes completely upon itself, resulting in the light-producing high intensity discharge. The excitation source in the preferred lamp design comprises an excitation coil disposed outside the outer lamp envelope and connected to a power supply through an impedance matching network. The spacing between the arc tube and outer envelope members in a preferred lamp device can be occupied by thermal energy barrier means, such as metal baffles or quartz wool, or even a vacuum. Such thermal barrier means desirably reduce heat loss from the lamp, which would otherwise be considerable due to the more elevated lamp operating temperatures and isothermal manner of lamp operation being achieved in this construction.

In another preferred embodiment of the invention, a discharge lamp of the electrodeless type is provided that is powered by RF energy. However it should be understood that the principles of the present invention apply equally well to discharge lamps having internal or external electrodes.

Fig. 1 illustrates an electroded embodiment of a lamp according to the invention shown in side elevation and operated at 50Hz. The lamp vessel 11 of quartz glass has an elliptical shape and a volume of approximately 1 cm . Tungsten electrodes 13 are present in the discharge vessel, about 10 mm spaced apart. Current supply conductors 14 to the electrodes penetrate into the discharge vessel. The lamp has a filling comprising a buffer gas for aiding in starting and buffering the discharge and a molecular radiator compound 12, selected from the group of metal halides of ruthenium(III), osmium(III,IV), rhodium(III) and rhenium(III), wherein the halide is selected from the group of chloride, bromide and iodide and combinations thereof. The lamp vessel 11 is mounted within an outer envelope 15, which is provided with a lamp base 16.

These embodiments can be improved by depositing a thin, non-conductive infrared reflective coating on the outside walls of the discharge vessel. The reflective coating is deposited either by evaporation, spraying, painting or other method. The material used is tin oxide or a similar reflective material. The function of the coating is to reduce the infrared radiation loss of the walls of the vessel and thereby increase the wall temperature of the vessel or achieve the same temperature at lower electrical input power of the lamp.

Irrespective of the mode of igniting and maintaining the discharge, the discharge vessel encloses a discharge area containing a gas fill for sustaining an electrical

discharge that includes a molecular radiator, selected from the metal halides of ruthenium(III), osmium(III,IV), rhodium(III) and rhenium(III), and a buffer gas but does not include mercury or mercury compounds. The halide is selected from the group of chloride, bromide and iodide and combinations thereof. Halides of ruthenium and especially RuCl3 are preferably used as the radiating species in the fill components because these halides show a favorable vapor pressure in the discharge and provide visible whitish radiation of high efficacy.

In some embodiments, these halides may be susceptible to degradation through reactions with oxygen and water vapor. Therefore, the invention provides two strategies to deal with the phenomenon in addition to the most basic approach, which is simply to remove oxygen and water vapor from the discharge vessel by vacuum or by inert gas flushing, or both, prior to filling the radiative fill. This later approach is well known in the art, but might not be sufficient to prevent oxidation of the halides of ruthenium(III), osmium(III,IV), rhodium(III) and rhenium(III) by remaining traces of oxygen and water. Therefore, the fill according to the invention may further comprise a regenerative additive in addition to the radiative species.

Suitable regenerative additives are selected to be competitively oxidized relative to the radiative species, while not reducing the metal halides to the elemental stage. Formulation of the composition of the fill according to the invention is facilitated by incorporating the regenerative additive as a volatile compound, such as sulfur, selenium, sulfur dioxide and carbon monoxide. Alternatively, the regenerative additive may be selected from solids of the group of metallic titanium, silicon and aluminum.

Advantageously, the metallic solids are provided in a form wherein the surface to volume ratio is high, so that a maximum amount of the metal surface may be exposed and is available for reaction. The metals are preferably in the form of powder or flakes, preferably powder. Powdered metal having an average particle diameter in the range of 1 to 10 microns are advantageously used, especially titanium.

The regenerative additives may prevent the reaction of the radiative species with impurities in the fill, which are apt to be introduced into the discharge vessel along with the other dosing constituents. In accordance with these embodiments of the invention, the regenerative additive may prevent both, oxidation, followed by a substantial build-up of oxidized, non-radiative species, and de-oxidation, followed by deposition of deoxidized species. A controlled chemical behavior of the regenerative additive is advantageous, a behavior that prevents both oxidation and deoxidation. By using a regenerative additive

according to the invention that is only mildly reductive, degradation of the fill may be positively influenced. In this case, the regenerative additive may prevent deposition of ruthenium, osmium, rhodium and rhenium as metals which, otherwise, may shorten the service life of the lamp. Since the addition of the regenerative additive may serve for reducing chemical reaction between the fill ingredients and the impurities in the fill, the quantity of additive fill in relation to the radiative species is based on experimental results and will vary over a wide range from 1 :1 to 1 :5.

The amount of molecular radiators is typically in a range of 10 ^~10 mole/cm ³ to 10 ^~5 mole/cm ³ . It should be understood that the absolute amount of the molecular radiator component in solid form that is used in the discharge vessel may vary depending on which substance is used, but the amount always will be such to produce the desired pressure range at operating temperature, i.e., at the temperature profile inside the discharge vessel during nominal operation. The discharge vessel typically also contains a buffer gas, which is inert to the extent that it does not adversely effect operation of the lamp. By using such an inert gas the starting characteristics are improved. Rare gases are suitable buffer gases. Although any rare gas will work to some extent, preferred gases are argon (Ar), helium (He), krypton (Kr), xenon (Xe), and mixtures thereof, with argon and mixtures thereof with other rare gases being particularly preferred.

The buffer gas may also serve the purpose of affecting the performance of the plasma by changing the thermal conductivity of the plasma.

The buffer gas typically has a partial pressure at nominal operation in the range of 10 to 500 hPa, preferably at 20 hPa. Those skilled in the art know that a discharge can be designed to be either dose limited (unsaturated) or vapor pressure limited (saturated) or a combination of dose and vapor pressure limited. A dose-limited design requires the entire molecular radiator present to be vaporized during operation of the arc. A vapor pressure limited design requires a portion of each molecular radiator to be present as condensate during operation of the arc. During operation a non-uniform temperature distribution is formed in the discharge vessel.

Typically, at least one hot region and at least one cold region are formed resulting in thermal gradients inside and across the discharge vessel. Typically, the molecular radiators in the discharge vessel migrate to the coldest part of the discharge vessel ("Cold Spot") and may condense on the wall if their vapor pressure exceeds their saturation vapor pressure.

Thus, in a vapor limited lamp design the total mass of the molecular radiator fill in the lamps is greater than that of the molecular radiator in the vapor phase at nominal operation, which is required to achieve the desired color and efficacy. 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 composition of condensed phase of the fill, and consequently the composition of the vapor phase, due to the differences in the thermo-chemical properties of the components of the gas fill, clearly depends on the temperature of the coldest spot in the discharge vessel of the lamp.

The value of this coldest spot temperature depends on the physical characteristics of the discharge vessel itself as well as on the variations in characteristics of the discharge maintaining means of the lamp.

The design of the lamp according to the invention typically is of the vapor pressure limited type.

When a discharge lamp is ignited, the means for igniting and maintaining a discharge produce an electric field inside the discharge vessel and start a discharge in the buffer gas. In the preferred embodiments using inductive operation, RF current in the inductive coil provides a time- varying magnetic field, which produces a solenoidal electric field in the discharge vessel. Current flows through the fill as a result of this solenoidal electric field, producing a toroidal arc discharge in the discharge vessel. The discharge may quickly progress from a glow discharge (low power) to an arc-discharge (high power) and a significant amount of molecular radiators is vaporized.

The electric field ionizes also the buffer gas within the discharge area. The electrons stripped from the buffer gas atoms and accelerated by the electric field collide with radiating species of molecular radiators. As a result, some species become excited to a higher energy state without being ionized. As the excited species fall back from the higher energy state, they emit photons, ultraviolet (UV) photons and/or visible photons.

In those embodiments comprising a phosphor, the UV photons interact with the phosphor in the phosphor layer of the lamp to generate visible light.

The intensity of the visible light generated by the lamp depends on the partial pressure of the vaporized molecular radiator in the discharge vessel. The visible light reaches its maximum intensity and the lamp operates at maximum efficacy at an optimum molecular radiator's partial pressure. At a partial pressure less than the optimum pressure, the light intensity of the lamp is less than maximum because the excited species produce less photons. At a pressure greater than the optimum pressure, the light intensity of the lamp is also less

than maximum because some of the species collide with the photons generated by other species and these photons get re-absorbed and do not generate UV or visible radiation. The following example is illustrative:

In a specific embodiment, a roughly spherical discharge vessel is made from fused silica, having an outer diameter of 36 mm and an inner diameter of 32.5 mm. The discharge vessel is evacuated and simultaneously a dose of 0.5 μmole/cm ³ ruthenium chloride is added. Also argon is introduced at a pressure of 20 Pa at ambient temperature. In operation, 400 to 600 Watt of microwave energy are applied at about 2.4 GHz to the fill from an external microwave source causing a solenoidal arc discharge. The fill is transferred to a plasma state that emits a molecular spectrum in the visible region.

Figure 2 shows the spectral emission of a RuCl3 discharge at 600W microwave power.

In comparison to the optical spectrum of a mercury emission the emission intensity in the visible range of the electromagnetic spectrum between 400 and 700 nm is significantly increased. The emission is densely populated with atomic and molecular transition lines. The correlated color temperature CCT of the spectrum is 9458K, the chromaticity coordinates are x=0.2746 and y=0.3117. The general color-rendering index is Ra ₈ = 85.

The correlated color temperature (CCT) value of a light source is defined as the temperature of a black body radiator, which would appear to have the same color as the light source in question. The unit of measurement is in Kelvin (K), which determines the warm or cool appearance of a light source. The lower the color temperature the warmer or more yellow is the appearance. The higher the color temperature the cooler or bluer is the appearance. The CCT measured is over the CCT range of interest (e.g. 3000-6500K) for general illumination

Chromaticity coordinates x and y in the chromaticity diagram of the Commission Internationale de l'Eclairage ("CIE") standardize the color of the light. The neutral white point is found at the center of the diagram at CIE x,y-coordinates, 0.33, 0.33. The color points of the discharge lamps according to this embodiment are in the cool white spectral range.

The color rendering ability of a light source is measured with the color rendering index CRI. CRI measures the difference between the appearance of test colors under artificial light to be measured and the appearance of the same test colors when seen by light from a blackbody source having the same color temperature as the tested light source.

The differences in value, chroma and hue of the light reflected under the light source to be measured and the light source are obtained and summed, the square root of the sum is taken, multiplied by a constant, and subtracted from 100. This calculation for the Ras is performed for 8 different color standards. The color rendering index for each of these standards is designated Ri, where i=l, . . . , 8. The general color rendering index Ras is defined as the average of these eight indices Rl - R8. The procedure has been chosen such that Ras for a standard warm white fluorescent tube is approximately 50. For better illustration, a Ras value of 100 corresponds to a "perfect" light source, i.e., one under which a color sample appears exactly as it would appear when illuminated by a blackbody source light source having the same color temperature. Acceptable to good CRI values are in the range of 80 - 86.

The given spectrum demonstrates that harmful UV radiation beyond 350 nm, which is a problem with mercury dosed lamps, is greatly suppressed. This provides an important advantage for the enhancement of safety for humans in general lighting applications and for the protection of exhibits in museums and art galleries.