EP0334355B1 - Wall-stabilized high-pressure discharge lamp - Google Patents

Description

The invention relates to a wall-stabilized high-pressure discharge lamp according to the preamble of claim 1, and in particular to a wall-stabilized high-pressure discharge lamp with high lighting efficiency and very good color properties, which is suitable for general lighting purposes in a compact structure, due to the good ability of the radiation to concentrate and the immediate Luminous flux output after switching on, but can also be used in motor vehicle headlights and for lighting tasks in scientific and medical device construction, and which can in particular also be designed for very low powers.

There are numerous high-pressure discharge lamps for general lighting purposes in which high luminous efficacy and good color rendering properties, ie a spectral emission which is well distributed in the area of the sensitivity curve, are aimed for. It is known that a light source with ideal color rendering can achieve a maximum light yield of about 250 lm / W if it is possible to suppress heat and electrode losses and to limit the spectral emission distribution to the range of visible radiation. This optimal emission distribution in the spectral range between 400 and 700 nm is proportional to the spectral emission of a black radiator with a temperature that is equal to the desired color temperature of the light source and is zero outside this spectral range.

In addition to the unavoidable heat and electrode losses, the light yield of a high-pressure discharge lamp via the spectral emission distribution of the radiating plasma is given by the composition and pressure of the discharge medium, the dimensions of the discharge tube and the spatial temperature distribution of the discharge, and can only be optimized within certain limits. For example, in a mercury vapor atmosphere at a pressure of 0.1 to 10 MPa, a gas discharge can be operated which is in the visible spectral range known mercury lines 405, 436, 546 and 577/579 nm emitted. The very intense radiation at 313 and 365 nm can be transformed into the visible spectral range using a suitable phosphor. Such high-pressure mercury lamps have found widespread use especially in outdoor lighting; their luminous efficacy is a maximum of 55 lm / W, and the general color rendering index is 50 (cf. Elenbaas, W .: High-pressure mercury-vapor lamps and their application; Philips Technical Library, 1965).

It was possible to fill in the incomplete mercury spectrum by introducing suitable, easily vaporizable lighting additives, with the result that both the luminous efficiency and the color rendering have improved considerably. Such metal halide lamps achieve a light output of 60 to 80 lm / W at a higher power conversion with a general color rendering index of 60 to 90, the high values of the light efficiency generally being combined with low values of the color rendering index and vice versa. With small power conversions down to 35 W, however, the luminous efficiency generally drops considerably to 50 lm / W and below. Such light sources have found widespread use in the lighting of interiors, production rooms and sales facilities, but their use is limited by the significantly reduced lifespan, the dependence on the burning position and the very disturbing instability of the color temperature due to specimen scatter and aging (cf.Rochlin, GN: metal halide lamps ; Energiya Moscow 1971).

The invention of alkali-resistant envelope materials and closure materials has made it possible to use sodium in metallic form as a carrier for light emission. High pressure sodium vapor lamps currently take among all known high pressure discharge lamps in terms of their luminous efficacy a top position and have found widespread use in outdoor lighting. The range of available power types ranges from 35 W with a luminous efficacy of 75 lm / W to 1000 W with 150 lm / W. Because of the very low color temperature of around 2000 K and the unsatisfactory color rendering index of around 20, however, such lamps are not accepted for more demanding lighting tasks, especially in indoor lighting.

All of the high-pressure lamps mentioned so far have in common that they answer any attempt to reduce the power conversion and to develop conventional power units in the living area with drastic losses in light output. The reason for this is that, according to the mechanisms of action of a high-pressure lamp, small luminous fluxes can only be generated in small volumes, the ratio of surface area to volume increasing disadvantageously. If the axis temperature of the discharge is predetermined by plasma-physical optimization, the temperature gradient of the discharge and thus the heat losses caused by heat conduction and particle diffusion from the hot zones of the discharge to the wall inevitably increase when the discharge volume is reduced. On the other hand, the use of metal vapors or halides as a radiating medium requires a minimum inner wall temperature of the discharge vessel, which ensures the required vapor pressures. These temperatures are relatively high and also result in unavoidable heat losses, which are particularly significant when the lamps have low power conversions.

Furthermore, all of these technical solutions for evaporating the luminous metal additives require a considerable start-up time in the minute range. In principle, the use of gases as a radiating medium eliminates the start-up problems and the need to keep the burner temperature high. Ignition problems that occur can be kept within limits using various ignition aids (see, for example, DE 1 638 977 and 3 323 603). Unfortunately, the radiation yields of gas plasmas in the visible spectral range are, without exception, very low with technically feasible power conversions. Therefore, only a single high-pressure light source based on atomic or molecular fill gases has been developed so far, the xenon high-pressure or ultra-high pressure lamps. They show a radiation emission similar to sunlight with a continuous spectral distribution, which results from the recombination continuum of the xenon. Such lamps are built for stationary operation with powers of 50 to 50,000 W and are characterized by excellent color rendering combined with a color temperature in the range of 6000 K. Regrettably, at low powers between 30 and 50 W, the luminous efficacy is only 15 lm / W and cannot be increased by more than 20 lm / W even with high power densities up to 800 W / cm (AT-PS 222 223). It can only increase to 35 lm / W with power in the kW range. The reason for this is, among other things, that in the area of stationary outputs this continuum is overlaid by an intense line radiation, which is emitted predominantly in the infrared range and considerably limits the achievable light yield. For this reason, stationary xenon discharges are not used as an effective light source for general lighting purposes.

Ultra-high pressure lamps use xenon or mercury vapor as a radiant medium at pressures of more than 1 MPa. The high energy concentration realized in them allows the radiation to be generated in a very small volume, which results in the radiation being able to be focused well. However, their light output is rather less than that of the corresponding ones High pressure lamps.

High-pressure capillary lamps with halide filling have already been proposed for use in motor vehicle headlights (DE 3 341 846). For this purpose, however, they have the major disadvantage of requiring a start-up time of around one minute to reach their full luminous flux.

Attempts have also already been made to improve the radiation yield in the green and yellow mercury lines (US Pat. No. 3,624,447) by means of a time-controlled energy feed in high-pressure mercury lamps with a pulse duty factor of 1: 4 to 1:20. In the case of high-pressure sodium lamps, the width of the resonance lines should be increased and the blue and green spectral components should be increased using the same technique (US Pat. Nos. 4,137,484, DE 2 657 824 and 2 825 532), in order to improve the color rendering and to increase the color temperature . Discharge instabilities and acoustic disturbances in the environment can be prevented by constructive measures on such lamps (DE 2 733 168 and 2 773 170) and special operating modes (DE 2 335 589, 2 704 311, 2 847 840 and 3 122 183). However, the color temperatures that can be achieved in this way are still too low for use in interior lighting, especially in the living area, because values of 3000 K and more are desirable for this. The tests were also abandoned because the undesirable side effects, such as increased self-absorption of the resonance lines of sodium and a disproportionate increase in the mercury lines, question the desired effect and further increases in color temperature are only possible with a major loss of light output (DE 2 657 824).

It is known for xenon discharges that they develop a higher lighting efficiency than with stationary operation with pulsed energy supply with high instantaneous power. This fact is used for many applications in the form of photo flash lamps. Operation with pulse sequences is also known and is used in stroboscopic devices (for example DE 1 924 162 and 2 134 544) and in drying and curing processes for paints and printing inks (DE 2 120 777).

Even as a lighting device for indoor and outdoor lighting, noble gas-filled flash tubes of high intensity have already been proposed, which are operated by means of an electronic control unit with a flash frequency sufficient to overcome the physiological inertia of the human eye (DE 2 724 101). However, this application has the major disadvantage that an ignition process is required for each pulse and the resulting extraordinarily high number of ignitions in the continuous operation of the lamp life is highly detrimental, so that additional measures to increase the life and reliability of such a lighting device in Form of cooling devices for the lamp or by using several redundant lamps had to be taken. For the same reason, one was forced to keep the flash frequency as low as possible, e.g. B. with 40 ... 50 Hz to produce a just flicker-free light. Visual physiological stresses cannot be excluded in this operating mode, which is why the use of such a lighting device also for special applications, such as traffic signs / advertising signs and the like. Like., Must remain limited, which are observed only briefly by the human observer in general.

All discharge light sources contain as energy converting Media electrically heated plasmas, the electrical properties of which change depending on the energy supplied. In particular, when the operating voltage increases, the power conversion increases, as a result of which the charge carrier density increases. This increases the electrical conductivity and the power consumption continues to increase. In order to counteract the resulting instabilities, discharge lamps must be operated via ballasts, which in particular contain current-limiting components. The mass and scope of these current-limiting components make it difficult to use such light sources, particularly in the living area. In order to achieve a current-voltage characteristic with a positive increase, the use of additives of certain metals or metal salts was tested in xenon and other noble gas high-pressure lamps (DE 2 236 973 and 2 525 408). However, there are considerable doubts about the effectiveness of the claimed effects; Nothing is known about their practical use. The positive current and voltage characteristic curve shown in AT-PS 222 223 can only be realized at very high powers in the discharge tube, which leads to a rapid destruction of the enveloping material. US Pat. No. 3,707,649 is concerned with a lamp in which the anode and cathode are at such a distance that a spark gap is created between them. These electrode-stabilized discharges have the disadvantage of high proportionate electrode losses and large gas dynamic losses due to shock waves. In addition, the tactile ratios mentioned in the patent disadvantageously result in high color temperatures of 9000 K and correspondingly low light yields as a result of the emission maximum at the edge of the UV range. No statements are made about the resulting low light yields.

The invention has for its object a high-pressure discharge lamp to develop, which has very good color rendering properties even at low power conversions, has a high luminous efficacy and emits the full luminous flux immediately after switching on.

In a compact design, it should preferably ensure a good ability of the radiation to be focused by optical components. In addition, the power supply to the lamp should preferably be made as simple as possible. Such a light source should be suitable for general lighting purposes as well as usable in motor vehicle headlights and is therefore suitable as an alternative to incandescent lamps with considerable savings in electrical energy or with a significant increase in luminous flux and as an alternative to metal halide lamps with the advantage of immediate luminous flux output after switching on. It can also be used for lighting tasks in scientific and medical device construction and, as an alternative to incandescent lamps and electrode-stabilized xenon high-pressure lamps, leads to a significant reduction in the thermal loads on the object or patient and in the device itself.

The technical task of developing such a light source is still to find a medium which, when excited in an electrical discharge, is able to emit an intensive continuum in the visible spectral range from a small volume as soon as the light source is switched on Medium to find a plasma state in which both this continuum radiation is generated with high yield and good bundling ability and the discharge has a positive current-voltage characteristic, and finally that specify technical means for realizing such a plasma state.

According to the invention, this object is achieved by the wall-stabilized high-pressure discharge lamp with the characterizing features of claim 1.

Advantageous further developments can be found in the dependent claims.

According to the invention, the discharge plasma is heated up to such an extent by powerful electrical impulses with a high repetition frequency and small duty cycle that the recombination continuum distributed over the visible spectral range is emitted with high intensity, and that this clearly exceeds the intensity of the line radiation in the spectrally ineffective spectral ranges.

For this purpose, a momentary power is supplied to the discharge during the pulses, which exceeds the mean power N permissible in continuous operation of the discharge tube in continuous operation by a factor of 5 or more. The discharge between the pulses is advantageously operated with the aid of a holding current of at most 50% of the average current, so that it does not go out and does not have to be restarted at the beginning of each pulse. The pulse repetition frequency is chosen to be equal to or greater than 100 Hz, so that it is sufficiently far above the flicker fusion frequency of the human eye and visual physiological problems in this regard are avoided.

The pulse length can then be calculated from the condition that the time average power fed in by pulse and holding current is equal to the average permissible in continuous operation Power N is. However, both the instantaneous power during the pulses and the pulse repetition frequency should be limited in such a way that the calculated in this way.

Pulse lengths should not be less than 10 »s, because otherwise the intended effect is only incompletely achieved due to the thermal inertia of the discharge.

The technically realized increase in the plasma temperature T _p above the temperature T _s by this type of lamp operation the discharges operated stationary with direct current or quasi-stationary with sinusoidal alternating current, without increasing the wall temperature T _{w of} the discharge vessel compared to stationary operation, brings about an unexpectedly strong increase in the continuum radiation in the visible range, so that there is a high visual efficiency. This high visual benefit is further enhanced by the fact that the short, but intense power pulses significantly reduce the thermal losses that occur in each gas discharge. A further improvement in the efficiency can be achieved if the ratio of the inside diameter of the discharge vessel to the electrode spacing does not exceed the value of 0.3, since then the conductance of the discharge path is not increased by a radial expansion of the discharge channel in the high-current phase.

Another advantage of this operating mode is that with pulse powers that are a factor of 5 and more above the steadily permissible power, the electron density no longer increases significantly with a further increase in power. Therefore, the electrical conductivity increases only slightly, and the discharge gains a positive characteristic. In this way, the space-consuming, heavy and lossy components for current limitation that are customary in high-pressure discharge lamps are dispensable, and the power supply devices for such lamps are particularly simple in construction.

The color temperature of the lamp according to the invention can be reduced and adapted to the requirements for interior lighting by surrounding the discharge tube with an outer bulb, the inner wall of which is coated with a phosphor which emits in the wavelength range between 550 and 650 nm and is excited by long-wave ultraviolet radiation can be. This phosphor transforms the short-wave portions of the recombination continuum below a wavelength of 450 nm into the yellow-red spectral range with the result of an effective reduction in the color temperature to a value of 4000 K or below, which is also associated with an increase in the luminous efficacy. At the same time, the luminous density of the lamp is greatly reduced by the light-scattering effect of the phosphor layer, and thus the risk of glare caused by it is avoided.

The invention will be described below in two exemplary embodiments which are designed for an electrical power of 35 W and are explained in more detail in FIGS. 1, 2, 3 and 4.

Fig. 1 shows an embodiment in which the discharge lamp is formed by a cylindrical discharge tube 1 made of silica glass with an inner diameter of 1.5 mm and with two electrodes 2 opposite one another at a distance of 10 mm and power supply lines 3 melted in a vacuum-tight manner. It is filled with xenon at a cold pressure of 300 kPa. An ignition probe 4 attached to the outside of the discharge tube serves to facilitate the ignition of the lamp.

The lamp is operated by a power supply device 5 in such a way that after ignition by a high-voltage pulse, 200 current pulses per second with a pulse duration of t = 100 »s and a maximum current strength of 50 A are supplied to it. These pulses are superimposed on a holding current in the form of a direct current of 0.1 A with the same polarity. Fig. 2 shows the course of the lamp current as a function of time.

The high-pressure xenon discharge lamp, which is operated with powerful pulses, emits a luminous flux of 1400 lm at an average power consumption of 35 W. Its color temperature is 6500 K and its general color rendering index is 95. Its average luminance is 12 000 cd / cm 'and is a good prerequisite for focusing the radiation through optical components. The current-voltage characteristic formed by the discharge is shown in FIG. 3; it has the desired positive increase in the area of the pulse current.

Another embodiment is shown in FIG. 4. It contains a discharge tube according to FIG. 1, which is surrounded by an evacuated outer bulb 6, a frame structure 7 taking over the mechanical mounting of the discharge tube and the electrical connection of the power supply lines to the base 8. The outer bulb is coated on its inner wall with a phosphor layer 9 made of Ca halophosphate: Mn. Discharge tube with ignition probe, frame structure, outer bulb and base together form the discharge lamp. It is operated in the same way as the lamp according to exemplary embodiment 1 and, with an average power consumption of 35 W, emits a luminous flux of 1450 lm at a color temperature of 4000 K and a color rendering index of 90.

Claims (7)

1. Wall-stabilised high-pressure discharge lamp, consisting of a light-transmitting discharge vessel with two fused-in electrodes and filled with a discharge medium, characterised in that a combination from use of xenon and/or krypton as the discharge medium is used with power supply of the lamp with pulses having a power density of 100 to 10,000 W/cm³, a maximum peak current intensity in the pulse of 50 A and a pulse frequency of 100 to 1000 Hz, wherein the ratio of the pulse width to the interval between the individual pulses is between 0.01 and 0.15, the ratio of the instantaneous power in the pulse to the mean time value of power is greater than 5:1, and the discharge between the pulses is maintained by a holding current to the amount of not more than 50% of the mean time value of the current at nominal power.
2. Wall-stabilised high-pressure discharge lamp according to claim 1, characterised in that the discharge medium of xenon and/or krypton has a cold pressure of 10 to 500 kPa.
3. Wall-stabilised high-pressure discharge lamp according to claim 1, characterised in that the pulse width is not less than 10 »s and not more than 1 ms and the ratio of pulse width to pulse interval is preferably between 0.02 and 0.1.
4. Wall-stabilised high-pressure discharge lamp according to claim 1, 2 or 3, characterised in that the peak current intensity in the pulse is selected such that the ratio of the instantaneous power in the pulse to the mean time value of power yields a positive build-up of the current/voltage characteristic.
5. Wall-stabilised high-pressure discharge lamp according to any of the preceding claims, characterised in that the discharge between the pulses is maintained by a holding current which is smaller than the mean time value of the current at nominal power.
6. Wall-stabilised high-pressure discharge lamp according to claims 1 to 5, characterised in that the ratio of the inside diameter of the discharge tube to the electrode spacing does not exceed a value of 0.3.
7. Wall-stabilised high-pressure discharge lamp according to claims 1 to 6, characterised in that the lamp is surrounded by an evacuated or gas-filled outer envelope of which the inner wall is coated with a luminous substance which emits in the wave region from 550 to 650 nm.

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