Plasma lamp as a radiation source in an apparatus for artificial weathering

An apparatus for artificial weathering or lightfastness testing of samples or for simulating solar radiation, the apparatus comprises a weathering chamber, an electrodeless lamp provided in the weathering chamber and comprising a bulb filled with a composition that emits light when in a plasma state, and a radio frequency source being arranged so that it radiate a radio frequency field into the bulb to generate a luminous plasma for emitting a radiation comprising spectral emission characteristics similar to natural solar radiation.

RELATED APPLICATIONS

The present disclosure claims priority to European Patent Application 21152377.4, filed on Jan. 19, 2021, which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an apparatus for artificial weathering, an apparatus for lightfastness testing, or a sun light simulator apparatus, such apparatus comprising as a light source an electrodeless discharge lamp in which a luminous plasma is generated by radio frequency or microwave energy for providing visible and/or infrared and/or UV radiation.

BACKGROUND

Artificial weathering or sunlight simulator apparatuses are intended to estimate the lifetime of materials which are constantly exposed to natural weather conditions during their use, and which therefore suffer from climatic effects such as sunlight, solar heat, moisture and the like. In order to obtain a good simulation of the natural weathering situation, the spectral energy distribution of the light generated in the device should correspond as closely as possible to that of natural solar radiation, for which reason xenon radiators are used as radiation sources in such devices. An accelerated ageing test of the materials is essentially achieved by much more intense irradiation of the samples compared with natural conditions, which speeds up the ageing of the samples. In this way, a prediction of the long-term ageing of a material sample can be made after a comparatively short time.

A large number of the samples studied in artificial weathering devices consist of polymeric materials. Their deterioration due to weathering is essentially caused by the UV component of solar radiation. The primary photochemical processes which take place during this, that is to say the absorption of photons and the generation of excited states or free radicals, are independent of temperature. The subsequent reaction steps with the polymers or additives, however, may be temperature-dependent so that the observed ageing of the materials is also temperature-dependent.

A xenon lamp is normally used as the radiation source in the weathering testers of the prior art. Although such a lamp is known to be able to simulate the solar spectrum very well, the emitted radiation nevertheless has a relatively high spectral component in the infrared spectral range, which needs to be suppressed by filters in order to prevent excessive heating of the samples. Furthermore, a commercially available xenon radiation source only has a lifetime of about 1500 hours.

A halogen lamp may also be used as the radiation source, although this has the disadvantage that it is not adjustable, or can only be adjusted to a minor extent. The same applies to fluorescent lamps, which likewise have already been used as radiation sources in weathering testers and which also have the disadvantage of a relatively short lifetime.

SUMMARY

An aspect of the present disclosure relates to an apparatus for artificial weathering or lightfastness testing of samples or for simulating solar radiation, the apparatus comprising a holder configured to hold samples to be analyzed, and an electrodeless lamp comprising a bulb filled with a composition that emits light when in a plasma state, and a radio frequency source being arranged so that it radiate a radio frequency field into the bulb to generate a luminous plasma for emitting a radiation comprising spectral emission characteristics similar to natural solar radiation.

High intensity discharge lamps (HID lamps) are widely employed in lighting thanks to their excellent luminous efficiency and color rendition. They consist, in many instances, of a transparent envelope containing a gas that is brought in a luminous state by an electric discharge flowing across two electrodes. An electrodeless lamp is a form of lamp in which a transparent bulb, filled with an appropriate composition, is heated by radio frequency or microwave energy.

Electrodeless lamps tend to exhibit a longer lifetime and maintain better their spectral characteristics along their life than electrode discharge lamps. While requiring a radio frequency power supply, they use bulbs of very simple structure without costly glass-metal interfaces. Moreover, the absents of electrodes allows for a much greater variety of light-generating substances to be used than in traditional discharge lamps. Sulphur, Selenium, Tellurium, among others, are popular fills whose use is limited to electrodeless lamps, because they are not chemically compatible with metal electrodes.

Electrodeless lamps are an interesting alternative to conventional HID lamps in general lighting application, and in all fields in which high efficiency and excellent spectral characteristics are called for like photography, movie recording, agriculture, testing of photovoltaic equipment, and artificial weathering, among others.

In an embodiment which will be shown and described below, the bulb comprises a spherical form, wherein the radio frequency source is arranged to radiate the radio frequency field into the bulb so that radio frequency field optimally fills the space within the bulb. In particular, the radio frequency source can be comprised of a magnetron emitting microwave radiation in the open 2.45 GHz band.

According to an example of the electrodeless lamp, the lamp further comprises an electrically conductive enclosure which surrounds the bulb. The enclosure can be realized by an electrically conductive sheet, layer, or mesh. In case of a sheet or layer it is supported by a suitably transparent, translucent, or light-transmitting substrate on which a thin electrically conductive layer is deposited. According to a further example thereof, the enclosure is connected with the radio frequency source. More specifically, the radio frequency source comprises an output terminal in the form of a waveguide, wherein the enclosure is connected to the waveguide. As will be seen and described in embodiments further below, the waveguide may comprise a central conductor and an outer sleeve-like conductor surrounding the central conductor, both the central conductor and the outer sleeve-like conductor extending from a main surface of the radio frequency source in the direction of the bulb. The enclosure can be connected with the outer sleeve-like conductor. In one example, a dielectric rod can be connected between the central conductor and an outer wall of the bulb and in another example there is an empty space provided between the central conductor and the outer wall of the bulb, in particular no element like a dielectric rod is provided between them.

According to an example of the lamp, the construction of the lamp is such that in normal operation of the lamp any formation of standing waves of the radiated electric fields does not take place.

According to an example of the lamp, the lamp does not contain any reflecting walls outside of the electrically conducting enclosure, in particular no reflecting walls which may form a cavity causing the build-up of standing waves between the reflecting walls in normal operation of the lamp.

According to an example of the apparatus, the apparatus comprises an additional light concentrator in which the lamp may be disposed. The light concentrator may have reflective walls in order to concentrate the light generated in the bulb into a beam of a desired aperture which may then be directed in a desired manner onto specific samples to be examined.

DETAILED DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of aspects and are incorporated in and constitute a part of this specification. The drawings illustrate aspects and together with the description serve to explain principles of aspects. Other aspects and many of the intended advantages of aspects will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference signs may designate corresponding similar parts.

In addition, while a particular feature or aspect of an example may be disclosed with respect to only one of several implementations, such feature or aspect may be combined with one or more other features or aspects of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “include”, “have”, “with” or other variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprise”. The terms “coupled” and “connected”, along with derivatives may be used. It should be understood that these terms may be used to indicate that two elements or layers co-operate or interact with each other regardless whether they are in direct physical or electrical contact, or they are not in direct contact with each other which means that there can be one or more intermediate elements disposed between them. Also, the term “exemplary” is merely meant as an example, rather than the best or optimal. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims.

FIG.1comprisesFIGS.1A and1Band shows an apparatus according to the present disclosure for the artificial weathering of samples.

The apparatus100ofFIG.1comprises a weathering chamber1and a holding frame2which is mounted so that it can rotate in the weathering chamber1. The holding frame2comprises a closed ring shape and samples3or work-pieces can be held on appropriate holding platforms or sockets provided on the holding frame2. The holding frame2can have, in particular, a cylindrical form and a circular lateral cross section as can be seen inFIG.1B.

The apparatus100ofFIG.1further comprises an electrodeless lamp10provided in the weathering chamber1and comprising a bulb11filled with a composition that emits light when in a plasma state, and a radio frequency source12being arranged so that it radiates a radio frequency field13into the bulb11to generate a luminous plasma for emitting a light radiation15comprising spectral emission characteristics equal or similar to natural solar radiation.

The electrodeless lamp10can be mounted so that its central axis falls together with the cylinder axis of the holding frame2. Even more the holding frame2can be rotated so that the rotation axis coincides with its cylinder axis and with this central axis of the electrodeless lamp10. As a result, when rotating the holding frame2the samples3move on a circular path around the electrodeless lamp10so that the distances between the samples3and the electrodeless lamp10are therefore at all times equal among themselves as well as constant in time.

In a manner which is known per se, the weathering chamber1may also have other artificial weathering instruments, for example moisture generators or the like, although these do not play an essential part in the present disclosure and will not therefore be discussed in detail. For example, an air flow may also be blown into the weathering chamber1and sweep past the samples3in a vertical direction.

As shown in the example ofFIG.1, the bulb11comprises a spherical form. However, the bulb can also have an elongate form comprising two opposing ends wherein the radio frequency source is arranged at one of the opposing ends.

The bulb11may be filled with a chemical composition that is suitable for producing light when it is ionized and heated to a plasma state. Several compositions can be used as fill in the frame of the present disclosure including, for example, Mercury, Sulphur, Selenium, Tellurium, metal halides and mixtures thereof, in an inert atmosphere. The composition may, for example, contain mercury together with one or more of the other mentioned components mentioned above. The composition may be such that it does not contain mercury alone. Moreover, the composition should be such that the radiation emitted by the lamp approximates the spectral characteristic of solar radiation. Otherwise the present disclosure is not limited to a particular chemical composition.

The bulb11may be realized by any kind of transparent material capable to withstand the high temperatures and internal pressures that are reached during the functioning of the lamp, and chemically compatible with the fill composition. In a typical realization the operating temperature of the bulb11will be in a range from 600° C. to 900° C., and the internal pressure at operation will be in a range from 0.1 MPa to 2 Mpa. For example, fused quartz (also fused silica, SiO2) can be used as material for the bulb11.

According to the desired power of the emitted radiation, the size of the bulb11may vary between 0.5 cm3and 100 cm3, in particular between 10 cm3and 30 cm3.

FIG.1shows only one electrodeless lamp in the weathering chamber. It is, however, also possible to arrange two or more lamps in the weathering chamber.

FIG.2shows a partial longitudinal sectional view of another example of an electrodeless lamp assembly to be used in a weathering apparatus.

As can be seen inFIG.2, the electrodeless lamp20may further comprise an electrically conductive enclosure24surrounding the bulb21. The electrically conductive enclosure24may be configured in the form of an electrically conductive mesh as it is indicated inFIG.2. The enclosure24may surround the bulb21in such a way that the enclosure24comprises cylinder symmetry and a central longitudinal axis of the bulb21falls together with a central cylinder symmetry axis of the enclosure24.

The enclosure24can also be realized in the form of a sheet of a suitable transparent, translucent, or light-transmitting substrate on which a thin and transparent electrically conductive layer is disposed.

A lateral diameter of the enclosure24can be in a range from 5 cm to 30 cm, and can in particular be constant over its entire length.

More specifically, the electrodeless lamp20as shown inFIG.2comprises a bulb21filled with a composition that emits light when in a plasma state. The electrodeless lamp20further comprises a radio frequency source23being arranged so as to radiate a radio frequency field23.1into the bulb21at its lower end. The radio frequency source23may be comprised of a magnetron emitting in the open 2.45 GHz band. The electrodeless lamp20further comprises a metallic mesh24which encloses the bulb21in such a way that both the bulb21and the mesh24are cylindrically symmetric having a common cylinder axis. The radio frequency source23comprises or is connected to a waveguide26which forms an output terminal for outputting the microwave radiation at the upper surface of the radio frequency source23. The waveguide26is configured like a coaxial transmission line and comprises a central conductor26.1and an outer conductor26.2which surrounds the central conductor26.1in a sleeve-like manner. The lateral diameter of the waveguide26can be in a range from 5 cm to 30 cm, and the lateral diameter of the enclosure24can be the same as the lateral diameter of the waveguide26. The lateral diameter of the central conductor26.1can be in a range from 1 cm to 10 cm, and can be essentially the same as the lateral diameter of the bulb21. The length of the waveguide26can be in a range from 2 cm to 10 cm.

As shown inFIG.2, the enclosure, in particular the mesh24, can be connected to the outer sleeve-like conductor26.2of the waveguide26and thus comprises essentially the same lateral diameter. The mesh24has the function of confining the radio frequency field. No element is disposed between the upper surface of the central conductor26.1and the lower surface of the bulb21.

FIG.3shows a lower section of an electrodeless lamp30in a longitudinal sectional view.

The electrodeless lamp30as shown inFIG.3comprises a bulb31, a radio frequency source32, a metallic mesh34, and a waveguide36, all of these elements being configured in the same manner as with the electrodeless lamp20ofFIG.2. As an additional element, however, a dielectric rod37is disposed between the central conductor36.1of the waveguide36and the bulb31. The waveguide36may comprise a metallic cup36.3which covers the central conductor36.1and which comprises a central opening in its upper surface in which opening the dielectric rod37is inserted. The dielectric rod37extends either from the opening or from an upper surface of the central conductor36.1through the opening to the lower surface of the outer wall of the bulb36.1. In this way the electric field is nearly completely guided through the dielectric rod37and the dielectric rod37acts as a dielectric waveguide and channels the microwave energy directly into the inner volume of the bulb31. Accordingly, as indicated by the field lines33.1of the electric field, the channeling of the electric field into the bulb31may be improved as compared with the electrodeless lamp20ofFIG.2.

In both examples of electrodeless lamps ofFIGS.2and3the radio frequency source was shown to be arranged completely outside the bulb and to radiate the radio frequency field from outside into the bulb. However, it is also possible to integrate at least a part of the radio frequency source into the bulb as, for example, by inserting the central conductor into the bulb through an opening of a wall of the bulb.

Starting the electrodeless lamp such as that shown and described in connection with either one ofFIGS.1to3, may be done in two steps. At the beginning, when starting the lamp, due to Paschen law, there is a discharge at a low pressure inside the bulb that produces UV light. This discharge contains principally the rare gas present in the bulb. The rest of the active materials is still at solid or liquid state. This discharge then heats these other active materials. The discharge is constituted of low ionized molecules and the temperature is still too low to obtain ionized molecules. When a certain level of temperature is reached inside the bulb, the rest of active material will be evaporated. At this moment, these materials are mixed inside the discharge. The discharge changes its behavior and switches to a thermal plasma. At this moment, the impedance of the discharge changes to match with the rest of the system. The reflection of the microwave radiation is considerably reduced and the power can be transmitted to the discharge to produce a lot of light. The composition of the discharge is completely different in comparison with the discharge at the beginning (during the first 10-15 seconds), the molecules evaporated are now in the majority, and the molecules of rare gas used to initiate the discharge are in the minority (less 1%).

FIG.4comprisesFIGS.4A and4Band shows an example of an artificial weathering device of the static type with fixed samples.

The apparatus200for artificial weathering as shown inFIG.4comprises a weathering chamber201which is configured so that a plurality of samples3or work-pieces can be arranged on a bottom surface of the weathering chamber201. The apparatus200ofFIG.4further comprises an electrodeless lamp210provided in the weathering chamber201and comprising a bulb211filled with a composition that emits light when in a plasma state, and a radio frequency source212being arranged so that it radiates a radio frequency field213into the bulb211to generate a luminous plasma for emitting a light radiation215comprising spectral emission characteristics similar to natural solar radiation.

The electrodeless lamp210further comprises a light concentrator214which concentrates the light generated in the bulb211into a beam of a desired aperture which may then be directed in a desired manner onto specific samples to be examined. In the example ofFIG.4the light concentrator214is formed of a cone shaped light reflector214which comprises reflective inner walls to collect as much of the light emitted by the bulb211and direct it onto the samples203as indicated by the arrows.

Otherwise the apparatus200and the electrodeless lamp210may comprise any feature that was described above in connection with the apparatus100ofFIG.1or the electrodeless lamps ofFIGS.2and3.