INFRA-RED LIGHT SOURCE

Abstract: A method for increasing the spectral stability of cold cathode discharge infra-red light sources is described, especially for sources operating with a carbon dioxide fill. A catalyst is added inside the lamp envelope to induce recombination of the dissociation products of molecules of the fill gas broken down by the action of the electrical discharge, thus reducing the changes with time in the concentration of excited gas molecules in the lamp active in emitting to ground state levels. Maintenance of the level of self absorption of the lamp emission results in a concomitant maintenance of the spectral shape of the lamp emission.

Description: INFRA-RED LIGHT SOURCE FIELD OF THE INVENTION The present invention relates in general to the field of infra-red radiation sources, and in particular to sealed-off molecular gas discharge sources BACKGROUND OF THE INVENTION The technology underlying the operation and production of electrode-less cold gas discharge infra-red lamp sources has been described in U S Patent No 5,300,859, entitled "IR-Radiation Source and Method for Producing Same" to S Yatsiv et al , hereby incorporated by reference m its entirety One of the important advantages of such lamp sources is that they emit very narrow spectral lines at discrete frequencies characteristic of the molecular rotational-vibrational to ground state transitions of the excited gas species contained in the lamp This is achieved in a source which is sealed-off, is compact, has a good level of conversion efficiency from electrical to optical power, and has a long life compared with previously available sealed-off lamps sources Because of their unique spectral properties, such lamps have been widely used as sources in non dispersive infra-red (NDIR) spectrometry instruments, and especially in gas analyzers for use in medical applications The narrowness of the emission lines provides such gas analyzers with high levels of selectivity, sensistivity and stability, which are many times better than gas analyzers of similar complexity, which use alternative technology sources, such as hot blackbody sources The other advantages mentioned above enable the production of compact and cost effective instrumentation using such sources The lamp sources described in U S Patent No 5,300,859 have found particularly advantageous applications as sources of the C0 2 spectral emission lines, for gas analysis of exhaled breath, to determine the levels of C0 2 therein Such C0 2 sources have been used to great advantage in capnography and breath testing instrumentation Since the mechanisms and dynamics of the electrical excitation of C0 2 and of its ensuing spontaneous emission spectrum are similar to those used in carbon dioxide lasers, there exists a very large body of knowledge and prior art concerning this technology In U S Patent No 5,300,859, there is a thorough discussion regarding the parameters affecting the lamp emission rise and decay time, efficiency, excitation and output, and the lamp lifetime as a function of chemical methods used to clean the lamp before sealing On the other hand, the question of the spectral stability of the lamp source is not addressed However, when used as a frequency selective source in NDIR spectroscopic applications, spectral stability may be even more important than the above parameters Intensity changes over time can easily be monitored and corrected by using a reference path, since lamp intensity is a single valued quantity On the other hand, spectral changes can not be easily monitored or corrected for, because there is virtually infinite information in a spectrum Changes in the lamp spectrum cause changes in the absorption cell absorption characteristics If these changes are not known, then it is impossible to accurately measure gas concentrations using such lamp sources There therefore exists a serious need for a method of maintaining a high level of spectral stability in electrode-less cold gas discharge infra-red lamp sources of the type described in U S Patent No 5,300,859 The disclosures of all publications mentioned in this section and in the other sections of the specification, are hereby incorporated by reference, each in its entirety SUMMARY OF THE INVENTION The present invention seeks to provide a new method of producing cold gas discharge infra-red lamp sources with improved spectral stability, especially those operating with a carbon dioxide fill The spectral stability of the lamp source is of particular importance for use in such applications as in the gas analyzer of breath test instrumentation, where high resolution, sensitivity and selectivity are required to accurately determine the concentration of one isotopic species in the presence of another Lack of good spectral stability may cause small changes occurring in the source spectrum to be erroneously interpreted as intensity changes resulting from changes in isotopic concentration It is to be understood though, that such lamps may be used in any gas analyzer application to provide high resolution, sensitivity and selectivity A number of factors affect the spectral stability of the electrode-less cold gas discharge infra-red lamp source, of the type used in the breath test instrumentation according to the present invention These factors include temperature, electrode position and time The first two are factors of the operating conditions and geometry of the lamp, and if well understood, can be well controlled The third factor mentioned, namely change over time, is much more problematic, since such changes are due to long term changes in the composition of the gas fill It is thought that a major cause for long term changes in the lamp output and spectrum is the result of the gradual break down of the IR-active molecule in the discharge In the case of the carbon dioxide lamp, the carbon dioxide dissociates into carbon monoxide and oxygen In the course of the first few minutes of lamp operation, an equilibrium of the above molecules is reached, but over a longer period of operation, this equilibrium level changes as CO and 0 2 are adsorbed on the walls of the lamp envelope Impurities also reduce the C0 2 level still further Up to now, the majority of the prior art on the subject of the changes with time in the operating conditions of electric discharges in carbon dioxide has been primarily concerned with the change in power output which occurs as the C0 2 level changes This has been the mam point of interest because of the importance of avoiding a decay over time in the power levels of C0 2 lasers, and especially of sealed-off C0 2 lasers However, in the case of discharge lamps, whose emitted radiation is a result of non-coherent, spontaneous emission from an excited state to the ground state, changes in C0 2 concentration also affect the emission spectrum by means of a process known as self-absorption In C0 2 lasers, on the other hand, this phenomenon is. for all practical purposes, virtually non-existent The phenomenon of self-absorption operates in the following way The CO? molecules in the lamp not only emit radiation when excited, but they also absorb radiation when in the ground state, by means of induced absorption The lamp itself thus operates as an absorption cell to its own emitted light as this light passes through the lamp's own gas fill to the output window The C0 2 lines are absorbed at their centers, and their shapes thus change by means of the process known as self-absorption The effect of self absorption on NDIR spectroscopy measurements can be significant Even when Doppler broadened, the lines emitted from the lamp are much narrower than the absorbing lines in a gas sample at atmospheric pressure As a result, the region of coincidence on the absorbing lines is of approximately constant magnitude Hence, although there is no change in absorption characteristics for any individual emission line for different degrees of self-absorption, the change in the distribution of the individual line intensities does cause an overall change in the absorption characteristics As a result, changes in self-absorption create a change in the emitted line strength distribution of the first order band group, 1 e the weaker absorbing lines of the Boltzman distributed intensities traverse the lamp with little attenuation, even in the presence of the high C0 2 concentrations inside the lamp, but the strongly absorbing lines of the Boltzman distributed intensities are strongly attenuated These changes in line intensity distribution are similar in their effect on the absorption characteristics to changes in distribution resulting from changes in relative group/order (isotope) strength, or Boltzman distribution changes resulting from changes in temperature Expressed mathematically, the lamp output radiation after absorption in the gas cell is given by where I] and I 2 are for the first and second order respectively, and are defined over their line intensities I nj This relationship follows from the well-known Beer-Lambert law Defining a distribution ratio X = Σ Ii, /(ΣIj, + ∑I 2| ), which is the output ratio between first and second order of line distribution j, the transmission t (c) is given by: t( C ) = ti( C ) X + t 2 ( C ) ( 1 -X) for any given concentration c. When the lamp C0 2 concentration changes with time, this distribution ratio changes and hence, also the absorption characteristics of the complete optical system, consisting of the emitting lamp together with the absorbing cell. Under these conditions, it becomes difficult to distinguish such changes from the changes in gas concentration being measured. Although self-absorption can be anticipated from theory, it is difficult to demonstrate directly. This is mainly because the typical line width of the lamp is less than 0.006cm "1 . The only instrument currently available which is capable of resolving such a narrow width is an FTIR (Fourier transform infra-red) spectrometer, with a mirror movement of at least 2 meters. The self absorption fine structure is even more difficult to resolve. Experiments have been performed to observe the effect in discharge lamps, by using an FTIR spectrometer with possibly one of the optimum resolutions currently attainable. Comparisons between aged and new lamps for changes in line shape clearly show the effect. Other effects such as changes in Boltzman distribution, or changes in the ratio of first and second order lines, are shown to be negligible in comparison. It is well-known in the art that one method of encouraging the recombination of dissociation products of molecules broken down under the effect of electrical discharges, to reproduce the parent gas molecule from which they originally dissociated, is by the use of catalysts. In the case of the CO 2 discharge, the carbon monoxide and oxygen molecules within the lamp envelope can be recombined under the influence of suitable catalysts, to reform the C0 2 molecules from which they dissociated. In sealed-off laser C0 2 laser technology, such catalysts are widely used to maintain the level of C0 2 in the laser cavity, in order to prevent a decay in the laser power output which would occur if the percentage of dissociated C0 2 were to increase with time As mentioned above, the effect of self-absorption is effectively non-existent in C0 2 laser discharges, as also in other systems not lasing directly to the ground state In the C0 2 laser, the stimulated emission of the laser light is produced by a decay transition from a metastable state down to a short-lived excited state It is this transition which produces the familiar C0 2 10 6μm wavelength radiation Since the lasing transition is not to the ground state, the large population of ground state molecules do not absorb the lasing transition energy, and for this reason, self-absorption is effectively non-existent in such laser discharges In gas discharge lamps of the type disclosed in U S Patent No 5,300,839, on the other hand, the IR radiation is created by spontaneous emission in the wavelength region of 4 3μm, by transition of the IR active molecules from rotational-vibrational excited states, directly to the ground state No metastable states are involved in this transition scheme Since a high proportion of the C0 2 molecules populate the ground level, the spontaneous radiation associated with the transition to this ground level is readily absorbed by these ground state molecules, by the process of induced absorption This results in appreciable self-absorption of the radiation In this respect, discharge lamps are significantly different from the stimulated emission of laser discharges operating in the same molecular system, in that they show a significant self absorption effect The previously known catalyst technology used m laser discharges is aimed exclusively at maintaining the laser gam, the laser efficiency and the power output level of the emission from the laser To the best of the Applicants' knowledge, no mention has been made or suggested, that such catalysts be used to maintain the spectral stability of the discharge, since the mechanism of self-absorption by which this could be performed, is not applicable to lasers Furthermore, to the best of the Applicants * knowledge, catalysts have never been used, or their use suggested for stabilizing the emission spectra of gas discharge lamps There is thus provided, in accordance with a preferred embodiment of the present invention, a method for increasing the spectral stability of cold gas discharge infra-red lamp sources, by the use of a catalyst to reduce the changes with time in the concentration of excited gas molecules in the lamp active in emitting to ground state levels, thereby resulting in a reduction in the changes in self-absorption In accordance with yet another preferred embodiment of the present invention, there is provided a method of constructing an improved, electrically excited, gas discharge lamp, whose output is characteristic of spontaneous emission of at least one IR-active gas species to a ground state, consisting of the steps of constructing a lamp envelope, cleaning the lamp envelope, and filling the envelope with a gas mixture consisting of the at least one IR-active gas species, the improvement being the additional step of including a catalytic material within the lamp envelope In accordance with a further preferred embodiment of the present invention, there is also provided a method of constructing an improved electrically excited gas discharge lamp, whose output is characteristic of spontaneous emission of at least one IR-active gas species to a ground state, consisting of the steps of constructing a lamp envelope, cleaning the lamp envelope, and filling the envelope with a gas mixture consisting of the at least one IR-active gas species, the improvement being the additional step of including a catalytic material within the lamp envelope, such that the volume of the lamp can be decreased There is even further provided in accordance with another preferred embodiment of the present invention, a method of constructing an improved electrically excited gas discharge lamp, whose output is characteristic of spontaneous emission of at least one IR-active gas species to a ground state, consisting of the steps of constructing a lamp envelope, cleaning the lamp envelope, and filling the envelope with a gas mixture consisting of the at least one IR-active gas species, the improvement being the additional step of including a catalytic material within the lamp envelope, such that the IR-active gas concentration can be decreased There is also provided in accordance with another preferred embodiment of the present invention, an improved electrically excited gas discharge lamp, whose output is characteristic of spontaneous emission of at least one IR-active gas species to a ground state, comprising a lamp envelope containing a gas mixture consisting of the at least one IR-active gas species, and electrodes external to the envelope for exciting the at least one IR-active gas species, the improvement being a catalytic material located within the lamp envelope There is further provided in accordance with yet another preferred embodiment of the present invention, any of the methods described above, wherein the catalytic material is operative to increase the spectral stability of the lamp by reducing changes with time in the level of self-absorption in the gas mixture Furthermore, in accordance with yet another preferred embodiment of the present invention, there is provided a lamp as described above, wherein the catalyst is operative to increase the spectral stability of the lamp by reducing changes with time in the level of self-absorption in the gas mixture Furthermore, in accordance with yet another preferred embodiment of the present invention, there is provided a lamp as described above, wherein the IR-active gas species is carbon dioxide The concentration of the carbon dioxide may be less than approximately 5% In accordance with still more preferred embodiments of the present invention, the catalytic material may be coated on an inside wall of the envelope, and may be gold, silver, rhodium, indium, palladium, platinum or nickel There is provided in accordance with yet a further preferred embodiment of the present invention, any of the methods described above, wherein the lamp volume is less than approximately 6 millihters BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawing in which Fig 1 is a schematic drawing of an improved electrically-excited gas discharge lamp, constructed and operative according to a preferred embodiment of the present invention DETAILED DESCRIPTIONS OF PREFERRED EMBODIMENTS A number of catalysts are known for reproducing C0 2 from CO and 0 2 , such as platinum with tin oxide, sputtered gold and silver coatings The method of providing the catalytic coating on the interior of the discharge lamp envelope, in order to increase the lamp's spectral stability, depends on the material used for the catalyst According to one preferred embodiment of the present invention, the method consists of sputtering the gold m a finely divided form onto the inside of the lamp envelope, such that it forms a non-conducting film, with a very high surface to volume ratio Other metals such as Indium, Rhodium, Palladium and Nickel can also be used as catalysts Other methods of applying the catalyst besides sputtering may also be used, such as chemical or vapor deposition Reference is now made to Fig 1 , which is a schematic drawing of an improved electrically-excited cold-cathode gas discharge lamp 10, constructed and operative according to another preferred embodiment of the present invention The basic elements of the lamp construction are similar to those disclosed U S Patent No 5,300,859 The envelope 12 of the lamp defines an internal volume 14 which contains a gaseous mixture 16 including one or more IR-active gaseous species The gas is preferably excited by two electrodes 18, 20, disposed outside of the envelope, and supplied with RF-excitmg power via a pair of cables 22 After being raised to an excited state, the IR-active gaseous species decays by means of a spontaneous emission directly to the ground state, and the emitted radiation is output by means of an optically transparent window 24 at one end of the lamp envelope According to this preferred embodiment of the present invention, on the inner wall of the lamp envelope is a layer of catalyst 26, operative to maintain a constant level of the dissociation products of the IR-active gaseous species, such that the self absorption of the emitted radiation is kept at a constant level, and spectral stability of the lamp output maintained, as explained in detail hereinabove The catalyst is chosen to ensure an equilibrium between the IR-active gaseous species and its dissociation products In this respect, it should be noted that unlike prior art use of catalysts in lasers, where the catalyst is optimally operative to keep dissociation of the IR-active species to a minimum, in order to keep laser output power to a maximum, according to the present invention, the catalyst need only maintain a constant equilibrium level of the IR-active species in order to achieve its aim of maintaining constant self-absorption and hence constant spectral stability The catalyst may be applied by any of the methods known in the art, by chemical, sputtered or vapor deposition, or by any other suitable means The catalyst may be of any of the types mentioned hereinabove Since the catalyst reduces the breakdown of C0 2 , it is possible to reduce the need for a large ballast volume of gas, and thus to produce a lamp of considerably reduced size Such a lamp is advantageous for use in portable systems Such a smaller lamp has a better surface to volume ratio with respect to the active media and the activated 0 2 molecules require a shorter path length to reach the coating, and hence have a higher probability of reaching the catalytic coating in the required activated state In U S Patent No 5,300,859, lamp volumes of the order of 60 ml are disclosed This volume was required to ensure an adequate reservoir volume to ensure long lamp life If lamps of smaller volume were used, the increased effect of the absorption of dissociated carbon monoxide and oxygen on the increased surface of the walls in relation to the gas volume, would result in the lamp discharge decaying very rapidly The use of a catalyst on the lamp walls, according to a further embodiment of the present invention, allows significantly smaller lamps to be constructed, without negatively affecting their lifetime or spectral stability It is possible to achieve lamp volumes of 6ml or less, which provide similar lifetimes to those of the prior art 60ml volume lamps, while maintaining the same level of spectral stability This is of significant advantage for use in the breath tester, where two lamp sources are typically required, whose mutual stability is dependent on the maintenance of both of them under similar environmental conditions, something that is simpler to achieve with more compact lamps Since the catalyst reduces the breakdown of C0 2 , the use of a catalyst according to the present invention, makes it feasible to produce a lamp with a lower C0 2 pressure In U S Patent No 5,300,859, C0 2 percentages of the order of 10% are recommended for optimum output and lifetime considerations According to preferred embodiments of the present invention, the use of a catalyst allows the operation of the lamp with lower C0 2 concentrations in the lamp gas mixture, down to 5% This results in lower self-absorption effects, with consequent higher intensity central lines of the absorption spectrum, as explained hereinabove This provides very deep absorption curves with a high extinction ratio in the gas analyzer measurement cell, permitting use of a shorter cell path and a more compact instrument without losing detection sensitivity or selectivity This is of high importance when measuring the very low concentration levels of the minority isotopic component in a breath test It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove Rather the scope of the present invention includes both combinations and subcombmations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not m the prior art

Claims: CLAIMS We claim 1 A method of constructing an improved, electrically excited, gas discharge lamp, whose output is characteristic of spontaneous emission of at least one IR-active gas species to a ground state, comprising the steps of constructing a lamp envelope, cleaning said lamp envelope, and filling said envelope with a gas mixture comprising said at least one IR-active gas species, the improvement being the additional step of including a catalytic material within said lamp envelope 2 The method according to claim 1, wherein said catalytic material is operative to increase the spectral stability of said lamp by reducing changes with time in the level of self-absorption in said gas mixture 3 The method according to claim 1 wherein said catalytic material is coated on an inside wall of said envelope 4 The method according to claim 1 , wherein said catalytic material is chosen from a group comprising gold, silver, rhodium, indium, palladium, platinum and nickel 5 A method of constructing an improved electrically excited gas discharge lamp, whose output is characteristic of spontaneous emission of at least one IR-active gas species to a ground state, comprising the steps of constructing a lamp envelope, cleaning said lamp envelope, and filling said envelope with a gas mixture comprising said at least one IR-active gas species, the improvement being the additional step of including a catalytic material within said lamp envelope, such that the volume of said lamp can be decreased 6 The method according to claim 5, wherein said lamp volume is less than approximately 6 millihters 7 A method of constructing an improved electrically excited gas discharge lamp, whose output is characteristic of spontaneous emission of at least one IR-active gas species to a ground state, comprising the steps of constructing a lamp envelope, cleaning said lamp envelope, and filling said envelope with a gas mixture comprising said at least one IR-active gas species, the improvement being the additional step of including a catalytic material within said lamp envelope, such that the IR-active gas concentration can be decreased 8 The method according to any of the previous claims wherein said IR-active gas species is carbon dioxide 9 The method according to claim 8, wherein the concentration of said carbon dioxide is less than approximately 5% 10 An improved electrically excited gas discharge lamp, whose output is characteristic of spontaneous emission of at least one IR-active gas species to a ground state, comprising a lamp envelope containing a gas mixture comprising said at least one IR-active gas species, and electrodes external to said envelope for exciting said at least one IR-active gas species, the improvement being a catalytic material located within said lamp envelope 1 1 The lamp according to claim 10, wherein said catalyst is operative to increase the spectral stability of said lamp by reducing changes with time in the level of self-absorption in said gas mixture 12 The lamp according to claim 10, wherein said catalytic material is coated on an inside wall of said envelope 13 The lamp according to claim 10, wherein said catalytic material is chosen from a group comprising gold, silver, rhodium, indium, palladium, platinum and nickel 14 The lamp according to any of claims 10 to 13, wherein said IR-active gas species is carbon dioxide

Inventors: COLMAN LEWIS, COLMAN, LEWIS
Applicants: COLMAN LEWIS, ORIDION MEDICAL 1987 LTD, COLMAN, LEWIS, ORIDION MEDICAL (1987) LTD.
Application year: 2000
Main classification: H01J61
IPC codes: A61B5, G01N33, H01J9, H01J61, H01J65