HYDROGEN PEROXIDE CONCENTRATION

Technical Field

The present invention relates to a device and a method for in-situ online

measurements of hydrogen peroxide concentration, in particular concentration of vaporized hydrogen peroxide concentration.

Technical Background

In machines where packaging material or packaging containers are handled use is often made of hydrogen peroxide as a sterilizing agent. It may be that the packaging containers (or the packaging material) should be sterilized prior to being filled (or prior to being formed into packaging containers). The sterilizing agent may be applied to the packages in a gaseous/vaporized state or in a liquid state. For some systems the sterilizing agent is applied in a gaseous form and thereafter it condenses onto the packaging containers, i.e. it assumes a liquid state.

The hydrogen peroxide is usually provided in the form of an aqueous solution, which means that gaseous/vaporized hydrogen peroxide and water will be present in the same volume.

The concentration of gaseous hydrogen peroxide is an important control parameter, and for that reason several systems for measuring that particular concentration exists today. Two common approaches are various types of optical measurements utilizing the absorption spectrum of hydrogen peroxide. One major issue during such measurement is the presence of water vapour, which easily may bias measurement results if due care is not taken. The impact of the biasing effect will also vary with temperature, which makes the situation still more complex. One partial solution towards minimizing the adverse effect of water vapour is described in US 7 880 887, including the use of multiple wavelengths for measurements. Another approach is to avoid the use of optical measurements altogether, e.g. by using a sensor based on a catalyst, wherein the temperature profile of the catalyst may be indicative of the hydrogen peroxide concentration in the gases passing through the catalyst.

Summary

In a device and method according to the present invention and in particular embodiments thereof the above drawbacks are addressed, and their negative impact is eliminated or at least alleviated, and further advantages are achieved.

The present invention provides a method for remote measurement of hydrogen peroxide concentration, comprising the steps of: providing a source of ultraviolet laser radiation,

injecting the radiation from the source into a first optical fibre,

guiding the radiation to a measurement region comprising hydrogen peroxide vapour, where the optical fibre debouches,

passing the radiation through the measurement region,

collecting the radiation to second optical fibre and leading it to a photo detector,

determining the concentration of hydrogen peroxide by means of comparing the transmitted radiation (¾) without hydrogen peroxide in the measurement region with the transmitted radiation (Ic) with hydrogen peroxide in the measurement volume.

In this context "injecting radiation" refers to the process of transferring the laser radiation from the laser into the optical fibre. This is usually solved using an optical lens of suitable diameter and focal length. When entering the optical fibre the laser radiation should have a convergence/divergence within a critical interval in order to avoid significant losses when passing through the optical fibre. Parameters of the optical fibre and the characteristics of the laser radiation determine what is considered "suitable" and "a critical interval".

The use of radiation in the ultraviolet enables measurements in a wavelength region where water vapour does not absorb to any significant extent, which will increase the reliability of the measurements. Also, in the ultraviolet region the cross section of hydrogen peroxide presents several advantageous features the most important being that the cross section increases exponentially with decreasing wavelength and that the temperature dependence decreases with decreasing wavelength. The increased cross section makes it possible to use a shorter absorption length (or measurement length), which increases the spatial resolution of the measurement etc. The use of a laser source enables the use of an extremely well defined and narrow wavelength interval, basically a single wavelength with a sub-nanometre width. This means that the value used for the cross section will constant and reliable, adding to the reliability of the measurement results. The use of a single wavelength also enables efficient discrimination of unwanted radiation, such as thermal radiation or background radiation (e.g. from ambient light) which otherwise could interfere with the measurements. Using an optical fibre enables the positioning of the laser and the photo detector in a position remote from the measurement region. This has several advantages one being that they may be localized in an area with some thermal stability, such that variation in temperature does not affect the measurement. From a more practical standpoint space, and the use of fibres makes it possible to position the laser and detector in any suitable position while leading the radiation to and from the measurement region in a safe way.

In one or more embodiments the laser source is pulsed, i.e. instead of delivering a seemingly continuous laser beam it concentrates the available energy in time and delivers pulses of radiation. The use of pulses increases the intensity of the radiation used for measurement and raises the relevant measurement above the background noise/radiation significantly. Also, it enables the use of temporal filtering of measurements, which enhances the measurement results even further. It also enables more elaborate measurement schemes, some of which will be described in the detailed description.

In one or more embodiments the method may comprise the additional steps of injecting the radiation into several optical fibres, wherein the subsequent steps of guiding, passing collecting and determining may be performed for each optical fibre. This enables for measurements to be performed in several locations simultaneously while using only one source of radiation.

In one or more embodiments the wavelength of the radiation is 224.3 nm with a FWHM below 1 nm, and in other embodiments 248 nm or 266 nm may be used, both being standard wavelengths for lasers.

In one or several embodiments the first optical fibre may debouch in a measurement cell defining the measurement region and positioning a downstream end of the first optical fibre and an upstream end of the second optical fibre.

The downstream end of the first optical fibre and the downstream end of the second optical fibre may be positioned at opposite ends of the measurement cell, yet in some embodiments they may be positioned at the same end measurement cell, in which case a reflective surface may be arranged in the other end of the measurement cell for direction of radiation from the first optical fibre to the second optical fibre. The use of a reflective surface in the measurement cell enables for the upstream and downstream end to be positioned according to several alternative configurations.

In one or more embodiments a second source of radiation may be used in addition to the first. This second source of radiation may in fact be the same laser, or another laser or lightsource, the important feature being that it radiates at another wavelength than the one used for absorption in hydrogen peroxide. In fact, the wavelength is preferably chosen such that it has a very low absorption in relation to hydrogen peroxide. Further it is preferred that the second wavelength is guided along the same optical path as the first (i.e. as described above and below). By monitoring the behaviour of this second wavelength it may be possible to better account for changes along the optical path which are not related to a varying concentration of hydrogen peroxide. Such changes may include, but are not limited to, fouling of optical surfaces, condensation of stabilizers used for the hydrogen peroxide, again on optical surfaces. By including these types of changes in the calculation of the hydrogen peroxide concentration, the assessment may be improved, in particular in a practical case.

Since two different wavelengths are used it will be possible to spectrally discriminate the first wavelength from the second by simple means, such as spectral filters arranged at suitable locations of the optical path, i.e. downstream a beam splitter or just upstream a measurement sensor. In the embodiments using a first source of radiation providing a pulsed output the second source of radiation may provide a pulsed output too. In such a case there will be an additional way of discriminating the first wavelength from the second, namely to inject the pulses at different temporal positions, such that only one of them reaches a measurement sensor at any given moment.

A typical wavelength radiated from the second source of radiation may be 385 nm and upwards, yet given the explicit task of finding a suitable wavelength under the given circumstances the skilled person should be able to find other examples where hydrogen peroxide exhibits little or no absorption.

The present invention also relates to a machine for sterilizing packaging containers of packaging material utilizing the device or the method according to the present invention or embodiments thereof. Said "machine for sterilizing..." may preferably form a section of a machine for forming and filling packaging containers.

Brief Description of Drawings

Fig. 1 is a graph illustrating the H ₂ O ₂ vapour absorption cross section as a function of wavelength for a number of temperatures.

Fig. 2 is a schematic drawing illustrating the experimental setup according to a first embodiment of the present invention.

Fig. 3 is a schematic drawing illustrating the experimental setup according to a second embodiment of the present invention.

Fig. 4 is a schematic drawing illustrating a measurement cell in closer detail.

Fig. 5 is a schematic drawing illustrating a measurement setup in accordance with a third embodiment of the present invention.

Fig. 6 is a schematic side view of a part of a filling machine including a sensor arrangement.

Detailed Description of Embodiments

The present invention aims at providing a simplified and versatile system, enabling reliable and repeatable measurement results and multi -point measurement of hydrogen- peroxide gas concentration.

During the development of the inventive device and method several aspects have been found as being important. The uttermost important parameter is the absorption cross section of hydrogen peroxide, since the cross section is used when determining the concentration from an absorption measurement.

The absorption cross section varies with wavelength, and if a source such as a LED or a lamp emitting light in a wavelength spectrum is used some sort of "effective mean" cross section has to be deduced, at least in situation where a measurement sensor such as a photomultiplier or a photodiode is used. This is a laborious procedure if performed by theoretical modelling of tabulated values. To avoid this, it is common practice to perform one or several calibration measurements, i.e. several absorption measurements for several concentrations of hydrogen peroxide are performed, after which an effective mean cross section is deduced empirically. This may sound as a straight forward process, yet the properties of H202 may complicate such calibration measurements since there are many factors to consider: Not all H ₂ 0 ₂ becomes vaporized, some is still present in aerosol form; the H ₂ 0 ₂ will start to decompose, both effects resulting in that the expected concentration is not the actual concentration. Such error factors will affect the reliability of the deduced value of the cross section, which still makes the method a laborious and not necessarily reliable one. The term "absorption coefficient" is sometimes used instead of "absorption cross section" and these terms may be used in an analogue way though they are not equivalent entities.

The next problem that may be encountered is that the cross section may vary with temperature, and to make matters worse this variation is wavelength dependant too. To make the situation even more complex the absorption cross section of water behaves in a similar manner, and the absorption spectrum of water has a significant overlap with the absorption spectrum of hydrogen peroxide. This means that a detected absorption must be analyzed thoroughly and compensated for the prevailing conditions, such as temperature, pressure, humidity etc. before a likely value for the hydrogen peroxide concentration may be evaluated. It is obvious that any necessary calculations may be performed in a processing unit, by e.g. inputting data to a computer performing the necessary calculations, in a fully automated manner. However, even if all calculations are performed accurately, with the best available data each uncertainty will add to the total error margin and thus to the reliability of the deduced result.

The graph of Fig. 1 [adapted from Nicovich, J. M., and P. H. Wine (1988),

Temperature-dependent absorption cross sections for hydrogen peroxide vapor, J. Geophys. Res., 93(D3), 2417-2421] illustrates the absorption cross section of hydrogen peroxide vapour as a function of wavelength for a number of temperatures. Note the logarithmic scale of the y-axis.

The temperature in a machine used for sterilization of packaging containers may range from about 80 to 140°C. Fig. 1 reveals that measurements in a machine region where the temperature varies with a radiation source emitting in a broad wavelength interval may affect the reliability of the results significantly. In a theoretical situation where all parameters are given, full compensation for the variations may be accomplished. In practice, however, the situation may be more complex, one example being variation in the spectral intensity profile over time which may be more difficult to account for without complex measurement equipment. A varying temperature may also affect the reliability of the measurement in that the spectrum provided by the source of radiation as well as the intensity of the emitted radiation may shift with temperature, i.e. parameters not coupled to the environment in a measurement region. Also, the response of the sensor, generally the photodetector, may vary with temperature (thermal drift). Some of these issues are discussed in US 6 269 680. Measurement Equipment and Methodology

In order to ensure sufficient disclosure one example of measurement equipment is described in the following, referring to Fig. 2. The light source 2 is a pulsed HeAg deep UV laser providing radiation at 224.3 nm with an oscillation bandwidth of about 3 GHz, and a repetition rate of about 1-30 Hz. The pulse width (FWHM) is variable between about 5 and 200 μβ. The use of monochromatic radiation has several advantages, one being that all emitted radiation may be used in the measurement, as oppose to a solution where filters are used for selection of a particular wavelength interval from a broader spectrum. The laser radiation is collected and injected into a first (silica) optical fibre 4 allowing transmittance of UV radiation, which guides the radiation to a measurement cell 8 arranged in the

measurement position in a machine, indicated at reference numeral 6. A suitable collection lens may be used for injection purposes. The measurement cell 8 is positioned in the location where the measurement is to be made, and has an open configuration allowing entrance of gas mixture to the measurement region. The radiation passes the measurement region once (or twice if a reflector is used) after which it is injected in a second optical fibre 10 of the same type as the first 4. The second optical fibre 10 transports the collected radiation to a photomultiplier 12, and the output of the photomultiplier is forwarded to a processing unit (not shown). The triangular arrows indicate the travel direction of the radiation through the system, and the measurement length, i.e. the distance over which the radiation is expected to interact with H ₂ 0 ₂ is indicated by the letter 1.

The entire measurement procedure may be controlled and monitored using a dedicated control unit CU, which may also comprise a CPU for performing the actual evaluation of data. The CU may be a commercial product, such as NI Lab VIEW yet tailor-made solutions may also be developed without it requiring any inventive skills.

To keep track of variations in laser output it is preferred that a photodiode (not shown) is arranged to measure the output of the laser pulse by pulse and transfer a measure of the output to the CU. In many commercial laser systems this type of sensor is a standard component arranged within the laser housing, and it is not illustrated in the appended drawings. Further, in the basic configuration the photomultiplier is gated such that

measurement data is acquired in a temporal region surrounding the laser pulse in order to suppress the influence of background radiation. If needed measurement data may be acquired clean between laser pulses too in order to gain data in regard of variations in background radiation. Measurement data indicates that the latter procedure is not needed since the suppression of the background in relation to the laser radiation achieved by the gating around the laser pulse. This obviously depends on the measurement situation. Since the laser radiation is truly and highly monochromatic an optical filter, such as an interference filter or bandpass filter may be used to discriminate all other wavelengths than the laser wavelength carrying the relevant information. Such a filter may be arranged in front of the

photomultiplier.

In total four various measurement parameters coupled to radiation may be used as input to the CU on a pulse to pulse basis:

I _re f- A value of the laser output. This measure basically normalizes all other measurements and may be measured for each laser pulse.

Io - The output from the photomultiplier as a response from a laser pulse without any H202 in the measurement region. This parameter is measured before any H ₂ 0 ₂ is introduced in the measurement region.

I _H202 - The output from the photomultiplier as a response from a laser pulse with H ₂ 0 ₂ in the measurement region.

hack - The output from the photomultiplier in a time interval between laser pulses, which may be used to account for any significant background radiation.

In the above equation σ is the absorption cross section, / the measurement length, and Nthe density of absorbing particles, i.e. a measure of the concentration. Since Nis the only unknown parameter a measure of the concentration may be readily deduced from a

measurement. The parameter 7 _re may typically be used such that each measurement value of I _H202 and I ₀ is normalized (divided by the correlating measurement value for I _re j). The above equation, the Beer-Lambert law, is well known for the skilled person as is the processing steps necessary to obtain a result. If hack is of a magnitude such that it affects the

measurement result compensation is performed by subtraction of hack from the correlated value for lmo ₂ and I ₀ respectively.

In a measurement sequence the measurement cell or probe is arranged in the desired location inside the filling machine. After that heating of the interior of the machine is initiated. Heating is performed since any cold surfaces will be potential condensation areas for H ₂ 0 ₂ vapour. In some instances condensation may be desired for sterilization purposes yet for the present measurement method it is of some importance that condensation does not occur on the optics of the measurement probe since it would bias measurements.

During the heating stage there is no hydrogen peroxide in the measurement region, and therefore it is a suitable time for starting the laser and the rest of the measurement equipment in general, and to acquire a value of I ₀ in particular, once the heating is adequate and before H ₂ 0 ₂ is introduced in the measurement region. Following the heating stage the machine is subject to machine sterilization, after which sterilization of packages or packaging material may commence. At this stage measurements may be performed continuously. Measurement results may be evaluated in real-time processing, and as such the results may be used as a control parameter. The control parameter may be used to control the supply of H2O2 to the machine automatically; analogous to how a lambda sensor is used to control the fuel-to-air ration in a combustion engine. The control may also be less elaborate, such that a measurement results outside of an expected interval trigger a visual or audible warning to an operator, for further assessment and possible actions. Measurement results may be logged and stored in correlation with relevant parameters (identifiers) for the corresponding sequence of packages or packaging material, contents of the package, etc. which enables quality assurance and back-tracking.

From studying the above description it is envisaged that the skilled person could practise the present invention.

As an additional feature a second wavelength may be used for measurements. This second wavelength may be provided by the same radiation source 2 as the first one, or by another radiation source. Even if the actual radiation source is the same as for the first wavelength the radiation source of the second wavelength may be referred to as the second radiation source. The purpose of the second wavelength is to measure absorption not caused by the concentration of sterilization agent. Examples of such absorption have been given previously in the application, and also of a suitable wavelength that may be used. By being able to extract that information the deduced concentration of sterilization agent as calculated in accordance with what is previously described may be further refined. This may be an important feature in a practical installation, e.g. in commercial filling machine.

As such this second wavelength may be performed as a continuous beam, preferably laser beam, or a pulsed laser beam. In case the same laser in used the parameters for the second beam may be similar to those of the first, apart from the obvious difference of the wavelength.

It is preferred that the second beam follows the beam path of the first beam, in particular that it passes the same measurement cell, since the most probable cause of absorption not directly related to the concentration of sterilization agent relates to fouling of optical surfaces inside the measurement cell. This is also true for the embodiments to follow.

In Fig. 3 a schematical setup representing a second embodiment of the present invention is illustrated. Corresponding components have been given the same reference numeral in Figs. 2 and 3, and in all the drawings for that matter. The setup differs from the setup in Fig. 2 in that the measurement cell 8' has a reflector 7 in the remote end, in relation to the radiation inlet end. Laser radiation enters the cell 8', makes one pass, reflects of the reflector7, makes a second pass and is collected to a second optical fibre 10 downstream of which the setup is similar to that of Fig. 2. A more detailed view of a measurement cell which may be used in the embodiment presented in Fig. 3 is illustrated in Fig. 4. The measurement cell 8' or probe is formed from a hollow cylinder with the optical fibres 4, 10 entering from one end and the reflector 7 being arranged at the other end. Hydrogen peroxide is prevented from leaving the measurement area through the measurement probe by having the probe sealed at some suitable position. The choice of sealant will vary with the position, e.g. with the amount of thermal load, yet the material in the sealing position should not decompose or degrade when in contact with H ₂ O ₂ . Depending on the position and the type of product sterilized it may also have to be stable at elevated temperatures, and have the adequate approvals to be used in food-processing. In the illustrated embodiment the measurement cell has a through-hole 9 of racetrack-shaped cross section allowing gases to enter the measurement region. The skilled person realizes that the through-hole 9 may have any other suitable other cross sections, that the measurement cell 8' may comprise more than one through-hole 9 etc. Due to the fortunate choice of wavelength, and the comparably high absorption cross section, the measurement length 1 may be kept quite short, in the order of a few centimetres, which results in a good spatial resolution. A beneficial side-effect is that the probe 8' may be kept small, having a low mass, which in turn results in that probe readily assumes the ambient temperature, which reduces the

condensation of hydrogen peroxide once the rest of the machine has reached operational temperature.

In the illustrated embodiments a photomultiplier has been used for detection of the transmitted laser intensity. There are other types of photosensors, more complex or less complex which may be used instead depending on the application. Therefore the present invention should not be limited in this respect.

The suggested type of laser is beneficial due to its "performance to price" ratio. Laser diodes or other types of solid state lasers do not seem to be able to compete with the present type of laser, at least not at the time being. However, it has been a historical fact that within the field of quantum electronics and over time performance is going up while prices are going down. The present invention should not be limited by the particular type of laser mentioned in the detailed description, even if some advantages of pulsed operation, such as the options to suppress background radiation etc, will still be a fact.

Fig. 5 illustrates a third embodiment of the present invention, in which the laser radiation 16 which exits the laser is split into four (i.e. more than one) beams 16', e.g. by using semitransparent reflectors 18. Each beam 16' may then be injected into one optical fibre 4 each. Radiation is collected and injected into each fibre 4 in a manner as described previously, and is lead to and from the measurement region 6 as described in relation to

Fig. 2 or Fig. 3 (or Fig. 4). The photomultipliers 12 may be arranged within the same housing 20 as the semitransparent reflectors 18, whereby the housing 20 may be arranged in front of the laser as a plug-and-play unit. It should be noted that even though only one measurement region 6 is indicated the individual probes 8 are located at different measurement positions.

In a case where N measurement probes 8 are used and where the semitransparent reflectors 18 are perfect in the sense that the sum of their individual transmittance and reflectance is 1 (i.e. no losses) the reflectance of the n ^th mirror could preferably be about R=l/(N+l-n), such as to provide a balanced measurement system where comparable laser energies are injected into each optical fibre. In the illustrated embodiment this would correspond to a reflectance of 0.25 for the first mirror (reflector), 0.33 for the second, 0.5 for the third, and 1 for the fourth, starting with the upstream mirror. The fourth optical fibre may be moved to a location where it collects the radiation transmitted by the third mirror directly, making the forth mirror obsolete. In a case where there is an abundance of laser energy all fibres may instead be collected in a bundle such that the laser injects radiation into all fibres simultaneously. In such a case the laser beam 16 may be expanded or condensed, e.g. by using a telescopic lens system, prior to it illuminating the optical fibres.

Instead of using the 224.3 nm laser wavelength other characteristic laser wavelengths may be used, such as 248 nm or possibly even 266 nm.

In any of the above embodiments the described light source 2 may be accompanied by a second light source (not shown), providing a second source of radiation. The purpose of this second light source is to provide light of a second wavelength, differing from the first (as generated by the first light source 2). Several lasers functions such that they emit radiation at a certain frequency, which then may be frequency-doubled, -tripled or mixed in other ways. Therefore the second source of radiation may actually be the same as the first source of radiation, while still fulfilling the purpose of providing a second wavelength. Further, it is preferred that this second wavelength is guided along essentially the same optical path as the first one, in particular in a measurement region, since this is where the purpose of the second wavelength is expected to have the most impact. The purpose of the second wavelength is to account for absorption sources other than hydrogen peroxide. These absorption sources mainly, almost solely, relates to fouling of optical surfaces, such as windows, reflectors, prisms etc. If such absorption sources are significant, they may result in an error in the calculated concentration of hydrogen peroxide. If they are a result of a gradual build-up they may cause an overestimation of the concentration of hydrogen peroxide, and if they vary over time in another way they will in any case cause an uncertainty. Deposits or buildup on optical surfaces may be caused by stabilizer agents included in the hydrogen-peroxide mixture, yet other impurities may also be an issue. The exact wavelength for this second wavelength is not crucial, yet for achieving its intended purpose it should be wavelength differing from the first and a wavelength at which the absorption cross section of hydrogen peroxide is insignificant. In one or several embodiments the wavelength emitted from this second source of radiation may be found in the visual region. The present invention, in any embodiment thereof, may preferably be used in a filling machine, preferably a filling machine capable of providing carton-based packages enclosing liquid food. In such a filling machine the device of any embodiment of the present invention may be arranged to provide a value of hydrogen-peroxide concentration in a particular measurement region, e.g. by arranging the measurement cell 8, 8' in the desired location. A measure of the concentration may be provided to a control unit. The control unit may then compare the measured value with a desired value, and as a consequence of that comparison the control unit may increase or decrease the supply of hydrogen peroxide from a hydrogen peroxide supply. The control unit may also deduce that the supply of hydrogen peroxide should be maintained at a constant level. Further, if the concentration of hydrogen peroxide passes a certain threshold value (upper or lower) the control unit may cause a visual or audible alarm, or communicate an undesired operation condition by taking another action.

An example of this is illustrated in Fig. 6, showing a schematic view of a section of a filling machine in which a measurement cell 8, 8' is arranged. The section is illustrated as a sterilization unit 24 in which ready-to-fill packages 22 are introduced by means of a carrier system 32. The packages 22 may preferably be ready-to-fill packages consisting of a sleeve and a top 28 sealing one end of the sleeve. The top 28 may comprise a folded portion of the sleeve, e.g. forming a gable-top package, or the top 28 may comprise an injected moulded plastic arrangement, as is the case for the present example.

The sterilization unit 24 includes a sterile gas manifold 25 enclosing the gaseous sterilization substance (e.g. H202 with or without added stabilizers) and comprises a number of discharge nozzles 26 configured to eject a spray distribution of the sterilization substance to the interior of the packages 22.

The measurement cell 8, 8' is arranged within the manifold 25 for measuring and determining the concentration of the sterilization substance. The measurement cell 8, 8' is therefore configured to operate continuously during operation of the filling machine in order to accurately determine the quality and robustness of the sterilization unit 24. Preferably, the measurement cell 8, 8' measures a value of the concentration of the sterilization agent, a measure of which is reported to a control unit 30. The control unit 30 may comprise several operational sub-units, and it is only included for illustrative purposes. The control unit 30 may in turn, directly or indirectly communicate or control a substance supply (a source of sterilization agent and associated equipment for supplying the sterilization agent to the sterilization unit) for increasing or decreasing the amount of sterilizing agent within the manifold 25, i.e. for control of the sterile substance supply.

Such feed-back loop may be implemented in various applications, whereby a determined concentration is compared with a reference value corresponding to a desired concentration. The difference between the determined concentration and the reference value may thus be converted to a corrected operating parameter, such as flow rate, of a sterilizing agent supply being connected to the sample. Hence, the control unit of the concentration measurement device may thus be configured to transmit a signal to the substance supply in order to increase or decrease the concentration of the substance within the sample.

Though hydrogen peroxide has been used as the preferred example another sterilizing agent may be used instead.

As used herein deep -ultraviolet (DUV) denotes wavelengths shorter than about 300 nm, which basically corresponds to the term UVC, denoting wavelengths between 100 and 280 nm. For wavelengths shorter than about 200 nm oxygen will absorb radiation in a way that makes such wavelengths less useful for the present application.