WO2015024094A1

WO2015024094A1 - Uv apparatus and method for air disinfection

Info

Publication number: WO2015024094A1
Application number: PCT/CA2013/050641
Authority: WO
Inventors: Ashraf Najeeb KHALID; Alexander Novikov
Original assignee: Khalid Ashraf Najeeb
Priority date: 2013-08-20
Filing date: 2013-08-20
Publication date: 2015-02-26

Abstract

There is provided an air disinfecting system comprising a UV light source which comprises a laser to generate a fundamental beam of predetermined frequency and harmonic frequencies generators for producing UV light beam using the fundamental beam, an air flow structure, and optical elements for optically coupling the UV light beam to the air flow structure to inject substantially dispersed UV light therein, the substantially dispersed UV light having sufficient energy to inactivate pathogens.

Description

UV APPARATUS AND METHOD FOR AIR DISINFECTION

Technical Field

This invention relates generally to UV germicidal irradiation. More specifically, this invention relates to the use of UV light for air disinfection.

Background

The knowledge that electromagnetic radiation with wavelengths in the Ultra Violet C (UVC) band (210 nm to 280 nm) kills bacteria is well established with the first recognition of this discovery resulting in a Nobel prize awarded to Niels Ryberg Finsen for Physiology or Medicine in 1903. It is now known that DNA of living organisms is highly absorbent at 260nm and that at sufficient dose, UV light can cause molecular breaks resulting in inactivation of pathogens.

Pathogens can be a serious issue in public buildings and especially in hospitals where over 100,000 persons die every year in the USA because of pathogens infections. Pathogens are spread from infected individuals through touch and through circulating air. Removal of pathogens is very difficult. Air filtration is commonly used but does not inactivate pathogens; filtration makes matters worse by providing sites for the pathogens to multiply as filtration alone does not kill them. Early attempts at removing pathogens from the air by filtering them out was disclosed by M. WILEY et al in US patent 3,347,025. HEPA filters were disclosed by Mark A. Tuckerman et al in US patent 5,616,172. Periodic replacement of the filters can solve the problem some of the problems but at a cost of very frequent maintenance. Furthermore smaller pathogens, like viruses, cannot be removed by filtering.

Life on this planet evolved in the absence of UVC. The ozone layers shields the planet form these wavelengths of electro-magnetic radiation. All life forms are extremely sensitive to these wavelengths and cells are destroyed with small doses of this radiation. In simple organisms, this means immediate death, in complex organisms, it means various cancers and cellular destruction leading to death. The use of UVC to inactivate pathogens is widespread. Traditionally UVCs are generated through mercury arc lamps and through gas discharge tubes. These sources produce a complex spectrum of electromagnetic radiation not all of them efficient for pathogen inactivation. Furthermore UVC generated using mercury and fluorescent lamps are costly, produce ozone, and maintenance and heat generated by these lamps are barriers to large scale adoption.

Further improvement on the filter based method has been disclosed by William Morrow et al. in US patent 5,656,242, where UVC produced from light tubes is used to kill the pathogen that is deposited on the filter surfaces. Although an improvement, the method requires a large number of light tubes, regular replacement of these light tubes as they have rapidly declining power curve as a function of time.

Another method disclosed in US patent 7,407,633 uses UV light to create ozone to kill pathogens. While effective in killing most forms of bacteria, ozone still needs to be removed. US 2005/0249630 A1 by Olubunmi Ayodele Odumuye et al. discloses a method wherein UV light produced by light tubes is used to purify the air or other carriers of pathogens such as water. This method suffers from the drawbacks of all of the above methods, namely it is costly and produces ozone.

US 2010/0320440 A1 teaches producing electromagnetic radiation in UVC, specifically 365 nm, aimed at killing pathogens. This method is effective provided there is sufficient time to overcome the low power of these devices and is confined to purifying water in small containers over hours.

In US patent 6,555,01 1 by Zamir Tribelsky et al., claim is made of an apparatus whereby the gas or liquid to be disinfected is contained in a parabolic chamber that is then exposed to electromagnetic radiation of many types.

US 201 1/0142725 A1 by Xuanbin Liu et al. discloses an air purification method that uses electromagnetic radiation through a glass but requires a coating of photo catalyst. In view of the above there is a need for improved UV germicidal irradiation. Summary

The invention relates to air disinfecting UV light source comprising a diode-pumped infrared laser to generate an IR fundamental beam, a second and fourth harmonic frequencies generators for producing a UV light beam using the fundamental beam, and optical elements for optically coupling the UV light beam to an air flow structure to inject substantially dispersed UV light into the air flow structure, the substantially dispersed UV light having sufficient energy to inactivate pathogens.

In another aspect of the invention there is provided an air disinfecting system comprising an air disinfecting UV light source comprising a diode-pumped infrared laser to generate an IR fundamental beam, a second and fourth harmonic frequencies generators for producing a UV light beam using the fundamental beam, and optical elements for optically coupling the UV light beam to an air flow structure to inject substantially dispersed UV light into the air flow structure, the substantially dispersed UV light having sufficient energy to inactivate pathogens and an air flow structure optically coupled to the UV light source.

In yet another aspect of the invention there is provided an air disinfecting system comprising a solid state UV light source substantially ozone-free producing for generating a UV light beam, an air flow structure optically coupled to said solid state UV light source, and optical elements for optically coupling the UV light beam to the air flow structure to inject substantially dispersed UV light into the air flow structure, the substantially dispersed UV light having sufficient energy to inactivate pathogens.

There is also provided a method for inactivating pathogens in an air flow structure by providing UV light generated using a UV light source comprising a laser to generate a fundamental beam of predetermined frequency and harmonic frequencies generators for producing a UV light beam, and injecting the light within an air flow structure. Brief Description of the Drawings

The invention will be better understood by way of the following detailed description of embodiments of the invention with reference to the appended drawings, in which:

Figure 1 is a schematic representation of the UV air disinfecting apparatus in association with an air duct.

Figure 2 is a schematic diagram of a configuration of optical components of the harmonic frequencies generator.

Figure 3 A is a cross-section of an air duct showing a light injection port with a circular geometry. Figure 3 B is a cross-section of an air duct showing a light injection port with a rectangular geometry.

Figure 4 is a schematic representation of a system in which the harmonic UV light generator is coupled with an air duct with an optic fiber.

Figure 5 is a flow chart diagram of the circuit for feedback control of light intensity using air flow structure environment parameters measurements .

Figure 6 is a cross-section view of an air duct showing two UV light generator opposite each other.

Figure 7 is a schematic representation of an insertable air flow structure optically coupled to a UV light generator.

Detailed Description

The air disinfecting apparatus of the present invention provides an efficient way of delivering UV radiation to an air flow structure, such as an air duct or HVAC, to kill pathogens and therefore substantially disinfect the air rendering it more healthy to breathe. The apparatus 1 is schematically represented in FIG.1 when coupled to an air duct 2 through light injection ports 3.

In one aspect of the invention the UV light is generated through a two-stage frequency doubling optical assembly using a fundamental infrared (IR) beam. Figure 2 schematically represents an exemplary embodiment of the various components of a UV light generator of the invention that generally comprises a light source (laser) to produce a fundamental beam that is then transmitted through optical components to generate harmonic frequencies resulting in shorter wavelengths beams. An IR light pumping source 10 generates a pumping beam 1 1 directed to a mirror 12 positioned at an angle to the beam and that is highly reflective for the fundamental beam and transparent for the pumping beam. The IR pumping source is preferably a laser diode or a plurality of laser diodes such as to provide a collimated beam. The beam 1 1 from the IR pumping source 10 passes through the mirror 12 into a laser medium 13. The laser medium 13 can comprise Nd:YAG, Nd:YLF, Nd:YV0₄, Ti:Sapphire or other laser media, and preferably operates at about 1064 nm. It will be appreciated that other laser media, with different wavelengths, may also be used. Preferably the laser is a solid-state laser. The light passing through the laser medium 13 strikes mirror 15 that is highly reflective for the fundamental beam. Mirrors 12 and 15 form a resonator for the fundamental beam. The fundamental beam from the laser medium 13 is reflected by the mirror 12 to second harmonic mirror 16 which is highly transparent for the fundamental beam and highly reflective for the second harmonic. Then the fundamental beam passes second harmonic mirror 16 and enters second harmonic generator 18 where a portion of the beam is converted to a second harmonic beam having a wavelength of approximately 532 nm. The output mirror 19 is at least partially transparent to the second harmonic beam and for the fundamental beam and a portion of the fundamental beam is reflected back into the second harmonic generator 18 where a further portion of the fundamental beam is converted to the second harmonic beam. The mirror 16 transmits the fundamental beam for further amplification in the laser medium 13. Mirror 19 can be partially reflective to second harmonic beam so as to create a resonant cavity between mirror 16 and mirror 19. The second harmonic beam and the unconverted fundamental beam then travel through the beam shaping optics 20 and 22. The shaping optics can be one or more lenses, mirrors, prisms or any other beam shaping optics to optimize the shape of the second harmonic beam and fundamental beam prior to their entry into the fourth harmonic generator 23 where a portion of the fundamental beam and a portion of the second harmonic beam are converted to a fourth harmonic beam having a wavelength of approximately 266 nm. The resulting beams which are the fundamental, second harmonic and fourth harmonic are then separated by output mirror 24 which is reflective for the fundamental and second harmonic beams and at least partially transparent to the fourth harmonic beam. The beam of interest, in this case the fourth harmonic, is directed and injected into an air duct through output 25. Mirror 24 can be partially reflective to the fourth harmonic beam to reflect part the fourth harmonic within the resonant cavity created by mirror 24 and mirror 19. Output 25, can be one or several optical components used to couple the UV light generator to an air flow structure and may at the same time be selected such as to provide a beam geometry having dispersion characteristics suitable for dispersing light throughout a desired volume within the air flow structure.

The laser can be operated either in continuous wave mode or pulse mode. In the case of pulse mode, a Q switch 14 is used to produce pulses. It will be appreciated that pulses with high peak power may be generated as known in the art. In the pulse mode, an optional optical delay line 21 can be used to enhance the efficiency of the resonant cavity created by mirror 24 and 19 as would be known in the art.

The optical configuration described above generates light at approximately 532 nm after the pass in the second harmonic generator and the wavelength is further reduced to 266 nm when passing through the fourth harmonic generator. It will be appreciated that the wavelength of the beam generated by the second and fourth harmonic generators can be determined by the selection and the nature of the laser medium and pumping source.

Thus, as a result of the harmonic frequency generation, light having a substantially pure wavelength of about 266 nm or a small bandwidth around 266 nm is produced. While the UV generator can be configured to generate harmonics in different regions of the UV spectrum, in a preferred embodiment the UV source of the present invention generates UV radiation in the UVC region which spans wavelengths between 210 nm and 280 nm. More preferably the wavelength(s) generated is between about 250 and 270 nm and more preferably about 266 nm Such narrow bandwidth procures a great advantage over traditional UV light sources used to disinfect air in that light energy is focused at the wavelength, or relative narrow bandwidth, which is most efficient in inactivating pathogens. By opposition, traditional UV sources exhibit broader bandwidth and therefore have their energy spread out among the wavelengths some of which being wasted on wavelengths that are not efficient at killing pathogens.

The intensity (power per unit area) of the harmonic frequency increases with the square of the laser pump intensity according to

P harmonic ^— MP pump

Where μ is depends on characteristics of the non-linear material used in the generation of the harmonic such as length of the crystal, effective non-linearity and the like. It can be appreciated that the power of the UV beam resulting from the harmonic frequency generation can be controlled at several levels. Conversion efficiency of the intensity varies according to the mode of laser operation. High intensity conversion efficiency can be achieved with pump light delivered in pulses because of higher peak power of pulses. In continuous-wave mode intensity can be adjusted (increased) using intra-cavity frequency doubling (laser resonator). External resonant cavity may also be used. Other factors, known in the art, such as beam divergence and acceptance angle, also contribute to intensity conversion efficiency and are encompassed as means of intensity modulation for the apparatus and systems of the invention.

The generator is preferably configured to deliver UV radiation doses of between approximately 1 to 1000 mW sec/cm². Such doses can inactivate most bacteria, viruses and parasites. The UV radiation doses that can be delivered is dependent on the power of the laser diode and the efficiency of the frequency generator(s). Low power diodes have power output in the mWatts range and high power diodes have output generally higher than 1 Watts. Therefore the power of the laser diode(s) can be selected to provide sufficient power to inactivate pathogens. The power output required to achieve pathogens inactivation will depend on the size of the air flow structure, the flow rate and the like.

Frequency doubling (or more generally harmonic frequencies generation) is normally used to produce a source of electromagnetic radiation that is highly collimated (low dispersion), thus requiring high quality optical components especially the non-linear crystals that need to be without structural defects in their crystal lattice. It will be appreciated that light injected in an air conduit such as an air duct, for the purpose of inactivating pathogens, should reach all or substantially all cross-sectional volume elements in a predetermined section of the conduit to ensure that all pathogens flowing through this cross-section receive a dose sufficient to inactivate them. Accordingly, the optical elements and the choice of materials for these optical elements can be selected so as to optimize beam dispersion and/or relax the requirement for purity of the harmonic generation crystals. Harmonic generators, as is well known in the art, are made of non linear materials. Crystals are generally used and certain organic polymeric materials can also be used. Example of crystals include lithium triborate (LBO), BBO ( β -barium borate), KDP (potassium di-hydrogen phosphate) KTP (potassium titanyl phosphate) and lithium niobate). The type and characteristics of nonlinear material used can affect beam dispersion as, for example, crystal length and the purity of the crystals. In this respect beam dispersion is dependent on optical homogeneity (as measured by homogeneity of refractive index Δη ) of the crystals. Non-linear crystals with high levels of homogeneity producing more collimated beams and lower levels of homogeneity producing more dispersion. For the present invention and as explained above some dispersion of the beam can be desirable. Accordingly non-linear crystals with Δη of up to about 5 x 10^"5/cm and more preferably between about 2 X 10^"5/cm and 5 X 10^"5/cm can be used in the present invention advantageously allowing the use of less expensive crystals. Beam dispersing elements can also be appropriately positioned in the harmonics generator. Because high collimation quality optical components are not required, the cost of the harmonics generator/pathogens inactivation apparatus of the present invention may be reduced when compared to traditional harmonics generators. Thus the present invention combines the advantages of UV harmonic generators such as narrow bandwidth, ozone free UV light generation, inexpensive UV light production with the advantages of using low cost optical components that provides desirable beam dispersion characteristics.

Furthermore, the single wavelength or narrow bandwidth UV beam generated with the present system facilitates a shaping of the beam using known optical components to generate a beam of light with the desired geometry and dispersion characteristics when entering the air duct. For example diffraction gratings, diffusion lens and the like can be used to optically couple the UV beam generator and the air flow structure such as to propagate light throughout a predetermined volume.

UV light injection ports can be designed to optimize light propagation within the air flow structure. As will be appreciated, the size and shape of the aperture will be function of the size and shape of the air duct as well as the spatial light energy distribution needs. For example a circular aperture 30 or slit design 31 will generate different light propagation patterns as shown in FIG 3 A and 3 B. The former generating a more circular light propagation pattern 32 while the latter produces a more rectangular pattern 33. Other injection port geometries can be designed to adapt light propagation within the air flow structure. Optical elements may also be positioned either inside or outside, or both, the air flow structure to that effect.

In another aspect, the design of the UV light generator can be advantageously exploited by optically coupling light transmission elements such as optic fibers 40 as schematically represented in FIG 4.

Air ducts in ventilation systems, by their design, often have twists and bends that can create volume elements "invisible" to light. Thus a single light injection port within a duct may not allow light to reach every volume element of a cross-section of an air flow therefore allowing some pathogens to pass without being irradiated. In another aspect of the invention there is provided an air disinfecting system comprising a UV light source as described above and an air flow structure to which the UV light source is optically coupled. The further embodiments of the system described below solve many of the inherent problems associated with air flow disinfecting. In one embodiment, two or more light injection ports are provided at predetermined positions to optimize the illumination of a given volume within an air duct. For example, two such UV Radiation Generators 1 may be installed in an HVAC (heat ventilation and air conditioning) conduit opposite each other, as is shown in FIG 5. It will be appreciated that other light injection ports configurations are possible depending on the design of the air conduit and that more than two light injection ports may be used. While UV sources may be installed at each light injection port, it is also possible to use light wave guides such as optic fibers to transmit the light to the injection ports as shown in FIG 4. Using wave guides such as optic fibers offers greater flexibility and allows injection ports to be located at difficult to reach positions. The use of multiple injection ports also provides a mean to adjust the desired level of the total dose of radiation in a given volume.

The environment within the air flow structure influences the dose required to inactivate pathogens. By environment it is meant the light intensity, the air flow, the temperature, the humidity or any other such environment parameter that can affect the survival of pathogens. In another embodiment of the system of the present invention air flow structure environment parameters detectors are provided to obtain measurements of such parameters that are used in a feedback circuit to control light intensity as a function of the parameters values. UV detectors may be positioned within the air duct and connected to a feedback control electronic circuit that controls the power of the excitation source in the UV generator(s). Thus the radiation energy density is maintained through the operating life of the system. This ensures the effectiveness of the apparatus and makes inactivation of the pathogens predictable and thus reliable. Dose of radiation is a function of the intensity of the radiation and the time of radiation exposure. Shorter exposure time may be compensated by higher light intensity in order to achieve a similar degree of inactivation. Therefore the speed of the air flow within an air conduit will influence the dose received by airborne pathogens. Optionally air flow detectors may be positioned at or near the region of light propagation within the air flow structure.

Pathogens are sensitive to the environmental parameters such as temperature and humidity and these parameters can contribute to level of sensitivity of pathogens to UV irradiation. Therefore, temperature and humidity detectors can also be incorporated in the air flow structure to provide measurements that would than be used to adjust light intensity.

As shown in schematic diagram of an embodiment of the system of the invention in FIG 5, measurements of air flow 50, light intensity measurements 51, temperature 52 and humidity 53 can be channeled to a processor 54 which will determine the necessary light intensity adjustments as a function of air flow fluctuations and adjust light intensity generated by UV generator 1 to inject light with proper intensity in air flow structure 2.

With respect to the control of light intensity the UV generator of the present invention, when coupled to an air flow structure provides a greater flexibility than conventional UV sources. For example light intensity can advantageously be adjusted by using the UV generator in the pulse mode. The pulse mode enables the control of pulse peak power as well as the frequency modulation of the pulses either individually or together and therefore offers a finer control over the output beam power. Furthermore, optical components within the UV generator can also be adjusted to provide yet another level of light intensity adjustment.

In another aspect of the invention, the internal surface 60 of the air flow structure is coated with UV reflective material. In addition to ensuring that a given volume of space is filled by UV light by virtue of the multiple reflections of the light, it also provide a means to deliver increase dose by repeated irradiation (through reflection) of pathogens and therefore increasing the cumulative dose to which they are exposed.

As shown in FIG 7 the air flow structure may consist of an insertable section 70 optically couple to the UV light generator 1 that can be position in a ventilation system such that circulating air 72 is forced through the insertable section. The optical configuration of the insertable section may advantageously be designed to provide selectable light injection configurations. For example multiple injection ports 3 may be installed and when in used, a certain combination of these ports may be selectively activated to optimize the irradiation parameters. Also, in such a design, light guiding means can be installed to propagate light in the frame prior to being injected inside the insertable section. Such an insertable structure may be rigid or flexible depending on the needs.

In yet another aspect of the invention there is provided an air disinfecting system that comprises a solid state UV source substantially free of ozone production that is optically coupled to an air flow structure such as an air duct. The UV source can be a solid state UV laser diode or an UV light emitting diode or it can be generated by the frequency doubling method described above using a diode-pumped IR source to produce a UV beam. It will be appreciated that such a system provides a great advantage over traditional UV irradiation in that the absence of ozone production makes it widely applicable to air circulation systems used in housing and public building.

It will be appreciated that the apparatus and system of the present invention can be used in conjunction with other pathogen inactivation means such as filters.

There is also provided a method for inactivating pathogens comprising providing a UV light source from a harmonic frequency generator with sufficient light power to inactivate pathogens, and injecting the UV light in an air flow structure.

In an embodiment the UV light beam generated by the harmonic frequency generator is shaped so as to deliver a substantially dispersed beam within the air flow structure. The shaping of the beam may be accomplished by optimizing the various optical components of the harmonic generator as described above.

In yet another aspect of the method, light intensity within the air flow structure can be controlled by detecting the intensity and providing a feedback control electronic circuit to adjust light intensity. To that effect, environmental parameters within the air flow structure such as light intensity, air flow, temperature, humidity and the like can be measured and monitored and light intensity adjusted as a function of these parameters.

The description of the present invention illustrates certain embodiments not intended to be limited to the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art.

Claims

What is claimed is:

1 . An air disinfecting UV light source comprising: a diode-pumped infrared laser to generate an IR fundamental beam, a second and fourth harmonic frequencies generators for producing a UV light beam using the fundamental beam, and optical elements for optically coupling the UV light beam to an air flow structure to inject substantially dispersed UV light into the air flow structure, the substantially dispersed UV light having sufficient energy to inactivate pathogens.

2. The air disinfecting UV light source of claim 1 wherein the laser is a solid state laser.

3. The air disinfecting UV light source of claim 1 or 2 wherein the air flow structure is an air duct.

4. The air disinfecting UV light source of any one of claim 1 to 3 wherein the harmonic frequencies generators comprise non-linear crystals having an optical homogeneity, Δη, of between about 2 X 10^"5/cm and about 5 X 10^"5/cm.

5. The air disinfecting UV light source of any one of claim 1 to 4 wherein the optical elements are selected from gratings, lenses, mirrors, optic fibers and combinations thereof.

6. The air disinfecting UV light source of any one of claim 1 to 5 wherein the UV light beam has a wavelength that is between about 210 and 280 nm.

7. The air disinfecting UV light source of any one of claim 1 to 6 wherein the wavelength is about 266 nm.

8. The air disinfecting UV light source of any one of claim 1 to 7 wherein the UV light beam is a fourth harmonic beam.

9. An air disinfecting system comprising: an air disinfecting UV light source according to any one of claim 1 to 8; and an air flow structure optically coupled to the air disinfecting UV light source.

10. The air disinfecting system of claim 9 wherein the air flow structure is an air duct.

1 1 . The air disinfecting system of claims 9 or 10 further comprising at least a second UV light beam and wherein UV light is propagated within the air flow structure from two or more UV light ports.

12. The air disinfecting system of claim 1 1 wherein the at least second UV light beam injects light from a substantially opposite direction from the UV light beam.

13. The air disinfecting system of any one of claim 9 to 12 wherein the air flow structure has an internal surface that is UV reflective.

14. The air disinfecting system of any one of claim 9 to 13 wherein the air flow structure is an insertable air flow structure, adapted to be inserted in an air flow structure so that substantially all air flowing in the air flow structure also flows in the insertable air flow structure.

15. The air disinfecting system of claim 14 wherein the insertable air flow structure is a flexible structure.

16. The air disinfecting system of any one of claim 9 to 15 further comprising one or more air flow structure environment parameter detectors within the air flow structure to provide one or more environment parameter measurements.

17. The air disinfecting system of claim 16 wherein the one or more air flow structure environment parameter detectors are selected from light intensity detectors, air flow detectors, temperature detectors, humidity detectors and combination thereof to provide a light intensity measurement.

18. The air disinfecting system of claim 16 or 17 further comprising a processor to receive the one or more environment parameter measurements and adjust the light intensity within the air flow structure to optimize pathogens inactivation.

19. An air disinfecting system comprising: a solid state UV light source substantially ozone-free producing for generating a UV light beam, an air flow structure optically coupled to said solid state UV light source, and optical elements for optically coupling the UV light beam to the air flow structure to inject substantially dispersed UV light into the air flow structure, the substantially dispersed UV light having sufficient energy to inactivate pathogens.

20. The air disinfecting system of claim 19 wherein the solid state UV light source comprises an air disinfecting UV light source as claimed in any one of claim 1 to 8.

21 . A method for inactivating pathogens in an air flow structure comprising providing a UV light from an air disinfecting UV light source as claimed in any one of claims 1 -8 with sufficient light intensity to inactivate pathogens, and injecting the UV light in an air flow structure.

22. The method of claim 21 wherein the step of providing comprises dispersing the UV light so as to substantially fill a cross-sectional volume of the air flow structure.

23. The method of claim 21 or 22 further comprising a steps of measuring one or more air flow structure environment parameters to generate one or more air flow structure environment parameter measurements, providing a processor to receive the one or more measurements, and adjusting light intensity based on the one or more measurements.

24. The method of claim 23 wherein the one or more air flow structure environment parameters are selected from light intensity, air flow, temperature, humidity and combination thereof.

25. The method of claim 23 or 24 wherein the air disinfecting UV light source is operating in pulse mode and wherein the step of adjusting light intensity comprises adjusting pulse peak power.

26. The method of any one of claim 23 to 25 wherein the air disinfecting UV light source is operating in pulse mode and wherein the step of adjusting light intensity comprises adjusting pulse frequency modulation.