WO2002090950A1

WO2002090950A1 - Optically energised & interrogated sensors

Info

Publication number: WO2002090950A1
Application number: PCT/GB2002/001988
Authority: WO
Inventors: Douglas Fraser Clark
Original assignee: The University Court Of The University Of Paisley
Priority date: 2001-05-04
Filing date: 2002-05-01
Publication date: 2002-11-14
Also published as: GB0110967D0

Abstract

Sensors which are energised and interrogated by optical means. The sensors employ a layer of light absorbent (typically semiconductor) material (14) and are energised by light (18) of at least one wavelength that is absorbed by the semiconductor and interrogated by detecting and analysing light reflected from the semiconductor. The absorbed energising light heats the semiconductor to a greater or lesser extent, dependent on the input optical power and the thermal characteristics of the surrounding medium. Variations in the optical properties of the semiconductor are detected from the reflected light. Certain embodiments relate to an improved catalytic microcalorimeter, employing a layer of catalyst (16), for detecting combustible gases. Other embodiments comprise semiconductor type gas sensors in which variations in electrical properties of the semiconductor are detected optically. Still other embodiments comprise self-heating temperature sensors that can be used for measuring temperature, thermal conductivity and gas pressure.

Description

Optically Energised & Interrogated Sensors

The present invention relates to sensors which are energised and interrogated by optical means. The sensors employ a layer of optically absorbing material and are energised by light of at least one wavelength that is absorbed by the optically absorbing material and interrogated by detecting and analysing light reflected from the material. The absorbed energising light heats the material to a greater or lesser extent . The heating of the material depends on the thermal properties (conduction and convection) of the medium or material surrounding or in contact with the sensor. Variations in the optical properties of the material are detected from the reflected light. Certain embodiments of the invention relate to an improved catalytic microcalorimeter for detecting combustible gases. This sensor comprises a silicon based microcalorimetric gas sensor which is optically activated and interrogated. Other embodiments comprise semiconductor type gas sensors. Still other embodiments comprise self-heating temperature sensors that can be used, for example, for measuring temperature, thermal conductivity and gas pressure.

The invention may be used in a broad range of sensing and measurement applications including but not limited to, for example, applications relating to gas detection for emission and process control, diagnostic monitoring and health and safety control, and, with modification, for temperature and pressure measurement.

Gas detection forms an important measurement aspect in a wide range of industrial applications connected to health and safety or to process control and monitoring. Many gas sensors exist or have been proposed using operating principles based on a wide variety of interaction mechanisms between the gas and transducer, the most common involving chemical, biological, electrical and optical reactions. Ideally, the output from the sensor should give a signal related to the gas constituents and quantity present^' in an atmosphere.

For the detection of combustible gases, the most common commercially available device is the calorimeter. Calorimeters are one type of gas sensor based upon measuring the temperature rise from the reaction between the gas and oxygen at the surface of a catalyst. One type of commercially available calorimetric gas sensor, called the "pellistor", consists of a ceramic body impregnated with a suitable catalyst with a platinum resistance thermometer embedded inside. Commercially available pellistors are formed from a thin coil of platinum wire that acts both as the heating element and the temperature sensor (the resistivity of the coil varies in an approximately linear manner with temperature) . The coil is coated with an alumina compound containing the catalyst using a dip coating process. The alumina compound is porous, presenting a large surface area of catalyst and allowing the gas to diffuse through the structure. The catalyst is heated, e.g. by means of an electric current applied to the platinum thermometer coil (or by a separate heater) , to the "light off" temperature of the combustible gas; i.e. the temperature at which a heat-generating (exothermic) reaction takes place at the surface of the catalyst between the gas and any oxygen present in the atmosphere. The identity of the gas is determined by the light off temperature, which is detected by a sudden increase in temperature (measured by the platinum thermometer) when the exothermic reaction begins. The amount of heat generated is proportional to the concentration of the gas, which is determined by the temperature change above the light off temperature as measured by the platinum thermometer.

In order to take account of f luctuations in the ambient atmospheric conditions , a pair of calorimetric sensors is used in tandem, one with activated catalyst (s) and the other with deactivated catalyst (s), so that the heat of combustion is measured relative to the ambient conditions. In this configuration, each sensor operates independently at an elevated temperature, but only the activated sensor is energised by the presence of a combustible gas. The change in resistance of the platinum thermometers may be measured as a voltage change by connecting the sensors in a heatstone bridge configuration.

The output from calorimetric sensors is approximately proportional to the number of carbon atoms in a particular gas molecule because the heat of combustion is proportional to the carbon content of the molecule (assuming the ratio of air is stoichometric) . The signal is also proportional to the number of carbon atoms present at the surface of the catalyst, which is a function of the gas concentration, and is also dependent on the number of vacant sites available on the surface of the catalyst. The sensitivity of the sensor to different gases varies as a function of the operating temperature and depends also on the type and amount of catalyst employed.

It has been observed that increasing the temperature of the catalyst element above that at which the catalytic reaction initiates results in a plateau being reached in the sensor output where the catalytic reaction reaches an equilibrium with the available oxygen, combustible gas and active catalytic sites. Restricting the amount of combustible gas reaching the catalyst surface results in the reaction becoming substantially diffusion limited. By restricting the amount of combustible gas and oxygen at the catalyst surface, the equilibrium state is reached at a lower power level allowing the sensor to operate at lower powers. The benefits of diffusion limited operation are that the effective life of the catalyst may be extended and heat loss by convection greatly reduced. The response time of the sensor may be reduced by the presence of a diffusion limiting membrane. If the membrane is placed close to the catalyst the diffusion time delay will be negligible. In practice, where protection is required to prevent flashback from the device, a porous or sintered cap is placed over the sensor.

A significant disadvantage of pellistor sensors is their high electrical power consumption. Using silicon micromachining techniques, microminiature pellistors can be made with reduced power consumption. New catalysts with enhanced catalytic properties have been developed to compensate for the reduced surface area of microminiature devices of this type. However, even microminiature pellistors suffer the disadvantage of requiring substantial electrical power to energise and interrogate the sensor. In many situations it is undesirable to have electrical conductors present. Other known types of gas sensor include semiconductor type sensors, based on measuring changes in conductivity of a semiconductor sensor element caused by adsorption of a gas on the surface of the semiconductor element . These sensors are used for measuring small concentrations of reactive gases in air, typically using Sn0₂ as the active material, which offers high sensitivity at a relatively low operating temperature. However, semiconductor sensors of this type still require to be operated at an elevated temperature (typically 300°C to 500°C) and the conductance characteristics are very sensitive to operating temperature. Other semiconductor oxides have been investigated with a view to reducing the required operating power and increasing sensitivities by mixing dopants such as antimony oxide into the semiconductor material.

Semiconductor gas sensors can be classified into two classes. The first involves changes in surface conductance, whilst the second involves changes in the bulk conductance properties. Materials which sense small concentrations of reactive gases in air rely on changes in surface conductance and detect a displacement, induced by the gas, from an equilibrium condition. Conventionally, the conductivity is measured by electrical means. In view of this and the requirement to operate such sensors at an elevated temperature, semiconductor sensors suffer similar disadvantages to microcalorimeters since they require the use of electrical means for both energisation and interrogation.

A variety of sensor types, typically temperature sensors, are also known that use the optical transmission characteristics of bulk semiconductors. These are generally energised by light having a wavelength close to the band-edge absorption wavelength of the semiconductor material and interrogated by detecting and analysing the light transmitted through the bulk semiconductor. The degree of light attenuation is a function of the temperature of the semiconductor.

The present invention concerns sensors that are optically energised and interrogated using a semiconductor film illuminated by light of at least one wavelength that is strongly absorbed by the semiconductor film.

In accordance with a first aspect of the present invention, there is provided a sensor including a layer of light absorbent material adapted to be heated by light of at least one predetermined wavelength incident thereon and having variable, temperature-dependent optical properties detectable by optical interrogation of said light absorbent material.^'

In accordance with a second aspect of the invention there is provided a method of operating a sensor including a layer of light absorbent material adapted to be heated by light of at least a first predetermined wavelength incident thereon and having variable, temperature-dependent optical properties detectable by optical interrogation of said light absorbent material, comprising illuminating said light absorbent material with light of said at least first predetermined wavelength, detecting light reflected from said light absorbent material, and processing signals representing said reflected light to detect variations in said optical properties.

These and other aspects and preferred features and applications of the invention are defined in the claims appended hereto.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic side view of an embodiment of a device in accordance with the invention; and

Fig. 2 is a schematic plan view of the device of Fig. 1.

Referring now to the drawings, embodiments of the invention will be described firstly with particular reference to microclorimetric gas sensors. An embodiment of a microcalorimetric gas sensor in accordance with one aspect of the present invention comprises a support membrane 10 of low thermal conductivity, supported around its periphery by a substrate 12, an island of light absorbent material 14 (typically a semiconductor material) formed in the centre of the membrane 10 and a layer of catalyst material 16 formed on top of the material 14. In use, the catalyst material 16 is surrounded by a fluid having particular thermal characteristics. The sensor is energised by light 18 transmitted from a light source 22 through an optical fibre 20. The sensor may also be interrogated by the same optical fibre 20, or by separate means. Where the sensor is energised and interrogated using the same fibre, the fibre or light source would incorporate directional coupling means 24 to tap off the light reflected from the sensor. In any case, it will be understood that the light reflected from the sensor is directed to a suitable optical sensor 26 or the like and output signals from the optical sensor processed by any suitable data processing means 28 according to the particular sensor function.

The light absorbent material 14 is substantially thermally isolated or insulated from the remainder of the sensor structure by minimising thermal paths so as to be capable of being heated to the light off temperature of gases of interest by means of intense light 18, such as pulses of laser light, transmitted into the material 14 via the optical fibre 20. The material 14 also has optical properties which are temperature dependent, so that the temperature of the material 14 may be determined by optical interrogation. The material 14 is typically a thin film of material a few microns in thickness, having ^• as a physical property the ability to strongly absorb light over a particular band of wavelengths. Semiconductor materials with such a property include silicon, gallium arsenide, III-V compounds etc. where incident light on the semiconductor below the bandgap of the material is absorbed. Generally speaking, for the purposes of the present invention, the light used to energise the sensor includes light of at least one wavelength that is less than the band-edge absorption wavelength of the material 14. For practical purposes, the light absorbent material 14 might typically have an absorption coefficient α>100/cm for the wavelength (s) of interest. The absorbed radiation generates lattice phonons that contribute to the heating up of the material.

The low thermal conductivity of the membrane 10 minimises heat loss from the material 14 by conduction, with the surface area optimised to reduce heat loss by convection, allowing the material 14 to be heated to the required temperature by the incident light 18. The operating temperature of the material 14 is also dependent on the thermal conductivity of the fluid surrounding the sensor. The membrane material 10 should have a very low absorption coefficient (i.e. be substantially transparent) for wavelength (s) of incident light applied to the material 14, so that the light 18 passes through the membrane 10. The membrane 10 might typically comprise a thin film of silicon nitride or silica, of the order of 0.1 to 5 microns in thickness .

The substrate 12 might suitably be silicon, typically of the order of 100 to 1000 microns in thickness.

The catalyst 16 preferably comprises an oxidation catalyst such as a group VIII metal, especially Pt, Pd, Rd, Ir and Rh and combinations of these materials in organic or non-organic formulations to provide specific characteristics (e.g. resistance to poisoning, large surface area, etc.) .

Microminiature gas sensors in accordance with the invention may be made by means of any of a variety of known semiconductor fabrication techniques, such as silicon processing technology. A preferred method of fabrication uses silicon as the base technology. The substrate 12 has a thin film of membrane material 10 deposited or grown thereon (using any standard deposition technique such as sputtering, LPCVD (low pressure chemical vapour deposition), etc.). The light absorbent material 14 is then deposited or grown on top of the membrane material 12. Alternatively, standard silicon-on-insulator (SOI) materials might be used in the manufacture of the sensors. A window is etched through the back of the substrate 12 to expose the lower surface of the membrane layer 10. Preferential etching techniques may be employed using photolithography to delineate the desired window pattern on the back f ce of the substrate 12. An etch resistant layer (e.g. silicon nitride or silicon oxide) which is resistant to the bulk etchant (preferably KOH) used for etching the substrate material 12 is deposited onto both surfaces of the substrate to prevent etching in undesired regions. The bottom face of the substrate 12 is then coated with photoresist which is exposed, baked and developed to open up windows in the resist, exposing the KOH etch resistant materials. Another etchant is used to etch away the KOH resistant layer until the underlying silicon is exposed. The substrate is then immersed in the silicon etchant solution to etch away material back to the membrane layer 10.

Processing then reverts to the top face to define rectangular areas of the light absorbing material 14 smaller than the window etched from the back. The shape of the material 14 is again defined using photoresist spun on to the top face and exposed through a photolithographic mask, developed and baked to provide the correct geometry, in this case to leave material of a rectangular geometry. Etching of the sacrificial layer then allows access for etching of the top of the semiconductor material 14 to leave a rectangular section of the material 14 supported on the membrane 10. The process described thus far may be used in fabricating sensors in accordance with any of the various embodiments of the invention.

For the purposes of microcalrometric gas sensor embodiments of the invention, the catalyst 16 is deposited on top of the material 14 using photolithography to define the areas of catalyst deposition. The catalyst 16 may be deposited by any of a variety of means including, but not limited to, electro-deposition, evaporation, electroless deposition, silk screen printing, physical vapour deposition etc.

Sensors in accordance with the invention could be made in other configurations and by other means. For example, a similar sensor could be made by applying a layer of silicon or other suitable material 14 directly onto the end of an optical fibre and (for a microcalorimetric gas sensor) applying a layer of catalyst 16 on top of the material 14. The material 14 would be thermally isolated from the fibre by an intervening layer of material having low thermal conductivity and suitable optical transmission characteristics, such as silica.

It is desirable for the membrane 10 to be as thin as possible and to have as large an area as possible (optimised to reduce heat losses by conduction and convection) relative to the island of material 14, in order to minimise heat loss by conduction to the substrate 10. However, the thinner and larger the membrane, the more vulnerable it becomes to physical damage. Suitably, the area of the island 14 may be of the order of tens to hundreds of microns on a side and the area of the membrane of the order of hundreds to thousands of microns on a side.

Sensors in accordance with this aspect of the invention enable gas detection by monitoring the temperature of the sensor in a manner analogous to conventional calorimetric gas sensors such as pellistors, as discussed above. However, in the case of the present invention, the sensor is energised and interrogated optically rather than electrically.

It is known to heat semiconductor materials optically, e.g. using laser light, for processing applications. In the present case, light of suitable wavelength (s) and intensity from a laser is delivered to the material 14 by means of the optical fibre 20. The output 18 from the fibre 20 is either collimated or focussed onto the material 14, by means of suitable optics such as a lens arrangement, through the supporting membrane 10. The material 14 absorbs the light 18 increasing its temperature and that of the catalyst 18, until the light off temperature of the combustible gas is reached. The heating efficiency is optimised by locating the material 14 in the middle of the membrane 10 of low thermal conductivity material so as to thermally isolate the material 14 from the surrounding substrate 12. If the thin film of light absorbent material 14 is of sufficient quality in terms of parallelism and smoothness of its faces then, when subject to an incident electromagnetic wave, multiple reflections will occur within the layered structures of the material 14. The reflection characteristics of such multi-layered structures exhibit resonant. characteristics dependent on the optical properties of the films forming the structure (as in Fabry-Perot cavities) . These resonances can be enhanced by forming multilayer structures involving combinations of the light absorbent material with metal films and other optical films (generally on top of the light absorbent material so as to increase reflection) . If the optical properties of one or more of the films constituting the structure vary with temperature (for example optical absorption, refractive index) then the reflective properties over a band of wavelengths will vary. The change in reflective characteristics can then be used as a measure of the temperature change, in this case caused through the catalytic reaction. Techniques for measuring temperature on the basis of temperature-dependent changes in the optical properties of materials have been reported, for example in "Fiber-optic temperature sensing with ultrathin silicon etalons", L. Schultheis et al . , Optics Letters, Vol. 13, No. 9, September 1988.

In the context of the present invention, two wavelengths or spectrums of wavelengths of light may preferably be used to operate the sensor. One wavelength or spectrum is chosen so that the light absorbent material absorbs incident photons (i.e. a wavelength below the band gap, or less than 1.1 microns for silicon) and so is the source of heating. The same wavelength or spectrum could be used to interrogate the sensor, but it is preferred that another wavelength or spectrum is used above the bandgap (or greater than 1.1 microns for silicon) to form the interrogation signal to determine the operating temperature of the sensor by changes in reflectivity. Alternatively, broadband illumination may be used spanning the bandgap. For example, either a change in the amplitude of a single wavelength may be monitored or a spectroscopic technique (such as a monochromator) may be employed if a spectrum of wavelengths is used to determine the change in reflection properties of individual wavelengths. In all situations the wavelengths present may share the same optical fibre 20. Sources of the light may include but are not limited to semiconductor lasers, gas lasers, light emitting diodes or intense non-coherent sources.

The operation of the device may be controlled to provide, for example, enhanced selectivity of gases; e.g. the incident light may be pulse modulated so that the average operating temperature of the device is effectively determined by the mark/space ratio of the pulse modulated light. In a microcalorimetric gas sensor, the mark/space ratio may be tuned to a particular gas or gases or may be varied to scan through a series of gases .

In other embodiments of gas sensors in accordance with the invention, the semiconductor material 14 may be of a type having electrical properties, such as conductivity, which vary in the presence of a gas to be detected; e.g. by adsorption of the gas on the surface of the material 1 . In such cases the catalyst 16 would be omitted but the sensor could otherwise be similar to the embodiments described above. These embodiments of the invention are comparable to conventional gas sensors the type in which the gas-sensitive material would be heated to an elevated operating temperature by electrical means and changes in its electrical properties would be detected by electrical measurement. By contrast, in the case of the present .invention, the material 14 is heated to its operating temperature (typically 300°C to 500°C) by optical energy and interrogated optically by detecting changes in the optical properties of the material arising from the same carrier generation that changes the conductivity of the material (i.e. changes in conductivity are effectively detected by optical means) . Small changes in the optical properties of the semiconductor are enhanced by using a thin film Fabry-Perot cavity incorporating the gas sensitive film, in a manner similar to the microcalorimetric sensors described above. Changes in the conductivity of the semiconductor film, which effectively forms the reflecting boundary of the Fabry-Perot cavity, will produce small changes in reflectivity that will significantly alter the reflection characteristics of the Fabry-Perot cavity.

The sensor can be energised and interrogated, and the output signal analysed, in a manner similar to the preceding embodiments. By choosing a suitable interrogation wavelength it is possible to control the temperature of the sensing film and characterise its sensitivity to a variety of gases.

The basic sensor structures and operating methods described above (minus the catalyst 16 of the microcalorimetric gas sensor embodiments) provide a type of self-heating temperature sensor, which is influenced by the incident optical power and the thermal characteristics of the surrounding medium, having applications besides gas sensing applications. Temperature measurements are affected by the power of the incident light, which has the effect of heating the sensor, in a manner that might be considered as being analagous to a self-heating electrical thermistor. For example, the sensor may be used simply as a temperature sensor, or for measuring thermal conductivity of materials and fluids in thermal contact with the sensor, by calibrating the sensor to monitor temperature changes as a function of the input optical energy. Thermal conductivity measurements of this type might further be employed as a means of measuring gas pressure, since the thermal conductivity of a gas varies with its pressure. Depending on the application, the degree of heating of the semiconductor layer by the energising light may vary.

Accordingly, the invention provides sensors in which optical energy is used to energise the sensors and in which the sensors are interrogated by detecting changes in the optical properties of the light absorbent material.

Sensors in accordance with the invention require no electrical input or output at the actual sensor location (electrical power would of course normally be required for the light source, light sensor and data processing means, but these may be remote from the sensor location) , are immune to radio frequency interference and electromagnetic interference, can be far removed from processing electronics and can safely be used in hazardous environments where electrical connections would be inappropriate.

The gas sensor embodiments of the invention enable the composition and presence of gases to be measured in a wide variety of applications, particularly (but not exclusively) in the measurement and control of gases that pose potential health and safety or environmental hazards. One particular application would be in monitoring exhaust gases generated by internal combustion engines with a view to monitoring and controlling engines for optimal operation. Another potential application is in monitoring gases such as methane in landfill sites and mines .

Sensors in accordance with the invention can be manufactured using standard semiconductor technology, are low in cost and small in size (tens to thousands of microns) enabling their use in areas where space is limited.

Improvements and modifications may be incorporated without departing from the scope of the invention as defined in the appended claims .

Claims

1. A sensor including a layer of light absorbent material adapted to be heated by light of at least one predetermined wavelength incident thereon and having variable, temperature dependent optical properties detectable by optical interrogation of said light absorbent material.

2. The sensor of claim 1, wherein said light absorbent material strongly absorbs light of said at least one predetermined wavelength.

3. The sensor of claim 1 or claim 2, wherein said light absorbent material comprises a semiconductor material such as silicon or gallium arsenide or III- V mixture.

4. The sensor of any preceding claim, including means for transmitting light to said light absorbent material.

5. The sensor of claim 4, including at least one light source for generating light to be transmitted to said light absorbent material via said light transmitting means for heating and/or interrogating said light absorbent material.

6. The sensor of claim 5, wherein said at least one light source comprises a laser such as a semiconductor laser, gas laser or light emitting diode, or an intense non-coherent light source.

7. The sensor of claim 5 or claim 6, wherein the at least one light source is adapted to generate light for energising the sensor including light of at least one wavelength less than the bandgap of said light absorbent material.

8. The sensor of claim 5, claim 6 or claim 7, wherein the at least one light source is adapted to generate light for interrogating the sensor including light of at least one wavelength greater than the bandgap of said light absorbent material.

9. The sensor of any preceding claim, wherein the layer of light absorbent material is formed on a central portion of a support membrane having low thermal conductivity.

10. The sensor of claim 9, wherein said membrane is substantially transparent to light of wavelengths used to heat and/or interrogate said light absorbent material.

11. The sensor of claim 9 or claim 10, wherein said membrane comprises silicon nitride or silica.

12. The sensor of any of claims 9 to 11, wherein said membrane is supported around its periphery by a substrate.

13. The sensor of any of claims 9 to 12 , including light transmitting means arranged to transmit light through said support membrane .

14. The sensor of any preceding claim, whereby the identity of a gas in contact with said sensor is determined by optical interrogation of said light absorbent material .

15. The sensor of claim 14, adapted for use as a microcalorimetric gas sensor, further including a layer of catalyst material in thermal contact with said layer of light absorbent material .

16. The sensor of claim 15, wherein said catalyst material comprises an oxidation catalyst such as a group VIII metal.

17. The sensor of claim 15, wherein the catalyst material comprises Pt, Rd, Pd, Ir or Rh or a combination of two or more of these materials in organic or non-organic formulations.

18. The sensor of any preceding claim, wherein said light absorbent material has temperature dependent optical properties whereby the temperature of said light absorbent material is determined by optical interrogation.

19. The sensor of any of claims 1 to 14, wherein the light absorbent material comprises a semiconductor material and at least one electrical property of the semiconductor material varies in the presence of a gas to be detected, and the optical properties of the semiconductor material vary with said variable electrical property such that said gas may be identified by optical interrogation.

20. A method of operating a sensor including a layer of light absorbent material adapted to be heated by light of at least a first predetermined wavelength incident thereon and having variable, temperature dependent optical properties detectable by optical interrogation of said light absorbent material, comprising illuminating said light absorbent material with light of said at least first predetermined wavelength, detecting light reflected from said light absorbent material, and processing signals representing said reflected light to detect variations in said optical properties.

21. The method of claim 20, wherein said at least first predetermined wavelength is a wavelength less than the band gap of said light absorbent material.

22. The method of claim 20 or claim 21, wherein the sensor is interrogated by illuminating the light absorbent material with light of at least a second predetermined wavelength.

23. The method of claim 22, wherein said at least second predetermined wavelength is a wavelength greater than the band gap of said light absorbent material.

24. The method of any of claims 20 to 23, wherein said signals are processed to determine the temperature of the light absorbent material .

25. The method of claim 24, wherein the signals are processed to identify a combustible gas by detecting the light off temperature of the gas.

26. The method of claim 24, wherein the signals are processed to determine the thermal conductivity of a material or fluid in thermal contact with the sensor.

27. The method of claim 26, wherein the thermal conductivity is used to determine the pressure of a gas.

28. The method of any of claims 20 to 23, wherein the light absorbent material is a semiconductor material and said signals are processed to determine an electrical property of the semiconductor material.

29. The method of claim 28, wherein the electrical property of the semiconductor material is used to identify a gas in contact with the semiconductor material.

30. The method of any of claims 20 to 29, wherein the light absorbent material is illuminated with pulsed light.