WO2011064806A2

WO2011064806A2 - Method and apparatus for measurements of luminous isotropic radiation as obtained by means of laser spectroscopy techniques, in particular for sub- micrometric particulate measurements

Info

Publication number: WO2011064806A2
Application number: PCT/IT2010/000462
Authority: WO
Inventors: Francesco Cignoli; Giorgio Zizak
Original assignee: Consiglio Nazionale Delle Ricerche
Priority date: 2009-11-25
Filing date: 2010-11-22
Publication date: 2011-06-03
Also published as: WO2011064806A3; ITRM20090617A1

Abstract

The invention concerns a method for the measurement of the spectrum of isotropic luminous radiation of a gas excited by a laser light, utilizing an apparatus comprising at least a transparent measurement chamber (41 ), and performs the following steps: A. Making said gas pass in said at least a measurement chamber (41); B. Sending a laser beam (21) on said gas in said at least a measurement chamber (41 ). The method uses an integrating sphere (42) substantially enclosing said at least a measurement chamber (41), as well as one or more photo - detectors for measuring the luminous signal collected by the integrating sphere. Before step A a calibration step is performed and, subsequently to step B, the following further steps are performed: C. collecting the light diffused by said gas hit by said laser beam (21 ), by means of said integrating sphere (42), thus obtaining a measurement luminous signal; D. transforming the measurement luminous signal into an electrical signal by means of suitable photo - detectors; E. analysing said electric signal in order to obtain the measurement of the gas spectrum. The invention further concerns a measurement apparatus implementing the method according to the invention.

Description

METHOD AND APPARATUS FOR MEASUREMENTS OF LUMINOUS ISOTROPIC RADIATION AS OBTAINED BY MEANS OF LASER SPECTROSCOPY TECHNIQUES, IN PARTICULAR FOR SUB- MICROMETRIC PARTICULATE MEASUREMENTS

The present invention concerns a method and apparatus for measurements of luminous isotropic radiation as obtained by means of laser spectroscopy techniques, in particular for sub-micrometric particulate measurements.

More in detail, the present invention concerns a method and apparatus for measurements of laser-induced spectroscopy of gas fluxes even in the presence of ultra-fine particulate by using an integrating sphere to improve the sensitivity. Such a method can be used for example for the measurement of carbon particulate in combustion systems and for the environmental monitoring by means of the laser-induced incandescence. The developed apparatus can allow the measurement of the particles dimensions, making reference to mathematical models that have been already developed in the scientific community. The present invention is the result of the work and experiments of a research founded by the Energy and Transport Department of the National Research Council (CNR) in the framework of the project "clean carbon" of the Ministry for the Economic Development.

The presence of particulate in the environment stirs up great concern in the population of the big cities and the public administrations for the several consequences connected to the quality of the air and the health of the citizens. The harmfulness of the powders depends on their dimensions, besides their chemical composition.

With the abbreviation PM10 one identifies the fine powders that are present in the atmosphere in the form of microscopic particles whose diameter is equal or smaller than 10 pm (10,000th of millimetre). Such powders enter the oral and nasal cavities. The fine powders (PM₂,s) penetrate the bronchus whilst the ultra-fine powders (PM-i) reach the lung alveoluses. There are various sources of the fine powders that can be both natural, such as the soil erosion, the woodland fires, the pollen dispersion and the sea salt, and connected to the human activity such as the various processes of combustion in motors, heating plants, in the industrial activities and thermoelectric plants. In the urban areas, the vehicular traffic weights for around 30% upon the production of PMi₀. The regulations for the control of the air quality and the emissions are based essentially on the use of a reference method of the gravimetric type. In Italy, the limiting values as defined by the law decree number 60 of April 2, 2002 establish two acceptable limits for the PM10 in the atmosphere. The first limit is of 50 pg/m3 as mean value measured in the 24 hours, not to be exceeded for more than 35 times per year, the second is of 40 g/m3 as annual mean value. The gravimetric method, on the other hand, is not apt to determine the particles dimensions, which is steady is an important parameter for evaluating the harmfulness of the powders. Moreover, it is of limited accuracy especially if he used to measure the powders emitted by low emission vehicles.

There exists a great interest for the development of methods up to measure the concentration of the particles and their average diameter with accuracy and higher spatial and time resolution.

Different techniques and commercial equipments are known which are able to satisfy various needs of sensitivity and particles dimensions range. In particular, the techniques MOUDI (micro-orifice uniform deposit impactor) and ELPI (electrical low-pressure impactor) have been developed, which gives the dimensions of the particles as an aerodynamic diameter; further, tools based on the measurement of the electric mobility such as the DPMS (differential mobility particle sizer), SMPS (scanning mobility particle sizer) and the nano-DMA (nano-differential mobility analyser) exist, which give the measurement of the equivalent diameter of electric mobility.

Such equipments are not able to distinguish particles of different nature and, in the case of sub-micrometric particles aggregates, they give only the measurement of an equivalent diameter rather than the value of the diameter of the primary particles composing the aggregates.

Recently, with the development of the laser-induced incandescence, or Lll, the measurement of the concentration of the nanometric carbon particles received great impulse by the studies on the mechanism of formation of the soot. Such a technique, which is based on the use of laser light, is able to provide information on the carbon particles concentration and the dimensions of the primary particles with excellent spatial and time resolution.

The laser-induced incandescence is a experimental technique mainly used in the laboratories for studying the flames, the combustion processes and the exhaust from various combustion systems. The technique consists in the irradiation of soot particles with an intense pulsed laser radiation, that is apt to be absorbed by the particles and therefore provokes a strong heating. Each particle behaves almost as a black body and, according to the Plank's principle, emits a radiation whose spectrum depends on the temperature that has been reached by the same particle. Since the temperature of the soot particles can easily reach 4000 K (which is the sublimation temperature of the carbon), the Lll signals can be easily isolated from the radiation of the surrounding environment. By simple optical arrangements one obtains measurements with high spatial and time resolution. According to the principles of the Lll, the intensity of the emitted radiation is proportional to the volumetric concentration (or volume fraction) of the particles whilst the decay profile is connected to the dimensions of the primary particles.

The phenomena that come into play in such a technique are however complex and occur on the nanosecond scale. This concerns mainly the absorption of the laser radiation and the heating of the particles, the thermal exchange with the surrounding gases, the light emission and various sublimation phenomena at the highest temperatures. The Lll technique has been known for a long time, however some experimental difficulties and the still not complete comprehension of the physical phenomena ruling it has confined the use of the technique mainly to the research laboratories. To date, no more than 2-3, commercial realisations are present on the market.

In particular, it is found that LI²SA (laser-induced incandescence soot analyser) produced by Esytec (www.esvtec.com) of Erlangen is one of the spin-offs of the thermodynamics Department of the University of Erlangen. It is based on different patents having as inventors Leipertz et al. (WO9730335, US6496258, US20006256330), wherein one proposes to measure the thermal radiation in two different points of the decay curve in order to determine the diameter of the primary particles.

The experimental apparatus can be used for the carbon black and the metallic oxides particles. It can also be implemented with other techniques, such as the light scattering, and used in a modular system also for the analysis of the motor exhausts.

A difficulty inherent to the used techniques is in the fact that the measurement relies on mathematical models that describe the temperature decay and therefore the Lll signal. Such models are not yet completely reliable. Moreover, given the used optical arrangement, the signals are rather weak and noisy. The laser fluence (energy spatial density) must be as much uniform as possible in order to allow a uniform distribution of the temperature of the particles irradiated in the measurement volume and therefore a right determination of the temperature.

Other commercial realisations are the LII200 and LII300 of Artium (www.artium.com) of Sunnyvale, California. Such a company has acquired the patents of the National research Council of Canada having as inventors Snelling et al. (CA2272758, US6154277, US6181419, US6809820). In such patents, one proposes to measure the absolute value of the intensity of the signal Lll that can be directly correlated to the concentration of the particles in the case one wishes to know the temperature of the same particles. The temperature can be determined by mathematical models or by other experimental means, in particular by using the two-colours pirometry technique. One stresses also the necessity of using a spatial profile of the laser as much uniform as possible that is obtained by a special optical arrangement. In particular, in the patent US6809820 the accuracy of the measurements is improved thanks to the fact of using a laser that is not at the maximum fluence, so as to limit the interference of the sublimation and evaporation process of the particles on the decay of the Lll signal. The mathematical model describing the Lll phenomenon is an integral part of the patents.

Such an arrangement is based on a series of theoretical assumptions and needs particular and complex optical arrangements. As a consequence, such a technique is not sufficiently reliable and is still too much expensive.

So far, the disadvantages of the traditional techniques for some specific measurements of interest have been illustrated.

The problem can however be set up in a more general way.

Indeed, in various techniques of laser spectroscopy, such as for example the laser-induced fluorescence (LIF) and the Lll, the phenomenon of the "saturation" occurs wherein the light signal to be measured does not further augment above a certain applied power laser level. For example, it is already well-known in the scientific community that the laser fluence value for which one obtains the saturation of the Lll signal is of around 250 mJ/cm² for a laser irradiation at wavelength of 1064 nm, corresponding to the fundamental emission of a laser Nd:YAG.

The general problem of increasing the sensitivity of the laser spectroscopy^' techniques sets up, so that one can for example measure very low concentrations of particulate or micro-pollutants as required by the environment monitoring.

To this end, in the prior art systems are proposed that use particular optical arrangements, not sufficiently adaptable and in any case bulky.

The integrating spheres are known in the prior art.

The integrating sphere is an optical component consisting mainly in a cavity, typically spherical, whose interior is covered by a high-reflectivity material with some small apertures that are needed for the input and output of the luminous radiation.

Its most important property is the ability to diffuse the light, by means of multiple reflections, distributing it uniformly in such a way to minimise the effect of the initial direction of the light. An integrating sphere can be thought as a diffuser which maintains the luminous power but destroys the spatial information. If the reflectivity of the covering at the various wavelengths is high and the apertures are small, the integrating sphere can provide a high optical efficiency.

The integrating the spheres are normally used for a variety of optical measurements, photometric and radiometric measurements such as the measurement of all in the light irradiated by a lamp, the measurement of the surfaces reflectivity, the formation of the luminous source with uniform intensity and the measurement of the power of laser beams independently from the form and direction of the beam incidence.

Some non-conventional applications of the integrating sphere are described in some international patents. The patents US43209785 in 1979 describes the realisation of a high-sensitivity turbidimeter. A cylindrical measurement cell is placed through the integrating sphere and a collimated light beam is sent through the cell. When a water flux wherein particles are dispersed is made flow through the measurement cell, the light diffused by the particles is collected by the integrating sphere and measured. The intensity of the light signal is correlated to the quantity of the suspended particles that make the water cloudy. The system can be calibrated with the standards of turbidity NTU (Nephelometric Turbidity unit). The patent US4942305 filed in 1989 describes the realisation of a detector of aerosols particulate by means of the technique of the laser light scattering. A fine flux of aerosols is conveyed at the centre of the integrating sphere by a duct and collected at short distance by another duct. In the space between the two ducts a laser light beam is made passing. A particle that passes in the measurement area diffuses the laser light. Some photo-detectors placed on the surface of the integrating sphere provide a signal that is proportional to the dimensions of the particle but is independent from the form and orientation of the particle with respect to the laser beam.

The patents of US7173697 B1 filed in 2005 describes the realisation of a nephelometer with low truncation losses. A nephelometer measures the whole scattering signal, i.e. the diffused component of an extinction signal. To this end, it is necessary to integrate the light diffused in all the directions. This can be done by an integrating sphere and the cited patent describes an arrangement that reduces the losses due to the introduction of the sample and improves the angular response of the tool.

In the document to US 6,441 ,387 B1 an integrating sphere is used in a trigger device that detects them our presence of a biological aerosol that is harmful for the health. The previous problems of alignment to the interface of the laser beam are eliminated by using of an integrating sphere to direct the scattering and the fluorescence on detectors. The sensitivity of the composite system is not affected by the misalignment of the laser beam within the air flux thanks to the use of the integrating sphere to couple the elastic and inelastic scatters on detectors.

The document WO2005/001436 describes an analogous method for the detection of sub-micrometric particles that uses two concave mirrors or an integrating sphere and at least two detectors for the measurement of the elastic scattering and fluorescence bands to better determine the presence of particles of different nature.

It is observed that all the above-mentioned applications use of the integrating sphere is substantially limited to the diffusion of light to the same wavelength of the luminous source, to avoid alignment problems, and, in the case of fluorescence, to provide a signal of trigger of the presence of particular fluorescence particles carried by the gaseous flux without the ability to provide a quantitative measurement of the concentration of such particles. It is object of the present invention to provide a method for performing a high-sensitivity laser spectroscopy measurement, such as for example laser-induced incandescence for the measurement of the sub- micrometric carbon particulate, that solves the problems and overcomes the inconveniences of the prior art.

It is further specific object of the present invention an apparatus for performing high-sensitivity laser-induced incandescence measurements, such as for example laser-induced incandescence of the sub-micrometric carbon particulate, that solves the problems and overcomes the inconveniences of the prior art.

It is specific subject-matter of the present invention a method for the measurement of the spectrum of isotropic luminous radiation of a gas excited by a laser light, which uses an apparatus comprising a measurement chamber, and performs the following steps:

A. Making said gas pass in said at least a measurement chamber;

B. Sending a laser beam on said gas in said at least a measurement chamber;

The method being characterised in that it uses an integrating sphere substantially enclosing said at least a measurement chamber, as well as one or more photodetectors for measuring the luminous signal collected by the integrating sphere, before step A being performed a calibration step AO and in that, subsequently to step B, the following further steps are performed:

C. collecting the light diffused by said gas hit by said laser beam, by means of said integrating sphere, thus obtaining a measurement luminous signal;

D. transforming the measurement luminous signal into an electrical signal by means of suitable photo-detectors;

E. analysing said electric signal in order to obtain the measurement of the gas spectrum.

It is specific subject-matter of the present invention a method for the measurement of the spectrum of isotropic luminous radiation of a gas excited by a laser light, utilizing an apparatus comprising at least a transparent measurement chamber, and performs the following steps: A. Making said gas pass in said at least a measurement chamber;

B. Sending a laser beam on said gas in said at least a measurement chamber; The method being characterised in that it uses an integrating sphere substantially enclosing said at least a measurement chamber, as well as one or more photo-detectors for measuring the luminous signal collected by the integrating sphere, before step A being performed a calibration step AO comprising the following sub-steps:

A0_1. Positioning a calibrated, spectral irradiance lamp at a calibration distance from a proper calibration door of the integrating sphere, which can assume a opened or closed configuration, in such a way that the light of the calibration lamp illuminates the interior of the integrating sphere and therefore said at least a measurement chamber when the calibration door is open;

A0_2. Measuring the power of the signal exiting the integrating sphere, by means of the setup of step A0_1 ;

A0_3. For each photo-detector, calculating the calibration constant:

C_a(X) = V_ca/(X)/S Rcal(X) ΔΧ wherein λ is the wavelength at which the calibration of the photo- detector is done, V_cai(X) is the signal as measured by the photo- detector, S is the section of the calibration door of the integrating sphere, Rcal(X) the irradiance of the calibration lamp at the calibration distance and AX is the spectral band of the photo- detector;

and in that, subsequently to step B, the following further steps are performed:

C. collecting the light diffused by said gas hit by said laser beam, by means of said integrating sphere, thus obtaining a measurement luminous signal, for example Lll, both with the calibration door open and closed;

E. calculating the corrected calibration constant:

C(X) = C_a(X) LII/LIl_a wherein LII is the value of the measurement with opened calibration door and LII_a is the measurement value with closed calibration door;

F. utilizing the corrected calibration constant for analyzing said electric signal and obtaining the corrected measurement of the gas spectral emission.

It here clear that there is a substantial difference between the measurement chamber and the integrating sphere containing it: in the measurement chamber the gas is made pass which is hit by the laser light; the integrating sphere is instead used to collect the light diffused by the gas in the transparent measurement chamber. The calibration of step AO does not concern only the integrating sphere but the whole apparatus, since the integrating sphere collects the light that is diffused inside the measurement chamber. The calibration therefore does not concern the calibration of the integrating sphere as if this was separated from the rest of the apparatus, but of the measurement apparatus itself and such a calibration is absolutely necessary in order to perform the measurement method, since it is not here to say whether there is gas or not, rather to measure quantitatively and precisely its emission spectrum.

Preferably according to the invention, step F implements the two- colours incandescence measurement technique.

Preferably according to the invention, during step F one takes into account the time decay of the luminous signal of the integrating sphere, which follows a law of the type e^"^, wherein:

2 D 1

r = ^;

3 c \n p

wherein D_s is the diameter of the integrating sphere, c is the light speed and p is the average reflectivity of the walls of the integrating sphere.

Preferably according to the invention, during step F one takes into account also the time constant of the photo-detectors, the time response of the whole system comprising the integrating sphere and the photo- detectors being given by the convolution of the time responses of the integrating sphere and the photo-detectors.

It is further specific subject-matter of the present invention an apparatus for the measurement of the spectrum of isotropic luminous radiation of a gas excited by a laser light, comprising:

- means for supplying gas into

- at least a measurement chamber; - means for evacuating the gas from the measurement chamber;

- at least a laser source generating a laser beam passing through said at least a measurement chamber;

and characterized in that it further comprises:

- an integrating sphere through which said at least a measurement chamber is made passing;

- optical means suitable to collect the measurement luminous signal collected by said integrating sphere and to convey it to

- a system for analyzing the measurement luminous signal.

Preferably according to the invention, said optical means are constituted by an optical fibre connected at an end to an aperture of said integrating sphere to collect a measurement luminous signal, and at the other end to said analysis system.

Preferably according to the invention, said optical means are constituted by a dichroic mirror, one or more interference filters placed in series with corresponding photo-multipliers, said optical means being connected directly to the exit of the integrating sphere.

Preferably according to the invention, said means for supplying gas comprise a drawing probe and a conduit.

Preferably according to the invention, said system for analyzing the measurement luminous signal comprises:

means for the separation of the same signal in two optical beams belonging to two suitable spectral bands, as well as

means for detecting said two optical beams, in order to implement the two-colours incandescence measurement technique.

Preferably according to the invention, said at least a measurement chamber is constituted by a transparent tubelet made of pirex or quartz, and passing diametrally through the integrating sphere.

Preferably according to the invention, for laser-induced fluorescence measurements, with particular respect to the determination of substances subjected to wide-band fluorescence like the aromatic polycyclic hydrocarbons, said system for analyzing the measurement luminous signal comprises a spectrograph with CCD intensified detector.

Preferably according to the invention, the apparatus is for the measurement of isotropic gas radiation by means of laser-induced breakdown spectroscopy, wherein before the entrance of the measurement chamber a suitable lens is placed, which allows the focusing of the beam at the centre of the integrating sphere.

The invention will be now described by way of illustration but not by way of limitation, with particular reference to the drawings of the enclosed figures, wherein:

- figure 1 shows a scheme of the measurement apparatus according to the invention;

figure 2 shows in greater detail the integrating sphere as inserted in the scheme of figure 1 ;

figure 3 shows a scheme of the arrangement necessary to the calibration of the apparatus.

In the following, the method according to the invention will be indicated for brevity with the acronym SILS ("Sphere-Integrated Laser Spectroscopy").

It is to be specified at once that the method according to the invention is intended for the measurement of isotropic luminous radiation obtained by laser spectroscopy techniques. The method measures therefore a luminous spectrum, which can be utilised according to the known technique to determine the composition and/or property of the gas. The method will be illustrated in the application of the LI I technique. As previously noted, the signal deriving from the laser-induced incandescence or "LM" derives from the luminous emission of sub-micrometric particles previously heated by laser impulse. Such particles can reach a maximum temperature that, in the case of soot carbon particles, is of around 4000 K. By further heating the particles by more powerful laser impulses, the temperature does not increase because the particles sublimate, reducing their mass. It is evident that, given the particle concentration, the peak of the Lll Signal reaches a maximum and cannot increase further on, exhibiting a typical phenomenon known with the name of "saturation".

As previously recalled, the "saturation" value is of about 250 mJ/cm² for laser irradiation at wavelength of 1064 nm, corresponding to the fundamental emission of a Nd:YAG laser.

Therefore, to increase the sensitivity of the spectroscopic techniques, in order to being able to measure for example very low concentration of particulate or micro-pollutant as required by the environment monitoring, the present invention realizes an increase of the measurement volume and the solid angle of reception of signals.

The apparatus according to the invention is essentially composed by four elements: a sampling probe, a measurement cell with integrating sphere, a pulsed laser (for example Nd:YAG) that produces a laser beam and a measurement spectroscopic system (for example two-colour spectroscopy^' system) coupled by optical fibre to the integrating sphere. Each of these elements has precise features to maximize the result.

In the case of nanometric particulate measurements by means of the LI I technique, the scheme of the apparatus 100 according to the invention can be realised as reported in figure 1 showing the different above-mentioned elements.

First of all, a probe 10 is present for sampling the gases produced for example by a combustion process 200. The use of a probe for sampling the gases containing the particulate to be analysed offers numerous advantages. In the first place, it is possible to perform a controlled dilution to perform measurements also in conditions of high concentration of particulate wherein the absorption phenomena could invalidate the measurements. There exists on the market several probes that are realized according to regulations standards, that are to be used in the various conditions of use of the apparatus according to the invention. In particular, for industrial applications it is possible to insert filters apt to cut the fraction of particulate greater than PM-i , whilst for applications in the combustion field one can use isokinetic probes.

The probe 10 is connected to a measurement section 40 by means of a suitable connection 1 1 and the gases, after having crossed the measurement section 40, are drained away by means of a pump 30 with adjustable flow rate through a conduit 31. A filter (not shown) provides the reduction of the particles before they go through the pump 30.

An advantage of the use of the pump 10 is in the fact that the flow rate of the gas to be analysed can be sufficiently small because all the gas containing the particulate is analysed in the measurement section 40. One obtains therefore an apparatus of reduced dimensions and easily transportable.

The measurement section 40 comprising an integrating sphere is reported in greater detail in figure 2. It comprises a measurement chamber, for example a simple transparent tubelet 41 in pirex or quartz, passing through an integrating sphere 42.

A laser source 20 produces a laser beam 21 that excites the gas particles in the measurement camber 41 (tubelet). At the exit of the measurement chamber, the laser beam is intercepted by a beam dump 22.

In all the described application of the prior art, the use of the integrating sphere is substantially limited to the diffusion and measurement of light at the same wavelength of the luminous source.

On the contrary, in the present invention none uses the integrating sphere for measurements of laser spectroscopy wherein the luminous radiation is at wavelengths different from that of the source. The phenomena that produce wavelength shifts are different and very well known in the literature.

Since the integrating sphere is an almost perfect diffuser, it can be applied according to the invention to measurements of phenomena that provide an isotropic diffusion of the light and with lifetimes of the order of nanoseconds, which is the characteristic time of the light multiple reflections inside the integrating sphere, or larger. Various techniques can be used with the methodology proposed by the present invention. One recalls here the particulate incandescence, the molecular fluorescence and the breakdown spectroscopy that is produced as a consequence of a discharge caused by intense focalisation of a laser beam.

Coming back to the embodiment of the figures, the choice of the material of the tubelet 41 depends on the wavelength of the signals to be measured. For LI I measurement in the visible and near infrared, the pyrex is sufficient. For fluorescence measurement in the UV one needs the quartz. Through the tubelet 41 (measurement section 40) the flux of the gas to be analysed is made flowing, which in the case of Lll measurements, contains the particulate.

Two further entrances 48a and 48b allows the passage of a slight air flux for the possible clearing of the doors placed at the ends of the tubelet in such a way to allow the crossing of the cell by the laser beam.

In the case of Lll measurements, the measurement volume is determined by the diameter of the laser beam and the tubelet length 41 inside the integrating sphere. A typical value of the diameter of a commercial laser beam is of about 6-7 mm.

At the ends of the tubelet 41 two closure connectors 44, connected in turn to two respective windows 45 for the laser beam and two respective closure perforated plugs 46.

The integrating sphere has the aim of collecting the largest part of the signal generated inside the tubelet and emitted by the particles in every direction. The integrating sphere, besides the two circular apertures that are needed to let the measurement tubelet pass, is provided with two further circular apertures 43a, 49 placed at 90° on a plane that is perpendicular to the axis of the tubelet. These apertures are used one, 43°, for the collection of the luminous signal (Lll or other), and the other one, 49, normally closed, for the calibration procedure that will be described later in this description (figure 3).

Concerning laser 20 for the excitation of Lll signals, the choice of the laser depends from the type of measurements to be performed. In the case of Lll measurements in general one uses a pulsed laser of the Nd:YAG type that emits in the infrared at a wavelength of 1064 nm.

As one has previously highlighted, it is not necessary to use a high- energy laser since, in order to reach saturation of the Lll signals, a laser fluence of around 250 mJ/cm² is needed. By using larger fluence values, sublimation phenomena occurs on a timescale of nanoseconds. Such phenomena are not yet completely known and are still subject of scientific research. It is also known that commercial Nd:YAG lasers exists that are constructed with a configuration that can present a near field energy distribution very similar to a uniform distribution ("top hat").

By coupling directly the laser exit at the measurement cell and by selecting, by a circular opening, a laser beam section, it is possible to obtain an uniform illumination of the particles without the necessity of using special optical arrangements for the realisation of a uniform beam. All this is to the advantage of simplicity and compactness of the experimental arrangement. However, other laser types can be utilised.

There is still the spectroscopic measurement system 50, according to the invention, as described in the following. At an exit of the integrating sphere 43a, an optical fibres bundle 51 with wide angle of collection is preferably connected by means the connection 43 of optical fibre. The main part of the emitted light from the gas particles inside the cell, after different reflections on the integrating sphere walls, falls on the optical fibres beams, that send it at the spectroscopic measurement system 50. In general, it deals with analysing the intensity of the luminous signals at the different wavelengths. This can be done by means a spectrograph having a suitable detector. In the case of particulate measures with the Lll two- colour technique, the realisation of the measurement system can be done in the following manner (figure 1 ). After the optical fibre 51 , the luminous signal is divided in two optical beams by means a dichroic mirror 52. Two interferential filters select two suitable spectral bands and the luminous signals are detected with two photo-multipliers 53a, 53b. So, the technique of two-colours incandescence is realized ("two-colour 11"), widely described in the literature for absolute measurements of particulate concentration and of nanoparticles average dimensions by means of the detection of the temporal decay of the LI I signals. The spectral zone to be used for the measurement have to be away from possible spectral interferences. The regions around 400 and 700 nm offer a good compromise between photomultipliers sensibility and distance from possible interferences.

Another application provides for the connection of the dichroic mirror 52, the two interferential filters and the two photo-multipliers, 53a, 53b, directly at the exit 43a of the integrating sphere, avoiding the use of the optical fibre 51 .

Calibration

In order to obtain absolute measurements of volumetric concentration of particulate, it is necessary to perform the calibration of the measurement system.

It can be realised by a methodology that use a standard calibrated lamp of spectral irradiance. These lamps provide the irradiance, R_cai( ) [mW/nm cm²], that arrives on a surface placed at a well-known distance from the lamp (in general 50 cm) at the different wavelengths. The calibration procedure is the following, referring to the scheme of figure 3.

The measurement system (composed by the chamber 41 , the integrating sphere 42 and the photo-detectors 53a, 53b, with or without optical fibre 51 ) is placed at the calibration distance from the lamp 81 with the calibration aperture 49 of the integrating sphere 42, that is opened in such a way that the light of the calibration lamp 81 illuminates the interior of the integrating sphere. A perforated screen 83 allows the lamp light to reach the integrating sphere, avoiding that spurious reflexions and other sources of light could influence the calibration procedure. The diameter of the calibration aperture 49 is known, therefore it is possible to calculate the entering power in the integrating sphere at the different wavelengths.

The signal measured on a photo-multiplier is given by the expression: V_cal { ) = G_PMTZS\^X)R_cal {X)®_PMT {X)T{X)d wherein G_PMT is the photo-multiplier gain, Z is the measurement impedance, S is the calibration aperture section of the integrating sphere, η(λ) is reflectivity of the integrating sphere including the tubelet inserted in it, ΘρΜτ(λ) is the spectral response of the photo-multiplier [A/W], and τ(Χ) is the optical trasmissivity of the measurement filter before the photo- multiplier.

If the passing band of the filter, Δλ, is sufficiently narrow that the other quantities can be considered constant, the photo-multiplier signal becomes:

V_cai ( ) = C_a ( )SR_cal ( )A wherein X is now the measurement wavelength. The relation permits the calculation of the calibration constant C_a(X) [V/mW] with the calibration door in the opened position: C_a(X) = V_cal(X)/S Rcal(X) ΔΧ

The SILS spectroscopy signals are however measured with the calibration opening that is closed. The new calibration constant C(X) is derived by performing two measures, for example LII with closed door, LI I , and with opened door, LII_a. The correct calibration constant is given by the simple relation:

C(X) = C_a(X) LII/LII_a wherein the ratio LII/LII_a takes into account the loss across the same opening. This calibration procedure has to be repeated for all the wavelengths necessary for the measurements. It is to be noticed that in the previous technique the doors on the integrating spheres are used for detecting luminous energy, and not for the calibration. With the present invention, these doors are utilised also for the calibration, since it is not possible to insert a lamp inside the integrating sphere with a tubelet passing through the same, and there are not known solutions to this problem.

Application of the methodology according to the invention to the particulate concentration measurements using the Lll technigue (SILIIS).

The incandescence signal emitted by a single soot particle, LII_P [mW/nm], is given by the well-known relationship:

wherein h is the Plank constant, c is the light speed, k is the Boltzmann constant, X is the measurement wavelength, E(m) is the absorption function depending on the complex refraction index, m, d_p is the soot particle diameter and T_soo, is the temperature of the soot nanoparticle. Assuming that all the particles inside the measurement volume have the same diameter, the total LII signal emitted inside the measurement cell is calculated as:

LII_tot = LU_pn_pV wherein n_p is the particle concentration [#/cm³] and ν=πΦ²1/4 [cm³] is the volume of the small pirex tube inside the integrating sphere. The volume fraction of the soot, /v, is given by the relationship:

Starting from the measured signal LII, Vu_/β) [mV] at the wavelength X, and utilising the calibration constant C(X), it can be obtained the following: he 7*S²L

exp E{m)fv Αλ

k T s, oot

The soot temperature can be calculated by the ratio between the LII signals at two different wavelength, Xi and X₂, by means the classic formula of the two-colours pirometry: ν_ιη {λ_λ ) C(A₂ )AA₂

Once determined the particles temperature, the volume fraction is given by:

The laser fluence to be used for the measurements has to be selected with some other additional considerations.

Utilizing low fluences (for example up to 100 mJ/cm² in the IR) the Lll signals can be weak and rather noisy, on the other hand high fluences (over 500 mJ/cm²) cause sublimation phenomena of the nanoparticles with diameters shortening and loss of materials, that can invalidate the measurement. The "best practice" suggested by the researchers is to remain at the boundaries of the Lll signal linearity zone, in the range of 150-200 mJ/cm², or to arrive to the "saturation" beginning zone in the range of 250-300 mJ/cm², thus limiting the sublimation phenomena. The final choice of the laser fluence to be used obviously depends on the nanoparticles concentrations and therefore on the Lll signals intensity.

In the case of use of a high laser fluence, with the possibility of sublimation phenomena, it is necessary to consider the gas travelling speed inside the tubelet of the measurement cell in order to avoid that more laser impulses excite the same gas volume. Calling L, the distance between entrance and exit (7a e 7b of figure 2) of the measurement gas and Φ, the internal diameter of the pirex tubelet, the gas volume in the measurement section is V_t = πΦ,²Σ/4 [cm³]. By utilizing an aspiration pump with a flow rate Q, [cm³/s] to guarantee that the full section of gas is interested by a single laser impulse, the laser impulses frequency has to respect the condition: / < 4Q/n0?L, [Hz]. For example, for a gas flow rate of 1 l/min, a diameter of the tubelet of 6 mm and a length L_t equal to 8 cm one obtains a maximum laser impulses frequency of 7 Hz.

The present description of the patent has been, until now, limited to the use of the integrating sphere coupled to the Lll technique for measurements of carbon particulate concentration.

Nothing prevents to use the same Lll technique and the invention apparatus also for measurements of the particulate average dimensions. To this end, the scientific community has already elaborated some rather complex theories, that consider the different physical-chemical phenomena that occur during the interaction of the laser beam with the nanometric particles. The equations that describe such phenomena regard mass and energy balance including the laser energy absorption, the heat losses due to vaporisation, the heat conduction with surrounding gases, the radiation and the variations of the particles internal energy.

Different calculation programs have been developed from the scientific community. In particular, in the website www.liisim.com there is an interface that allow the user to select between different general configurations in order to solve the mass and energy balance equations taking into account the particles diameters distribution and their aggregation degree. In particular, by utilising the experimental data of the temporal decay curves of the Lll signal, it is possible to determine the average diameter of the particles by means a procedure of "best fit" taking into account also the temporal response of the experimental apparatus.

It is important to note that the integrating sphere introduce a time constant in the temporal response to the input photons. This effect is due to the transfer time increase of the photons reflected inside the sphere, that travel along different paths before reaching the light detector. The temporal decay of the luminous signal of an integrating sphere is described by an exponential-type relation, β^'ντ , wherein the time constant is given by the relation:

2 D 1

r = -— ^:—

3 c In p wherein D_s is the diameter of the integrating sphere, c is the light speed and p is the average reflectivity of the sphere walls. Assuming a reflectivity value of 0,98 with a sphere diameter of 5 cm, one obtains a time constant τ = 5,5 ns. Also the photo-multipliers, that are used for detecting the luminous signals, present a time constant that is not negligible and of the nanosecond order of magnitude. The temporal response inside the detection system is given therefore by the temporal responses convolution of the integrating sphere and of the photo-multiplier.

An experimental measurement of the time constant of the overall system can be done by injecting in the measurement cell a low-power pulsed laser beam (in order to avoid incandescence phenomena) and measuring the temporal form of the impulses of the scattering light. The time constant of the Lll signals measurement system is therefore used for the signal analysis according to the previously described procedure. NOTE ON THE SILS TECHNIQUE FOR FLUORESCNCE MEASUREMENTS (SILIFS) AND BREAKDOWN SPECTROSCOPY (SILIBS)

In particular, the SILS technique can be applied to laser-induced fluorescence measurements (SILIFS) with particular regard at the determination of substances subjected at wide-band fluorescence such as polycyclic aromatic hydrocarbons (PAHs). One can therefore extend the applicability of the patent application No. MI97A000240 filed on 07.02.1997 whose inventors are F. Cignoli, S. Benecchi and G. Zizak, of whom two ones are also inventors of the present patent.

Through the adduction means 10, 1 1 , the gas containing the substances to be detected is sent to the measurement chamber 41 according to the previously described scheme.

An suitable laser 20, for example a Nd:YAG in fourth harmonic that emits UV radiation at 266 nm, passes through the measurement chamber, as above described. The substances, such as the PAHs, that are present in the measurement chamber, absorb the UV radiation and emit wide-band fluorescence in the field of UV-VIS.

The fluorescence spectrum depends on the composition and concentration of the chemical species that are present in the measurement volume. By means of the optical fibre 51 , the fluorescence signal is directed to the measurement apparatus such as in the above-described scheme.

In this case, dealing with structured wide-band spectra, it will be appropriate to provide for the use of a spectrograph with a CCD intensified detector, instead of the two-colour system. The procedure for the acquisition, the analysis of the spectra and the numerical deconvolution for the recognition of the gases has been described in the above-mentioned 1997 patent.

Another application of the SILS technique can concern the application of the well-known "laser-induced breakdown spectroscopy" for the analysis of the atomic composition of the aerosols (SILIBS). This technique utilises a laser, typically a pulsed Nd:YAG laser in IR, focalised so as to generate a discharge that breaks the molecules contained in the aerosols that pass in coincidence with the laser impulse. Due to the high electric field, the atoms are excited and emit a luminous radiation that permits the identification. The spectral analysis of the radiation furnishes therefore the atomic composition of the aerosols. The technique is particularly useful in the detection of contaminants such as heavy metals.

In order to realise the SILIBS apparatus, it is necessary to substitute the entrance window 45 in the measurement cell 41 by an opportune lens that permit to focalise the beam at the centre of the integrating sphere. The sampling system 10, 1 1 , the laser 20 are identical to what has been described, while the measurement system 50 can be realised such as for the technique SILIFS, with a spectrograph and an intensified CCD chamber.

The preferred embodiments have been above described and some modifications of this invention have been suggested, but it should be understood that those skilled in the art can make variations and changes, without so departing from the related scope of protection, as defined by the following claims.

Claims

1 . Method for the measurement of the spectrum of isotropic luminous radiation of a gas excited by a laser light, utilizing an apparatus comprising at least a transparent measurement chamber (41 ), and performs the following steps:

A. Making said gas pass in said at least a measurement chamber (41 );

B. Sending a laser beam (21 ) on said gas in said at least a measurement chamber (41 );

The method being characterised in that it uses an integrating sphere (42) substantially enclosing said at least a measurement chamber (41 ), as well as one or more photo-detectors for measuring the luminous signal collected by the integrating sphere, before step A being performed a calibration step AO comprising the following sub-steps:

A0_1. Positioning a calibrated, spectral irradiance lamp (81 ) at a calibration distance (83) from a proper calibration door (49) of the integrating sphere (42), which can assume a opened or closed configuration, in such a way that the light of the calibration lamp (81 ) illuminates the interior of the integrating sphere (42) and therefore said at least a measurement chamber (41 ) when the calibration door is open;

A0_3. For each photo-detector, calculating the calibration constant:

C_a(X) = V_cal(X)/S Rcal(X) ΔΧ wherein λ is the wavelength at which the calibration of the photo- detector is done, V_cai(X) is the signal as measured by the photo- detector, S is the section of the calibration door of the integrating sphere, Rcal(X) the irradiance of the calibration lamp at the calibration distance and AX is the spectral band of the photo- detector;

and in that, subsequently to step B, the following further steps are performed:

C. collecting the light diffused by said gas hit by said laser beam (21 ), by means of said integrating sphere (42), thus obtaining a measurement luminous signal, for example Lll, both with the calibration door (81) open and closed;

transforming the measurement luminous signal into an electrical signal by means of suitable photo-detectors;

calculating the corrected calibration constant:

C(X) = C_a( ) LII/LII_a wherein LII is the value of the measurement with opened calibration door and LII_a is the measurement value with closed calibration door;

2. Method according to claim 1 , characterized in that step F implements the two-colour incandescence measurement technique.

3. Method according to any claim 1 to 2, characterized in that during step F one takes into account the time decay of the luminous signal of the integrating sphere (42), which follows a law of the type e^"v wherein:

2 D, 1

3 c In p

wherein D_s is the diameter of the integrating sphere (42), c is the light speed and p is the average reflectivity of the walls of the integrating sphere (42).

4. Method according to claim 3, characterized in that during step F one takes into account also the time constant of the photo-detectors, the time response of the whole system comprising the integrating sphere and the photo-detectors being given by the convolution of the time responses of the integrating sphere (42) and the photo-detectors.

5. Apparatus (100) for the measurement of the spectrum of isotropic luminous radiation of a gas excited by a laser light, comprising:

- means (10, 11 ) for supplying gas into

- at least a measurement chamber (41 );

- means (30,31 ) for evacuating the gas from the measurement chamber;

- at least a laser source (20) generating a laser beam (21 ) passing through said at least a measurement chamber (41);

and characterized in that it further comprises:

- an integrating sphere (42) through which said at least a measurement chamber^'(41) is made passing;

- optical means (51 , 52, 53a, 53b) suitable to collect the measurement luminous signal collected by said integrating sphere and to convey it to

- a system (50, 60, 70) for analyzing the measurement luminous signal.

6. Apparatus according to claim 5, characterised in that said optical means are constituted by an optical fibre (51) connected (43) at an end to an aperture (43a) of said integrating sphere (42) to collect a measurement luminous signal, and at the other end to said analysis system.

7. Apparatus according to claim 5 or 6, characterised in that said optical means are constituted by a dichroic mirror (52), one or more interference filters placed in series with corresponding photo-multipliers (53a, 53b), said optical means being connected directly to the exit (43a) of the integrating sphere.

8. Apparatus according to any claim 5 to 7, characterized in that said means (10,1 1 ) for supplying gas comprise a drawing probe (10) and a conduit (1 1 ).

9. Apparatus according to any claim 5 to 8, characterized in that said system (50, 60, 70) for analyzing the measurement luminous signal comprises:

means (52) for the separation of the same signal in two optical beams belonging to two suitable spectral bands, as well as

- means (53a, 53b) for detecting said two optical beams, in order to implement the two-colour incandescence measurement technique.

10. Apparatus according to any claim 5 to 9, characterised in that said at least a measurement chamber (41) is constituted by a transparent tubelet made of pirex or quartz, and passing diametrally through the integrating sphere.

1 1 . Apparatus according to any claim 5 to 10, when not depending to claim 9, characterized in that, for laser-induced fluorescence measurements, with particular respect to the determination of substances subjected to wide-band fluorescence like the aromatic polycyclic hydrocarbons, said system (50, 60, 70) for analyzing the measurement luminous signal comprises a spectrograph with CCD intensified detector.

12. Apparatus according to claim 1 1 , for the measurement of isotropic gas radiation by means of laser-induced breakdown spectroscopy, wherein before the entrance of the measurement chamber (41 ) a suitable lens is placed, which allows the focusing of the beam at the centre of the integrating sphere (42).

13. Use of the apparatus according to any claim 5 to 12, for the measuring of the isotropic luminous emission spectrum of a gas excited by laser light.