RU2279663C2 - Device for determining composition of aerosols - Google Patents

Abstract

FIELD: analyzing or investigating materials.

SUBSTANCE: device comprises mirror-lens that focuses the light beam from the source within the space through which the aerosol particles flow. The re-emitting light fluxes are collected with an optical recording system and are directed to the field-effect diaphragm and, then, to photodetectors. The signals are then sent to the processing system.

EFFECT: enhanced sensitivity.

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Description

The invention relates to a measurement technique, in particular to devices for optical control of the fractionally dispersed composition of aerosol particles, and can be used, for example, in monitoring the state of the environment.

A device for analyzing the dispersed composition of aerosol particles [S.P. Belyaev, N.K. Nikiforova et al. Optoelectronic aerosol analysis methods. M .: Energoizdat, 1981. P. 24-74].

Using this device, an analysis of air pollution by aerosol substances in industrial and residential premises, as well as an open atmosphere, is performed. The ultimate goal of the analysis is the most accurate assessment of the dispersed and chemical composition of the controlled aerosol and the determination of the component-wise concentration of pollutants with high sensitivity.

The closest in technical essence to the proposed invention is the device described in the copyright certificate (prototype) [V.N.Kazanov et al. Scheme of a small-sized analyzer of the dispersed composition of aerosol particles. Optical-mechanical industry, 1976, No. 6, p. 50-54].

The device comprises a tube radiation source sequentially installed along the optical axis, a beam-forming system containing a lens focusing lens for generating the analyzed volume, an optical recording system containing a collecting lens, the optical axis of which does not coincide with the axis of the focusing lens, a field diaphragm for suppressing background illumination, and a photodetector . In this case, the exit pupils of the focusing and collecting lens are limited by round apertures, and their optical axes do not coincide.

The device operates as follows.

The light from the tube radiation source is collected by the beam forming system and focused by the lens focusing lens within the solid angle Ω into the analyzed volume through which the aerosol particles pass. When they interact with the beam, secondary reemitted streams of light arise, which are collected within the solid angle Ω by the optical recording system and directed by it to the field diaphragm and then to the photodetector. In order to avoid exposure of the photodetector to exciting radiation, the device is designed in such a way that the spaces of solid angles Ωв and Ω do not intersect, and the optical axes of the focusing lens and the optical registration system do not coincide. Next, the signals are sent to the processing system, which may be an internal or external unit to the device. Signal analysis allows us to conclude that the aerosol composition is fractional.

The disadvantage of this device is the low sensitivity when assessing the concentration of microparticles of aerosols of different fractions from the signals of fluorescence of the particles.

The reason for this drawback is that the optical elements used in the device, which focus the exciting radiation onto a jet with a sample and collect the signal reradiated by an individual particle, do not provide the optimal ratio of the apertures of the solid angles of the excitation and light collection spaces Ωв and Ω, respectively, during luminescent analysis.

The technical problem that the proposed device solves is the express analysis of multicomponent aerosols by light scattering and intrinsic fluorescence signals of individual small particles with the possibility of evaluating their component-wise concentration using a tube source of microparticle excitation.

Technical results obtained using the invention

- the achievement in luminescent analysis of high sensitivity by evaluating the component-wise concentration of aerosol microparticles of different fractions.

The specified technical result is achieved by the fact that in the device for analyzing the fractional dispersed composition of aerosol particles containing a tube source of excitation sequentially installed along the optical axis, a light beam forming system containing a focusing lens for generating the analyzed volume, an optical recording system containing a collecting lens, an optical the axis of which does not coincide with the optical axis of the focusing lens, the field aperture and photodetectors, focusing and collecting about injective performed so that the value of their outer half linear aperture angles α and β, respectively, measured with respect to the center of the analyzed volume, have the meanings

25 ° ≤α≤45 °

45 ° ≤β≤55 °

and mirror elements are introduced into the focusing lens, which ensure the formation of a cone-shaped light beam having an external cone-shaped surface bounded by the angle 2α and an internal surface of the beam bounded by a linear angle 2α2, with 0.1 · α≤α2≤0.55 · α

The author does not know the device in which the features that are distinctive in the proposed technical solution are used.

Figure 1 presents a schematic diagram of the proposed device. Figure 2 is a diagram of the mutual configuration of the outer generatrix of the angles of the focusing α and the recording β lenses and the center of the analyzed volume is indicated. Figure 3 - calculated curves of sensitivity (productivity) of the device at different levels of background illumination on the angle α, at which the light diameter of the output element of the focusing lens is visible from the analyzed volume of the device, as well as on the magnitude of the light collection angle β, figure 4 - ray path in a focusing lens containing a mirror element with a central mirror.

The proposed device consists of the following elements located in series along the optical axis of the device:

1 - the optical axis of the device, 2 - a tube radiation source, 3 - a system for generating a light beam, 4 - a focusing lens, 5 - an analyzed volume, 6 - nozzle for feeding a sample, 7 - registration system, 8 - collecting lens, 9 - field diaphragm, 10 - photodetectors. In this case, 11 is the optical axis of the collecting lens 8, 12 is the center of the analyzed volume 5, 13 and 14 are the boundaries along the Y axis, respectively, of the real analyzed volume, the dimensions of which are determined by the shape of the waist of the exciting beam and jet, and the entire space in the region of the analyzed volume, transmitted through the field diaphragm 9. The dimensions of the latter are blurred due to the limited depth of field of the collecting lens.

Curves 15-17 show the dependence on the recording angle β of the recording lens β of the analysis performance Q (α, β, Kn) at various levels of the illumination coefficient Kn and the focus angle α = 40 °.

Curves 18–20 show the dependence of Q (α, β, Kn) on os for different values of the recording angle β, with a light factor (Kn = 0.03).

Figure 4 presents a diagram of a focusing lens containing an inner mirror 21 and an outer 22, which form a diagram of angles consisting of a hollow cone, with the inner surface formed by the rotation figure of the half aperture angle α2.

The device is designed in such a way that the optical axes 1 and 11 of the focusing 4 and collecting 8 lenses respectively pass through the center 12 of the analyzed volume 5 and do not match. In this case, focusing angles α and light collection β are formed in space, respectively.

The device operates as follows:

The light of the lamp source 2 is collected by the system of formation of the light beam 3 and is formed in the form of radiation of the required spectral range. A focusing lens 4 focuses the generated radiation on the analyzed volume of the device 5.

Aerosol particles are fed into the analyzed volume using the nozzle for feeding sample 6. When the aerosol particle crosses the analyzed volume 5, a secondary stream re-emitted by the particle arises, which is converted, collected and formed by the optical recording system 7. In this case, the light reradiated by the particle is collected by a high-aperture collecting lens 8, the task of which is to form the light flux to the field diaphragm system 9. In this case field diaphragm 9 may be an inlet for a spectral device not shown separately in the figure. After the field diaphragm 9, the collected radiation enters the photodetectors 10 that generate the corresponding electrical signals. The dimensions of the field diaphragm 9 are consistent with the image of the analyzed volume 5, which is built in the plane of the first collecting lens 8. At the same time, the image of the segment including the center point of the analyzed volume 12, the edges of the real analyzed volume 13 is transmitted through the field diaphragm 9, along the axis 11 of the lens 8 and point 14, corresponding to the blurring of the boundary of the analyzed volume due to the finiteness of the depth of field of the lens 8, a segment from point 12 to point 14 in figure 2.

The principles of optimizing the optical system of a device with a tube source are as follows.

Measurements of any signals, including light, measured by photodetectors, are made with an error, which is usually characterized by the value of η signal-to-noise ratio. For a photodetector, this ratio can be expressed as

where N = fs · τ is the total number of useful signal photocounts obtained during the measurement of τ;

fs and fn are the average frequencies of occurrence during the passage of a particle through the analyzed volume of signal and noise samples of the photodetector, respectively.

In measurements, the ratio η should not be lower than a certain value.

The fluorescence flux in a good approximation is spatially isotropic. Therefore, the intensity of the recorded fluorescence signal fs in the selected spectral range is proportional to:

fs = Kr · ρв · σф · Ω,

where Ω is the value of the solid angle within which the particle fluorescence flux is collected by the optical registration system;

ρв is the flux density of the exciting radiation in the analyzed volume of the device;

σф - the value of the cross-sectional area of the fluorescence of the particle;

Kr is the transmittance of the corresponding recording channel.

As you know, the value of the maximum radiation flux density, which can be obtained in a certain volume when using a radiation source having an overall brightness J, is:

ρв = J

where KPR is the transmittance of the optical system of beam formation;

 Ωв is the value of the solid angle at which the radiation focused from the source converges to the image of the luminous body of the source, which in the device coincides with the analyzed volume. Now the expression for fs can be rewritten as:

fs = K1 · J · σf · Ω · Ωв,

where K1 is the coefficient of proportionality

Similarly, you can express the intensity of the noise signal fn:

fn = fz + ft,

where fz and ft are the output frequencies of the noise reading electron associated with insufficiently suppressed radiation from the background light in the device and the dark current of the photodetector, respectively.

Obviously, the intensity fz is generally proportional to the flux F1 of the exciting radiation of source 2, introduced through the analyzed volume 5 into the analytic part of the device.

F1 ~ ρв · Sв = J · Ωв · Sв,

where Sb is the cross-sectional area of the analyzed volume in the direction perpendicular to the exciting radiation.

It is also obvious that the fz value is proportional to the amount of background radiation F2 transmitted to the photodetector through the field diaphragm 9, which is a blurred image of the analyzed volume 5 in the direction of the axis 11 of the recording lens 8. It is easy to show that

F2 ~ Sp

where Sp is the cross-sectional area of space in the region of the analyzed volume 5, the image of which fits in the dimensions of the field diaphragm 9.

Based on this, you can write:

fз ~ J · Sв · Sp · Ωв · Ω

When analyzing aerosols, in order to ensure the necessary sensitivity of the device to the concentration of the determined aerosol fraction, it is necessary to have a certain amount of microparticles pass through the device during the analysis T. The rate of analysis of microparticles is determined by the performance of the device according to the volumetric flow rate of the sample

Q = v

where v is the linear velocity of the jet with a breakdown;

Sp - the cross-sectional area of the analyzed volume of the device in the direction perpendicular to the aerosol jet.

The velocity V can be expressed in terms of the duration of the glow pulse τ, as

where Z is the size of the analyzed volume of the device along the jet of sample.

Then the expression for the volumetric flow rate is converted to:

where ϑ is the size of the analyzed volume.

Obviously, to maximize the performance and sensitivity of the analysis, sample flow v should be accelerated. However, this will decrease the time τ of the passage of the particle through the analyzed volume. But, according to the expression for η, the time τ cannot be made arbitrarily small, since the signal-to-noise ratio will drop and their error will increase. From the expression for η we can write:

In the above formula, the value of η should be understood as a given minimum value of the signal-to-noise ratio, the level of which should be maintained during the measurement process to enable the analysis of microparticles by their glow signals.

Given the last formula, the expression for Q can be reduced to:

where the values of fn and fs are determined by the above expressions for them;

Ks is the coefficient of proportionality.

Obviously, the maximum performance and sensitivity of the analysis will be provided by the device at the maximum possible value of Q.

However, the possibilities for increasing the Q function are limited. Getting its maximum value requires optimizing the configuration of the optical circuit of the device.

Consider the conditions for obtaining optimal ratios of elements.

We restrict ourselves to the case when the role of dark currents in a noise signal is negligible compared to the noise associated with the background radiation of the illumination. It can be shown that the presence of dark currents will not affect the conclusions drawn below. However, in order to simplify the analysis, we assume ft = 0.

This condition is often fulfilled by choosing a photodetector in which dark currents do not affect the measurements.

Based on the foregoing, we can write:

where Kn is the proportionality coefficient characterizing the noise level of the device due to background illumination.

As can be seen from figure 2,

Sv = (Z · X).

The dimensions of the analyzed volume along the X and Y coordinates approximately coincide.

In this case, Sp is the cross-sectional area of the space near the analyzed volume corresponding to the size of the field diaphragm 9, which is a blurred image of the cross section of the analyzed volume 5. The image in the plane of the diaphragm of volume 5 is blurred due to the finite depth of field of the lens 8, as well as because of its aberrations, which rapidly increase with increasing aperture, that is, the angle β of the lens.

From figure 2 it can be seen that even for the ideal [A.S. Dubovik. Applied Optics. M .: Nedra, 1982, pp. 32-61] of the lens, the blurring of the analyzed volume due to the finiteness of the depth of field of the first will lead to the size of the cross-sectional area transmitted to the photodetectors:

Sp ~ Z Y (1 + tg ² (β)) ²

The dependence of aberrations on the magnitude of the angle can be introduced taking into account the fact that spherical aberrations at small angles are of the 3rd order of smallness, that is, they are proportional to the angle to the 3rd degree. Lenses have compensated aberration. However, the value of residual aberrations also increases rapidly with an increase in the aperture angle of the lens. The growth of general aberrations accelerates as the image field increases. Therefore, we introduced the influence of lens aberrations in the form:

Sp ~ Z · Y · (1 + tg ² (β)) ² · (1 + 0.018 · β ³ ) ² .

It is easy to see that the aberration factor, the last on the right side, in the expression for the image area of the diaphragm Sp, corresponds to a blur of the linear size at an angle β = 30 ° by ~ 16%. With a characteristic size of the analyzed volume Z ~ 0.1 mm, this amounts to 0.016 mm, which characterizes the lens as very high quality. Based on the foregoing, we can write:

Based on the foregoing, you can write for performance:

where Kc is the coefficient of proportionality.

The expression for Q also takes into account the effect on the signal of the magnitude of the aberrations of the focusing lens depending on the angle α.

It can be seen from the obtained expression that in order to realize the maximum productivity, and hence the sensitivity of the device, it is necessary to maximize the value of the analyzed volume ϑ. At the same time, in order to reduce the effect of noise, it is necessary each time to strive for the minimum size Z of the analyzed volume in the cross section transverse to the probe light beam.

It is necessary to observe the condition:

Co · ϑ≪1,

where Co is the upper limit of the total concentration of the studied aerosol.

In this case, the maximum value of the analyzed volume

θ ~ 0,1 · Co ^-1

independent of the values of Ω and Ωв.

The spaces of solid angles in which the exciting radiation is focused, Ωв, and the light reradiated by the particle, Ω, are collected, should not have mutual intersection. Otherwise, the exciting radiation, whose power is many orders of magnitude higher than the power of the light reradiated by the particle, will fall into the optical paths of the photodetector and saturate them with high-intensity background noise, which completely suppresses a weak pulsed useful signal.

The value of the solid angle in the space of the exciting radiation

 Ωв = 2π (1-cos (α)),

where, in accordance with figure 2, α is the half of the aperture linear angle bounding the conical space of the solid angle Ωв, in which the exciting radiation is focused.

In accordance with the description of the operation of the prototype device for the solid angle of the collecting lens, you can write:

 Ω = 2π (1-cos (β)) - f (α),

Where

For each Z, the maximum analyzed volume will be obtained when the condition:

ϑ ~ Z · X · Y ~ Z ³ · ctg (α).

From here

Z ~ (ϑ · tg (α)) ^(1/3) .

Based on the foregoing, the expression for analysis performance can be reduced to the following:

Figure 3 curves 15-17 shows the dependence on the aperture external half linear angle of the recording lens β of the analysis performance Q at various levels of illumination coefficient Kn and focus angle α = 40 °.

It can be seen from the presented dependence that only in the absence of illumination, Kn = 0, the analysis performance monotonically increases with increasing light collection of the recording lens β. This case is not realizable in practice. In the presence of noise, there is a maximum in the dependence of Q on β, which corresponds to the optimum angle of light collection

β ~ (45-55) °

In curves 18–20, the dependence of Q on α also has a maximum. The indicated dependence at different values of the recording angle β with a small illumination coefficient (Kn = 0.03) is shown in Fig. 3.

It can be seen that the maximum in the dependence of Q on α is observed at the angle

α ~ (25-45) °.

Our studies have shown that similar conclusions can be drawn for other really valid flare values, Kn.

The dependences of 18-20 Q (α) on α presented in FIG. 3 indicate that in the region of the maximum this dependence is weak. So, in the area

25 ° ≤α≤45 °

the value of Q (α) for all possible situations of analysis does not change by more than 30%. If argument α leaves the indicated range of values, the sample productivity may decrease by an order of magnitude or more.

Thus, we can conclude that in order to obtain the highest sensitivity of the device to individual fractions, it is necessary to use an optical system in which the solid angle, within which the exciting radiation is focused into the analyzed volume of the device, must be limited outside by a linear angle, half of which is in the limits

25 ° ≤α≤45 °.

In this case, the value of half the linear angle limiting the solid angle of the light collection of the registration system should have a value:

45 ° ≤β≤55 °.

The above conditions indicate that both lenses in an optimized device — forming and collecting — must have large angular apertures α and β, respectively, or, which is the same, a large aperture.

So, for example, an angular aperture

2α = 60 °

corresponds to aperture

R = 1.7.

In this case, the lenses must build an image of an object with a size (0.1-0.5 mm) that is not blurred due to aberrations in the wavelength range of 200-400 nm. Lenses work from a finite distance to the subject at a finite distance to the image.

As is known from the literature (L.A. Zapryagaeva, I.S.Sveshnikova. Calculation of lenses. M. MIGAiK, 1980. 129 pp. 56-62), in order to achieve the specified quality for a lens, up to 10 lenses are necessary . Therefore, lenses of this quality have a low transmittance and a high level of background illumination arising from the scattering of light on the lenses. To ensure achromaticity in the lenses, it is advisable to use materials such as crowns and flints, which are opaque in the ultraviolet region, the work in which is necessary for the device in question, because this region provides the most universal excitation of the aerosol present in the atmosphere.

At the same time, it is known [B.N. Begunov, N.P. Zakaznov and others. Theory of optical systems. M .: Mashinostroenie, 1981, pp. 268-270], that lenses using mirror elements such as the Cassegrain lens provide high image quality with large angular apertures and a low level of scattered light.

The Cassegrain lens forms a cone-shaped light beam having an outer cone-shaped surface bounded by a linear angle of 2α. In this case, the inner surface of the beam, which is formed due to the shielding action of the small mirror of the lens 21, is limited by a linear angle of 2α2. For maximum lens transmission, the half angle α2 should be as small as possible. The Cassegrain lens is characterized by the ratio:

0.1 · α≤α2≤0.5 · α.

Thus, it can be written that for an optimal device, the focusing lens should contain an element of two mirrors with a central screening of the beam. In this case, the aperture angle of the lens is bounded externally by a conical surface, the generatrix of which has an angle

25 ° ≤α≤45 °,

and from the inside - a conical surface bounded by an angle

0.1 · α≤α2 <0.5 · α.

You can evaluate how many times the specified technical solution will increase the analysis performance compared to the prototype.

Let us set the angles: α = 40 °, α2 = 10 °, β = 45 °. The exposure level is set by the value Kn = 0.03.

From the graphs of figure 3 it can be seen that these parameters correspond to a value of productivity Q ~ 0.12 conventional units.

Applicable to calculate the value of the catch used in the prototype device:

α = 14 °, angle β = 20 °.

It is easy to calculate that these parameters correspond to a value of productivity Q less than 0.01 conventional units. It can be seen that productivity has decreased by more than 10 times. It should be remembered that a multi-lens lens should be used in the prototype device. For a focus angle of 2α ~ 80 °, the specified lens must contain at least 5 lenses. Given that approximately the same lens should be mounted on the light gathering near the radiation source, it is clear that the transmission, for example, in the ultraviolet region for such an optical system will be ~ 10%. Thus, the performance of the prototype will be more than an order of magnitude worse than the performance of the proposed device. The proposed device can be used in the areas of production and life, where rapid and sensitive analysis of the fractional composition of a multicomponent aerosol is required, for example, when controlling emissions of chemical and microbiological industries, air purity in industrial and populated areas, while protecting against biological terrorism and in space research with the purpose of detecting life in the atmosphere of other planets.

Claims (1)

A device for analyzing the fractional dispersed composition of aerosol particles, containing a tube source of excitation sequentially installed along the optical axis, a light beam forming system containing a focusing lens for generating the analyzed volume, an optical recording system containing a collecting lens, the optical axis of which does not coincide with the optical axis of the focusing lens, field aperture and photodetectors, characterized in that the focusing and collecting lenses are made in such a way then the values of their aperture external half linear angles α and β, respectively measured relative to the center of the analyzed volume, have values 25 ° ≤α≤45 °; 45 ° ≤β≤55 ° and mirror elements are introduced into the focusing lens, which ensure the formation of a cone-shaped light beam having an external conical surface bounded by an angle of 2α and an internal surface of the beam bounded by a linear angle of 2α2, with 0.1 · α≤α2≤0.55 · α.

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