Abstract:
An optical system design for measuring the velocity of fluids flowing through pipes or other conduits is disclosed. The optical system is comprised of a means for delivering two beams through a window in the wall of the pipe, focused to two points aligned along an axis of the pipe and separated by a known distance, and means for detecting light that is scattered by particles carried in the fluid stream through a second window, that is disposed on the opposite side of the pipe. By measuring the time delay between detected signals, the velocity of the fluid can be determined. The delivered light beams are focused in a shallow cone of light and are blocked by an obstruction disposed behind the second window. The scattered light passes through an aperture behind the second window that surrounds the obscuration, and is focused on to a detector surface.

Description:
BACKGROUND OF THE INVENTION 
   The present invention generally relates to metering devices for measuring the velocity of fluid flowing in a pipe, and is typically used to determine the flow volume rate in industrial applications, such as the transport of natural gas. More specifically, the invention relates to an optical system that focuses two light beams in the pipe through a transparent window in the pipe wall, and detecting the time of flight delay of light scattered by small particles carried by the fluid, as the particles pass from one focal spot to the other. 
   In pipeline operations and other industrial processes, flow meters are used to measure the flow rate of gases or fluids moving through the pipeline. There are many mechanical methods for determining the flow rate in pipes, including orifice plates, pitot tubes, Venturi meters, vortex meters, coriolis effect meters, variable area meters, and turbine meters, but generally they require that obstructive structures be inserted inside the pipe, which is undesirable in many applications because it disrupts the fluid flow and creates a pressure drop. Furthermore, many mechanical based sensors require that substantial gas pressures or flow rates be attained to produce a measurable effect. This is problematic for some applications where the reservoir pressure is very low, such as coal bed methane production, or when the fluid is vented to atmosphere or a large storage vessel. 
   Ultrasonic based meters are also known, which measure the Doppler shift of the acoustic velocity of ultrasound beams that are directed diagonally or along the pipe axis. Many ultrasound meters require pockets in the pipe walls to seat the ultrasound transducers, which is undesirable because contaminants tend to build up in the cavities. Long sections of pipe are required to accommodate the ultrasonic beam paths, which can be awkward and expensive, especially for large pipe diameters. 
   Other versions of ultrasonic flow meters launch the ultrasonic waves through the wall of the pipe, using clamp on transducers, but the accuracy performance suffers at low operating pressures and low flow rates. 
   Optical techniques for measuring the flow rate of fluids in pipes are also well known, and generally fall into two categories. Laser Doppler Anemometers use a single coherent laser that is split into two beams that are directed to intersect at the measurement point. The intersecting laser beams create an interference light pattern of alternating light and dark bands along the axis of the fluid flow. Particles passing through the measurement zone scatter the light, which creates a periodic varying optical signal, whose modulation frequency is proportional to the velocity of the particle. This technique is useful when measuring complex flows, where there are many large scattering particles, but because the light is distributed over many intensity maxima, the detection efficiency is low and small particles do not scatter enough light to be measured effectively. 
   The velocity of fluids can also be measured using a technique, generally referred to as the Laser-Two-Focus method. This system involves an optical delivery system that directs the light from one or two laser beams to form two focus spots in the pipe, separated by a known distance along the pipe axis. Particles in the fluid stream that pass through the two focus spots, scatter the light which is directed on to a photodetector by an optical collection system. The resulting signal consists of short impulses, and by measuring the time delay between adjacent pulses, the velocity of the particle can be determined. Because the intensity of the delivered light is concentrated in only two spots, the sensitivity of the Laser-Two-Focus method is superior to the Laser Doppler Anemometer system. This is important in certain fluids, such as natural gas, which contain only very small particles that are often less than 1 micron in diameter. 
   The amount of light scattered by a particle at a given angle depends on many variables, including the size, shape, surface quality, transparency/opacity, refractive index, and conductivity of the particles. The combination of these effects is very complex and generalized theories such as Mie and Rayleigh scattering fail to predict real world results accurately, so empirical studies are most often used to characterize specific systems. Mie theory is useful however in gaining a basic understanding of general trends in scattering behaviour. For instance it predicts that the amount of light scattered by very small particles (approaching the wavelength of the incident light or smaller), is subtended mostly within a very small, forward scatter angle.  FIG. 1  shows that more than 90% of the light scattered by a spherical transparent droplet, with a diameter equal to 6 wavelengths of the incident light, occurs within a 10 degree forward angle cone. 
   The ability of a Laser Two Spot optical system to discriminate light scattered by a particle depends not only on the amount of light collected by the detection optics, but also by how much unscattered light is prevented from reaching the detector. For example, any light that is scattered at an angle less than the divergence cone of the incident light can not be effectively detected because the detector will be blinded by the unscattered light. The contrast or detectability of scattered light is fundamentally limited by the contrast ratio of detected scattered light to detected unscattered light. 
   Previous laser two spot optical flow meters, such as described by Kiel et al and Williamson et al, optimize the contrast of the detected light scatter signal, by shifting the optical axis of the collection optics away from the incident light axis, as shown in  FIGS. 2   a  and  2   b . This mininizes the signal bias caused by the unscattered light, but only a small amount of the scattered light is coupled into the collection aperture. In some cases, such as natural gas, where the size of naturally occurring scattering particles is very small, this can be a limiting factor and the signal to noise ratio will suffer due to weak detected scattered light levels. 
   Laser two spot anemometers are also known to characterize the flow of relatively large particles (greater than 10 wavelengths of the incident light) such as particulate dusts or aerosols. Hairston et al teaches a system for measuring the size and velocity of aerosols ejected by a nozzle, using the laser two spot method, with the collection aperture colinear with the incident beam axis. The unscattered light is blocked by a central obscuration located on the opposite side of the measurement zone, and light scattered at larger angles that pass into the collection aperture are focused on to a photo-detector. Because the particles are relatively large in this application, the detected light amplitude is not so much a concern, so a large central obscuration can be used without sacrificing sensitivity. 
   The optical systems described by Kiel, Williamson, Hairston et al., all feature telecentric or parallel optical systems, that generate delivery light beams that are directed perpendicular to the flow direction. This is important in some applications, particularly when the fluid is a gas under high pressure. Most low pressure gases have a refractive index vary near unity, but at high reservoir pressures, greater gas density causes a significant increase in the refractive index, which would change the optical refraction angle of any light passing into the medium. This can cause a parallax type shift in the spacing between the focus spots, if the optical axis is not perpendicular to the flow axis, resulting in a measurement error. 
   In some flow measurement environments, such as natural gas wells, a significant amount of water, liquid hydrocarbons, particulates, and other contaminants may deposit on the optical windows and degrade the efficiency of the transmitted light over time. This problem has not been effectively addressed in the prior art for applications where the optical metering apparatus is intended to be left in place for long periods of time. Optical windows are used in many other pipeline applications, particularly sight glasses, and there are a number of remedies that have been developed to allow for the windows to be cleaned from time to time. It is desirable however, to develop an optical system that both resists fouling and is tolerant of variations in the optical transmission efficiency. 
   Also, for many industrial applications, information on size and shape of the particles flowing in the pipe is highly desirable to characterize and monitor the quality of the fluid in the process. This data verifies, for example, the quality of filtering means used at a natural gas processing plant, condition of the pumps and corrosion level of the pipes. Specialized laser devices for measuring particle size are known, but their use has been largely limited to controlled laboratory environments and they are not considered suitable for in-field applications due to their sensitivity to vibration and misalignments. 
   SUMMARY OF THE INVENTION 
   Therefore, it is an object of the invention to provide a device for determining the flow velocity of a fluid in a pipe, by measuring the time of flight delay of light scattered by small particles carried by the fluid, from two focused beams of light aligned along the pipe axis with a known separation distance between them. 
   It is an object of the invention to provide a fluid flow meter device that can be inserted in a thin gap in a pipe, and has smooth walls matched to the inner diameter of the mating pipe sections, so as not to disrupt the flow of the fluid within the pipe. 
   It is also an object of the invention to maximize the sensitivity of the optical system detection. More specifically, it is an object of the invention to maximize the efficiency of detected scattered radiation and minimize the detection of unscattered incident light. 
   It is also an object of the invention to provide an optical system that can compensate for fouling of optical windows in the pipe walls, caused by a build up of contaminants transported in the fluid stream, by monitoring the transparency of the windows and adjusting the gain of the optical detectors. 
   It is also an object of the invention to provide an optical system that is insensitive to refractive index changes in the transported gases that are compressed at high operating pressures. 
   Yet another object is to provide an optical system that can also characterize the size and shape of the scattering particles carried in the fluid stream, by measuring the amount of light scattered at large angles relative to the amount of light collected over small forward angles. 
   To achieve these and other objects, there is provided an apparatus for measuring the velocity of small particles carried by a fluid flowing through a pipe. The velocity measurement is most commonly used to determine the flow rate of the fluid inside the pipe. The apparatus includes at least one light source and a first optical lens system to generate two beams of light and direct the beams through a first window in the pipe wall to form a pair of focus spots in the volume of the pipe, at the same location in the pipe cross-section but separated along an axis parallel to the flow direction. Small particles carried in the fluid stream which travel along a trajectory coincident with the two focal points, scatter the light in succession and the time delay between scatter occurrences is inversely proportional to the particle velocity. 
   A second window is also provided in the pipe wall approximately opposing the first window, whereby means are provided to collect a portion of the scattered light that passes through the second window aperture, and to direct the scattered light by means of a second optical lens system, on to a light detector means. To improve the signal detection integrity, two light detector means may be deployed in the focal plane of the second optical system, each aligned to accept light scattered by a respective focus spot. 
   Means are also provided to block the unscattered light using an opaque obscuration positioned to intercept the beams at or behind the second window to prevent the unscattered light from reaching the detector. The obscuration is sized to block most or all of the unscattered light, but is smaller than the aperture of the second window to maximize the contrast of the detected scattered light. 
   The first optical system, first window, the second window, opaque obscuration and second optical system are approximately centered on a common optical axis that is approximately perpendicular to the pipe flow direction. The two beams of light are directed along the same common optical axis but the focus spots are separated laterally at the focal planes by approximately equal distance from the central axis. The apparatus is further characterized as having a pipe axis which is parallel to the flow direction, and a transverse axis which is perpendicular to both the optical axis and the pipe axis. Means are also provided to reduce the beam convergence of the light entering the pipe, in the transverse axis, to widen the focal spots and present a larger scattering cross-section to particles traveling in the fluid stream. 
   Further means are provided to convert the detected light into electrical signals that are approximately proportional to the incident light intensity. As a scattered particle passes through each focal point a pulse of light is scattered and received by the respective detector, and generates an electrical pulse. Means are provided to electronically determine the time delay between electrical pulses, and the velocity of the particles can be calculated if the distance between the focal spots is known. Further means are provided to determine the flow rate, which is approximately proportional to the particle velocity. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a further appreciation of the above and other features and advantages of the invention, reference is made to the following detailed description and drawings, in which: 
       FIG. 1  is a diagram of the scattering light efficiency as a function of forward scatter angle of a 3 micron oil droplet, using a light wavelength of 650 nm.; 
       FIGS. 2   a  and  2   b  are cross-sectional views, respectively along and perpendicular to the pipe axis, of an optical system used in the prior art showing the collection optical system axis intersecting the delivery optical system axis at an angle; 
       FIG. 3  is a system diagram showing the a preferred embodiment of the present invention with separate opto-mechanical head, electro-optical assembly connected by an fiber-optic extension cable; 
       FIGS. 4   a  and  4   b  are cross-sectional views, respectively perpendicular to the pipe axis and along the optical axis, of the opto-mechanical head assembly portion of the present invention; 
       FIGS. 5   a  and  5   b  are cross-sectional views, respectively along and perpendicular to the pipe axis, showing the delivery optical system of the preferred embodiment, exaggerated in size relative to the pipe cross-section, for greater clarity; 
       FIGS. 6   a  and  6   b  are cross-sectional views, respectively along and perpendicular to the pipe axis, showing the collection optical system of the preferred embodiment, exaggerated in size relative to the pipe cross-section, for greater clarity; 
       FIG. 7  is an illustration of the shape of the opaque obscuration that may be used with the preferred embodiment of the present invention, comprised of a union between a thin rectangular strip and a circular central spot; 
       FIGS. 8   a  and  8   b  are two views of the optical reference collection assembly, comprised of a central reflector and two reference collection fibers, disposed in front of the opaque obscuration of a preferred embodiment of the present invention; 
       FIG. 9  is a system diagram of the present invention, comprised of separate opto-mechanical head, electro-optical assembly and extension cable, showing a block diagram of the electronics processing circuits and the flow of signals used to compensate for optical system efficiency losses; 
       FIGS. 10   a  and  10   b  are cross-section views, respectively along and perpendicular to the pipe axis, showing an alternate embodiment of the present invention, comprised of a modified ferrule with facets polished at angle relative to the optical system axis, and a single coupling lens provided to focus the delivery beams and maintain the chief rays parallel with the optical axis through the pipe section; 
       FIGS. 11   a  and  11   b  are cross-section views, respectively perpendicular to the pipe axis and along the optical system axis, showing an alternate embodiment of the present invention, providing a second collection optical system to detect light scattered at large angles, such that the amplitude of the detected light signal is used to determine the size of the scattering particle; 
     And  FIG. 12 , showing an alternative embodiment of the optical collection system shown in  FIG. 12 , where the second collection optical system is collinear with the first collection optical system axis. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   There is shown in  FIG. 3  an optical flow meter constructed in accordance with the preferred embodiment to determine the flow rate of a fluid in a pipe, by measuring the velocity of small particles carried by the fluid stream. The flow meter apparatus is comprised of an opto-mechanical head  10  inserted between adjacent flanges  11  in the pipe  12 , an electro-optical assembly  13  which contains two laser light sources  14 , two scatter signal photo-detectors  54  and  55 , and processing electronics  15 , and a fiber-optic extension cable conduit  16  which connects the electro-optical assembly  13  to the opto-mechanical head  10 . In this configuration, the electro-optical assembly  13  can be located remotely so that no electrical potentials are present at the opto-mechanical head  10 , which is desirable when the fluid passing through the pipe can be ignited by sparks or short circuits, such as natural gas. Furthermore, the fiber-optic terminations at the opto-mechanical head  10  are compact, robust, and dimensionally stable because they are removed from thermal loading from the electronics and the light sources. 
   The opto-mechanical head  10  of the preferred embodiment, shown in further detail in  FIG. 4 , is comprised of a rigid plate housing  17  that can be clamped between flanges in the pipeline, with a central bore  18  having a diameter approximately matched with the inner pipe diameter and thus forming a contiguous sealed passageway for fluid to flow through without disruption. The central bore  18  contains two windows  19  and  20 : the delivery window  19  allows delivery light beams  21  and  22  to enter the central bore  18 ; and the collection window  20  on the opposite side of the central bore  18 , allows both unscattered light beams  23  and  24 , and scattered light beams  25  and  26  to pass into the collection aperture  27 . In the preferred embodiment, both windows  19  and  20  are shaped to match the cylindrical curvature of the central bore  18  to form a smooth continuous surface to minimize disruption of the fluid flow and reduce the build up of contaminants, which may be present in the fluid stream. In a further embodiment of the invention, a length of optically transparent tubing with an inner diameter approximately matched to the inner diameter of the pipe wall, may be inserted into the central bore  18  of the housing plate  17  to seal the fluid flow and allow light to pass into and out of the pipe flow. 
   The delivery lens system  28  is contained in a delivery lens bore  29  in the opto-mechanical plate housing  17 , extending radially from the delivery window aperture  30  to the outer perimeter. The delivery lens system  28  is typically housed in a tubular mechanical housing  31  and collectively they establish an optical axis  32  for the system. The delivery lens system  28  is shown in  FIGS. 5   a  and  5   b , exaggerated in size relative to the pipe cross-section for greater clarity. Two delivery optical fibers  33  and  34  are terminated in a double bore ferrule  35 , and spaced equally from and on opposite sides of the optical axis  32 . The ferrule  35  is rotated around the optical axis  32  so that the tips of both fibers  33  and  34  are aligned with the pipe axis  36 . The delivery light beams  21  and  22  diverge from the delivery optical fibers  33  and  34  and are approximately parallel with the optical axis  32 . The delivery light beams  21  and  22  are collimated by the fiber coupling lens  37 , and then focused by the delivery objective lens  38  through the delivery window  19 , to form two primary focal points  39  and  40  inside the central bore  18 , separated along the pipe axis  36 . The separation distance between primary focal points  39  and  40  is determined by selecting the focal lengths of the delivery coupling lens  37  and objective lens  38 , according to the following formula:
 
 D=d   d   *f   do   /f   dc 
 
Where D is the axial separation between focal spots in the pipe; d d  is the axial separation between optical fibers  33  and  34 ; f do  is the focal length of the delivery objective lens  38 ; and f dc  is the focal length of the delivery coupling lens  37 .
 
   In the preferred embodiment, the optical path separation between the delivery coupling lens  37  and the objective lens  38  is selected to be equal to the sum of their focal lengths. This forms what is referred to in the art as a telecentric optical system, which means the chief ray at the focal plane remains parallel to the optical axis. The delivery light beams  21  and  22  are projected along axes parallel with the optical system axis  32 , and perpendicular to the pipe axis  36  as well as the surface of the delivery window  19 , as shown in  FIG. 5   a . This is important in applications where the refractive index of a compressable gas, such as natural gas, varies depending on the internal pressure. In the case of a non-telecentric system, the separation between focal spots would change due to the parallax caused by the refractive index change of the compressable fluid, resulting in a velocity measurement error. 
   Referring to  FIG. 5   a , a cylindrical lens  41  is disposed within the optical delivery system  28  to shift the beam focus away from the primary focal points  39  and  40  to form two secondary line foci  42  and  43  along the optical system axis  32 . This has the effect of broadening the width of the beam at the primary focal spots  39  and  40  into two sheets of light, and thus increasing the cross-sectional area that intercepts the fluid flow at the velocity measurement zone. In the preferred embodiment, the focal length of the cylindrical lens  41  is chosen so that the two beams form two collinear line foci  42  and  43  at a common point on the optical system axis  32 , located at or behind the surface of the collection window  20 . 
   At the secondary foci  42  and  43 , the cross-sectional area of the incident unscattered delivery beams  23  and  24  are a minimum. An optically opaque obscuration  44 , approximating a thin rectangular shape, sized to be slightly larger than the unscattered beam profile at the secondary focal plane, is positioned to intercept most or all of the unscattered delivery beams  23  and  24 . This allows for a maximum amount of scattered light to enter the unobscured collection aperture  27  to optimize the optical signal detection sensitivity. More specifically, the minimum extent of the beam profile subtended by the narrow dimension of the rectangular opaque obscuration  44  allows for the smallest possible angles of scattered light  25  and  26  to enter the collection aperture  27 , without bias from the unscattered beams  23  and  24 . 
   Referring back to  FIG. 4 , the collection lens system  45  of the preferred embodiment is contained in a collection lens bore  46  opposite to and approximately collinear with the delivery lens bore  29 , extending radially from the collection window aperture  27  to the outer perimeter of the opto-mechanical plate housing  17 . Referring to  FIGS. 6   a  and  6   b  the collection lens system  45 , shown exaggerated in size relative to the pipe cross-section for greater clarity, is comprised of an objective lens  47  and a fiber collection coupling lens  48 . Scattered light beams  25  and  26 , entering the collection aperture  27 , are focused into two corresponding scatter collection optical fibers  49  and  50 , positioned at the focal point and corresponding to each of the primary focal spots  39  and  40  in the central bore  18 . The core diameters of the scatter collection fibers  49  and  50  are sized to accept a substantial portion of the scattered light beams  25  and  26 , generated from points across the width of the light sheet at the velocity measurement zone, as determined by the following formula:
 
 W=w   c   *f   co   /f   cc 
 
Where W is the width of the acceptance scatter field at the primary focus points  39  and  40 ; w c  is the diameter of the core of the scatter collection optical fibers  49  and  50 ; f co  is the focal length of the collection objective lens  47 ; and f cc  is the focal length of the collection coupling lens  48 .
 
   The depth of focus of the optical system, which determines the length of the velocity measurement zone along the optical system axis  32 , is affected by both the length of the beam waist of the delivery light beams  21  and  22  and the depth of focus of the collection lens system  45 . Both parameters can be adjusted by selection of lenses, in accordance with the constraints provided above, but there are limits imposed by the physical size of components and housing dimensions. The depth of focus provided by the optical system is also affected by the size and shape of the opaque obscuration  44 . In some cases it is desirable to reduce the depth of focus of the collection lens system  45  to further restrict the extent of the scatter measurement zone, along the optical axis  32 . This can be achieved by increasing the size of the opaque obstruction  44  at the center of the optical axis  32 , resulting in a shape comprised of both a rectangular strip portion overlapped with a circular central spot, as shown in  FIG. 7 . 
   In a further preferred embodiment, a second pair of reference collection fibers  51  and  52  is positioned directly in front of the opaque obscuration  44  and means are provided to couple light from the unscattered light beams  23  and  24 , as shown in  FIGS. 8   a  and  8   b . A central reflector  53  is positioned at the center of the obscuration  44  which directs a portion of each incident unscattered light beam  23  or  24  into a corresponding reference fiber  51  or  52 , which are in turn routed behind the long dimension of the rectangular strip shaped opaque obscuration  44  and out the perimeter opening of the collection lens bore  46 . The signal level generated by the unscattered light beams  23  and  24  coupled into the reference fibers  51  and  52  can be used to monitor the overall strength of the incident light, which may degrade over time because of a build up of contaminants fouling the optical windows  19  (not shown) and  20 , or due to variations in the light source intensity. 
   Referring to  FIG. 9 , the scatter collection fibers  49  and  50 , and reference collection fibers  51  and  52 , are gathered with the delivery fibers  33  and  34 , and sheathed in a common extension cable conduit  16  connected to the opto-electronic assembly  11 , which may be located at some distance from the opto-mechanical head  10 . The collection fibers  49  and  50 , and  51  and  52  are coupled to corresponding scatter signal photodetectors  54  and  55  and reference signal photodetectors  56  and  57 . The light coupled into the scatter collection fibers  49  and  50 , and transmitted to the scatter signal detectors  54  and  55 , generates electrical pulses when particles pass through the velocity measurement zone, and processing electronics  15  are provided to determine the time of flight delay by correlating the electrical signals from the two scatter detectors  54  and  55 . The reference signals generated by the reference detectors  56  an  57  may be used to adjust the gain of the scatter detector circuit  60 , to compensate for optical signal loss due to fouling of the optical windows  19  and  20 . 
   In an alternative embodiment of the invention, shown in  FIGS. 10   a  and  10   b , the telecentric delivery lens system  28  provides a delivery coupling lens  61  (may be a compound lens) and a modified ferrule  62  with two facets  63  and  64 , polished on the face of the ferrule  62  and tilted equally away from the optical system axis  32 . The resulting delivery light beams  21  and  22  are refracted at an angle as they exit the delivery optical fibers  33  and  34 , and cross over the optical axis  32  at a distance in front of the delivery coupling lens  61  equal to its focal length, thus generating the same telecentric alignment with a more compact optical configuration. 
   Referring also to  FIGS. 10   a  and  10   b , a cylindrical delivery coupling lens  65  may also be added to the delivery lens system  28  disposed after the fiber ferrule  62  to collimate the delivery light beams  21  and  22  in the transverse axis. This allows for independent control of the width of the light sheet at the primary focus spots  39  and  40  by selecting the focal length of the cylindrical delivery lens  65 . The focal length of the cylindrical objective lens  41  must also be shortened (negative lens) to compensate for the focal shift introduced by the cylindrical delivery lens  65 , so that the secondary foci  42  and  43  remain coincident with the opaque obscuration plane  44 . 
   In another embodiment of the invention, shown in  FIG. 11 , a secondary collection lens system  66  is disposed behind a third optical window  67  in a third bore  68  in the opto-mechanical plate housing  17 , shown at an angle perpendicular to the optical system axis  32  of the delivery and primary collection lens systems  28  and  45 . The secondary collection lens system  66  does not incorporate any obscurations or reference fibers, but otherwise may be identical to the primary collection lens system  45 . A secondary pair of collection fibers  69  and  70  is disposed to accept the scattered light coupled into the secondary collection aperture  71 , connected to a second pair of measuring photodetectors (not shown). The amount of light scattered at large angles, away from the optical system axis  32  is strongly dependent on the size of the scattering particles, so by measuring the ratio of light signal intensities, information about the size of the scattering particles carried by the fluid stream can be determined. 
   Yet another embodiment of the invention, shown in  FIG. 12 , provides a second collection lens system  72  with an aperture  73  larger than the aperture of the first collecting system  27 , and both systems are disposed collinearly. The second collection lens system  72  collects the light that is scattered at larger angles within a solid angle defined by the difference between the apertures  73  and  27  of both systems. The comparison of the amount of light collected by each system gives the information about the size and shape of the particles.