CA1118869A - Optical smoke detector - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A smoke detector is disclosed using a dark field optical system in which an air sample is illuminated and forward scattered light is collected in a zone centered about but exclusing on-axis light. The collected light is then sensed to detect the presence of smoke. The system includes a self-contained light source and a photo detector suitable for operation by a dry cell. The optics are designed for high light gathering efficiency and the internal design minimizes scattered light to achieve maximum smoke detection sensitivity.

Description

6~3 The present invention relates -to smoke detectors in which the presence o smoke is sensed by directing a beam of light into the smoke and sensing the light scatteredO
The invention also relates to dark field optical systems in which light scattering is reduced to darken the field~
The invention also relates to the design of illuminating and light collecting optical elements.
Smoke detectors available for home use currently fall into two categories: Those which are of the ionization type, and those which are of the optical type. The present invention deals with a smolce detector of the optical type.
The conventional optical smoke detector contains a light source which illuminates a sample of air potentially containing smoke. If smoke is present, light is scattered in all directions from the scattering particles. In known optical systems, the collection is at an off-axis position to one side of the beam. It is known that the scattering flux is very much a function of the scattering angle. For instance, back scattering is relatively weak, side scattering reaches a minimum value, and forward scattering is relatively strong. The scattering efficiency varies through nearly two orders of magnitude as one goes from side scattering (perpendicular to the beam) to forward scattering in substantial alignment with the beam. Since this principle has been known, a system has been proposed in which the main illuminating beam is obscured by a stop and light is collected by a lens placed off the illuminating axis, beh:ind a stop. In such off-axis systems, the scattered light collection solid angles are small and the light detection efficiency low since only a small part of the scattered light is collected.

It is an object of the present invention to provide an ~ 8~ 35 EL-1386 improved smoke detector of the type which senses light scattered by particles of smoke.
It is still another object of the present invention to provide an improved smoke detector of the type which senses light scattered in a forward direction by particles of smoke and which uses a dark field optical system.
It is still another object of the present invention to provide a smoke detector of the type which senses scattered light and which has a self-contained light source, the detector being suitable for battery operation.
These and other objects of the present invention are achieved in a novel smoke detector using a dark field optical system in which an air sample is illuminated and forward scattered light is collected in a zone centered about but excluding on-axis-light. The collected light is then sensed to detect the presence of smoke. The smoke detector com-prises a measurement chamber into which airborne particles of combustion are admitted, and which excludes light except at an entrance aperture at one end and at a zonal exit aperture containing a central stop at the other end. The measurement chamber also has an internal aperture which to-gether with the other two apertures stop are oriented per-pendicularly to the axis of the detector and centered thereon.
The smoke detector also includes means ~or projecting a beam of li~h-t along the axis, through the entrance aperture and into the measurement chamber for illuminating any smoke particles present in the chamber, the beam passing through the internal aperture and being intercepted by the central stop. An output lens is provided arranged in the zonal exit aperture for collecting scattered light throughout the zone, the lens bein~ blocked to the rays of the beam by the :internal aperture and the stop, but collecting forward scattered light l3fi~
when airborne scattering particles are present. Finally, the smoke detector includes a light detector arranged on the axis, behind the output lens for sensing the scattered ligh-t collected by the output lens.
In accordance with another aspect of the invention, an anterior chamber is provided from which airborne particles of combustion are excluded and from which light is excluded except for an exit aperture opening into the measurement chamber. The beamforming means comprises a light emitting diode having a condensing lens which is formed integrally with the light emitting diode and which creates a divergent beam. The divergent beam is concentrated into a narrow beam by the beamforming lens. Preferrably, the integral condensing lens is set in a circular aperture for defining the edges of the light source. The deamforming lens forms an image of the bounded virtual source in the plane of the internal aperkure. In this way, the limits of the image of the virtual source are well defined, and when smaller than the internal aperture, illumination of the edges of the aperture are avoided. This edge illumination should be avoided to keep the field of the detector dark. The beam-forming lens is arranged to collect widely diverging light from the source. Since spheric aberration under these circumstances substantially enlarges the image of the virtual source, the beamforming lens is of an aspheric design.
~he exact curvature of the lens is calculated to correct spherical aberration in the indicated source and imaye positions so as to improve the sharpness of the image of the virtual source ~ormed in the internal aperture of the measure ment chamber. As will be shown, this leads to an increase in the sensitivity of the system.

To further darken the field, both the anterior and the l B~6~
measurement chambers, which are of generally cylindrical shape, have a low Light reflectance interior. Each contains at least one baffle extending inwardly from the cylindrieal walls to reduce the amount of light scattered into the beamforming lens or the output optics. As a further step in darkening the field, the central stop is conical, made entrant into the output lens, and given an opaque, low reflectance interior to reduce the amount of light scattered into the output lens.
In accordance with another aspect of the invention, the output lens is a three element annular lens of a hlgh power, having at least one aspheric surface to correct spherical aberration and sharpen the image of the scattered light focused on the photo detector. More specifically, the front surface of the first element of the output lens is of low power to facilitate collection of highly divergent light, and the back surface of the first element is as-pheric and of higher power than the front surface. The back surface of the second elemen~ is of low power to facilitate a large convergence angle toward the detector and the front surface of the second element is of higher power than the back surface. The third lens element of the output lens is a hemispheric immersion lens, coupling light over a wide angle to the small light detector.
The novel and distinctive features of the invention are set forth in the claims appended to the present applieation.
The invention itself, however, with further objects and advantages thereof may best be understood b~ referenee to the following deseription and aecompanying clrawiny. The drawing is a eross-sectional view of a novel optieal smo~e deteetor in whieh an air sample is illuminated and forward seattered light is eollected to deteet the presence of smoke.

Referring to the drawing, there is shown an optical smoke detector incorporating the invention In the smoke detector, light is projected into a chamber containing smoke and the for~ard scattered light is measured to a~ ccw ~c ~ ~s determine the smoke concentration. The optical ~3mp~
of the device are arranged in three light coupled, but generally light tight, coaxially arranged, cylindrical compartments 11, 12 and 13. A light source 14, an aperture 15 for the light source and a beam forming lens 16 are housed in the ~irst or anterior chamber 11. The anterior chamber is sealed to avoid the admission of smoke or dust. The measurement chamber 12 has two openings 10 to the outside air for the admission of airborne particles of combustion, i.e., smoke, and is shielded to avoid light coupling, except to the anterior and posterior chambers, by a housing, which consists of two u shaped o~erlapping box members. The measurement chamber 12 contains an aperture 17 for admitting light from the anterior chamber~ the aperture 17 being associated with the beamforming lens 16; an aperture 18 internal to the chamber; and an output light stop 19. The light stop 19 is an opaque, blackened, conical cavity at the center of a three element output lens assembly (20, 21, 22). The stop 19 and lens elemenk 20 form the boundary bet~een the measurement chamber and the posterior chamber.
The posterior chamber 13, which is also sealed against smoke or dust, i5 light coupled via the unstopped annular region of the lens element 20 to the measurement. The posterior chamber contains the three element output lens assembly (20, 21, 22) and the photodetector 23. As will be shown, the output lens assembly is masked to avoid direct rays ~rom the source (1~, 15) and light from the principal scattering surfaces. The unmasked area of t~e output lens assembly B6~
collects light scattered by smoke near the internal aperture of -the measurement chamber, and focuses it upon a photo-detector 23.
Significantly, the smoke detector utilizes forward scattered light collected over a large solid angle in a dark field system to achieve a high smoke detection sen-B sitivity. The o~ a optics (20,21 and 22) are positionedcoaxially of the beam and the beam forming optics, in the direction of travel of the beam. This locates the output optics, which are centrally stopped to prevent direct ill-umination by the beam, in the forward light scattering region where the scattered light from a given concentration of scattering smoke particles is one or two orders more intense than side or back scattered light. The second advantage of the coaxial arrangement is that the collection optics may surround the beam, and collect scattered light over the full annular area encircling the beam as opposed O_~S
to a small segment oE the ~5, when the collection lens is placed at a single off axis position. In the present arrangement, the output optics embraces a la~e solid angle than conventional and the solid angle embraces the region of greatest scattering for greatest light collection, and greatest sensitivity. The smoke detector works upon the dark field principle with the output lens collecting zero light ideally in the absence of smoke and appreciable li~ht i.n the presence of smoke, The operation of the smoke detector will now be treated in detail kogether w.ith a further kreatment of the individual components. The light sour~e 14, 15 is a pulsed, solid state light source designed for energy economy. The energy economy soughk is to achieve a year's operation powered by a small dry cell. The light source is a semiconductor diode which B~

emits light in the infrared or red portion of the electro-magnetic spectrum. The unit is typically 0.2 inches in diameter and includes an opaque base and self-contained optics including a reflector 26 and a condensing lens 27 for producing light over a solid angle whose cross section is approximately 40 degrees~ Since there is a + 7 un-certainty in the directivity of the emitted light with respect to the base of the "LED", the li~ht which is actually collected by the beam forming lens 16 is normally restricted to a small solid angle where light emission is substantially certain. This solid angle is typically a solid angle whose cross section is approximately 26. As noted above, the light source (14, 26, 27) is set into an ante-chamber 22 which is light tight except for an aperture ~15) for admitting light from the LED and an aperture (17, 28) for projecting light into the measurement chamber. The outer limits of the LED light source are precisely defined by a circular aperture 15 through which the light is admitted into the ante-chamber. The aperture 15 is of a reduced diameter (0.138 inches) and is arranged at the tip of the lens (27) integral with the LED source. Light from the LED is collected by the lens 16 and formed into a beam which is projected into the measurement chamber 12.
The beamEorming lens 16 is a lens oE moderate power and moderate numerical aperture for uniformly illuminating a well defined region in the chamber near the center of the internal aperture 18. The lends 16 is an aspheric lens having a flat first face aligned toward the LED light source and a convex second face aligned toward the smoke chamber.
The curvature of the lens 16 is designed to yield zero spheri-cal aberration. The computer program by which the curvature is calculated takes into account the positions of both the ' 36~
"virtual source" and the image of the virtual source. The convex ~ace of the lens is set into the aperture 17, 28, which ~orms the opening through which light is projected into the smoke chamber. The numerical aperture of the lens 16 is approximately .23. The lens 16 forms an image of the apertured source (14, 15) in the plane of the aperture 18, which is larger (.1875 inches) than the source (.138 inches) and located near the center of the internal aperture 18.
In accordance with conventional principles of illumination, it is not the light emitting diode itself but rather the surface of the LED's integral lens which is the virtual source, imaged in the plane of the aperture 18. This optical design technique has the effect of producing a soft, even illumina-tion in the plane of the aperture without loss in definition of the edge of -the illuminated area. The internal aperture is made larger in diameter (.275 inches) than the focused image (.1875) and has a knife-edge (low radius) opening to reduce edge reflections into the output optics. This pre-caution insures that no part of the beam impinges on the edges of the internal aperture 18 and reduces edge reflections into the output optics. The illuminating beam, whose marginal rays are illustrated by the dash-dot-dash-out lines 29 is intercepted at the end of the smoke chamber by the conical stop 19, the stop being substantially larger than the in-tercepted beam. The light output from the measurement chamber is gathered in an annular region outside this skop.
'rhe ~unction of the foregoing elements, :Lncluding the apertured LED light source (14, 15, 26, 27), the beamforming lens 16, the apertures 17, 28, 18 and the conical stop 19 is to illuminate a sample of air which may contain smoke in a manner suitable for dark field viewing. The elements are arranged along a common axis in such a manner that the beam of ~ 35-EL-1386 light formed by the lens 16 impinges on no surfaces that scatter any light into the annular output region surrounding the conical stop 19. IE no smoke is present in the chamber, the passage of the beam illustrated by lines 29 through the chamber is unmarked and no secondary light scatters in the beam are illuminated to deflect light outside the beam.
Ideally, the field will be black under these conditions and the photodetector will produce no output. If smoke is present in the smoke chamber, then the beam contains secondary scat-lQ terers in the path of the beam from the entrance aperture 17to the stop 19. These secondary scatters become secondary sources of light which make the path of the beam a source of general illumination. When this occurs, all portions of the beam will scatter light, albeit unevenly, through a full sphere. A portion of the illuminated beam, and in particular that in the vicinity of the internal aperture 18, will scatter light visible ~rom the ~iewpoint of the annular region surrounding the stop 19, where it will be collected by the output optics. The output field will be light and the photodetectors will produce an output.
Light scattering from sources other than smoke ordinarily does occur, and must be kept to a minimum to obtain a dark field in the absence of smoke. Scattering centers occur within and on the surface of the lens 16, on the edge of the entrance apertures 17, 28, and the edge of the internal aperture 18, the surface of the stop 19, and the walls of the an-terior and measurement chambers. These scatters become new sources of light, and if the interior o:E the measurement chamber iæ reflecti~e, and propagation paths exist, they will cause light to be reflected into the out-put lens. Any background light collected in the output lens tends to reduce the sensitivity of the system to low g _ smoke concentrations. As illustrated by the dotted lines 30, the apertures 16, 18 and conical stop 19 are placed to preclude any portion of the lens 17 from scattering light directly into the output optics.
The interior of the ante-chamber 11 and the measure-ment chamber 12, and the apertures and stops are designed to reduce internal reflections for increased smoke sensitivity.
The internal surfaces are normally black, and may be ridged or coated with flock. The stop 19, for instance, if not carefully designed, may be a principal secondary source of light scattered into the output optics. When illuminated by the beam, the stop 19 may reflect some light back toward the wall of the internal aperture 1~, where a second re-flection will convey that light to the output optics. Similarly, light scattered from the interior or either surface of the beam forming lens 16 may enter the output optics. While direct rays from the surfaces and interior of the lens 16 are masked from the output optics by the apertures 17, 18 and stop 19, as noted above, light scattered from the lens can illuminate a side wall of the chamber and after a single reflection be collected by the output optics. Back-ground light from both causes may be reduced to a level where the background light contributes less than 10% of the light output at the desired maximum ~typically 1~) smoke sensitivity~ For this reason, when the stop 19 is formed as a conical cavity, nonreflectively and opaquely coated, the back reflections into the chamber can be reduced below the critical level. Similarly re~lections along the cylindrical side walls o~ the ante-chamber, and the measure-ment chamber can be reduced below the critical lelvel by employing a low reflectance coating and annular baffles (two in each of these chambers). In the ante-chamber, two ~8~ 35-EL-1386 baffles 2~ will capture all single "bounce" reflections originating from scattering in the lens 16 and its aperture 17. Assuming that the ante-chambex is coated with low reflective material, this is normally adequate. Similarly, two baffles 25 in the measurement chamber 12 are usually adequate to capture most single "bounce" reflections originat-ing in the ante-chamber in the beam-Eorming lens 16 and its aperture 17. The baffles in eash case extend inwardly a fixed distance toward bot not touching the beam and terminate in a coaxial aperture, preferrably knife-edged (low radius) to avoid reflections.
The output optics consists of the three element ~ut-put lens 20, 21, 22, the photodetectox 23, and the aperture 18 and the stop 19, which define the zonal field of view of the output optics. The field of view of the output optics may be described as a polar zone of a sphere centered on the optical axis of the detector, in which a portion of the polar zone is removed from the field of view by a stop also centered on the axis. The output lens 20, ~1, 22 collects light throughout the annular or "zonal" surface outside the central stop and inside the cylindrical wall of the detector. The field of view of the output lens is illustrated by four pairs of lines 31 shown as a dash-dot-dot dash-dot-dot. As earlier noted, the output optics collects the forward scattered light from the beam, while at the same time being masked to avoid collecting light in -the beam per se or from the principal scatters. The light stop 19 is an opa~ue, blackened, conical cavity at the center of a three element output lens assembly (20,21,22).
The cone is of maximum cross section at the first lens element 20, of lessor cross-section at the second lens element (21), and the apex of the cone enters the last immersion element (22). The cone is dimensioned to leave a carefully defined annular surface area of the first lens element 20 unmasked for light collection, and to avoid in-~erference with the useful rays which have been collected as they pass through the initial (2) and the two succeeding element (21, 22) of the output optics.
The output optics (20, 21, 22) has a focal length designed to focus the illuminated airborne scatters in the plane of the aperture 18 on the photodiode 23. If there are no such scatters, there will be no lighted image to focus on the photodiode, and ideally no light output. The first element in the output lens is a double convex element 20 of high power. Since the central region of the lens 2~
is masked by the stop l9, only the annular region extending radially beyond the mask to the perimeter of the lens is active optically. The front surface of the element 20 is of spherical curvature and of low power rela-tive to the back surface to facilitate gathering light diverging widely from a~ially placed scatters near the lens. The unused central portion of the lens 20 may be of the same spherical curvature, or flat, or partially hollow, as shown.The active back surface of the lens 20, and in particular the annular region in the path of useful rays from surface, is aspheric.
The actual curvature of the back surface is calculated to ~ reduce spherical aberration to zero and of a higher power ; than the front lens surface. The second lens element 21 has a convex front face of annular shape that is of com-parable power to the back face of the first lens and which may be either aspheric or spheric. The optically active back surface of the second lens is flat. To reduce the overall axial depth of the lens, both the unused central portion of the first (20) and the unused central portion of a second (21) lens are flattened and the flattened faces are joined. ~s noted above, the conical stop passes through both the first and second lens elements. The third and final element in the output lens assembly is ~n immersion lens 22 into which the output photodiode is cast~ It is spheric, and maybe of a somewhat smaller solid angle than a hemisphere. An advantage of the immersion lens is that it avoids two air interfaces which cause light losses at the exit face of the immersion lens and at the entrance face of the photo voltaic diode 23.
In obtaining the necessary power, the three elements indicated are necessary. The initial lens 20 is designed such that a ray from the center of the aperture 17 diverg-ing 45 is deflected into a converging path (typically 15), while the ray from the lower edge of the aperture (as viewed in Figure 1) is brough lnto parallel with the axis, and the ray from the "upper" limit of the aperture is brought into a path converging 30 toward the axis.
The second lens element 21 produces an additional average convergence of about 30 so that all collected rays strike the surface of the final immersion element 22 at an average convergence angle of approximately ~5. A ray originating at the center of the aperture 18 will thus be bent 90 as it impinges on the surface of the immersion lens 23. The immersion lens is arranged to collect light over a large solid angle without substantially increasing the an~le of deflection. The immersion lens increases the apparent size of the photo diode to the converging rays from the lens element 21. The computer program hy which the lens surface were computed was designed to produce a zero spherical aberration for a source located in the plane of the aperture 18 imaged at the position of the photodiode.

35-EI,-1386 36~

The region of the measurement chamber near the aperture 18 is that which enters primarily into the smoke detection process. Light can only be collected from smoke particles that are within the confines of the light beam. This region is defined by the dash-dot-ted lines 29. In addition, light can only be collected from illuminated scatters that are within the field of view of the output optics. This field of view is defined by the dash-dot-dotted lines 31. ~inally, only those rays that have met the foregoing two criteria, and which enter the collection lens in a direction to strike the detector 23 will in fact be detected and measured. The image of the detector, using the reciprocal properties of the output lens (21, 22, 23) largely fills the aperture 18, being a square 0.2" by 0.2". Thus some of the rays of a scatter in the conjectured image position of the detector will impinge on the detector and be detected.
Scatters just outside the image position but in the same plane will generally not produce detectable rays. Similarly, rays originating in scatterers axially displaced from the image will produce some rays which will impinge on the detector and be detected and some that will not. In practice, these qualifications define a smoke sensitive region near the aperture 18, which extends axially both toward and away from the plane of aperture 18.
The optical design has been optimized for realization as an economical mass produced product. In reaching that design, it was taken to be essential that the dimensions of the optical system should remain compati.ble with con-ventional enclosures now common in the market place.
These set a maximum thickness of 1" to 1/2" and a maximum overall length of 5" to 6" on the optical elements. In addition, the lenses should be capable of low cost mass fabrication, i.e., be cast plastic lenses and the photo diode, which is a major item of cost and whose cost is proportional to size, be of minimum size (i.e., 2.5 mm by

2.5 mm).
Assumi~g that the maximum diameter of the lens system is a primary design constraint, the size of the photo detector is minimized by immersing the detector in an optical material of a high index of refraction (injection molded "SAN, n = 1.57) and by maximizing the angular subtense of the output lens as seen by the detector. At the same time r on the front surface of the output lens toward the smoke chamber, the angular subtense of the lens as seen from the internal aperture 18 where the illuminating beam is most concentrated, will also be maximized. An iterative com-puter program demonstrated that the scatter light collection efficiency is maximized if scatterers in the plane of the internal aperture (18) are imaged on the detector by the output optics. The program includes a factor for the scatter efficiency as a function of the scat-tering angle.
Given a constraint on the total length of the system (e.g., ~ 5 inches), and after the axial length of the output optics from internal stop l9 to photodetector 23 has been reserved (e.g., l~ inches), the distance from source to beamforming lens 16 and from the lens 16 to the internal aperture 18 were selected to minimize the size o~ the stop (l9) on the output optics. This latter factor again maximizes the solid angle over which light scattered ~rom the smoke is collected. The latter dimensional con-straint dictates the focal length o~ the beamforming lens (0.6 inches). Since the beam forming lens should on the average, collect light over an angle that will be illuminated, if diodes with a 7 to 9 directional error are to be used 6~
interchangeably, the angular subtense of the beamforming lens should be reduced from -the ~0 angular cross section available from a given diode, to the 20 to 26 angular cross section that all diodes will illuminate.
A final and very important factor in the lens design is the correction of spherical aberration. The effect of an oversized image on the photodetector is to waste the scattered light and thus reduce the sensitivity of the system. At the output optics (2~, 21, 22), an uncorrected lens of the correct power creates a blurred immage on the output diode, which is three times the siæe of the corrected image. The back face of the first lens element is of greatest concern and must be a calculated aspheric surface. The front face of the second lens element may be spheric, although a more concentrated image will be formed if the lens is aspheric. The output element may be of a simple spherical section. In the input optics, a similar problem occurs.
The input optics are designed to collect the most light and concentrate it without impingement on the internal aperture 1~ to the smallest size on the stop 19. A spherically uncorrected lens creates an image of double the size. If the beam at the aperture 18 is doubled, then the aperture 18 must be doubled, and the circular stop 19 must also be approximately doubled to mask the output optics from the scattering surfaces of the lens 16. If the stop 19 is doubled, an interolerable reduction in the area available to the output optics results.
The optical elements may have the following dimensions:
Lens 16 (optical material styrene acrylonitrile (SAN) n = 1.57 Front surface flat Back Surface:

~ 16 -~ 8~3 35-EL-1386 Distance to Axis Thickness 0.350 0.
0.320 0.029 0.290 0.056 0.260 0.080 0.230 0.102 0.200 0.122 0.170 0.139 0.140 0.154 0.110 0.165 0.080 0.175 0.050 0.181 0.020 0.185 Outp~t Optics Lens Element 20 loptical material SAN n = 1.57 Front surface 2.38" radius (spherical) Baek Surfaee:
Distance to Axis Thickness _ 0.550 0.
0.520 0.052 0-490 0.102 0.460 0.151 0.430 0.197 0.400 0.241 0.370 0.282 Lens Element 21 (n = 1.57) Front sur~aee .78" Radius (spherical) Baek sur~aee ~lat Lens Element 22 (n = 1.57) 0.3" Radius (spherical) The ~oregoing optical design represents a particularly sensitive smoke detection arrangement. The basic design ~ 8~ 35-EL-1386 allows one to collect forward scattered light in a zone centered about the axis of -the beam, where the scattering ls of greatest intensity. In addition, the arrangement of the light beam and the collection optics on a common axis, allows the light scattered by individual scatters to be gathered around the zone in an annular region, per-mitting a larger solid angle than ordinarily feasible if the light is collected from a single off axis position.
While a single embodiment has been shown and described, it should be evident that certain modifications may be made without departing from the inven;tion. In particular, the first two elements of th~ output lenses may be back to back Fresnel lenses arranged around the central stop. The disadvantage in using Fresnel lenses in a low cost embodiment are that even if molded they must be mounted with considerable concentric accuracy in relation to the ruling interval. This problem of registration ma~
be avoided if one uses a Fresnel lens and a non-Fresnel lens. The optical system may also use a narrow beam LED
light source, but unless the orientation of the light source is adjustable much of the light may be lost in a given clevice.
If the LED is adjustable, then a separate beamforming lens may not be needed, providing one is willincJ to pay a penalty of about 10% in the "quality" o the integral beamforming optics. If the 10~ increase in efficiency is desirable, a beamforming lens should be employed, and it will then re-:image a "virtual source" consisting of the surface of the LED and its defining aperture. Assuming a beamforming lens is used~ and that alignment is impractical, one should normally use a wide beam LED. The selection of measures to reduce internal reflections are largely dictated by the size and configuration of the enclosure. If large dimensions are tolerable than those indlcated, then the baffles to avoid reflections may be unnecessary.

Claims (6)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A particle detector of optimized optical efficiency having restricted axial dimensions and using a dark field optical system in which a gaseous sample is illustrated and forward-scattered light is collected in a zone centered about but excluding on-axis light and is sensed to detect the presence of suspended particles in the sample, comprising:
(A) a measurement chamber into which a gaseous sample is admitted, said chamber containing an entrance aperture, a zonal exit aperture having a central stop, and an internal aperture, said chamber otherwise excluding light, said apertures and said stop being perpendicular to the detector axis and centered thereon;
(B) beamforming means comprising:
(1) a narrow-band light source, and (2) a beamforming lens at said entrance aperture for projecting a beam of light from said source along said axis for illuminating suspended particles present in said chamber, said beamforming lens imaging said source in the plane of said internal aperture to a size smaller than said internal aperture to preclude beam impingement and to allow beam inter-ception by said central stop, said beamforming lens being of an aspheric design calculated to correct spherical aberration for a point source at predeter-mined object and image distances and produce a sharp image of said light source;
(C) said entrance aperture, said internal aperture and said central stop being arranged to prevent light scattered from said beamforming lens from impinging on said zonal aperture;
(D) an output lens arranged in said zonal aperture for collecting scattered light, said output lens being blocked to the light of said beam or light scattered by said beamforming lens but collecting forward-scattered light when airborne scattering particles are in said measurement chamber, said output lens being a three-element annular lens of high power having at least one aspheric surface calculated to correct spherical aberration for a point source at predetermined object and image distances and produce a sharp image of scattering particles present in said internal aperture, the central region of the more powerful face of each element of said output lens being truncated approximately to the obscuring diameter of said central stop to reduce the axial extent of said output lens, and (E) a light detector for sensing the forward-scattered light collected by said output lens, said image of scattering particles being focused upon said light detector and being of the approximate size of said light detector, said aspheric lenses permitting large numerical aper-tures and short focal lengths for maximum optical efficiency within a given axial dimension.

2. The particle detector of claim 1, wherein the front surface of the first element of said output lens is of low power to facilitate collection of highly divergent light, and the back surface of said first element is aspheric and of higher power than said front surface, and the back surface of the second element of said output lens is of low power to facilitate a large convergence angle, and the front surface of said second element is of higher power than said back surface.

3. The particle detector of claim 2, wherein said light detector is of small extent, and the third element of said output lens is a hemispheric immersion lens coupling light over a wide angle of said light detector.

4. The particle detector of claim 3, wherein the first two elements of said output lens each have one aspheric surface calculated to correct spherical aberration and focus light deflected from scattering particles present in said internal aperture on said detector.

5. The particle detector of claim 1, wherein:
an anterior chamber is provided from which air-borne particles are excluded, said anterior chamber excluding light except for an exit aperture opening into said entrance aperture of said measurement chamber, said exit aperture being perpendicular to said axis and centered thereon; and said narrow-band light source comprises:
a light-emitting diode, and a condensing lens adjacent and immersing said light-emitting diode for forming emitted light into a divergent beam, said diode and said condensing lens directing light through said anterior chamber along said axis and toward said beamforming lens at said entrance aperture.

6. The particle detector of claim 5, wherein:
said condensing lens is set in a circular aperture for defining the edges of said light source, and said beamforming lens collects a major portion of light in the beam formed by said condensing lens and forms an image of the surface of said condensing lens bounded by said circular aperture in the plane of said internal aperture, the bounded image being smaller than said internal aperture to avoid illumination of the edges of said internal aperture.