Note: Descriptions are shown in the official language in which they were submitted.
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CROSS REFERENCE TO RELATED APPLICATIONS
The basic concept of this invention which comprises the
improvement of the usefulness of a known integral planar geomet-
ric configuration of p~otovoltaic detectors whereby it is hiyh1y
flexible and is rendered capable of measuring most forward or
backward scattering angles and in all azimuths is independent
of the two applications which are identified below.
In a preferred form of the invention, however, an arrange-
ment is utilized by substitution for the integralplanargeometric
configuration of photovoltaic detectors which enables the fabri-
cation of a highly effective and economical device without the
configuration. This preferred form of the invention utilizes
teachings of the following patent applications: "Apparatus and
Method for Measuring the Distribution of Radiant Energy Produced
in Particle Investigation Systems", Serial No. 342,924 filed
January 2, 1980 and "Apparatus and Method for Measuring Scatter~
ing of Light in Particle Detection Systems", United States Patent
No. 4,286,876 granted September 1, 1981.
The applicants herein are the applicants in both of the
above refexred to applications and both of the said applications
together with this one are owned by the same assignee.
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FIELD A~D B~CKGROUND OF THE INVENTION
This invention is concerned generally with the measure-
ment of the spatial distribution of radiant energy such as that
of reradiated light produced by scattering and fluorescence.
More particularl~ the invention is concerned with the measurement
of the energy and direction of light flux or rays produced and
reradiated or distributed by particles passing through an optical
sensing zone whereby to enable the identification of the par-
ticles and/or their characteristics.
One problem with known measurement systems (not includingthose which are disclosed in the said copending applications) is
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;
that they are limited considerably in the range of polar angles
that can be measured. For the purposes of this discussion the
optical axis of reradiation may be considered the line oP the
incident light beam projected at a sensing zone where a particle
intersects the same. Using conventional definitions, the polar
angles are those defined by the angles of the optical axis with
lines centered at the sensing zone or point and radiating ~rom
that ~one, while the azimuthal angles are those measured around
the optical axis.
One attempt has been made to evolve an arrangement w~ich
can measure multiple angles by means of an integral, planar,
geometric configuration of photovoltaic detectors, but the prob-
lem with this device is that it can only measure polar reradiat-
ing angles from about 1 to somewhat less than 25. Any attempt
to measure the energy distribution in most polar angles in the
forward (0 to 90) and all angles in the backward direction
relative to the sensing or scattering zone and the incidentlig~t
direction including all azimuthal angles, fails because the
sensitive area of the device is too small. ~his is because all
of the photovoltaic elements which "see" the energy must be
mounted on the same plane within the available space which is
limited. Accordingly the utility of the device is limited.
The particular device which is referred to is in the form
of a concentric ring and wedge photovoltaic detector. Ik is
described in considerable detail in Patent 4,070,113 and in U.S.
Patent 3,689,772 and in two articles entitled "Light-Scattering
Patterns of Iosolated Oligodendroglia" by R.~. Meyer, et al in
The Journal of Histochemistry and CV~--h _l~try, Vol. 22, No. 7,
pp 594-597, 1974 and a second article entitled "Gynecological
Specimen Analysis by Multiangle Light Scattering in a Flow System"
by G. C. Salzman et al in the same journal, Vol. 24, No. 1,
pp 308-314, 1976. In these articles reference is made to the
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same or a similar detector device which is commerciallyavailable
and which is identified as a Recognition Systems, Inc. detector
(RSI).
The configuration of detect:ors which has been mentioned
above will be referred to hereinafl:er as a pLanar configuration
of detectors. As known at this time the one mentioned in the
above references is expensive, difi-icult to manufacture, deli-
cate, inefficient and slow-acting because of its relatively
large area considering the number of detectors which it carries.
The inefficient optical design results in a poor signal to noise
ratio.
Notwithstanding these disadvantages, such a configura-
tion and in general any radiant-to-electrical energy transducers
configured in a geometric assemblage which is planar are and can
be useful within the field they occupy, hut according to the
invention herein, this usefulness is materially increased. The
planar configuration of detectors at the minimum can serve the
purpose of helping to find a location at which some desired set
of polar reradiating angles are the optimum for a given optical -
system and for a particular family or type of particles being
studied.
Once an optimum position has been achieved in a given
system, the planar configuration of photovoltaic detectors may
be removed from the system and a more economical device sub-
stituted therefor, this latter device comprising a composite
deviating lens or reflector which is formed of a large number
of elements such as prisms each oriented to deviate or reflect
a certain geometric portion of the reradiated energy area being
studied to different and spaced apart commonly available and
highly economica.L photodetecting devices such as small photo-
cells. The measurements from all of the photocells give the
information desired.
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Prior art patents which may be of interest are:
U.K. Patent 137,637 of 1920 to Pollard a~d Frommer U.S. Patent
3,248,551.
SUMMARY OF THE INVENTION
A system for measuring reradiated radiant energy which
has been scattered or reradiated by Eluorescence which includes
a flow cell in which a particle or a biological cell is passed
through a sensing zone which additionally comprises the focus of
a reflector on the interior thereof. The reflector may comprise
a concave geometric shape symmetrical around its axis and adapt-
ed to capture reradiated light (e.g. scattered or fluorescent)
from wide and narrow polar angles and all azimuthal angles and
to project the same out of the reflector. The reflector is a
shape of revolution of a geometric law about its optical axis.
As used herein light means any e~ectromagnetic radiation whichis
capable of being detected by transducers of the type utilized in
this art.
Means are provided for bringing the particles, cells or
scatterers as they are often called herein, into the reflector
as for example in a li~uid ~low, the reflector interior being
enclosed and having a compatible liquid sealed therein.
The rays of light projected by the reflector may be
either focussed or directed without focussing onto an integral
planar configuration of photovoltaic detectors of a construction
known in the prior art thereby increasing the usefulness of that
device.
One aspect of the invention provides for theconfigura-
tion of detectors to be located at a place along the axis of the
geometric reflector which provides the most favourable informa- ~-
tion in the particle system being investigated. Another aspect
has the configuration movable to find such a place. A third
aspect teaches that once the best location has been found, a
different structure is substituted for the configuration which
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comprises a composite lens or a composite re~lector which cap-
tures the rays of reradiated light and either deviates them or
reflects them in a plurality of different directions relative
to the axis of the geometric reflector. An individual photocell
of economical construction is then located to intercept each
respective one of these rays and the investigation is carried
out by monitoring all of the photocells.
The apparatus may be used for fluorescent light measure- ;
ments by using filters to separate scattered and fluorescent
light produced at the same sensing zone.
BRIEF DESCRIPTION OF TEIE DRAWINGS
.
Figure 1 is a diagrammatic view of a light reradiating
flow cell system constructed in accordance with the invention
and using as an element thereof an integral planar coniguration
of photovoltaic detectors;
Figure 2 is a fragmentary view of the left hand por~ion
of the system of Figure 1 but showing how a fresnel lens and
individual photocells are substituted for the configuration of
photovoltaic detectors of Figure l;
Figure 3 is a view similar to that of Figure 1 but
illustrating a form of the invention using a hyperboloidal
reflector instead of an ellipsoidal reflector for certain speci-
fic measurementst
Figure 4 is a simplified diagram of a modified form of
the invention illustrated in Figure 3;
Figure 5 is a simplified diagram of another form of the
invention using a paraboloid;
Figure 6 is a simplified diagram of a form of the inven-
tion for additionally measuring fluorescence; and
Figure 7 is a simplified diagram of another form of the
invention which is the same as Figure 6 but symmetrically `~
reversed about a dichloric mirror in order to permit the use
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of commercially more available mirrors.
MENTS
Basically the invention comprises a flow cell in which
there are means for collecting radiant energy around the first
focal zone where a parkicle or other small body, hereinafter
called a scatterer, passes through a beam of light and causes
secondary radiation from said particle; the collecting means
causes a reflection of said secondary radiant energy to a second
focal zone and to detector means capable of responding to at
least one of several multiple angles of projected radiation. In
one form, the detector means comprise a known concentric ring
photovoltaic detector and it is adjuste~ along an axis which
passes through both the first and second focal zones. The opti-
mum position for detecting the desired informatlon from the
scatterer ls used. In an lmproved ~orm, the detector means
comprise a fresnel prismatic lens or other optical element of a
composite nature which deviates or reflects the incident radiant
energy flu~ in accordance with a plurality ofdifferentgeometric
areas to different, spaced apart, independent, small, phototec-
tor devices in a highly economical manner.
Other forms of the invention provide for back scattering.
In Figure 1 there is illustrated a flow cell system inwhich there is a source of light 12 which projects a beam 14
through a suitable optical train which is called op~ics 16 to a
partial reflector 18 on the optical axis 20. Thepartial reflec-
tor or beam splitter projects the incident light along a~is 20
at a reduced intensity, now designated 22, through the spherical
closure window 24 of the flow cell 26 to the sensing zone 28.
Transmitted light passes undeflected through the beam spli~ter18
as shown at 19 to other uses.
The flow cell 26 can be constructed using conventional
techniques for sealing and the like and it comprises a reflector
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29 that is a portion of an ellipsoid, having the window24sealed
to its front and open end as at 30, having a rear window 32, an
entering conduit 34 and a discharging conduit 36. The cell 26 is,
filled with a fluid 38 that is compatible with the liquid which
carries the particles and has the same index o refraction as
that liquid. The particle source 40 moves the scatterers in the
fluid mentioned to the entering conduit 34 from which they pass
through the sensing zone 28 into the entrance of the discharging
conduit 36 and pass into the reservoir 42. By techniques ~nown
as sheath flow the fluid with scatterers may be caused to pass
in a straight path across the sensing zone 28 and be discharged.
The beam of light 22 intersects the flow of scatterersat
the sensing zone 28. That light not scattered plus that light
scattered into small polar angles in the forward directions pass-
es on through the window 32 into the light dump 44. Instead of
such light dump there could be another dekecting system as dis-
closed in the applications and patent referred to on page 3.
The sensing zone is chosen to be centered in the region
of the first focal point of ellipsoid of which the reflector 29
is a part. The interior surface 46 of the reflector 29 ispolish-
ed or mirrored and all light or radiant energy which originates
at its focal point 28 will be reflected from the surface 46 to
the second focal point 48 of the ellipsoid of which the reflec-
tor 29 is a part. A typical ray of light is designated 50 and
its path may be traced from the first focal point 28 upward and
to the left to reflect from the surface 46 and then directly to
the second focal point 48. ~ ;
All of the reflected rays of the reradiated energy from
the reflector 29 are focussed at the focal point 48 and will
thereafter (to the left of the point 48) diverge. An aperature 54,
through which the radiant energy beams may pass and whichelimin-
ates most stray light, is located in opaque barrier 52 and atthe
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focal point 48. The plane of the opaque barrier 52 i8 de.signat-
ed 56, this being a plane which is normal to the axi~ 20 and the
beam 22. It will be noted that the radius 58 of the spherical
transparent window 24 which may be of glass is centered at the
focal point 48 to eliminate any refractive bending in the beam
50 or any others which emerge from the reflector 29.
At a distance d from the plane 56 there is provided the
planar configuration of detectors 60 which has previously been
mentioned. The planar face 62 of the configuration 60 has the
photosensitive elements and thls face 62 is parallel with the
plane 56. Accordingly its several rings and wedges if it has
such wedges, will respond to the portions of the radiant energy
which fa.ll upon them, respectively. As will be noted, the angles
which can be captured by the reflector surface 46 can be as large
as 140 for example. By itself, as previously mentioned, a
device such as 60 has only been capable of collecting scattering
ang].es to a maximum of 25 or so. Thus, the detector 60 has had
its utility increased. By virtue of the invention herein it is
made capable of responding to many more scattering angles and
providing much more lnformation than if it received the scatter-
ing light directly from the sensing zone 2~ with no ellipsoidal
reflector in place.
By moving the configuration 69 right and left through
the medium of a motor 65 or the like as indicated by the double
arrow 64 the optimum position can be obtained for determining
the information desired. This, of course will depend upon the
type of scatterers, what information is desired of them, etc.
Actually, by moving the configuration of detectors 60 over a
range of the dimension d information can be obtained ~rom a
variety of locations, keeping in mind, of course, that the
largest scattering angle alling on the detector will decrease
as the distance d is increased.
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Once a given course of investigation ls to be followed
and the optimum dopt has been ascertained and the corresponding
scattering or reradiating angles have been calculated, instead
of leaving the planar configuration of detectors 60 in place,
fresnel lens elements 66 and 69 (Figure 2) made up of portions
of prismatlc wedges or segments or rings oriented in different
directions relative to the axis ~0 and/or tilted relative to
said axis 20 may be substituted for the device 60. The fresnel
prismatic lens deviates the converging rays of radiant energy
to different spaced apart photocells such as shown at 68, 70, 72
and 74. The latter are small, sensitive, economical elements
easily obtained commercially and easily replaced. They can be
separated by distances to prevent interference between them,
often refarred to as "cross-talk". The rays of radiant energy
projected from the reflector surface 46 (Figure 1) are focussed
again at the second focal point 48 of the aperture 54 (Figure 2)
and the typical ray 50 is directed at the fresnel prismatic lens
66. Just in front of this lens 66 a focussing lens 69 has been
located for the purpose of converging the rays ~rom the reflect-
or 29 before they impinge on the prismatic elements of the lens
66. The focussing lens 69 can be a conventional ground or mold-
ed lens or could be a fresnel focussing lens as illustrated.
The concept of utilizing the type of prismatic lens 66
which has been described in connection with Figure 2 and the
details and benefits thereof are disclosed ln the cases referred
to on page 3.
Thus, an instrument can be built using the planar con-
figuration 60 in which the detector is moved by some mechanical
means 65 or even manually to provide flexibility; an instrument - ;
can be built in which the configuration 60 has been fixed in
place after adjustment in the factory to a particular distance d
for a specific purpose; an instrument can be huilt of the latter
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type in which a very economical fresnel lens which has thee~uiva-
lent function of the optimum arrangement has been substituted for
the configuration 60 after the best location dopt has been deter-
mined.
The exact mechanical construction of the components of
the system lO may be left to those skilled in the art who would
understand that the structure for assembling the flow cell 26 and
its parts must provide for filling, bubble relief, etc.
It should be appreciated that the increased utility o
the integral planar configuration o~ detectors 60 over that for
which it is at the present used comes about by virtue of thefact
that in the invention, each ring of the device 60 can correspond
to a large range of scattering angles, certainly larger than the
range of angles that is represented by the present conventional
way of using the said device 60.
Back scattering angles can be measured in the apparatus
10 simply by disposing the light dump 44 below the mirror 18 at
the location occupied by the optics 16 and light source 12 and by
disposing the optics 16 and light source 12 where the light dump
44 is located in Figure 1. In this way, the beam 22 willhave its
arrows reversed, light coming in by way of the window 32 and
passing from right to left as viewed in Figure l. In all other
respects the system lO will not be aItered. - ;
The system admits of variations and uses in addition to
the one described above without departing from the basic concept
of the invention. For example, the location of the light source ~ -
12, optics 16 and mirror 18 need not be where shown but could be
at a location bet:ween the plane 56 and the detector 62. This
location is indicated at 76 in Figure 1, this being the line
along which the beam 14 would be projected toward the previous
position of the mirror 18. The mirror 18 need not be fully
reflecting but could be semi-transparent. Fluorescent light
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reradiation can be measured with the apparatus 10 if slightly
modified as explained hereinafter.
In Figure 3 there is illustrated a system 100 in which
the object of the apparatus is to investigate scattering angles
closer to the forward direction than can be obtained through the
use of the ellipsoidal reflector 29 of Figure 1. Th~ flow cell
126 in this instance is comprised of a hyperboloidal reflector
129 whose axis is designated 120 and having a first focal point
128 and its second or virtual focal point at 128'. As in the
case of the cell 26, the interlor o~ the cell 126 is provided
with a fluid 138 which may be held in place by a glass or other
material transparent spherical closure 124 who~e radius of
curvature 158 is centered on the virtual focal point 128'. Par-
ticle source 140 provides the scatterers in a liquid which flows
into the interior of the cell 126 by way of the entrance conduit
134 through the sensing zone-focal point 128 and out by way of
the discharge conduit 136 to the reservoir 142. :
Light from the source 112 is directed as a beam 114 to ~:
the beam shaping optics 116 and thence as the beam 122 to the
folding mirror 118 on the axis 120 through the front of the - ~
closure 124 to the sensing zone-focal point 128 and out the win- . -
dow 132 to the beam dump 144. Typical light rays or fluxes
resulting from the scattering are shown at 150 and 151 and it
will be noted that these diverge, rather than coverge, giving
information on the type of polar angles mentioned above, thatis, .~;~
smaller forward angles.
These beams, as the others which are not shown, are r~
captured directly in Figure 3 by the configuration of detectors ` :.
160 which can be moved in the direction of the arrow 164 by a
motor such as 165 or by manual means to find the best location
along the axis 120 relative to the cell 126~ When the optimum
distance has been located, collecting optics, a fresnel lens
. .
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and individual photocalls may be substituted for the device 160.
It will be noted that the direction along which the beams of
scattered light 150 and 151 extend are extensions of lines 150'
and 151' respectlvely from the virtual focal point 128'. In
this case no stray-light suppressing aperture 54 is used.
In the system 200 of Figure 4, the light flux or rays
emerging from the hyperboloidal reflector 229 are not used in
their diverging form but are focussed 50 that they may be passed
through an aperture for suppressing the stray light before being
measured. Thus, the light rays 250 and 251 which are typical
of scattered light reradiated from the sensing zone-focal point
228 are captured by the lens 280, focussed on the aperture 254
provided by the opaque barrier or iris 252 at the ~ocal point
248 and then are brought to the device 260 which is the same
integral geometric configuration of photovoltaic detectors which
was described above. One good choice of the distance of the
lens 280 from the focal point 228~ on the axis 220 is twice the
focal length of the lens. Then the aperture 254 will be located
at the focal point 248, which is twice the focal length of the
lens 280 from the lens.
Again, as in this structure, it is feasible to have the
device 260 movable, to adjust it for optimum dlstance dopt from
the aperture 254 and to substitute collection optics and a fres-
nel lens or a composite mirror in its place.
Instead of the lens 66, in which the light or radiant
energy passes through ~he deviating means, it is feasible to use ;
a composite mirror of multiple re~lecting surfaceswhichreceives
the beams of light and reflects them to a plurality of spaced -
individual locations so that the separate photocells may be
located thereat.
In Figure 6 there is illustrated the application of the
invention to a system 300 in which fluorescent reradiation is
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measured in addition to reradiation ~y scattering. In this case
there is a flow cell 326 which is basically built out of an
ellipsoidal reflector 3~6 but of course this could be any con-
figuration of reflector of the types explained herein, The
reflector 346 llas a window 332 through which the light beam 322
from the light source 312and optics 316 enters. The light
source 312 could be a sultable laser and the beam 314 therefrom
is applied to beam-shaping optics 316.
The beam 322 passes through the first focal point 328 of
the reflector 346 where it encounters particles or cells which
are entering the reflector along the path 334 from the particle
source 340. At the first focal point 328 light will be reradiat-
ed in accordance with the character of the particle and this
light will be reflected from the reflector 346 towards thesecond
focal point 34~ which lies in the plane 356 normal to theoptical
axis 320. A typical ray is lndicated at 350 and this ray is
shown striking the interior of the reflector 346 at the top
thereof in the view and being deflected tGward a mirror 318
which intercepts the ray~
The mirror 318 which is here chosen is a dichroic ele-
ment in that light is transmitted or reflected according to the
wavelength of that light. The ray 350 includes componentswhich
are fluorescent and other components of visible light. The ;~
dichroic mirror 318 is constructed to have substantially no
effect upon light of the laser wavelength and hence those com-
ponents of the beam 350 pass through the mirror 318 without
deflection albeit somewhat diminished. These move toward the
second focal point 348 as the ray 350' and impinge on the sensi-
tive surface 362 of the integral planar configuration of photo-
voltaic detectors 360 which is the equivalent of the prior art
device 60 previously described.
The direct rays of light at 322 pass into a light dump
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344 after being reflected by small mirror 372.
The light rays which focus at the second focal point 348
are purified by the opaque barrier or iris 352 which lies in the
plane 356 that is normal to the axis 320 and passes through the
second focal point 348. Light passes the barrier 352 by way of
a center aperture 35A in the barrier. Ayain the distance d be-
tween the plane 356 and the face 362 can be adjustedbysuitable
means to be optimum and when this is achieved a collector and
fresnel prism element with separate photodetectors substituted
therefor as explained.
~ hose components of the ray 350 which have wavelengths
to which the action of the dichroic mirror 318 will become effec-
tive are reflected by the mirror as the ray 350" to focus at the
point 357 along the axis 320~ which axis is shown perpendicular
to the axis 320. At the focal point 357 there is a purifying
opaque barrier 353 with an aperture 355 so that the clear ray
extends below the barrier. It is there intercepted byasuitable
transducer such as a photomultiplier tube or element 361 so that
measurements can be made and compared against known information
to identify or characterize the particle. The combined informa-
tion from the device 360 ~or any system of fresnel lens and
photodetectors substituted therefor~ and the transducer 361 can
be channeled to a computer 370 where previously knowninformation
is stored and against which the new information can be compared.
The rays such as 350' which pass through the dichroic
mirror 318 will have less radiant energy than the original rays ;
350. Accordingly it is desired for maximum information to have
as sensitive a detector at the position of the device 360 as
feasible. The cases referred to on page 3 teaches how this can
be done by substituting the prismatic fresnel lens system and
its detectors for the low sensitivity device 360.
Figure 6 has been included to illustrate apparatus based
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on the principle of collecting scattered and fluorescent light
separately through the use of a dichroic mirror. To facilitate
explanation, the version in which the scattered light is trans~
mitted and the fluorescent light is reflected is shown. As a
practical matter, however, due to manufacturing problems of the
dichroic mirror, the inverse arrangement, in which fluorescent
light is transmitted while the scattered light is reflected, is
actually the preferred embodiment. This latter is illustrated
in Figure 7 and is identical to Figure 6 except for the t~ans-
position of the respective elements.
In the course of the explanations given herein and in the
claims, reference is made to focal points and to specific geo-
metric configurations of reflectors such as ellipsoids, parabo-
loids, etc. The manufacture of instruments which embody and
use all of the benefits and advantages of the invention would
call for providlng components which are formed with precision;
however, fox economy the configurations are certainly capable
of being formed as approximations. Thuswhen speci~ic configura-
tions are mentioned it should be taken to mean that in addition ; `
to the precise geometric configurations substantial and/or
approximate approaches to such configurations are intended to be
included herein. Focal points may not always be precisely a
point but may be a sort of zone but will still give the desired
results. Reference to two focal points in the discussion and
claims is not intended to exclude paraboloidal reflectors inas-
much as the second focal point thereof is considered to be at
infinity.
Such a paraboloidal reflector is illustrated in connec-
tion with the apparatus 400 of Figure 5. This apparatus in-
cludes a flow cell 426 which has a parabolic reflector 446 whose
front opening is closed off by a suitable closure 424 which is
planar because the reradiated radiant energy which emerges from
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the reflector 446 emanates along lines which are parallel tothe
optical axis 420. The light source 412 in this instance is
behind the reflector 446 as shown and it projects a beam of
incident radiant energy along the path 422 toward the folding
mirror and onto the light dump 444, this beam of incident ra-
diant energy passing through the sensing zone which coincides
with the focal point 428 of the parabloid 446. Particles ori~
ginating at the source 440 move along the path to the focal
point 428 and at that point which comprises the sensing zone
they intercept the beam 422 and reradiate some of the radiank
energy of said beam. The rays move outward of the focal point
428 as for example along the line 450, engage the inner surface
of the reflector 446 and thereafter are projected parallelto the
axis 420, as stated above, toward what may be considered the
second focal point of the reflector 446 at infinity.
In order to utilize the benefits of the invention, the
groups of rays emerging from the front of the reflector 446 are
captured by a focussing lens 469 which may be of conventional : ~.
construction or fresnel construction and diverted along the ,
lines such as 450~ to another focal point at 448. This focal : ~
point is located relative to the reflector 446 in accordance .
with the optical specifications of the lens 469 (or a lens sys-
tem used in lieu thereof) rather than as a result of the geo- `
metric law which defines the paraboloid~
At this focal point 448 there is an opaque barrier or ~: ;
iris 452 having a central aperture 454, these both lying on a
plane 456 which is normal to the axis 420. At a distance d from
the plane 456 there is located a measuring device which com-
prises an integral planar configuration of photovoltaic detec-
tors 460 whose sensitive surface is in the front plane thereof
at 462. The ray groups 450' emerging from the reflector 446
and focussed at the focal point 448 diverge afterbeingpurified:
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~L~L3597~L
of stray light at the focal point by the barrier 452 andimpinge
against the sensitive surface 462 of the measuriny device 460.
It will be recognlzed that this measuring device is the same as
those which have been referred to herein and identified as 60,
160, 260 and 360.
Again in this case, the measuring device 460 may be moved
back and forth parallel to the ax:ls 420 but without changingitS
disposition relative to the axis to vary the distance d. Once
the optimum distance and hence the optlmum polar angles have
been determined, the known measuring device 460 may be removed
and the measuring devices which are disclosed in the copending
applications for increased sensitivity and better data may be
substituted for collecting light at these angles. ,~
Many other vaxiatlons are capable of being made without
departing ~rom the spirit or scope of the invention as defined
in the appended claims.
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