An aircraft velocimeter comprises means for projecting three pairs of independently generated ribbon-shaped laser beams to a measurement volume located at a predetermined distance from the aircraft with a predetermined separation being maintained between the two ribbon-shaped laser beams of each pair at the measurement volume. The times of flight of atmospheric aerosol particles between the two ribbon-shaped laser beams of corresponding pairs of laser beams are measured as the aerosol particles pass through the measurement volume. From the predetermined distance between the two ribbon-shaped laser beams of each pair and the measured time of flight of an aerosol particle across that predetermined distance, a component of velocity of the aerosol particle relative to the aircraft is calculated for each pair of ribbon-shaped laser beams. From the three components of velocity corresponding to the three pairs of ribbon-shaped laser beams projected to the measurement volume, a vector measurement of the velocity of the aircraft relative to atmospheric particles in the surrounding atmosphere is obtained using an on-board computer programmed with a conventional algorithm.

TECHNICAL FIELD 
This invention relates generally to velocimeters, and more particularly to 
a velocimeter that continuously measures aircraft velocity relative to 
atmospheric aerosol particles in the aircraft's vicinity by making 
time-of-flight measurements of three-dimensional motions of individual 
aerosol particles relative to the aircraft. 
BACKGROUND OF THE INVENTION 
Investigations of the earth's atmosphere have shown that solid aerosol 
particles are distributed throughout the atmosphere in varying 
concentrations. In general, the concentration of any particular species of 
aerosol particles in the earth's atmosphere at any particular geographical 
location and altitude varies primarily with seasonal changes. A discussion 
of the origin and distribution of aerosol particles in the earth's 
atmosphere is provided in an article by J. M. Prospero et al. entitled 
"The Atmospheric Aerosol System: An Overview" published in Reviews of 
Geophysics and Space Physics, Vol. 21, No. 7, pages 1608-1629, (August 
1983). Other publications discussing the phenomenology of atmospheric 
aerosol particles include: 
(1) "Stratospheric Aerosol Measurements I: Time Variations at Northern 
Latitudes" by D. J. Hofmann et al., Journal of the Atmospheric Sciences. 
Vol. 32, pages 1446-1456, (July 1975); 
(2) "Stratospheric Aerosol Measurements II: The Worldwide Distribution" by 
J. M. Rosen et al., Journal of the Atmospheric Sciences, Vol. 32, pages 
1457-1462, (July 1975); 
(3) "Effect of the Eruption of El Chichon on Stratospheric Aerosol Size and 
Composition" by V. R. Oberbeck et al., Geophysical Research Letters, Vol. 
10, No. 11, pages 1021-1024, (November 1983); 
(4) "Satellite and Correlative Measurements of the Stratospheric Aerosol 
III: Comparison of Measurements by SAM II, SAGE, Dustsondes, Filters, 
Impactors and Lidar" by P. B. Russell et al., Journal of Atmospheric 
Sciences, Vol. 41, No. 11, pages 1791-1800, (June 1984). 
Techniques were developed in the prior art that make use of atmospheric 
aerosol particles for obtaining various kinds of air data measurements. 
For example, B. M. Watrasiewicz and M. J. Rudd showed in Laser Doppler 
Measurements. Butterworth & Co. (Publishers) Ltd., Section 3.8.4 at pages 
67-68, (1976), that an anemometric velocity measurement can be obtained by 
measuring the intensity of light scattered in three different directions 
by a moving particle passing through a focal volume. Photodetectors 
fixedly positioned at three different locations with respect to the focal 
volume collect light scattered by the moving particle, and generate three 
corresponding electrical signals proportional to Doppler shifts between 
the light scattered from the laser beam and the light scattered from the 
fixed scatterer in three different directions. From these Doppler shifts 
in three different directions, three corresponding components of the 
velocity of the moving particle relative to the photodetectors can be 
calculated. From these three velocity components, a vector velocity 
measurement for the moving particle can be obtained. 
In U.S. Pat. No. 4,506,979, the principle of laser Doppler velocimetry was 
applied by P. L. Rogers to the measurement of the velocity of an aircraft 
relative to the motion of aerosol particles in the surrounding atmosphere. 
According to the technique described by Rogers, three pairs of laser beams 
derived from a common source are focussed at a focal volume located at a 
predetermined distance from the aircraft. The two laser beams comprising 
each pair are coherent, and therefore interfere with each other to produce 
a three-dimensional fringe plane pattern. Three sets of three-dimensional 
fringe plane patterns are thereby obtained, corresponding to the three 
pairs of interfering laser beams focussed at the focal volume. A 
non-orthogonal three-dimensional coordinate system is defined by three 
unit vectors oriented along the respective directions of propagation of 
the corresponding three pairs of interfering laser beams. Each set of 
fringe plane patterns moves in a direction perpendicular to its 
corresponding unit vector. The motions of the three different sets of 
fringe plane patterns produced as an aerosol particle passes through the 
focal volume are characterized by different fringe spacings. 
According to the technique described by Rogers, each one of the three sets 
of fringe plane patterns produced as an aerosol particle passes through 
the focal volume modulates the intensity of light scattered in the 
direction of the corresponding one of the three unit vectors. The 
modulation of the intensity of the scattered light in each direction has a 
characteristic modulation frequency, whereby a component of the motion of 
the aerosol particle relative to the aircraft in the direction of any one 
of the three unit vectors causes an apparent shift in the modulation 
frequency of the light scattered in that direction. From the shifts in 
modulation frequency for the three directions defined by the three unit 
vectors, the magnitude and direction (i.e., the velocity) of the motion of 
the aerosol particle can be determined. 
Fringe pattern techniques developed in the prior art for modulating the 
intensity of light scattered from atmospheric aerosol particles were not 
readily adaptable to aircraft velocimetry, because coherent light sources 
of sufficient power were generally unavailable in the small-scale sizes 
and compact configurations required for practicable aircraft 
instrumentation. Furthermore, laser beam sources that were available in 
the prior art were generally unable to maintain coherent propagation under 
the extreme operating conditions and harsh environments routinely 
experienced by high-performance aircraft. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a velocimetric 
instrument for measuring air velocity of an aircraft in substantially real 
time from three-dimensional time-of-flight measurements of the velocity 
relative to the aircraft of aerosol particles in the surrounding 
atmosphere. 
An aircraft velocimeter according to the present invention comprises means 
for projecting three pairs of independently generated ribbon-shaped laser 
beams to a so-called "focal volume" located at a predetermined distance 
from the aircraft so that there is a predetermined separation between the 
two ribbon-shaped laser beams of each pair at the focal volume, and means 
for measuring the times of flight of atmospheric aerosol particles between 
the two ribbon-shaped laser beams of corresponding pairs as the aerosol 
particles pass through the focal volume. 
At the focal volume, the two ribbon-shaped laser beams of each pair are 
substantially parallel to each other, but are nonparallel and 
non-orthogonal with respect to the ribbon-shaped laser beams of the other 
two pairs. The time of flight of an atmospheric aerosol particle between 
successive penetrations by the aerosol particle of the two ribbon-shaped 
laser beams of each corresponding pair of laser beams is measured 
electronically. From the known separation between the two laser beams of 
each pair and the measured time of flight of an aerosol particle passing 
through the focal volume in a given direction between the two laser beams 
of the pair, the component of velocity of the aerosol particle in a 
direction normal to the pair of laser beams at the focal volume relative 
to the aircraft from which the laser beams have been projected can be 
determined. 
Simultaneously, from the known separations and the measured times of flight 
of other aerosol particles in the same given direction between the two 
parallel ribbon-shaped laser beams of each of the other two pairs of laser 
beams at the focal volume, the corresponding components of velocity of the 
aerosol particles in the directions normal to each of the other two pairs 
of laser beams at the focal volume relative to the aircraft from which 
each of the other two pairs of laser beams have also been projected can 
likewise be determined. Assuming that the aerosol particles passing 
through the focal volume at a given time are all travelling with 
substantially the same velocity relative to each of the pairs of laser 
beams at the focal volume (and therefore with substantially the same 
velocity relative to the aircraft from which the laser beams have been 
projected), the three different velocity components determined for the 
corresponding three different aerosol particles passing through the focal 
volume can be regarded as being equivalent to components in three 
different directions of the velocity of a single aerosol particle passing 
through the focal volume. 
Each pair of ribbon-shaped laser beams at the focal volume is 
non-orthogonal with respect to the ribbon-shaped laser beams of the other 
two pairs. From measurements made of the velocity components in three 
different non-orthogonal directions (i.e., in directions normal to the 
corresponding three different pairs of ribbon-shaped laser beams at the 
focal volume) of atmospheric aerosol particles all travelling in the same 
direction relative to the aircraft, a vector measurement of the velocity 
of atmospheric aerosol particles relative to the aircraft (or 
equivalently, a vector measurement of the velocity of the aircraft 
relative to the surrounding atmosphere) can be calculated in substantially 
real time by means of a computer on board the aircraft using a 
conventional mathematical algorithm. 
Occasionally, depending upon the direction of travel of the aircraft 
relative to the direction of travel of aerosol particles in the atmosphere 
(i.e., relative to wind direction), a single aerosol particle passes 
through all three pairs of ribbon-shaped laser beams at the focal volume. 
The penetration by a ingle aerosol particle of all three pairs of 
ribbon-shaped laser beams at the focal volume is called a "coincidence" 
event, which can be detected electronically. The three different 
components of velocity (i.e., the velocity components in the directions 
normal to the three different pairs of ribbon-shaped laser beams) measured 
for each coincidence event can be segregated and stored as especially 
precise measurement data. A velocity vector measurement obtained from the 
three velocity component measurements resulting from a coincidence event 
can be used as a reference measurement for assessing the statistical 
accuracy of the assumption that all atmospheric aerosol particles passing 
through the focal volume at a given time under a given set of atmospheric 
conditions are travelling at substantially the same velocity relative to 
the aircraft. Data based upon coincidence detection (i.e., the detection 
of coincidence events) is especially valuable when the focal volume is 
formed at a turbulence edge or a cloud edge, or when other nonuniformities 
are present in the focal volume, that would cause secondary motions of the 
aerosol particles and thereby introduce statistical errors. 
Measurements of the velocity of an aircraft relative to aerosol particles 
in the surrounding atmosphere can be made continuously by a velocimeter 
according to the present invention as the aircraft maneuvers through the 
atmosphere. These continuous velocity measurements can be used to generate 
electrical signals to operate mechanisms for continuously controlling 
flight characteristics of the aircraft. Preferably, a plurality of 
velocimeters according to the present invention are installed on the 
aircraft to provide redundant velocity measurements for purposes of 
reliability and safety. 
An aircraft velocimeter according to the prototype embodiment of the 
present invention disclosed herein uses six laser beam sources to produce 
three different pairs of parallel ribbon-shaped laser beams for measuring 
three corresponding components of the velocity of the aircraft relative to 
the surrounding atmosphere. It is not required that the two parallel 
ribbon-shaped laser beams of each pair be mutually coherent. In fact, it 
is advantageous for the two laser beams of each pair to be mutually 
incoherent in order to facilitate discrimination of light scattered from 
the respective beams by aerosol particles in the atmosphere. 
An aircraft velocimeter according to the present invention is preferably 
installed as a modular unit on the aircraft, and provides a velocity 
measurement directly (and in substantially real time) without requiring 
electronic components such as acousto-optical modulators to convert scalar 
measurements to vector measurements.

BEST MODE OF CARRYING OUT THE INVENTION 
In piloting a high-performance aircraft as depicted in FIG. 1, it is 
desirable to have an instrument capable of measuring air velocity (i.e., 
the velocity of the aircraft relative to the surrounding air mass) with 
high precision and in substantially real time. It is further desirable to 
have an instrument capable of generating electrical signals corresponding 
to instantaneous air velocity measurements in order to operate mechanisms 
for controlling flight characteristics of the aircraft. In accordance with 
the present invention, a prototype aircraft velocimeter has been developed 
that is capable of determining air velocity of an aircraft in 
substantially real time from time-of-flight measurements of components of 
the velocity (relative to the aircraft) of aerosol particles in the 
surrounding air mass in three non-orthogonal dimensions. 
In principle, only one velocimeter according to the present invention is 
needed on board an aircraft to provide substantially continuous air 
velocity measurements as the aircraft undergoes the most intricate 
maneuvers. However, for reliability and safety, it is generally desirable 
to install a plurality of independently operating velocimeters on board 
the aircraft (as indicated in FIG. 1) in order to obtain the advantage of 
overlapping operational envelopes of different velocimeters. Two or more 
independently operating velocimeters according to the present invention on 
board an aircraft would provide redundant air velocity measurements. A 
relatively large number of independently operating velocimeters on board 
the aircraft would ensure that continuous air velocity measurements can be 
made during time intervals when one or more of the velocimeters might not 
be able to produce a measurement. 
As illustrated in FIG. 1, four velocimeters (each operating independently 
of the others) are installed near the nose of the aircraft in an 
arrangement that enables independent measurements of the velocity of the 
aircraft to be made relative to aerosol particles located in the air mass 
above, below and on either side of the aircraft. When the aircraft is in 
straight level flight, the air velocity measurements obtained 
independently of each other at any given time by the four velocimeters 
installed on board the aircraft as indicated in FIG. 1 are substantially 
identical to each other. However, as the aircraft maneuvers so as to 
change direction and/or speed relative to the aerosol particles in the 
surrounding atmosphere (or equivalently, so that the aerosol particles in 
the surrounding atmosphere change velocity relative to the aircraft), 
measurements of components of the velocity of the aircraft relative to the 
aerosol particles correspondingly change so as to produce changing 
velocity vector measurements. Thus, when the aircraft turns to the right, 
the air velocity measurements made by one of the velocimeters relative to 
aerosol particles on the left side of the aircraft correspondingly 
increase, and the air velocity measurements simultaneously made by another 
of the velocimeters relative to aerosol particles on the right side of the 
aircraft correspondingly decrease. When the aircraft turns in any other 
direction, air velocity measurements made by different velocimeters 
relative to aerosol particles on opposite sides of the aircraft 
correspondingly change by opposite amounts. 
As the aircraft shown in FIG. 1 turns in any particular direction, 
electrical signals are generated by appropriate avionic means in response 
to the maneuvering of the aircraft. At any given time, electrical signals 
generated in response to the maneuvering of the aircraft can be combined 
with electrical signals generated from velocity measurements made by 
velocimeters according to the present invention on board the aircraft, 
thereby providing a net air velocity measurement that takes into account 
apparent changes in velocity attributable to the maneuvering of the 
aircraft. It is within the capability of presently available computer 
hardware and programming techniques to measure air acceleration and other 
flight parameters (e.g., true airspeed, angle of attack and side slip 
angle) of an aircraft, as well as to calculate update rates for each of 
these parameters, from sequential air velocity measurements. Furthermore, 
it is within the capability of conventional aeronautical engineering 
techniques to utilize measurements of air velocity, acceleration and other 
flight parameters to generate electrical signals that operate mechanisms 
for controlling flight characteristics of the aircraft. 
The prototype velocimeter disclosed herein is housed in a structure that 
was designed to be installed as a modular unit on board a testbed 
aircraft. A portion of a side wall of the housing structure is covered by 
an optically transparent plate that forms part of the exterior surface of 
the aircraft. As contemplated by the design of the prototype, a 
velocimeter according to the present invention need not have any 
components protruding outward from the exterior surface of the aircraft. 
In principle, it is desirable that aerodynamic capabilities of the 
aircraft not be compromised by structural features of the velocimeter. 
As illustrated in FIG. 2, the prototype velocimeter of the present 
invention is contained within a housing structure 10, which is configured 
for installation as a modular unit on board the aircraft. The location at 
which the velocimeter is to be installed on the aircraft is just one 
factor among the many factors that must be considered when the aircraft is 
being designed. Preferably, at least for purposes of prototype evaluation, 
the velocimeter should be installed near the front of the aircraft so that 
laser beams projected from the velocimeter can be focussed at a 
measurement volume (called a "focal volume") located at a predetermined 
distance from the aircraft outside the aerodynamic slip stream and 
turbulence associated with passage of the aircraft through the atmosphere. 
The housing structure 10 is structurally rugged, and has rigidly fixed 
walls forming a closed container within which optical components of the 
velocimeter are precisely positioned in fixed geometrical relationship 
with respect to each other. Preferably, the housing structure 10 is 
hermetically sealed in order to prevent entry therein of atmospheric 
aerosol particles, dust particles originating on the aircraft, or 
particles of other origin, which could cause spurious scattered-light 
signals. 
For aircraft velocimetric applications in which extreme changes in 
atmospheric pressure on the walls of the housing structure 10 would be 
likely to buckle the walls and cause concomitant misalignment of optical 
components supported thereon, a make-up gas bladder could be installed on 
board the aircraft in communication with the interior of the housing 
structure 10. The pressure within the housing structure 10 can be 
equalized automatically with respect to the external atmospheric pressure 
by means of the make-up gas bladder and appropriate valving. Techniques 
for maintaining equal pressure on both sides of the walls of a 
hermetically sealed structure (such as the housing structure 10) are 
well-known. 
In FIG. 2, the illustration of the housing structure 10 has been simplified 
to show a side wall 11 that is apertured to receive only two separate 
mountings, viz., a mounting 12 and a mounting 13. As will be explained 
more fully hereinafter, the mounting 12 supports a pair of optical trains 
for shaping a corresponding pair of laser beams, which are projected from 
the velocimeter so as to assume a ribbon-shaped configuration at the focal 
volume. The mounting 13 is shown (in simplified illustration) in FIG. 2 as 
supporting a pair of photodetectors for generating a corresponding pair of 
electrical signals responsive to the scattering of light from a 
corresponding pair of ribbon-shaped laser beams at the focal volume, and a 
single photodetector for generating an electrical signal responsive to 
background light. However, in the actual prototype velocimeter, the side 
wall 11 of the housing structure 10 is apertured to receive four separate 
mountings, viz., three separate (but substantially identical) mountings 
represented by the single mounting 12 shown in FIG. 2, as well as the 
mounting 13. Furthermore, in the actual prototype velocimeter, the 
mounting 13 supports not one but three separate pairs of photodetectors 
for generating three corresponding pairs of electrical signals responsive 
to the scattering of light from three corresponding pairs of ribbon-shaped 
laser beams at the focal volume, as well as the single photodetector for 
generating an electrical signal responsive to background light. 
Explanation of the present invention has been facilitated herein by not 
cluttering the drawing with repetitive illustrations of mountings 
identical to the mounting 12 for the two other pairs of optical trains for 
shaping the corresponding two other pairs of ribbon-shaped laser beams 
required for operation of the prototype velocimeter. Explanation of the 
invention has been further facilitated by not cluttering the illustration 
of the mounting 13 with repetitive details of two other identical pairs of 
photodetectors and associated optical components for detecting light 
scattered from the corresponding two other pairs of ribbon-shaped laser 
beams at the focal volume. Depiction in the illustration of the mounting 
13 of all three pairs of photodetectors and associated optical components 
required for detecting light scattered from all three corresponding pairs 
of ribbon-shaped laser beams at the focal volume in addition to the 
photodetector and associated optical components required for detecting 
background light (i.e., a total of seven photodetectors and associated 
optical components) would unnecessarily clutter the drawing with 
repetitious illustrations that would tend to obfuscate, rather than to 
clarify, the explanation of the invention. 
Since all three pairs of ribbon-shaped laser beams projected from the 
housing structure 10 to the focal volume are produced in the same way by 
three corresponding pairs of substantially identical optical trains 
supported on three substantially identical mountings represented by the 
mounting 12, it is only necessary to describe the single pair of optical 
trains supported on the mounting 12 in order to explain the functioning of 
all three pairs of optical trains. Similarly, since all three pairs of 
photodetectors and associated optical components supported on the mounting 
13 for detecting light scattered from the corresponding three pairs of 
ribbon-shaped laser beams at the focal volume are also substantially 
identical to each other and operate in the same manner and for the same 
purpose, it is only necessary to describe a single pair of photodetectors 
and associated optical components supported on the mounting 13 for 
detecting light scattered from a single pair of the three pairs of 
ribbon-shaped laser beams at the focal volume. Illustration in the drawing 
of three separate pairs of optical trains for projecting three 
corresponding pairs of ribbon-shaped laser beams to the focal volume, and 
illustration in the drawing of three pairs of photodetectors and 
associated optical components for detecting light scattered from three 
corresponding ribbon-shaped laser beams at the focal volume, would 
unnecessarily complicate the already complex drawing without providing any 
compensating advantage in terms of what is needed to describe the 
principle of operation of the invention. 
The photodetector and associated optical components for detecting 
background light (which in the phototype embodiment of the invention is 
mounted on the same mounting 13 as the three pairs of photodetectors and 
associated optical components for detecting light scattered from the three 
pairs of ribbon-shaped laser beams at the focal volume) measures the 
intensity of background light. It is necessary that the light that is 
scattered by an atmospheric aerosol particle from the ribbon-shaped laser 
beams at the focal volume be distinguishable from background light. 
In the simplified illustration of the prototype velocimeter shown in FIG. 
2, the mounting 12 for supporting one pair of optical trains for shaping 
one pair of laser beams to be projected to the focal volume is received in 
a corresponding aperture on the side wall 11 of the housing structure 10. 
Separate mountings (not shown in FIG. 2) for supporting the two other 
pairs of optical trains for shaping the corresponding two other pairs of 
laser beams projected to the focal volume are likewise received in 
corresponding other apertures on the side wall 11 of the housing structure 
10. The three mountings (only one of which is shown in the drawing) 
supporting the three pairs of optical trains for shaping the corresponding 
three pairs of laser beams are positioned on the side wall 11 with respect 
to reflective and refractive optical components mounted within the housing 
structure 10 (as described hereinafter) so that the three pairs of laser 
beams are focussed with ribbon-shaped configurations at the focal volume, 
and so that a predetermined spacing is provided between the two 
ribbon-shaped laser beams of each pair. 
The mounting 13, which supports the three pairs of photodetectors and 
associated optical components (only one pair of which is shown in the 
drawing) for detecting light scattered from the corresponding three pairs 
of ribbon-shaped laser beams at the focal volume, and which also supports 
the photodetector and associated optical components for detecting 
background light, is likewise received in a corresponding aperture on the 
side wall 11. In the prototype embodiment of the invention, the three 
pairs of photodetectors and associated optical components for detecting 
scattered light are disposed symmetrically on the mounting 13 around the 
photodetector and associated optical components for detecting background 
light, which is centrally disposed on the mounting 13. 
Each pair of ribbon-shaped laser beams projected from the housing structure 
10 to tee focal volume provides a measurement for a corresponding 
component in one dimension of the velocity of an atmospheric aerosol 
particle passing through the focal volume relative to the aircraft. To 
obtain a three-dimensional vector measurement for the velocity of the 
aerosol particle relative to the aircraft, three nonorthogonal components 
of the particle's velocity at the focal volume are measured 
simultaneously. The three non-orthogonal component measurements are 
converted to the velocity vector measurement by means of an on-board 
computer that is programmed using a conventional algorithm for the 
purpose. 
Referring to FIG. 2, the two ribbon-shaped laser beams produced by each 
corresponding pair of optical trains supported on the mounting 12 are 
directed along a folded optical path within the housing structure 10 (as 
described hereinafter) so as to emerge through an exit window 14 on an 
anterior wall 15 of the housing structure 10. Each pair of ribbon-shaped 
laser beams emerging from the exit window 14 is focussed so that the two 
beams have a predetermined separation from each other at the focal volume. 
In effect, the two ribbon-shaped laser beams of a particular pair form 
substantially parallel "walls of light" at the focal volume. As an aerosol 
particle passing through the focal volume penetrates first one and then 
the other of the two ribbon-shaped laser beams of a particular pair, the 
aerosol particle scatters light first from one and then from the other of 
the two beams of the pair. The distance between the two parallel 
ribbon-shaped laser beams of a particular pair at the focal volume is 
predetermined, and the time that elapses between successive penetrations 
by the aerosol particle of the two ribbon-shaped laser beams of the pair 
is measured. From these values for distance and time, a component of the 
velocity of the aerosol particle in a direction perpendicular to the pair 
of parallel ribbon-shaped laser beams at the focal volume can be 
calculated. 
Since there are three pairs of parallel ribbon-shaped laser beams at the 
focal volume, and since the ribbon-shaped laser beams of any one pair are 
non-parallel to the ribbon-shaped laser beams of each of the other two 
pairs at the focal volume, three components of the velocity of each 
aerosol particle passing through each pair of ribbon-shaped laser beams at 
the focal volume can be calculated with respect to the directions 
perpendicular to the three pairs of ribbon-shaped laser beams. Assuming 
that all of the aerosol particles in the air mass surrounding the aircraft 
have substantially the same velocity (viz., the wind velocity) relative to 
the aircraft at any given time, the three velocity components measured 
simultaneously for three different aerosol particles can be treated as 
being substantially equivalent to three components of the velocity of a 
single aerosol particle passing through all three pairs of ribbon-shaped 
laser beams at the focal volume. Occasionally, depending upon the 
direction of flight of the aircraft relative to direction of motion of the 
surrounding air mass (i.e., relative to the wind direction), a single 
aerosol particle actually does pass coincidentally through all three pairs 
of ribbon-shaped laser beams in passing through the focal volume. The 
occurrence of the passage of a single aerosol particle through all three 
pairs of ribbon-shaped laser beams at the focal volume is detected by 
conventional coincidence detection circuitry. The three velocity 
components measured when a single aerosol particle passes through all 
three pairs of ribbon-shaped laser beams at the focal volume are stored as 
especially accurate measurements, which can be used to evaluate the 
accuracy of a velocity vector measurement made using component 
measurements obtained from the passage of single aerosol particles through 
corresponding single pairs of ribbon-shaped laser beams at the focal 
volume. 
Determination of the appropriate spacing between the two ribbon-shaped 
laser beams of each pair at the focal volume involves a number of 
engineering trade-offs, including: (1) time-of-flight measurement 
resolution, (2) velocity measurement resolution, and (3) the effective 
rate for updating velocity measurements. If the separation between the two 
ribbon-shaped laser beams of each pair at the focal volume were too great, 
the maximum angle of approach that the aerosol particle could make 
relative to an axis normal to the two beams and still penetrate both beams 
when passing through the focal volume (i.e., the so-called "acceptance 
angle" of the focal volume) would be too small. It is necessary that the 
aerosol particle penetrate both ribbon-shaped laser beams in order for a 
time-of-flight measurement to be obtainable. Any aerosol particle having 
an angle of approach greater than the acceptance angle could not penetrate 
both beams, and hence could not provide a time-of-flight measurement. The 
angular distribution of aerosol particles passing through the focal volume 
that are capable of providing velocity measurements decreases with 
increasing separation between the two parallel ribbon-shaped laser beams 
of each pair at the focal volume. If, on the other hand, the separation 
between the two parallel ribbon-shaped laser beams of each pair at the 
focal volume were too small, the time of flight of an aerosol particle 
from one beam to the other when passing through the focal volume would be 
too short for the corresponding scattered-light photodetectors supported 
on the mounting 13 to be capable of distinguishing electronically between 
the light scattered from one beam and the light scattered from the other 
beam of each pair. Thus, there are electronic digitization considerations 
as well as optical considerations involved in determining an appropriate 
spacing between the two parallel ribbon-shaped laser beams of each pair at 
the focal volume. 
For purposes of prototype evaluation, it has been found that a separation 
of 10 millimeters between the two ribbon-shaped laser beams of each pair 
at the focal volume provides a sufficient time interval between successive 
illuminations of an atmospheric aerosol particle passing through the focal 
volume, so that a commercially available electronic device can be used to 
obtain an unambiguous measurement of the time interval between the 
successive illuminations. A separation of 10 millimeters between the two 
ribbon-shaped laser beams of each pair at the focal volume has also been 
found to provide an adequate effective rate for updating the velocity 
measurements of atmospheric aerosol particles having trajectories within 
the acceptance angle of the focal volume. 
The time that elapses (i.e., the time of flight) between successive 
penetrations of the two parallel ribbon-shaped laser beams of a pair of 
laser beams by an aerosol particle passing through the focal volume is 
measured by detecting the light scattered by the aerosol particle from 
each of the two laser beams of each pair. Three independent time 
measurements (i.e., one for each pair of ribbon-shaped laser beams at the 
focal volume) are obtained, from which a component of the velocity of each 
of three corresponding aerosol particles (i.e., a velocity component in 
each of the three different directions perpendicular to the three 
corresponding pairs of ribbon-shaped laser beams) is calculated. It is 
assumed that all atmospheric aerosol particles entrained in the air mass 
passing through the focal volume are homogeneously distributed throughout 
the focal volume, and have the same velocity as the air mass passing 
through the focal volume. Based upon this assumption, it is statistically 
acceptable to treat simultaneously obtained velocity components in three 
different directions for three different aerosol particles passing through 
the focal volume as being equivalent to three different components of the 
velocity of a single aerosol particle passing through the focal volume, 
inasmuch as all aerosol particles entering the focal volume simultaneously 
have substantially the same velocity. Light scattered from the 
ribbon-shaped laser beams at the focal volume by large non-entrained 
particles (e.g., rain droplets, ice, snow) present in the focal volume can 
be identified by a conventional electronic thresholding technique, and can 
be disregarded. 
In general, the two ribbon-shaped laser beams of each pair, which form 
"walls of light" that are parallel to each other at the focal volume, are 
tilted and/or canted with respect to the ribbon-shaped laser beams of each 
of the other two pairs of ribbon-shaped laser beams at the focal volume. 
The known tilts and cants of the three pairs of ribbon-shaped laser beams 
with respect to each other determine a three-dimensional (generally 
non-orthogonal) coordinate system in terms of which the components of the 
velocities of the aerosol particles passing through the focal volume are 
expressed. From the velocity components thereby obtained, a measurement of 
the velocity vector of an atmospheric aerosol particle relative to the 
aircraft (or concomitantly, a measurement of the velocity vector of the 
aircraft relative to the surrounding air mass) is mathematically 
determined by means of an on-board computer in substantially real time. 
A portion of the light scattered from each of the two ribbon-shaped laser 
beams of each pair of laser beams as aerosol particles pass through the 
focal volume is gathered through an entrance window 16 on the anterior 
wall 15 of the housing structure 10. Determination of an optimal distance 
from the aircraft for the focal volume to be located is based chiefly upon 
aerodynamic considerations, which are concerned principally with locating 
the focal volume in an undisturbed portion of the atmosphere outside the 
slip stream and concomitant turbulance produced as the aircraft passes 
through the atmosphere. The prototype velocimeter of the present invention 
has been designed for installation on a testbed aircraft, and a distance 
of eight feet between the velocimeter and the focal volume has been 
selected for prototype evaluation. At a distance of eight feet, the focal 
volume would be in the so-called "clear stream" outside the aerodynamic 
slip stream and associated turbulence. Furthermore, it is within the 
capability of presently available technology to provide: (a) laser devices 
with sufficient power, (b) photodetectors with sufficient sensitivity, and 
(c) optical systems with practical aperture sizes, so as to enable 
reflected light signals from eight feet away to be measured with 
sufficient precision to be useful for purposes of velocimetry. In the 
prototype velocimeter, the entrance window 16 has a 15-cm diameter and the 
exit window 14 has a 6.2-cm diameter. 
As indicated in FIG. 2, the housing structure 10 is installed on board the 
aircraft so that the anterior wall 15 is covered by a protecting panel 17, 
which forms a portion of the exterior surface (or "skin") of the aircraft. 
A region of the panel 17 overlying the exit window 14 and the entrance 
window 16 on the anterior wall 15 is apertured to receive a transparent 
glass cover plate 18, which is made of a low-absorptive optical glass such 
as Schott BK7 glass marketed by Schott Optical Glass Inc. of Duryea, Pa. 
The glass cover plate 18 preferably has a high-efficiency anti-reflective 
coating. 
In FIG. 3, the general direction of each of the three pairs of laser beams 
projected through the exit window 14 to the focal volume is indicated by a 
broken line 19. As atmospheric aerosol particles pass through the focal 
volume, light is scattered first from one and then from the other of the 
two ribbon-shaped laser beams of each pair. A portion of the light 
scattered from each ribbon-shaped laser beam of each pair travels back to 
the entrance window 16 in a direction indicated generally by a broken line 
20. Light scattered from each of the two ribbon-shaped laser beams of each 
pair is gathered at the entrance window 16, and the gathered light is 
directed along a folded optical path within the housing structure 10 to 
corresponding field stops (as described hereinafter) positioned at input 
ends of corresponding optical devices for transmitting light scattered 
from corresponding ribbon-shaped laser beams at the focal volume to 
corresponding photodetectors mounted within the housing structure 10. Each 
field stop discriminates between the light scattered from a particular 
ribbon-shaped laser beam and the light scattered from all the other 
ribbon-shaped laser beams at the focal volume. The field stops 
substantially prevent light scattered from any particular one of the 
ribbon-shaped laser beams at the focal volume from reaching photodetectors 
provided to detect light scattered from any of the other ribbon-shaped 
laser beams at the focal volume. Collectively, the field stops define the 
depth of the focal volume in the vicinity of the foci of the three 
projected pairs of ribbon-shaped laser beams. 
Background (i.e., ambient) light is also gathered at the entrance window 
16, and is directed to the field stops along the same folded optical path 
within the housing structure 10 as the light scattered from the 
ribbon-shaped laser beams at the focal volume. Each one of the field stops 
(except for a so-called "guardband" field stop) passes background light in 
addition to the light scattered at the focal volume from a corresponding 
one of the ribbon-shaped laser beams. The "guardband" field stop is 
positioned so as to pass only background light, and so as not to pass 
light scattered from any of the ribbon-shaped laser beams. The background 
light passed by the guardband field stop is therefore "uncontaminated" by 
light scattered from any of the ribbon-shaped laser beams. The 
"uncontaminated" background light is transmitted by dedicated optical 
components to the photodetector provided on the mounting 13 for the 
purpose of measuring background light. The background light measured by 
the background-light photodetector is subtracted in substantially real 
time from the scattered light measured by each of the corresponding 
scattered-light photodetectors, thereby providing precise measurements for 
the light actually scattered by aerosol particles from the ribbon-shaped 
laser beams at the focal volume. 
As indicated schematically in FIG. 3, the scattered-light photodetectors 
and the background-light photodetector supported on the mounting 13 
generate electrical signals that are processed by a signal processor 21 
(which may be of a conventional kind) to obtain a vector measurement of 
the velocity relative to the aircraft of an aerosol particle passing 
through the focal volume. The signal processor 21 includes a computer 
programmed to calculate the velocity measurement. 
In the simplified schematic illustration of FIG. 3, only three electrical 
signal inputs to the signal processor 21 are shown, which represent a pair 
of electrical signal inputs generated by a corresponding pair of 
scattered-light photodetectors, and an electrical signal input generated 
by the background-light photodetector. Actually, seven electrical signal 
inputs to the signal processor 21 are generated (viz., three pairs of 
electrical signal inputs generated by three corresponding pairs of 
scattered-light photodetectors, as well as the electrical signal input 
generated by the background-light photodetector) as aerosol particles pass 
through the focal volume. The signal processor 21 converts the electrical 
signal inputs generated simultaneously by the scattered-light 
photodetectors and by the background-light photodetector to the aircraft 
velocity measurement. The aircraft velocity measurement can be displayed 
by an airspeed indicator 22 mounted on an instrument panel of the 
aircraft, and can be used to generate signals for controlling avionic 
systems on the aircraft. 
FIG. 4 shows detailed features of portions of the mountings 12 and 13 that 
are visible outside the housing structure 10. The mounting 12 (which 
supports one pair of optical trains and a corresponding pair of laser 
devices and associated cooling devices) has a cover component 23 that is 
secured to an external surface portion of the side wall 11. As described 
hereinafter, the cover component 23 overlaps (but does not bear against) 
an annular portion of the external surface of the side wall 11 
circumjacent a circular aperture through which a major portion of the 
mounting 12 is inserted into the interior of the housing structure 10. The 
pair of laser devices is mounted on the cover component 23 so as to 
transmit a corresponding pair of laser beams through the corresponding 
optical trains, which are mechanically supported by components of the 
mounting 12 positioned inside the housing structure 10. The optical trains 
shape the pair of laser beams generated by the pair of laser devices 
mounted on the cover component 23. 
As shown in FIG. 4, the cover component 23 of the mounting 12 is secured to 
the side wall 11 by bolts 24. Attached to the cover component 23 so as to 
extend outwardly from the housing structure 10 are vane structures 25, 
which dissipate heat that is generated by the laser devices (and by 
thermo-electric cooling devices associated therewith) mounted on the cover 
component 23 to air flowing outside the housing structure 10 adjacent the 
side wall 11. 
The mounting 13, which is shown in the simplified illustration of FIG. 4 as 
supporting only a single pair of photodetectors for detecting scattered 
light as well as the photodetector for detecting background light, has a 
flanged portion 26 visible outside the housing structure 10. The flanged 
portion 26 overlaps and bears against an annular portion of the external 
surface of the side wall 11 circumjacent a circular aperture through which 
a major portion of the mounting 13 is inserted into the interior of the 
housing structure 10. The mounting 13 supports optical devices (described 
hereinafter) comprising fiber optical components and refractive components 
for transmitting light from the field stops (as mentioned above) to the 
corresponding scattered-light photodetectors and to the background-light 
photodetector, which are connected to electrical leads extending outside 
the housing structure 10. 
As shown in FIG. 4, the flanged portion 26 of the mounting 13 is secured to 
the side wall 11 by bolts 27. The scattered-light photodetectors are 
supported adjacent the flanged portion 26 of the mounting 13, and generate 
electrical signals that are transmitted to the signal processor 21 by 
means of electrical leads 28 and 29. The background-light photodetector, 
which is likewise supported adjacent the flanged portion 26 of the 
mounting 13, generates an electrical signal that is transmitted to the 
signal processor 21 by means of an electrical lead 30. Also shown in FIG. 
4, but in oblique view, is the anterior wall 15 of the housing structure 
10 on which the exit window 14 and the entrance window 16 are seen. In 
FIG. 5, a portion of the anterior wall 15 is shown on which the exit 
window 14 and the entrance window 16 can be seen in elevation view. 
FIG. 6 comprises two parts, which are set forth on separate sheets of the 
drawing as FIGS. 6A and 6B. When the drawing is perused, FIGS. 6A and 6B 
should be juxtaposed so that the line labelled "FIG-6B" in FIG. 6A 
coincides with the line labelled "FIG-6A" in FIG. 6B. 
In FIG. 6A, the interior of the housing structure 10 is shown in 
cross-sectional view along line 6A--6A of FIG. 4. A dividing wall 31 
partitions the interior of the housing structure 10 into a first 
compartment and a second compartment. The mountings 12 and 13 extend 
transversely across the first compartment between the side wall 11 and the 
dividing wall 31. The second compartment is furnished with reflective and 
refractive optical components that function to provide a first folded 
optical path whereby each pair of ribbon-shaped laser beams generated by 
the corresponding pair of laser devices and shaped by the corresponding 
pair of optical trains supported on the mounting 12 is projected through 
the exit window 14 and focussed at the focal volume, and to provide a 
second folded optical path whereby light scattered from each pair of 
ribbon-shaped laser beams at the focal volume and gathered at the entrance 
window 16 is focussed onto a corresponding pair of field stops at an input 
end of the mounting 13. Optical devices comprising fiber optical 
components and refractive components supported on the mounting 13 for 
transmitting light scattered from a pair of ribbon-shaped laser beams at 
the focal volume to a corresponding pair of photodetectors (and to no 
other photodetectors) are shown in the simplified illustration of FIG. 6A. 
As indicated in FIG. 6A (and as described in fuller detail hereinafter), 
the mounting 12 comprises three components that are attached together to 
form an integral structure, viz., the cover component 23 discussed above, 
a circularly cylindrical component 32 and an output-end component 33, 
which are all disposed coaxially with respect to each other along a common 
cylindrical axis. The output-end component 33 is secured to the 
cylindrical component 32, and the cylindrical component 32 is secured to 
the cover component 23. The cylindrical component 32 is solid with a pair 
of longitudinal bores 34 and 35 extending therethrough parallel to the 
cylindrical axis thereof. The bores 34 and 35 have substantially equal 
circular transverse cross sections, and are located diametrically opposite 
each other with respect to the circular transverse cross section of the 
cylindrical component 32. Each of the laser devices mounted on the cover 
component 23 has an optically active region aligned with a corresponding 
one of the bores 34 and 35, whereby the laser beam generated by each laser 
device is transmitted into a corresponding one of the bores 34 and 35 
through the cylindrical component 32. 
Substantially identical optical trains are positioned in the bores 34 and 
35 of the cylindrical component 32 to shape the laser beams generated by 
the corresponding laser devices. The output-end component 33 houses a 
prismatic device 36 (described in detail hereinafter), which geometrically 
combines the two ribbon-shaped laser beams shaped by the optical trains in 
the respective bores 34 and 35 of the cylindrical component 32. As shown 
in FIG. 6A, a proximal end of the output-end component 33 (i.e., proximal 
with respect to the cylindrical component 32) is open to receive the 
prismatic device 36, and is secured to a distal end of the cylindrical 
component 32 so that the laser beams shaped in the bores 34 and 35 of the 
cylindrical component 32 pass into the prismatic device 36. In the 
prototype embodiment, the distal end of the cylindrical component 32 is 
outwardly flanged to mate with an outwardly flanged portion of the open 
proximal end of the output-end component 33 after the prismatic device 36 
has been placed in the output-end component 33. The flanged open end of 
the output-end component 33 makes abutting contact with the flanged distal 
end of the cylindrical component 32, and the mating flanged ends are 
rigidly attached together by a conventional technique (as by screws). 
A distal end of the output-end component 33 terminates in an end wall 37, 
which has a circular aperture whose center is aligned with the cylindrical 
axis of the cylindrical component 32. A circularly cylindrical tubular 
portion 38 integral with the end wall 37 extends outward from around the 
aperture in the end wall 37, and is dimensioned so that a tip thereof is 
received with a tight fit in an aperture in the dividing wall 31. The two 
ribbon-shaped laser beams that are shaped in the bores 34 and 35, 
respectively, are re-directed by the prismatic device 36, and are combined 
so as to pass via the aperture in the end wall 37 through the tubular 
portion 38 into the second compartment of the housing structure 10. 
In fabricating the prototype velocimeter, the cylindrical component 32 
(with the pair of optical trains positioned in the corresponding bores 34 
and 35 thereof) and the output-end component 33 (with the prismatic device 
36 housed therein) are coupled together, and are inserted as a unit into 
the first compartment of the housing structure 10 through the aperture 
provided therefor in the side wall 11 so that the tip of the tubular 
portion 38 of the output-end component 33 is inserted into the aperture 
provided therefor in the dividing wall 31. A proximal end of the 
cylindrical component 32 (i.e., proximal with respect to the side wall 11) 
comprises an outwardly flanged portion 39, which bears against the annular 
portion of the external surface of the side wall 11 circumjacent the 
aperture provided therein to receive the mounting 12. A thermally 
insulating spacer 40 of annular configuration is then placed over the 
outwardly flanged portion 39 of the cylindrical component 32, and the 
cover component 23 is placed over the spacer 40 so that the active regions 
of the laser devices mounted on the cover component 23 are aligned with 
the corresponding bores 34 and 35 of the cylindrical component 32. The 
spacer 40 provides a thermal barrier between the cover component 23 (on 
which the heat-generating laser devices are mounted) and the cylindrical 
component 32, which should be isolated as much as possible from thermal 
expansion effects because of critical dimensions and spacings of 
components of the optical trains positioned in the bores 34 and 35. The 
bolts 24 are then inserted in aligned holes through a peripheral portion 
of the cover component 23, through the dielectric spacer 40, through the 
outwardly flanged portion 39 of the cylindrical component 32, and through 
a portion of the side wall 11 circumjacent the aperture for receiving the 
mounting 12. The holes in the side wall 11 for receiving the bolts 24 are 
screw-threaded, and the bolts 24 are tightened therein to form an 
air-tight seal around the aperture in the side wall 11 through which the 
mounting 12 is received. 
Adjustment screws 41 and 42 extend through the cover component 23 so that 
an inner end of the screw 41 bears against the laser device aligned with 
the bore 34, and so that an inner end of the screw 42 bears against the 
laser device aligned with the bore 35. Outer ends of the screws 41 and 42 
extend outwardly from an exteriorly facing surface portion of the cover 
component 23, and can be readily accessed by means of a screwdriver to 
provide fine-tuning adjustments of the positions and orientations of the 
laser devices. In the prototype velocimeter of the present invention, the 
laser beam emitted by each laser device is actually a composite of forty 
overlapping component beams, whose illumination patterns on a surface are 
curiously called "footprints" in the laser device art. Each laser beam 
produced by the prototype velocimeter is generated by a linear array of 
forty GaAlAs phase-locked semiconductor laser stripes, whose footprints 
overlap to produce the laser beam. The type of laser device used for the 
prototype velocimeter is an edge-emitting laser diode marketed by Spectra 
Diode Labs of San Jose, Calif. under the catalog designation SDL-4550-C. 
The optical trains mounted in the corresponding bores 34 and 35 of the 
cylindrical component 32 transform the two beams generated by the two 
corresponding laser devices mounted on the cover component 23 into two 
laser beams of generally ribbon-shaped configuration. The two 
ribbon-shaped laser beams, after being geometrically combined by the 
prismatic device 36, impinge upon a beam-folding mirror 43 mounted on a 
posterior wall 44 within the second compartment of the housing structure 
10. The mirror 43 reflects (or "folds") the optical path of the two 
geometrically combined ribbon-shaped laser beams, so that the two laser 
beams can pass via a transfer lens system (described in detail 
hereinafter) through the exit window 14 to the focal volume. 
The optical trains in the bores 34 and 35 of the cylindrical component 32 
cause images of the laser stripes on the corresponding laser devices 
mounted on the cover component 23 to be formed on a plane (called the 
"intermediate ribbon field") 45 at the aperture in the dividing wall 31 
through which the combined ribbon-shaped laser beams enter into the second 
component of the housing structure 10. The transfer lens system, which is 
positioned adjacent the exit window 14, then re-images the images of the 
laser stripes onto a plane (called the "projection plane") at the focal 
volume with a magnification ratio of 6:1 (i.e., so that a transverse 
dimension of the ribbon is six times larger at the focal volume than at 
the intermediate ribbon field 45), as illustrated in FIG. 6B. 
The combined ribbon-shaped laser beams entering the second compartment of 
the housing structure 10 are confined within a light shield 46, which 
prevents light generated by the laser devices mounted on the cover 
component 23 from straying within the second compartment to the field 
stops at an input end of the mounting 13. The light shield 46 has a folded 
conical configuration conforming generally to the configuration of the 
envelope of the combined ribbon-shaped laser beams emerging from an output 
end (i.e., the tubular portion 38) of the mounting 12 through the aperture 
in the dividing wall 31 into the second compartment of the housing 
structure 10. Two other functionally identical light shields (not shown in 
FIG. 6A) are also provided in the second compartment to confine the 
corresponding two other pairs of combined ribbon-shaped laser beams 
emerging through two other apertures provided in the dividing wall 31 to 
receive output ends of two other mountings (also not shown in FIG. 6A), 
which are substantially identical to the mounting 12 shown in FIG. 6A. 
The transfer lens system mounted in the second compartment of the housing 
structure 10 adjacent the exit window 14 comprises a pair of lens elements 
(which serves to magnify and separate the two ribbon-shaped laser beams of 
each pair) and a pair of wedge-shaped prism elements forming a Risley 
prism (which serves to align the pairs of ribbon-shaped laser beams 
projected through the exit window 14 with images of the corresponding 
pairs of field stops that are formed on the projection plane at the focal 
volume). The prism elements comprising the Risley prism are rotatably 
adjustable in a conventional manner to achieve the required optical 
alignment of the laser beams with the corresponding field stop images. 
The transfer lens system focuses the two laser beams of each pair at the 
focal volume, so that each laser beam assumes a prescribed width and 
height on the projection plane, and so that both laser beams have a 
prescribed depth of focus with respect to the projection plane. The 
transfer lens system also causes the two laser beams of each pair to be 
separated from each other by a prescribed distance on the projection 
plane. An atmospheric aerosol particle passing through the focal volume 
scatters light from each ribbon-shaped laser beam that the aerosol 
particle penetrates. The scattered light is distributed (generally 
non-uniformly) over a solid angle, and a portion of the scattered light 
travels along a path indicated by a broken line 47 in FIG. 6B to the 
entrance window 16 in the anterior wall 15 of the housing structure 10. 
A collecting lens system is mounted in the second compartment of the 
housing structure 10 adjacent the entrance window 16 to gather the portion 
of the light scattered from the ribbon-shaped laser beams that reach the 
entrance window 16. The collecting lens system (which in the prototype 
velocimeter as shown in FIG. 6A comprises a lens triplet consisting of 
lens elements 48, 49 and 50) directs the gathered light onto the mirror 
43, which folds the optical path of the gathered light onto field stops 
formed on a field stop array plate 51, which is attached to the input end 
of the mounting 13. The collecting lens system focuses the gathered light 
that has been scattered from each particular ribbon-shaped laser beam at 
the focal volume onto a corresponding particular field stop on the field 
stop array plate 51. 
The mounting 13 (which is described in structural detail hereinafter) is 
inserted into the first compartment of the housing structure 10 through an 
aperture provided for the purpose in the side wall 11 so that the input 
end of the mounting 13 (with the field stop array plate 51 attached 
thereto) is received with a tight fit in an aperture provided for the 
purpose in the dividing wall 31. An output end of the mounting 13 extends 
beyond the flanged portion 26 thereof to support the scattered-light 
photodetectors and the background-light photodetector outside the side 
wall 11 of the housing structure 10. 
As indicated in FIG. 6A, light that has been scattered from a pair of 
ribbon-shaped laser beams at the focal volume and gathered by the 
collecting lens system at the entrance window 16 is focussed by the 
collecting lens system onto a corresponding pair of field stops on the 
field stop array plate 51. Light scattered from the other two pairs of 
ribbon-shaped laser beams at the focal volume is likewise gathered by the 
collecting lens system at the entrance window 16, and is focussed onto two 
corresponding other pairs of field stops on the field stop array plate 51. 
Background light gathered by the collecting lens system is focussed onto 
each of the field stops positioned to pass light scattered from 
corresponding ribbon-shaped laser beams at the focal volume, and is also 
focussed onto the "guardband" field stop. The light gathered by the 
collecting lens system is confined within a light shield 52, which 
substantially precludes multiple scatterings of the gathered light from 
the mirror 43 and from other reflective surfaces within the second 
compartment of the housing structure 10. The light shield 52 has a folded 
conical configuration, and conforms generally to the configuration of the 
envelope of the gathered light as focussed by the collecting lens system 
onto the field stops on the field stop array plate 51. 
Each field stop on the field stop array plate 51 is an elongate slit, which 
is aligned with an input end of a corresponding bundle of optically 
conducting fibers (hereinafter called a "fiber bundle") associated with a 
corresponding photodetector and associated optical components supported on 
the mounting 13. The input end of each fiber bundle is positioned in 
alignment with a corresponding field stop by means of a fiber-optics 
support structure 53, which is secured (as by screw-threaded engagement) 
in an axial bore at the input end of the mounting 13. The field stop array 
plate 51 is attached (as by screws) to the input end of the mounting 13 so 
as to cover the axial bore in which the fiber-optics support structure 53 
is secured. Thus, as shown in FIG. 6A, the input end of the fiber bundle 
54 is positioned by the support structure 53 in alignment with a 
corresponding field stop on the field stop array plate 51. Light scattered 
at the focal volume from the ribbon-shaped laser beam that was formed in 
the bore 34 is focussed by the collecting lens system onto that 
corresponding field stop, which passes the light into the optically 
conducting fibers comprising the fiber bundle 54. Similarly, light 
scattered at the focal volume from the ribbon-shaped laser beam that was 
formed in the bore 35 is focussed by the collecting lens system onto a 
corresponding other field stop with which an input end of a corresponding 
other fiber bundle 55 is aligned by the support structure 53. Background 
light is focussed by the collecting lens system onto the "guardband" field 
stop with which an input end of a corresponding fiber bundle 56 is aligned 
by the support structure 53. 
Scattered light focussed onto the field stop aligned with the input end of 
the fiber bundle 54 is conducted by the fiber bundle 54 to a corresponding 
relay lens system supported on the mounting 13. An output end of the fiber 
bundle 54 is coupled by means of a conventional connecting device 57 to 
the corresponding relay lens system. Similarly, scattered light focussed 
onto the field stop aligned with the input end of the fiber bundle 55 is 
conducted by the fiber bundle 55 to a corresponding other relay lens 
system supported on the mounting 13, and background light focussed onto 
the "guardband" field stop is conducted by the fiber bundle 56 to an 
axially disposed relay lens system supported on the mounting 13. An output 
end of the fiber bundle 55 is coupled to its corresponding relay lens 
system by means of a connecting device 58, and an output end of the fiber 
bundle 56 is coupled to the axially disposed relay lens system by means of 
a connecting device 59. The connecting devices 57, 58 and 59 are 
substantially identical. 
The relay lens system to which the fiber bundle 54 is coupled focuses the 
light scattered from the ribbon-shaped laser beam that was shaped in the 
bore 34 onto a corresponding photodetector 60. Similarly, the relay lens 
system to which the fiber bundle 55 is coupled focuses the light scattered 
from the ribbon-shaped laser beam that was shaped in the bore 35 onto a 
corresponding photodetector 61, and the axially disposed relay lens system 
to which the fiber bundle 56 is coupled focuses background light onto an 
axially disposed photodetector 62. The photodetectors 60, 61 and 62 react 
to the light that is focussed thereon by generating electrical signals, 
which are transmitted by the electrical leads 28, 29 and 30, respectively, 
to the signal processor 21. 
Input ends of two other pairs of fiber bundles (not shown in FIG. 6A) are 
positioned by the support structure 53 in alignment with two corresponding 
other pairs of field stops on the field stop array plate 51. Those two 
other pairs of field stops pass light scattered at the focal volume from 
two corresponding other pairs of ribbon-shaped laser beams, which were 
shaped in corresponding pairs of bores in two corresponding other 
mountings that are substantially identical to the mounting 12. Output ends 
of those two other pairs of fiber bundles are coupled to two corresponding 
other pairs of relay lens systems, which focus the light scattered from 
the two other pairs of ribbon-shaped laser beams onto two other pairs of 
corresponding photodetectors. The relay lens systems coupled to the 
corresponding fiber bundles for transmitting scattered light (i.e., light 
scattered from corresponding ribbon-shaper laser beams at the focal 
volume) to the corresponding photodetectors are all supported on the 
mounting 13 in a symmetrical arrangement around the coaxially positioned 
relay lens system that is coupled to the fiber bundle 56 for transmitting 
background light to the background-light Photodetector 62. 
FIG. 7 provides a perspective view at the focal volume of a pair of 
ribbon-shaped laser beams projected thereto from the velocimeter of the 
present invention. The depth of focus L of each pair of laser beams at the 
focal volume, and the width W of each of the two laser beams of the pair 
at the focal volume, are predetermined to provide a sufficient "crossing 
event rate" (i.e., the rate at which atmospheric aerosol particles cross 
the focal volume) to ensure reliable operation of the electronic circuitry 
associated with the photodetectors 60, 61 and 62. Optimum values for the 
depth of focus L and the beam width W are determined from performance 
trade-off studies and a conventional signal-to-noise (S/N) optimization 
analysis of the photodetector electronics. The optimum value for the 
closest separation d.sub.c between the two ribbon-shaped laser beams of 
each pair at the focal volume depends upon the crossing event rate and an 
optimization of the velocity resolution. The parameters L, W and d.sub.c 
define the focal volume for each pair of ribbon-shaped laser beams. 
Actually, the focal volume defined for each pair of ribbon-shaped laser 
beams is slightly displaced with respect to the focal volumes defined for 
the other two pairs of ribbon-shaped laser beams. However, the focal 
volume for each pair of beams overlaps the focal volumes of the other two 
pairs of beams to such an extent (as discussed in detail hereinafter) that 
it is appropriate to speak of a common focal volume for the three pairs of 
ribbon-shaped laser beams. More rigorously, it would be proper to speak of 
three focal volumes (i.e., one for each pair of beams) being formed at the 
measurement volume, and to describe the passage of atmospheric aerosol 
particles through the measurement volume. Cross sections of the two 
ribbon-shaped laser beams at the middle and at each end of the measurement 
volume (hereinafter also called the focal volume) are illustrated by 
cross-hatched shading in FIG. 7. 
FIG. 8 is a reprise of the perspective view of the pair of ribbon-shaped 
laser beams at the focal volume as shown in FIG. 7, but with an axis 
normal to the two laser beams at the focal volume also being shown. The 
maximum horizontal and vertical angles of approach that an atmospheric 
aerosol particle can have relative to the normal axis in passing through 
the focal volume and still penetrate both ribbon-shaped laser beams are 
indicated in FIG. 8 by the angles .alpha. and .beta., respectively. Thus, 
if the velocity vector of the aerosol particle passing through the focal 
volume were to have a horizontal component that makes an angle greater 
than .alpha. with respect to the axis normal to the two ribbon-shaped 
laser beams, and/or were to have a vertical component that makes an angle 
greater than .beta. with respect to the same axis, the aerosol particle 
would be unable to penetrate both ribbon-shaped laser beams. Arrowheads 
are shown in FIG. 8 on the lines indicating the maximum vertical and 
horizontal angles of approach (also called the "acceptance angles"). Small 
circles are shown at the tips of the arrowheads in FIG. 8 to represent 
aerosol particles. The velocimeter is operational only with respect to 
aerosol particles within the acceptance angles .alpha. and .beta.. 
In FIG. 9, the technique of the present invention for determining relative 
velocity of an aerosol particle passing through the focal volume with 
respect to the aircraft is depicted graphically. A first scattered-light 
signal is generated and detected as the aerosol particle penetrates the 
first laser beam of a pair of ribbon-shaped laser beams at the focal 
volume, and a second scattered-light signal is generated and detected 
after a measured time interval t.sub.m as the same aerosol particle 
subsequently penetrates a second laser beam of the same pair of 
ribbon-shaped laser beams at the focal volume. The time interval t.sub.m 
between sequential detections of the first and second scattered-light 
signals is measured electronically. The thickness .omega..sub.d of each 
ribbon-shaped laser beam of the pair is substantially diffraction-limited, 
but is not negligible. However, peak-to-peak detection of the sequential 
first and second scattered-light signals by conventional signal-processing 
electronics takes the thicknesses of the ribbon-shaped laser beams into 
account. The predetermined separation d.sub.c between the two beams of the 
pair is precisely known. Thus, from the equation V.sub.perp =d.sub.c 
/t.sub.m, the magnitude of the component V.sub.perp of the velocity vector 
of the aerosol particle in a direction orthogonal to the two ribbon-shaped 
laser beams of the pair can be determined. 
There are actually three pairs of ribbon-shaped laser beams focussed at the 
focal volume. Since the two laser beams of any one pair are nonparallel to 
the two laser beams of each of the other two pairs, three components of 
the velocity vector of an aerosol particle (or of three different aerosol 
particles all of which have substantially the same velocity relative to 
the aircraft) in the three different directions perpendicular to the three 
corresponding pairs of laser beams are non-orthogonal to each other. The 
magnitudes of the three non-orthogonal components of the velocity vector 
are all determined simultaneously and in the same manner independently of 
each other. From the magnitudes of the three components of the velocity 
vector, and from known relationships of the orientations of the three 
different pairs of ribbon-shaped laser beams with respect to each other at 
the focal volume, the velocity vector of an aerosol particle passing 
through the focal volume can be mathematically determined by means of an 
on-board computer using an appropriate mathematical algorithm. The 
algorithm may be conventional. 
The mounting 12 for a pair of optical trains that shape the laser beams 
generated by a corresponding pair of laser devices, as shown in FIG. 6A, 
is illustrated in enlarged detail in FIG. 10. Thermo-electric cooling 
devices 63 and the vane structures 25 are attached to exteriorly facing 
surface portions of the cover component 23 of the mounting 12 by a 
conventional technique. In the prototype embodiment, four solid-state 
thermo-electric cooling devices 63 are soldered onto a first flat surface 
portion of the cover component 23, and another four identical cooling 
devices 63 are soldered onto a second flat surface portion of the cover 
component 23. (In the cross-sectional view of FIG. 10, only two of the 
cooling devices 63 can be seen on each of the first and second flat 
surface portions of the cover component 23.) One vane structure 25 is then 
attached (as by screws) to the four cooling devices 63 on the first flat 
surface portion of the cover component 23, and another substantially 
identical vane structure 25 is attached in the same manner to the four 
cooling devices 63 on the second flat surface portion of the cover 
component 23. The cooling devices 63 may be of the type marketed by Marlow 
Industries, Inc. of Garland, Tex. as under catalog designation MI 
1064-04AC. 
Electrical leads 64 and 65 are connected to each of the cooling devices 63, 
whereby a temperature difference is constantly maintained between the 
cover component 23 and the vane structures 25. This temperature difference 
causes heat generated by the laser devices mounted on interiorly facing 
surface portions of the cover component 23 to pass by conduction through 
the cover component 23 into the vane structures 25. As illustrated in the 
enlarged detail of FIG. 10, an edge-emitting semiconductor laser device 66 
is mounted on a surface portion of the component 23 so that the emitting 
edge of its active regions is in alignment with the bore 34 in the 
cylindrical component 32. Similarly, an edge-emitting semiconductor a 
laser device 67 is mounted on a surface portion of the cover component 23 
so that the emitting edge of its active region is in alignment with the 
bore 35 in the cylindrical component 32. The vane structures 25 shed the 
heat generated by the laser devices 66 and 67 (and by the cooling devices 
63) to air flowing through a channel provided for the purpose outside the 
housing structure 10. 
In fabricating the mounting 12, the lens elements comprising the optical 
trains that transform the laser beams emitted by the laser devices 66 and 
67 into ribbon-shaped laser beams are mounted in tubular structures, which 
are inserted into the bores 34 and 35. Thus, as shown in FIG. 10, tubular 
structures 68, 69 and 70 with appropriately positioned lens elements 
mounted therein are inserted into the bore 34, and tubular structures 71, 
72 and 73 with appropriately positioned lens elements mounted therein are 
inserted into the bore 35. The tubular structure 68 is positioned at the 
proximal end of the cylindrical component 32 so that one end thereof 
extends out of the bore 34 to the vicinity of the laser device 66. The 
laser beam emitted by the laser device 66 has a central axis of 
propagation lying substantially on the cylindrical axis of the tubular 
structure 68 within the bore 34. The tubular structure 68 supports seven 
lens elements, five of which collectively function as a collimator group, 
and two of which are part of an anamorphic beam-expander group. The optic 
axis of the collimator group coincides with the cylindrical axis of the 
tubular structure 68. 
The tubular structure 69 is aligned coaxially with (but is separated from) 
the tubular structure 68 in the bore 34, and supports three lens elements 
that comprise the remaining elements of the anamorphic beam-expander 
group. The optic axis of the anamorphic beam-expander group coincides with 
the cylindrical axis of the tubular structure 69, and is coincident with 
the optic axis of the collimator group. The tubular structure 70 is 
aligned coaxially with respect to the tubular structures 68 and 69 in the 
bore 34, but is positioned nearer to the distal end than to the proximal 
end of the cylindrical component 32. The tubular structure 70 supports a 
group of three lens elements, which collectively function as an imager 
group. The optic axis of the imager group coincides with the cylindrical 
axis of the tubular structure 70, and hence is coincident with the optic 
axes of the collimator group and the anamorphic beam-expander group. 
As illustrated in FIG. 10, the collimator group, the anamorphic 
beam-expander group and the imager group of lens elements supported by the 
tubular structures 68, 69 and 70 in the bore 34 transform the laser beam 
generated by the laser device 66 into a laser beam having the desired 
ribbon-shaped configuration. In an identical way, the tubular structures 
71, 72 and 73 are axially aligned with each other within the bore 35 to 
support a collimator group, an anamorphic beam-expander group and an 
imager group of lens elements that transform the laser beam generated by 
the laser device 67 into a laser beam having the desired ribbon-shaped 
configuration. The two laser beams generated by the laser devices 66 and 
67, respectively, are geometrically combined by the prismatic device 36, 
and are re-directed thereby into the tubular portion 38 of the output-end 
component 33 of the mounting 13. Images of the laser beam sources (i.e., 
images of the linear arrays of semiconductor laser stripes on the emitting 
edges of the active regions of the semiconductor laser devices 66 and 67) 
are formed by the respective optical trains at the intermediate ribbon 
field 45 on a plane coinciding with the surface of the dividing wall 31 
facing into the second compartment of the housing structure 10. 
The images of the laser beam sources formed at the intermediate ribbon 
field 45 are projected outside the housing structure 10 to corresponding 
foci at the focal volume by the transfer lens system (discussed 
hereinafter) mounted adjacent the exit window 14. With reference again to 
FIG. 6A, the beam issuing from the laser device 66 is seen (at transverse 
plane 74) to assume a substantially circular transverse cross section 
after having passed through the collimator group and the anamorphic 
beam-expander group of lens elements. The beam then assumes a 
ribbon-shaped transverse cross-section after having passed through the 
imager group of lens elements. The laser beam issuing from the laser 
device 66 is geometrically combined by the prismatic device 36 with a 
substantially identical laser beam issuing from the laser device 67. The 
two geometrically combined laser beams become precisely coincident at a 
pupil 75 located in the second compartment of the housing structure 10 
between the intermediate ribbon field 45 and the beam-folding mirror 43. 
Referring to juxtaposed FIGS. 6A and 6B, the two geometrically combined 
laser beams begin to diverge from each other after passing through the 
pupil 75. As shown in FIG. 6A, the two combined laser beams have 
substantially circular cross sections whose centers are displaced from 
each other when the two laser beams pass through the exit window 14. 
However, the two laser beams progressively assume more definite 
ribbon-shaped configurations and begin to become distinct from each other 
(as indicated by cross-hatching in FIG. 6B) as the laser beams proceed to 
the focal volume. At the focal volume, the images of the laser beam 
sources formed at the intermediate ribbon field 45 are re-imaged on the 
projection plane. The "re-imaged" images at the focal volume are spaced 
apart from each other by a predetermined distance, and have a 
predetermined magnification. The prototype velocimeter of the present 
invention, which was designed for evaluation on a testbed aircraft, 
re-images the images formed at the intermediate ribbon field 45 so as to 
have a spacing of 9.28 mm from each other and a magnification ratio of 6:1 
on the projection plane. 
The prismatic device 36 is fabricated from two substantially identical 
pieces of glass, whose dimensions are critical (as discussed hereinafter) 
for re-directing the two ribbon-shaped laser beams into the tubular 
portion 38 of the mounting 12. As shown in FIG. 10, the two pieces of 
glass comprising the prismatic device 36 are positioned symmetrically with 
respect to each other about an axis coinciding with the cylindrical axis 
of the cylindrical component 32, and are cemented together to form a 
structure having a longitudinal cross section of generally truncated 
V-shaped configuration. A flat surface of the prismatic device 36 bears 
against a portion of the circular end wall 37 of the output-end component 
33 circumjacent the aperture therein leading into the tubular portion 38. 
Two branching "legs" of the prismatic device 36 terminate in flat 
surfaces, which are coplanar with each other, and which are dimensioned to 
abut a mating surface on the distal end of the cylindrical component 32. 
In the prototype embodiment shown in FIG. 10, the mating surface on the 
distal end of the cylindrical component 32 is flat with bevelled undercut 
portions that are dimensioned to receive the legs of the prismatic device 
36, which is made of Schott BK7 glass. 
In fabricating the mounting 12, optical components comprising the separate 
tubular structures 68, 69 and 70, and 71, 72 and 73, with their respective 
lens elements securely mounted therein, are slid into position in their 
respective bores 34 and 35. Screws 76 passing through corresponding holes 
in the wall of the cylindrical component 32 are received in corresponding 
holes in the tubular structures 68, 69 and 70, and 71, 72 and 73 to secure 
the lens elements in proper position within the bores 34 and 35. 
The laser beams produced by the laser devices 66 and 67, after having 
passed through the optical trains mounted in the bores 34 and 35, 
respectively, and after having been geometrically combined by the 
prismatic device 36, enter via the tubular portion 38 of the output-end 
component 33 of the mounting 12 into the second compartment of the housing 
structure 10, and are confined within the light shield 46 as illustrated 
in FIGS. 6A and 11. Each of the two laser beams is focussed at the 
intermediate ribbon field 45 so as to produce spaced-apart images of the 
linear arrays of semiconductor laser stripes (as described hereinafter) on 
the laser devices 66 and 67. The two laser beams progressively change 
shape to a more rounded configuration, and begin to overlap each other 
shortly after passing the intermediate ribbon field 45. At the pupil 75, 
the two laser beams assume circular cross sections that precisely coincide 
with each other. After passing the pupil 75, the two laser beams begin to 
diverge from each other, and the optical path of the two diverging laser 
beams is reflected (or "folded") by the mirror 43 to the exit window 14. 
The two laser beams do not become distinct from each other and re-assume 
recognizable ribbon-shaped configurations, until after exiting from the 
housing structure 10. 
The light shield 46 prevents light produced by other sources from 
contaminating the pair of geometrically combined laser beams originating 
at the laser devices 66 and 67, and prevents light originating at the 
laser devices 66 and 67 from being scattered within the housing structure 
10 to the field stops on the field stop array plate 51. As shown in FIG. 
11, the light shield 46 has a flanged end portion 77 that abuts the 
dividing wall 31 circumjacent the aperture in which the tip of the tubular 
portion 38 of the output-end component 33 of the mounting 12 is received. 
The flanged end portion 77 of the light shield 46 is secured by means of 
machine screws 78 to the dividing wall 31. In the prototype velocimeter 
built for purposes of evaluation, the light shield 46 is separated from 
the beam-folding mirror 43 by about 6.4 mm in order to avoid stresses on 
the mirror 43. 
A depiction of the extent to which the two laser beams originating at the 
laser devices 66 and 67, respectfully, diverge from each other after 
having been reflected from the mirror 43 is shown in FIG. 11 by the 
overlapping (generally circular) cross sections for the two laser beams at 
a position within the housing structure 10 indicated by reference number 
79, which is located between the mirror 43 and the transfer lens system at 
the exit window 14. Referring back to FIG. 6A, it is noted that the two 
diverging beams retain generally circular cross sections until after 
leaving the housing structure 10 through the exit window 14. As indicated 
in FIG. 6B, the two laser beams become distinct from each other and assume 
discernible ribbon-shaped configurations only in the vicinity of the focal 
volume. 
As illustrated in FIG. 12, the laser device 66 is a multiheterojunction 
GaAlAs laser diode having a linear array of forty phase-locked active 
stripes that together produce a composite laser beam, which is transmitted 
through the optical train in the bore 34. An electrical lead 80 provides 
power to the laser device 66. Adjustment screws on the laser device 66 
enable the position and angular orientation of the output facet of the 
laser device 66 (i.e., the origin of the composite laser beam) to be 
varied with four degrees of freedom, as indicated by arrows in FIG. 12. 
The laser device 67 is substantially identical to the laser device 66, and 
produces a substantially identical composite laser beam, which is 
transmitted through the optical train in the bore 35. 
In FIG. 13, transverse cross sections are illustrated for the composite 
laser beams issuing from the laser devices 66 and 67 at various locations 
along the optical paths through the bores 34 and 35, respectively. The 
linear arrays of semiconductor laser stripes on the laser devices 66 and 
67 are imaged as corresponding line images at the intermediate ribbon 
field 45, and are re-imaged at the focal volume with a 6:1 magnification 
ratio as indicated in FIG. 6B. The magnification of the images of the 
linear arrays of laser stripes occurring at the focal volume produces the 
predetermined dimensions for the ribbon-shaped laser beams as illustrated 
in FIGS. 7 and 8 at the focal volume. 
FIG. 14 shows a portion of the emitting edge the active region of any one 
of the semiconductor laser devices 66 and 67, or of any of the other 
semiconductor laser devices producing laser beams for projection to the 
focal volume. For the laser diode type of laser device used in fabricating 
the prototype velocimeter as shown in FIG. 12, there is a linear array of 
forty semiconductor laser stripes, each of which is about 6.5 microns in 
length with a spacing of about 3.5 microns between adjacent stripes. Each 
individual laser stripe generates a beam component (called a "footprint"), 
which combines with the "footprints" generated by all the other laser 
stripes in the linear array to produce the composite laser beam issuing 
from the laser device. The composite laser beam has a generally elliptical 
configuration with a major axis transverse to the linear array of laser 
stripes. 
The composite beam generated by the forty laser stripes of the laser device 
66 is illustrated in FIG. 15 by overlapping cross sections of three of the 
forty "footprints" comprising the composite beam, viz., a first 
"footprint" generated by a laser stripe at one end of the linear array, a 
fortieth "footprint" generated by a laser stripe at the other end of the 
linear array, and an intermediate (say, the twenty-first) "footprint" 
generated by a laser stripe at a corresponding intermediate (i.e., the 
twenty-first) location on the linear array. The laser beam comprising the 
forty "footprint" components then enters the bore 34 to be shaped into a 
ribbon-shaped configuration, and to be focussed at the intermediate ribbon 
field 45, by the optical train mounted in the bore 34. 
The lens elements comprising the optical train mounted in the bore 35 are 
illustrated in profile in FIG. 16. Corresponding lens elements of the 
optical trains mounted in the bores 34 and 35 have the same dimensions, 
are made of the same optical materials, and have the same relative 
positions with respect to the other lens elements of the same optical 
train. Preferably, each of the lens elements in each of the optical trains 
has a high-efficiency anti-reflective coating that is matched to the 
nominal output wavelength and bandwidth of the corresponding laser device. 
The optical trains in the bores 34 and 35 are achromatized for a pair of 
primary wavelengths, one of which is a selected infrared wavelength at 
which the velocimeter is designed to operate, and the other of which is a 
selected visible wavelength at which testing and alignment of the system 
can be performed. For the prototype velocimeter of the present invention, 
the selected operating wavelength is nominally 0.83 micron, and the 
selected visible wavelength is 0.6328 micron (i.e., the HeNe laser line). 
The operating wavelength nominally specified as 0.83 micron represents an 
average value in a wavelength band from 0.81 micron to 0.84 micron. 
As shown in FIG. 16, the collimator group mounted in the bore 35 comprises 
five lens elements 91, 92, 93, 94 and 95, which are designated from right 
to left because the direction of propagation of the laser beam from the 
laser device 67 through the lens elements is from right to left. The 
collimator group is itself achromatized at the operating wavelength (0.83 
micron in the infrared region) and the testing and alignment wavelength 
(0.6328 micron in the visible region). Design parameters for the 
collimator group are specified in Table I with reference to 0.83 micron as 
a base wavelength as follows: 
TABLE I 
______________________________________ 
Surface 
Radius Thickness 
No. (mm) (mm) Material 
.N.sub..83 (measured) 
______________________________________ 
0 .infin. 1.58 Air 
1 -11.549 10.77 LAFN2 1.731759 
2 -8.128 0.24 Air 
3 36.309 1.92 SF3 1.721101 
4 13.927 4.38 BAK1 1.565285 
5 -18.720 1.34 Air 
6 56.060 1.92 SF3 1.721101 
7 9.390 4.38 BAK1 1.565285 
8 -298.400 4.27 Air 
9 .infin. Air (Aperture Stop) 
______________________________________ 
where the surfaces of the lens elements and the other optically significant 
surfaces of the collimator group are numbered consecutively from right to 
left. 
The radius listed in the second column of Table I for each surface is the 
radius of curvature of the surface expressed in millimeters. The radius of 
curvature of any particular surface listed in the second column is 
positive if the center of curvature of the surface lies to the left of the 
surface, and negative if the center of curvature of the surface lies to 
the right of the surface. This designation of positive and negative radii 
of curvature is in accord with a conventional practice of designating the 
curvature of a surface as positive if the surface is convex with respect 
to the direction of propagation of light through the surface, and as 
negative if the surface is concave with respect to the direction of 
propagation of light through the surface. 
The thickness listed in the third column of Table I for each surface is the 
thickness of the lens element, or of the spacing, bounded on the right by 
the particular surface. Thickness is expressed in millimeters, and is 
measured along the optic axis of the lens group. The material listed in 
the fourth column of Table I for each surface refers to the type of 
optical material (i.e., the type of glass) used for making the lens 
element bounded on the right by the indicated surface. 
The heading "N.sub..83 (measured)" in the fifth column of Table I indicates 
the refractive index of the optical glass from which the lens element 
bounded on the right by the indicated surface is made, where the value of 
the refractive index is provided by the manufacturer of the optical glass 
from melt data. The parenthetical designation "measured" in the heading 
"N.sub..83 (measured)" indicates that the value for refractive index 
listed for each of the various types of optical glasses was interpolated 
for the nominal 0.83-micron infrared operating wavelength from a 
mathematically fitted curve in which measured values of refractive index 
were determined at a number of different wavelengths for the actual melt 
from which each particular optical glass was produced. 
Each of the optical glasses listed in Table I is identified unambiguously 
by the manufacturer's catalog number, which ordinatily is an alphanumeric 
designation or a purely numeric designation depending upon the 
manufacturer. The process of making any particular type of optical glass 
is generally regarded by the manufacturer as proprietary information, and 
is ordinarily not known in detail by the optical designer who uses the 
glass. Accordingly, it is practically universal practice among optical 
designers to identify optical glasses by manufacturer's catalog numbers 
rather than by chemical composition. 
The object plane of the collimator group of lens elements shown in FIG. 16 
is designated in Table I as Surface No. 0, and has an infinite radius of 
curvature. The lens element 91 (whose surfaces from right to left are 
designated as Surface No. 1 and Surface No. 2) is made of LAFN2 glass 
manufactured by Schott Optical Glass Inc. Lens element 92 (whose surfaces 
from right to left are designated as Surface No. 3 and Surface No. 4) is 
made of Schott SF3 glass. Lens element 93 (whose surfaces from right to 
left are designated as Surface No. 4 and Surface No. 5) is made of Schott 
BAK1 glass. Lens elements 92 and 93 are cemented together, and therefore 
can be considered as sharing a common surface, viz., Surface No. 4. Lens 
element 94 (whose surfaces from right to left are designated as Surface 
No. 6 and Surface No. 7) is made of Schott SF3 glass. Lens element 95 
(whose surfaces from right to left are designated as Surface No. 7 and 
Surface No. 8) is made of Schott BAK1 glass. Lens elements 94 and 95 are 
cemented together and share a common surface, viz., Surface No. 7. The 
aperture stop for the collimator group is shown in FIG. 18, and is 
designated in Table I as Surface No. 9. 
The anamorphic beam-expander group of lens elements is afocal, and 
comprises five lens elements 96, 97, 98, 99 and 100, which are likewise 
designated from right to left in FIG. 16. The anamorphic beam-expander 
group is also achromatized at the operating wavelength (0.83 micron in the 
infrared region) and the testing and alignment wavelength (0.6328 micron 
in the visible region). Design parameters for the anamorphic beam-expander 
group are specified in Table II with reference to the base wavelength of 
0.83 micron as follows: 
TABLE II 
______________________________________ 
Surface 
Radius Thickness 
No. (mm) (mm) Material 
N.sub..83 (measured) 
______________________________________ 
1 -9.000 2.000 SF11 1.763321 
2 .infin. 1.000 Air 
3 -9.000 2.000 SF11 1.763321 
4 .infin. 4.305 Air 
5 -34.595 1.600 LF7 1.564422 
6 .infin. 3.000 PSK3 1.545105 
7 -11.078 0.500 Air 
8 .infin. 3.000 PSK3 1.545105 
9 -20.487 Air 
______________________________________ 
where the surfaces of the lens elements and the other optically significant 
surfaces of the anamorphic beam-expander group are numbered consecutively 
from right to left according to the convention explained above for Table 
I. 
In the anamorphic beam-expander group of lens elements shown in FIG. 16, 
lens element 96 (whose surfaces from right to left are designated in Table 
II as Surface No. 1 and Surface No. 2) is made of Schott SF11 glass. Lens 
element 97 (whose surfaces from right to left are designated as Surface 
No. 3 and Surface No. 4) is likewise made of Schott SF11 glass, and has 
the same dimensions and configurations as lens element 96. Surface No. 1 
of lens element 96 of the anamorphic beam-expander group abuts (but is not 
cemented to) Surface No. 8 of lens element 95 of the collimator group. 
Surface No. 3 of lens element 97 abuts (but is not cemented to) Surface 
No. 2 of lens element 96. The aperture stop for the collimator group (i.e. 
Surface No. 9 in Table I) is located 2 mm to the left of lens element 97 
of the anamorphic beam-expander group (i.e., Surface No. 4 in Table II). 
Lens element 98, whose surfaces from right to left are designated as 
Surface No. 5 and Surface No. 6 in Table II, is made of Schott LF7 glass. 
Lens element 99, whose surfaces from right to left are designated as 
Surface No. 6 and Surface No. 7 in Table II, is made of Schott PSK3 glass. 
Lens elements 98 and 99 are cemented together and share a common surface, 
via., Surface No. 6. Lens element 100, whose surfaces from right to left 
are designated as Surface No. 8 and Surface No. 9 in Table II, is made of 
Schott PSK3 glass. 
The imager group of the lens elements comprises three lens elements 101, 
102 and 103, which are also designated from right to left in FIG. 16. The 
imager group is likewise achromatized at the operating wavelength (0.83 
micron) and the testing and alignment wavelength (0.6328 micron). Design 
parameters for the imager group are specified in Table III with reference 
to the same base wavelength of 0.83 micron as follows: 
TABLE III 
______________________________________ 
Surface 
Radius Thickness 
No. (mm) (mm) Material 
N.sub..83 (measured) 
______________________________________ 
1 46.563 4.000 SK11 1.556329 
2 -30.940 2.000 SF4 1.735245 
3 -1172.490 0.250 Air 
4 33.561 3.500 BK7 1.510288 
5 -680.212 (To image) 
Air 
6 .infin. Air (Image plane) 
______________________________________ 
where the surfaces of the lens elements and the other optically significant 
surfaces of the imager group are numbered consecutively from right to left 
in accordance with the convention used above for Tables I and II. 
In the imager group of lens elements shown in FIG. 16, lens element 101 
(whose surfaces from right to left are designated in Table III as Surface 
No. 1 and Surface No. 2) is made of Schott SK11 glass. Lens element 102 
(whose surfaces from right to left are designated as Surface No. 2 and 
Surface No. 3) is made of Schott SF4 glass. Lens elements 101 and 102 are 
cemented together and share a common surface, viz., Surface No. 2. Lens 
element 103 (whose surfaces from right to left are designated as Surface 
No. 4 and Surface No. 5) is made of Schott BK7 glass. The optical path of 
the laser beam shaped by the optical train mounted in the bore 35 is 
doubly folded by the prismatic device 36 (as indicated in FIG. 16) so that 
the linear array of laser stripes on the laser device 67 is imaged at the 
intermediate ribbon field 45 (which is designated in Table III as Surface 
No. 6.) with a 4.8.times. magnification in the elongate dimension of the 
linear array and with only a 1.6.times. magnification in the dimension 
transverse to the elongate dimension. As described above, the image of the 
linear array of laser stripes at the intermediate ribbon field 45 is 
re-imaged at the projection plane with a 6.times. magnification. 
In FIG. 16, the extent of overlap of the transverse cross sections of the 
first and the fortieth "footprints" forming two of the components of the 
laser beam transmitted through the optical train in the bore 35 is shown 
in exaggerated view at various positions along the optical path of the 
laser beam. In FIG. 17, the transverse cross sections of the overlapping 
"footprints" are seen to be elliptical within the collimator group of lens 
elements, and to assume a progressively more circular configuration in 
passing through the anamorphic beam-expander group of lens elements. As 
shown in FIG. 16, the transverse cross sections of the overlapping 
"footprints" are generally circular within the imager group of lens 
elements. At an entrance surface of the prismatic device 36, the 
transverse cross sections of the overlapping "footprints" are circular, 
and remain circular at two successive reflecting surfaces within the 
prismatic device 36. 
The laser beam shaped by the optical train in the bore 34 and the laser 
beam shaped by the optical train in the bore 35 are focussed as line 
images (which in the cross-sectional view of FIG. 16 appear as points) at 
the intermediate ribbon field 45. However, as shown in enlarged view in 
FIG. 16A, each of the line images formed at the intermediate ribbon field 
45 is actually a linear array of images of the forty laser stripes on the 
corresponding one of the laser devices 66 and 67. After passing the 
intermediate ribbon field 45, the two laser beams begin to overlap each 
other and eventually coincide precisely with each other at the pupil 75. 
After passing through the pupil 75, the two laser beams begin to diverge 
from each other. At the mirror 43, the two laser beams are reflected 
through the transfer lens system and through the exit window 14 to the 
focal volume. At the focal volume, the images formed at the intermediate 
ribbon field 45 are re-imaged on the projection plane with a magnification 
six times larger than at the intermediate ribbon field 45. As indicated in 
FIG. 16B, each of the re-imaged images formed on the projection plane is a 
linear array of images of the forty laser stripes on the corresponding one 
of the laser devices 66 and 67. The linear arrays of re-imaged images 
shown in FIG. 16B are six times longer than the corresponding linear 
arrays of images shown in FIG. 16A. At the focal volume, the two laser 
beams assume the ribbon-shaped configurations and are spaced apart from 
each other as indicated in FIGS. 7 and 8. 
The transfer lens system at the exit window 14 comprises a first lens 
element 104, a second lens element 105, a first prism element 106 and a 
second prism element 107. The lens elements 104 and 105, acting in 
combination with each other, separate the two ribbon-shaped laser beams 
reflected from the mirror 43, and magnify the images formed on the 
intermediate ribbon field 45 so as to be six times larger when re-imaged 
on the projection plane at the focal volume. Design parameters for the 
lens elements 104 and 105 are specified in Table IV with reference to the 
base wavelength of 0.83 micron as follows: 
TABLE IV 
______________________________________ 
Surface 
Radius Thickness 
No. (mm) (mm) Material 
N.sub..83 (measured) 
______________________________________ 
1 467.256 5.000 SFl 1.699628 
2 117.490 0.500 Air 
3 117.490 9.500 SKll 1.556329 
4 -164.160 (To image) Air 
______________________________________ 
where the surfaces of the lens elements of the transfer lens system are 
numbered consecutively in the direction of propagation of the 
geometrically combined laser beams from the mirror 43 through the transfer 
lens system to the focal volume (i.e., from bottom to top in FIG. 16). The 
lens element 104 (whose surfaces from bottom to top in FIG. 16 are 
designated as Surface No. 1 and Surface No. 2) is made of Schott SF1 
glass. The lens element 105 (whose surfaces from bottom to top in FIG. 16 
are designated as Surface No. 3 and Surface No. 4) is made of Schott SK11 
glass. Surface No. 2 of lens element 104 has the same curvature (but of 
opposite sign) as Surface No. 3 of lens element 105, but a gap of 0.5 mm 
(which is too small to see in the scale of FIG. 16) is provided 
therebetween in order to facilitate mounting. 
The prism elements 106 and 107 are thin wedge-shaped prisms of 
substantially equal power, which are spaced apart from each other by a 
small distance (about 2 mm), and which can be rotated by conventional 
means in opposite directions with respect to an axis perpendicular to a 
plane in the space between them. The prism elements 106 and 107, acting 
together, are optically equivalent to a single prism of variable power 
(called a Risley prism). The power of the prism elements 106 and 107 
acting in combination can be varied from zero to a value that is twice the 
power of either prism element 106 or 107 alone, as the prism elements 106 
and 107 are rotated relative to each other from an arrangement in which 
the narrow edges of the two prism elements are opposed to each other to an 
arrangement in which the narrow edges of the two prism elements are 
coincident with each other. The extent to which a prism deviates a beam 
depends upon the power of the prism. Consequently, by rotatably adjusting 
the orientations of the prism elements 106 and 107, the deviation of the 
two ribbon-shaped laser beams transmitted by the lens elements 104 and 105 
can be correspondingly adjusted so as to align the ribbon-shaped laser 
beams with the field stop images on the projection plane. 
For the prototype embodiment of the velocimeter of the present invention, 
each of the prism elements 106 and 107 as illustrated in FIG. 16 has a 
thickness of 4.00 mm at its center and a wedge angle of 2.19 degrees. 
Since the wedge angles are small, dispersion caused by the prism elements 
106 and 107 is small so that visible light can be used for preliminary 
adjustment. The thickness and "air wedge" angle of the gap between the 
prism elements 106 and 107 is not critical. The prism elements 106 and 107 
are made of Schott K5 glass, which is chosen for its thermal stability. It 
is desirable to minimize any changes in optical deviation with changes in 
temperature. 
The side view shown in FIG. 17 of the collimator group and the anamorphic 
beam-expander group of lens elements positioned in the bore 35 is an 
enlargement of a corresponding portion of the optical train shown in FIG. 
16. In FIG. 18, a top view is shown of the lens elements comprising the 
collimator group and the anamorphic beam-expander group positioned in the 
bore 35, as seen along line 18--18 of FIG. 16. 
FlG. 19 illustrates geometrical details of one of the two pieces of glass 
comprising the prismatic device 36. The angular dimensions indicated in 
FIG. 19 are critical in the context of the particular geometry specified 
in Tables I, II and III for the optical trains positioned in the bores 34 
and 35 of the cylindrical component 23 of the mounting 12. It is to be 
recognized, however, that alternative design parameters for the optical 
trains could require different angular dimensions for the prismatic device 
36. A perspective view of the prismatic device 36 is shown in FIG. 20. 
With reference back to FIGS. 6A and 6B, the ribbon-shaped laser beams 
combined by the prismatic device 36 are caused to become separated by the 
lens transfer system adjacent the exit window 14 and are projected to the 
focal volume. Light scattered from the ribbon-shaped laser beams by 
aerosol particles passing through the focal volume is gathered by the 
collecting lens system at the entrance window 16. In the prototype 
velocimeter, the collecting lens system is a triplet comprising lens 
elements 48, 49 and 50 as illustrated in FIG. 6A. Design parameters for 
the collecting lens system are specified in Table V with reference to the 
base wavelength of 0.83 micron as follows: 
TABLE V 
______________________________________ 
Surface 
Radius Thickness 
No. (mm) (mm) Material 
N.sub..83 (measured) 
______________________________________ 
0 .infin. 2450.708 Air 
1 229.283 24.000 BK7 1.509924 
2 -589.788 0.500 Air 
3 193.853 26.880 BK7 1.509924 
4 -309.321 0.800 Air 
5 -314.091 12.500 SF6 1.783366 
6 507.365 264.202 Air 
______________________________________ 
where the surfaces of the lens elements are numbered consecutively in the 
direction of propagation through the entrance window 16 toward the 
beam-folding mirror 43 (i.e., from top to bottom in the illustration of 
FIG. 6A) of the portion of the light scattered from the ribbon-shaped 
laser beams that enters the entrance window 16. The mirror 43 folds the 
optical path of the gathered light, so that the image plane of the 
collecting lens system is located precisely at the field stop array plate 
51. 
The object plane of the collecting lens system (i.e., the projection plane 
at the focal volume) is designated in Table V as Surface No. 0, and has an 
infinite radius of curvature. The lens element 48 (whose surfaces from top 
to bottom in FIG. 6A are designated in Table V as Surface No. 1 and 
Surface No. 2) is made of Schott BK7 glass. The lens element 49 (whose 
surfaces from top to bottom in FIG. 6A are designated as Surface No. 3 and 
Surface No. 4) is made of Schott BK7 glass. The lens element 50 (whose 
surfaces from top to bottom in FIG. 6A are designated as Surface No. 5 and 
Surface No. 6) is made of Schott SF6 glass. 
In FIG. 21, the mounting 13, which supports the scattered-light receivers 
and the background-light receiver as shown in FIG. 6A, is illustrated in 
enlarged detail. The raison d'etre of the mounting 13 is to support the 
photodetectors 60, 61 and 62. If presently available photodetector devices 
could have been readily mounted immediately adjacent the field stop array 
plate 51 (which was the preferred design approach), the elaborate 
technique involving optically conducting fibers and relay lens systems as 
described hereinafter for transmitting light from the field stops on the 
field stop array plate 51 to (he corresponding photodetectors 60, 61 and 
62 would not have been necessary. However, the preferred design approach 
for mounting photodetectors for the scattered-light and the 
background-light was regrettably not a practical option for building the 
prototype velocimeter. In order to be able to use commercially available 
photodetector devices for the prototype velocimeter, the technique 
described hereinafter for mounting the scattered-light photodetectors and 
the background-light photodetector was used. 
The mounting 13 as shown in FIG. 21 is a hollow structure of generally 
circularly cylindrical configuration, which comprises three cylindrical 
sections, viz., an inner-end section 108, a middle section 109 and an 
outer-end section 110. The inner-end section 108 has a narrower distal 
portion (i.e., distal with respect to the end wall 11) and a wider 
proximal portion. The distal portion of the inner-end section 108 is 
dimensioned to fit tightly in a corresponding circular aperture provided 
therefor in the dividing wall 31, and the proximal portion of the 
inner-end section 108 is joined to a distal end of the middle section 109 
by a conventional technique (as by screws). Similarly, a proximal portion 
of the middle section 109 is joined to a distal portion of the outer-end 
section 110 by a conventional technique (as by screws). 
The fiber-optics support structure 53 is a circularly cylindrical plug with 
a screw-threaded exterior cylindrical wall, which is dimensioned to be 
received in the correspondingly screw-threaded axial bore in the distal 
portion of the inner-end section 108 of the mounting 13. As shown in FIG. 
21, the fiber-optics support structure 53 is screwed into the bore in the 
distal portion of the inner-end section 108, so that a flat end of the 
fiber-optics support structure 53 is flush with a flat end of the distal 
portion of the inner-end section 108. The field stop array plate 51 is of 
rectangular configuration, and is positioned against the flat end of the 
distal portion of the inner-end section 108 so as to bear against and 
cover the flat end of the fiber-optics support structure 53. 
Corresponding channels of elongate rectangular transverse cross section 
extending longitudinally through the fiber-optics support structure 53 
receive end portions of the fiber bundles 54, 55 and 56, so that the input 
ends of the fiber bundles 54, 55 and 56 abut the field stop array plate 51 
adjacent corresponding field stops. A cylindrical locking pin 111 extends 
through a hole in the field stop array plate 51 into a recess provided 
therefor in the flat end of the fiber-optics support structure 53. The 
locking pin 111 aligns the field stop array plate 51 with the rectangular 
channels through the fiber-optics support structure 53, so that the input 
ends of the fiber bundles 54, 55 and 56 are fixedly positioned adjacent 
the corresponding field stops on the field stop array plate 51. 
A circular cover plate 112 is secured to the flat end of the distal portion 
of the inner-end section 108 of the mounting 13 by means of screws 113. A 
surface of the cover plate 112 that bears against the flat end of the 
distal portion of the inner-end section 108 is recessed to receive the 
field stop array plate 51, thereby retaining the field stop array plate 51 
against the input ends of the fiber bundles 54, 55 and 56. The cover plate 
112 has a central aperture through which the field stops on the field stop 
array plate 51 are exposed to scattered light and background light 
focussed thereon by the collecting lens system mounted adjacent the 
entrance window 16. In the simplified illustration of FIG. 21, only the 
three fiber bundles 54, 55 and 56 are shown. Actually, there are four 
additional bundles of optically conducting fibers (i.e., seven fiber 
bundles in all), including two pairs of fiber bundles not shown in FIG. 21 
corresponding to the two other pairs of scattered-light photodetectors not 
shown in FIG. 21 for detecting light scattered from the two other pairs of 
ribbon-shaped laser beams at the focal volume. Each of the optically 
conducting fiber bundles 54, 55 and 56, as well as each of the four other 
fiber bundles not shown in FIG. 21, comprises six individual optical 
fibers. Each optical fiber is preferably of the type having a square 
transverse cross-sectional configuration, which is manufactured by 
Collimated Holes, Inc. of Santa Clara, California. 
Each ribbon-shaped laser beam projected to the focal volume is positioned 
by means of the Risley prism elements 106 and 107 so as to be in precise 
alignment on the projection plane with the image of a corresponding field 
stop on the field stop array plate 51. In FIG. 21, an elongate rectangular 
aperture (i.e., a slit) 114 on the field stop array plate 51 functions as 
a field stop for light that is gathered by the collecting lens system from 
the light that has been scattered from the ribbon-shaped laser beam 
generated by the laser device 66 as an aerosol particle enters the focal 
volume. Other slits (i.e., field stops) on the field stop array plate 51 
transmit light scattered from corresponding other ribbon-shaped laser 
beams at the focal volume. There is a corresponding field stop on the 
field stop array plate 51 dedicated to each ribbon-shaped laser beam at 
the focal volume. 
In FIG. 22, the input ends of the six optical fibers comprising the fiber 
bundle 54 are seen to be arranged in a (6.times.1) linear array (indicated 
in phantom outline), which is aligned with the slit 114. The dimensions of 
the six optical fibers comprising the fiber bundle 54 are such that the 
linear array formed by the input ends thereof has a rectangular 
configuration with a length that is longer and a width that is wider than 
the corresponding length and width of the slit 114. The slit 114 transmits 
background light as well as light scattered from the ribbon-shaped laser 
beam generated by the laser device 66, but blocks light scattered from any 
of the other ribbon-shaped laser beams at the focal volume. In this way, 
"cross-talk" between non-corresponding laser devices and photodetectors is 
diminished. 
Similarly, the input ends of the six optical fibers comprising the fiber 
bundle 56 are positioned adjacent a slit 115 on the field stop array plate 
51. As indicated in phantom outline in FIG. 22, the input ends of the six 
optical fibers comprising the fiber bundle 56 are arranged in a 
(6.times.1) linear array aligned with the slit 115. The dimensions of the 
six optical fibers comprising the fiber bundle 56 are such that the linear 
array formed by the input ends thereof has a rectangular configuration 
with a length and a width that are greater than the corresponding length 
and width of the slit 115. The slit 115 functions as the guardband field 
stop, which is positioned to receive ambient (i.e., "background") light 
that is focussed thereon by the collecting lens system mounted adjacent 
the entrance window 16, but to block substantially all the light scattered 
from any of the ribbon-shaped laser beams at the focal volume. 
Likewise, the input ends of the six optical fibers comprising the fiber 
bundle 55 are positioned adjacent a slit 116 (not shown in FIG. 22, but 
shown and labelled in FIG. 21 and in FIGURES discussed hereinafter) on the 
field stop array plate 51. The slit 116 is also of elongate rectangular 
configuration, and the input ends of the six optical fibers comprising the 
fiber bundle 55 are arranged in a (6.times.1) linear array aligned with 
the slit 116. The dimensions of the six optical fibers comprising the 
fiber bundle 55 are such that the linear array formed by the input ends 
thereof has a rectangular configuration with a length and a width that are 
greater than the corresponding length and width of the slit 116. The slit 
116 transmits light scattered from the ribbon-shaped laser beam generated 
by the laser device 67 as well as background light, but blocks light 
scattered from any of the other ribbon-shaped laser beams at the focal 
volume. 
As indicated in FIG. 22, the input ends of the optical fibers comprising 
each of the fiber bundles 54, 55 and 56 (as well as each of the four other 
fiber bundles not shown in FIG. 21) are arranged in a linear array for 
each fiber bundle, because the corresponding field stops on the field stop 
array plate 51 with which the input ends of the optical fibers are aligned 
are configured as elongate slits. However, the output ends of the same 
fiber bundles are reformatted at the corresponding connecting devices 57, 
58 and 59 into more compact (3.times.2) rectangular arrays, as shown in 
FIG. 23. The more compact (3.times.2) rectangular-array format for the 
output ends of the fiber bundles 54, 55 and 56 (as well as the four other 
fiber bundles not shown in FIG. 21) facilitates numerical aperture 
matching and optical formatting required by relay lens systems (described 
hereinafter) mounted in the outer-end section 110 of the mounting 13. 
There are four other field stops (in addition to the three slits 114, 115 
and 116 shown in FlG. 21) on the field stop array plate 51, which transmit 
scattered light to four corresponding other photodetectors from the four 
corresponding other ribbon-shaped laser beams at the focal volume. 
However, only the three field-stop slits 114, 115 and 116 are shown in 
FIG. 21 in order to simplify the illustration. This simplification is 
continued in FIGS. 24, 25 and 26, which likewise show only three of the 
seven field-stop slits actually provided on the field stop array plate 51. 
FIG. 24 shows the field stop array plate 51 (on which only three of the 
seven field stops are illustrated) positioned against the fiber-optics 
support structure 53 so that the field-stop slits 114, 115 and 116 are 
aligned with corresponding linear arrays of input ends of the optical 
fibers comprising the fiber bundles 54, 55 and 56, respectively. In FIG. 
25, the field stops defined by the slits 114, 115 and 116 are shown in 
perspective view in alignment with the input ends of the optical fibers 
comprising the fiber bundles 54, 55 and 56, respectively. 
The technique used to position the input ends of the optical fibers 
comprising the fiber bundles 54, 55 and 56 (as well as the input ends of 
the optical fibers comprising the four other fiber bundles not shown in 
FIG. 21) of the prototype velocimeter of the present invention in 
alignment with corresponding field stop slits on the field stop array 
plate 51 is indicated in FlG. 26, which is simplified to show only the 
three fiber bundles 54, 55 and 56. (Actually, there are seven fiber 
bundles aligned with seven corresponding field stop slits.) The linear 
array of input ends of the optically conducting fibers comprising the 
fiber bundle 54 is shown at one extremity (i.e., at the top) of a 
rectangular bore extending axially through the circularly cylindrical 
fiber-optics support structure 53, and the linear array of input ends of 
the optically conducting fibers comprising the fiber bundle 55 is shown at 
another extremity (i.e., at the bottom) of the same rectangular bore, 
where the designations "top" and "bottom" have reference to the 
orientation of the drawing sheet on which FIG. 26 is presented. A spacer 
117 is positioned in the rectangular bore in contact with the fiber bundle 
54, and a spacer 118 is positioned in the rectangular bore in contact with 
the fiber bundle 55, whereby a channel is formed between the spacers 117 
and 118. The linear array of input ends of the optically conducting fibers 
comprising the fiber bundle 56 is then inserted into the channel formed 
between the spacers 117 and 118. 
ln the actual prototype velocimeter of the present invention, the input end 
of the fiber bundle 56, which transmits background light to a relay lens 
system associated with the background-light photodetector 62, is 
positioned at the center of the rectangular bore through the fiber-optics 
support structure 53 in accordance with the simplified illustration in 
FIG. 26. However, the input ends of three pairs of fiber bundles, which 
transmit scattered light to relay lens systems associated with three 
corresponding pairs of scattered-light photodetectors (and not merely the 
single pair of fiber bundles 54 and 55 associated with a single pair of 
corresponding scattered-light photodetectors as shown in FIG. 26) must be 
positioned within the rectangular bore through the fiber-optics support 
structure 53. In FIG. 26, the spacers 117 and 118 are shown as having a 
rectangular transverse cross-sectional configuration. However, actually 
three pairs of spacers are needed to support the fiber bundles. The 
spacers are made with precisely dimensioned trapezoidal cross-sectional 
configurations to maintain the input ends of the seven fiber bundles 
(i.e., the fiber bundles 54, 55 and 56 plus the two other pairs of fiber 
bundles not shown in FIG. 26) in alignment with corresponding field stop 
slits on the field stop array plate 51. 
The linear arrays of input ends of the optically conducting fibers 
comprising each pair of fiber bundles supported by the fiber-optics 
support structure 53 are parallel to each other, and are aligned with a 
corresponding pair of field-stop slits that are parallel to each other on 
the abutting field stop array plate 51. However, the linear arrays of 
input ends of the optically conducting fibers of any one pair of fiber 
bundles are nonparallel to the linear arrays of input ends of the 
optically conducting fibers of each of the other two pairs of fiber 
bundles, and are aligned with corresponding pairs of field-stop slits that 
are nonparallel to each of the other two pairs of field-stop slits on the 
abutting field stop array plate 51. The spacing between the two linear 
arrays of input ends of the optically conducting fibers of each pair of 
fiber bundles (and therefore the spacing between the two slits of each 
pair of field-stop slits) is substantially the same for all three pairs of 
fiber bundles (and therefore for all three pairs of field-stop slits). 
With reference to FIG. 21, the proximal portion of the inner-end section 
108 of the mounting 13 has substantially the same external diameter as the 
middle section 109 and the outer-end section 110. Adjacent ends of the 
inner-end section 108 and the middle section 109 are outwardly flanged to 
accommodate a conventional attachment technique (e.g., screws). Likewise, 
adjacent ends of the middle section 109 and the outer-end section 110 are 
outwardly flanged to accommodate a similar coupling A disk 119, which is 
of generally circular configuration with an aperture at its center and six 
symmetrically arranged notches on its perimeter, extends transversely 
across the inner-end section 108 at a mid-position, preferably where the 
inner-end section 108 widens from the distal portion thereof to the 
proximal portion thereof. The disk 119 is secured in a conventional manner 
to an annular lip 120 projecting inwardly from the interior wall of the 
inner-end section 108. Another disk 121, which is likewise of generally 
circular configuration with an aperture at its center and six 
symmetrically arranged notches on its perimeter, extends transversely 
across the Proximal portion of the inner-end section 108, preferably where 
the inner-end section 108 is coupled to the middle section 109. The disk 
121 is secured in a conventional manner to an annular lip 122 projecting 
inwardly from the interior wall of the inner-end section 108. The fiber 
bundles 54 and 55 pass from the fiber-optics support structure 53 through 
corresponding peripheral notches on the disks 119 and 121 to the 
corresponding connecting devices 57 and 58, which are mounted on a 
circular disk 123 positioned against a proximal end of the middle section 
109. The fiber bundle 56 passes from the fiber-optics support structure 53 
through the central apertures on the disks 119 and 121 to the connecting 
device 59, which is centrally mounted on the disk 123. 
The outer-end section 110 of the mounting 13 is a solid circular cylinder 
with seven bores extending longitudinally therethrough parallel to each 
other, viz., an axial bore 124, and six other bores arranged symmetrically 
around the axial bore 124. The disk 123, which is positioned against the 
proximal end of the middle section 109, has a central aperture and six 
other apertures arranged symmetrically around the central aperture. The 
apertures in the disk 123 are aligned with corresponding bores in the 
outer-end section 110, when a distal end of the outer-end section 110 is 
coupled to the proximal end of the middle section 109. Thus, the central 
aperture of the disk 123 is aligned with the axial bore 124, and each of 
the six other apertures in the disk 123 is aligned with a corresponding 
one of the six other bores in the outer-end section 110. 
As shown in FIG. 21, the disk 123 is fitted into an annular recess at the 
proximal end of the middle section 109, and is sandwiched between the 
coupled middle section 109 and outer-end section 110. The connecting 
devices 57, 58 and 59 are secured to the disk 123 in a conventional manner 
at the respective apertures therein, thereby aligning the output ends of 
the fiber bundles 54, 55 and 56 with corresponding relay lens systems 
positioned in corresponding bores in the outer-end section 110 of the 
mounting 13. Thus, the connecting device 57 couples the output end of the 
fiber bundle 54 to the relay lens system mounted in a bore 125, the 
connecting device 58 couples the output end of the fiber bundle 55 to the 
relay lens system mounted in a bore 126, and the connecting device 59 
couples the output end of the fiber bundle 56 to the relay lens system 
mounted in the axial bore 124. Four other connecting devices (not visible 
in FIG. 21) couple the output ends of four other fiber bundles to four 
corresponding other relay lens systems mounted in four corresponding other 
bores (not visible in FIG. 21) in the outer-end section 110. Light 
transmitted by the depicted fiber bundles 54, 55 and 56 (and by the four 
other fiber bundles not shown in FIG. 21) to the corresponding relay lens 
systems mounted in the corresponding bores in the outer-end section 110 is 
focussed by the respective relay lens systems onto the corresponding 
depicted photodetectors 60. 61 and 62 (and the four other photodetectors 
not shown in FlG. 21). 
The bores 124, 125 and 126 (and the four other bores not shown in FIG. 21) 
are circularly cylindrical with substantially the same diameter. 
Corresponding optical elements of the relay lens systems mounted in the 
bores 124, 125 and 126 (and in the four other bores not shown in FIG. 21) 
are substantially identical to each other, except for filter plates 
(described hereinafter) that are individualized. The relay lens system 
mounted in the bore 126, which is representative of all the other relay 
lens systems, is illustrated in detail in FIG. 27. Each of the relay lens 
systems is achromatized at the same nominal infrared wavelength of 0.83 
micron and the same visible wavelength of 0.6328 micron. 
Each of the relay lens systems supported by the mounting 13 comprises 
optical elements that are mounted in a conventional manner inside a tube 
inserted into the corresponding bore. As shown in FIG. 27 for the 
representative relay lens system mounted in the bore 126, the optical 
elements comprising the relay lens system are mounted in a tube 127 that 
is inserted into the bore 126. The tube 127 has regions of different 
internal diameters (as described hereinafter) to accommodate different 
diameters for the lens elements comprising the relay lens system. The 
optical elements of the relay lens system consist of a collimator group, a 
filter group, and an imager group. The collimator group comprises four 
lens elements 130, 131, 132 and 133, which are mounted coaxially within 
the tube 127. The filter group comprises a filter assembly 134 consisting 
of a plane-parallel filter plate mounted in a metal holder, which is 
inserted through a hole 135 provided for the purpose in the cylindrical 
wall of the output-end section 110 of the mounting 13, and through a hole 
aligned therewith in the wall of the tube 127, into the optical path of 
light passing through the relay lens system in the bore 126. The imager 
group comprises five lens elements 136, 137, 138, 139 and 140. 
The filter plate of the filter assembly 134 is a conventional dielectric 
multilayer narrowband filter having a center wavelength that is precisely 
matched to the measured output wavelength of the laser device 67, which 
produces the laser beam from which scattered light is transmitted by the 
field-stop slit 116 via the fiber bundle 55 to the relay lens system 
mounted in the bore 126. The bandpass of the filter plate of the filter 
assembly 134 is a relatively narrow 5 nm about the center wavelength. 
Similarly, the filter plate of each of the filter assemblies inserted into 
the corresponding other bores disposed around the axial bore 124 has an 
individualized center wavelength, which is precisely matched to the 
measured output wavelength of the corresponding laser device that produces 
the particular laser beam from which scattered light is transmitted by the 
corresponding field stop on the field stop array plate 51 to the relay 
lens system in the corresponding bore. The filter plate of each filter 
assembly is custom-made so as to have a center wavelength that precisely 
matches the measured output wavelength of its corresponding laser device. 
Consequently, whenever a laser device needs to be replaced, the 
corresponding filter assembly must also be replaced. The filter assembly 
134 is secured in the bore 126 by a set screw (not visible in the 
cross-sectional view of FIGS. 21 and 27), which is accessible to 
facilitate replacement of the filter assembly 134. Similarly, the filter 
assemblies inserted into the other bores disposed around the axial bore 
124 are secured in their corresponding bores by set screws, which are 
likewise accessible to facilitate replacement of the filter assemblies. 
As shown in FIG. 21, the filter group of the relay lens system mounted in 
the axial bore 124 consists of a permanently installed filter plate. The 
filter plate in the axial bore 124 has a center wavelength of 0.83 micron 
with a bandpass of 5 nm about the center wavelength, and reduces 
background radiation transmitted by the guardband field-stop slit 115 via 
the fiber bundle 56 to the relay lens system mounted in the axial bore 
124. Filter assemblies with filter plates that are customized to match 
specified wavelengths are available from Barr Associates, Inc. of 
Westford, Massachusetts. 
As indicated in FIG. 27, the internal diameter of the tube 127 is smallest 
at a distal end thereof through which light from the output end of the 
fiber bundle 55 passes into the relay lens system mounted in the tube 127. 
The lens elements 130 and 131 are cemented together, and are placed as a 
unit within the tube 127 so that a peripheral portion of the lens element 
130 abuts an annular ridge 141 formed where the internal diameter of the 
tube 127 widens from its smallest value. The lens elements 130 and 131 are 
secured in place between the ridge 141 and another annular ridge 142, 
which is formed where the internal diameter of the tube widens still 
further to accommodate marginal rays of the light passing therethrough. An 
air gap of wider diameter than the diameter of the lens elements 130 and 
131 intervenes between the lens element 131 and the lens element 132. The 
lens elements 132 and 133 are cemented together, and are secured in place 
as a unit within the tube 127 so that a peripheral Portion of the lens 
element 132 abuts an annular ridge 143 formed where the internal diameter 
of the tube 127 widens still further to accommodate marginal rays. An air 
gap having the same diameter as the lens elements 132 and 133 intervenes 
between the lens element 133 and the lens element 136. The tube 127 is 
positioned within the bore 126 so that the hole in the wall of the tube 
127 through which the filter assembly 134 is received is aligned with the 
hole 135 in the wall of the output-end section 110. The filter assembly 
134 is inserted into the air gap between the lens element 133 and the lens 
element 136. 
The lens element 136 is secured in place within the tube 127 so as to abut 
an annular ridge 144, which is formed where the internal diameter of the 
tube 127 widens still further to its widest value. A cylindrical tube 145 
in which the lens element 137 is mounted is then inserted into the tube 
127 with a tight fit so as to abut a peripheral portion of the lens 
element 136. A peripheral portion of the lens element 137 abuts an annular 
lip projecting inwardly from the interior wall of the tube 145 at a distal 
end thereof. The lip at the distal end of the tube 145 has a longitudinal 
dimension that provides a precisely determined spacing between the lens 
element 136 and the lens element 137. 
A cylindrical tube 146 in which the lens elements 138 and 139 are mounted 
is then inserted into the tube 127 with a tight fit so as to abut a 
proximal end of the tube 145. The lens elements 138 and 139 are cemented 
together, and are inserted as a unit into the tube 146 so that a 
peripheral portion of the lens element 138 abuts a lip at a distal end of 
the tube 146. The tube 145 has a length that, together with the 
longitudinal dimension of the lip at the distal end of the tube 146, 
provides a precisely determined spacing between the lens elements 137 and 
138. The lens element 140 is mounted in an end plug 147, which is inserted 
into the bore 126 adjacent the proximal end of the output-end section 110 
after the tube 127 (with the lens elements comprising the relay lens 
system mounted therein) has been positioned within the bore 126. A 
screw-threaded female extension on a distal end portion of the end plug 
147 matingly engages a correspondingly screw-threaded male extension on a 
proximal end portion of the tube 127, whereby the end plug 147 is secured 
to the tube 127. 
The end plug 147 has a bore of truncated conical configuration aligned 
coaxially with the cylindrical axis of the tube 127. The lens element 140 
is configured and dimensioned so as to be received within the bore of the 
end plug 147 at a distal end thereof, and a snap ring 148 is received in 
an annular groove provided on the conical surface adjacent the distal end 
of the end plug 147. The snap ring 148 bears against a peripheral portion 
of the lens element 140, and thereby retains the lens element 140 in 
position in the end plug 147. A proximal end portion of the end plug 147 
is configured to receive the photodetector device 61, as shown in phantom 
outline in FIG. 27. The photodetector device 61 is protected by a 
plane-parallel transparent glass window 149, which is provided by the 
manufacturer as an integral part of the photodetector device 61. An 
annular groove is provided on an interior cylindrical surface of the end 
plug 147 adjacent the proximal end thereof to receive a snap ring 150, 
which bears against the photodetector device 61 to retain the 
photodetector device 61 within the end plug 147. A snap ring 151 is 
received in an annular groove on an interior surface portion of the bore 
126 adjacent the proximal end thereof to retain the end plug 147 and the 
associated photodetector 61 securely within the bore 126. 
The relay lens system mounted in the bore 126, as shown in FIG. 27, 
collects and collimates the light emanating from the output ends of the 
compact (3.times.2) rectangular array of optically conducting fibers 
comprising the fiber bundle 55, spectrally filters the light so collected, 
and images the rectangular array of output ends of the optically 
conducting fibers onto the photosensitive region of the photodetector 
device 61. The collimator group and the imager group are each separately 
achromatized at the operating wavelength of 0.83 micron (in the infrared 
region) and the testing and alignment wavelength of 0.6328 micron (in the 
visible region). Design parameters for the relay lens system mounted in 
any one of the bores 124, 125 and 126 (and in any one of the four other 
bores not seen in FIG. 21) are specified in Table VI with reference to 
0.83 micron as a base wavelength as follows: 
TABLE VI 
______________________________________ 
Surface 
Radius Thickness 
No. (mm) (mm) Material 
N.sub..83 (catalog) 
______________________________________ 
0 .infin. 3.635 Air 
1 -18.217 2.500 SF14 1.741403 
2 18.217 4.550 BK7 1.510206 
3 -5.931 5.500 Air 
4 46.010 2.000 SF14 1.741403 
5 14.940 4.350 BK7 1.510206 
6 -14.940 1.000 Air 
7 .infin. 2.000 BK7 1.510206 
8 .infin. 1.000 Air 
9 48.750 3.485 BK7 1.510206 
10 -14.285 1.567 Air 
11 -11.146 1.600 SF4 1.735580 
12 -41.135 6.391 Air 
13 11.557 3.840 K10 1.494296 
14 -11.557 2.000 SF6 1.782732 
15 -19.431 3.178 Air 
16 4.770 3.907 LAFN2 1.731382 
17 6.840 0.500 Air 
18 .infin. 1.200 K10 1.494296 
19 .infin. 1.142 Air 
20 (Image) 
______________________________________ 
where the surfaces of the lens elements and the other optically significant 
surfaces of the relay lens system are numbered consecutively in the 
direction of propagation of light therethrough (i.e., from left to right 
in FIG. 27). The image plane of the relay lens system is located at the 
surface of the photodetector device 61. 
The object plane of the relay lens system shown in FIG. 27 is the surface 
of the field stop array plate 51 abutting the fiber-optics support 
structure 53. The object plane is designated in Table VI as Surface No. 0. 
In the collimator group of lens elements of the relay lens system, lens 
element 130 (whose surfaces from left to right in FIG. 27 are designated 
in Table VI as Surface No. 1 and Surface No. 2) is made of Schott SF14 
glass. Lens elements 130 and 131 are cemented together and share a common 
surface, viz., Surface No. 2. Lens element 131 (whose surfaces are 
designated as Surface No. 2 and Surface No. 3) is made of Schott BK7 
glass. Lens element 132 (whose surfaces are designated as Surface No. 4 
and Surface No. 5) is made of Schott SF14 glass. Lens elements 132 and 133 
are cemented together and share a common surface, viz., Surface No. 5. 
Lens element 133 (whose surfaces are designated as Surface No. 5 and 
Surface No. 6) is made of Schott BK7 glass. 
The filter plate of the filter assembly 134, which is positioned between 
the collimator group and the imager group, has substantially planar 
surfaces designated (from left to right in FIG. 27) as Surface No. 7 and 
Surface No. 8. Each of the filter plate surfaces is listed in Table VI 
with infinite radius of curvature. The filter plate is conventional, and 
preferably comprises a pair of glass substrates between which a dielectric 
multilayer narrowband filter coating is sandwiched. The substrates of the 
filter plates used for the prototype velocimeter of the present invention 
are made of Schott BK7 glass. 
In the imager group of lens elements shown in FIG. 27, lens element 136 
(whose surfaces from left to right are designated in Table VI as Surface 
No. 9 and Surface No. 10) is made of Schott BK7 glass. Lens element 137 
(whose surfaces are designated as Surface No. 11 and Surface No. 12) is 
made of Schott SF4 glass. Lens element 138 (whose surfaces are designated 
as Surface No. 13 and Surface No. 14) is made of Schott K10 glass. Lens 
elements 138 and 139 are cemented together and share a common surface, 
viz., Surface No. 14. Lens element 139 (whose surfaces are designated as 
Surface No. 14 and Surface No. 15) is made of Schott SF6 glass. Lens 
element 140 (whose surfaces are designated as Surface No. 16 and Surface 
No. 17) is made of Schott LAFN2 glass. 
The window 149 of the photodetector device 61, which is provided by the 
manufacturer as an integral part of the photodetector device 61, has two 
substantially planar surfaces (i.e., surfaces of infinite radius of 
curvature) that are listed in Table VI as Surface No. 18 and Surface No. 
19. The window 149 is made of Schott K10 glass. The image plane of the 
relay lens system, which coincides with the surface of the photodetector 
device 61, is listed in Table VI as Surface No. 20. 
In FlG. 28, the projection plane is depicted lying in the plane of the 
paper. The three pairs of ribbon-shaped laser beams projected from the 
velocimeter to the focal volume intersect the projection plane in three 
corresponding pairs of line segments, which are illustrated schematically 
in FIG. 28 by a first pair of line segments each of which is labelled (1), 
a second pair of line segments each of which is labelled (2), and a third 
pair of line segments each of which is labelled (3). The lengths of the 
line segments of the pairs (1), (2) and (3) are established by the 
predetermined dimension W for the height of the focal volume as indicated 
in FIG. 7. The Risley prism formed by the prism elements 106 and 107 are 
adjusted (as explained above) to align the line segments of the pairs (1), 
(2) and (3) with the images on the projection plane of corresponding pairs 
of field stop slits on the field stop array plate 51. 
Also illustrated in FIG. 28, but in phantom outline, is a line segment (4) 
representing background light that is "uncontaminated" by light scattered 
from any of the ribbon-shaped laser beams at the focal volume. Of course, 
background light is not focussed as a ribbon-shaped beam onto the 
projection plane, and hence does not actually intersect the projection 
plane in a line segment. However, the line segment (4) also represents the 
image on the projection plane of the guardband field-stop slit 115. 
Therefore, the "uncontaminated" background light that is gathered from the 
focal volume and focussed onto the guardband field-stop slit 115 for 
transmission via the fiber bundle 56 and the optical train in the axial 
bore 124 to the photodetector 62 can be represented schematically as the 
line segment (4) in FIG. 28 to indicate that the "uncontaminated" 
background light present at the projection plane is measured in the same 
way that the combination of scattered light and background light is 
measured for each of the ribbon-shaped laser beams intersecting the 
projection plane. To measure the light actually scattered from each of the 
ribbon-shaped laser beams by an aerosol particle passing through the focal 
volume, the measured "uncontaminated" background light is subtracted by 
conventional electronic means from the combination of scattered light and 
background light measured for each of the ribbon-shaped laser beams. 
The two ribbon-shaped laser beams of each pair projected from the 
velocimeter are parallel to each other at the focal volume. Thus, in FIG. 
28, the two line segments of the pair labelled (1) are parallel to each 
other, the two line segments of the pair labelled (2) are parallel to each 
other, and the two line segments of the pair labelled (3) are parallel to 
each other. Also, the spacing between the two ribbon-shaped laser beams of 
each pair is the same for all three pairs at the focal volume. Thus, in 
FIG. 28, the spacing between the two line segments of the Pair labelled 
(1) is equal to the spacing between the two line segments of the pair 
labelled (2), which is equal to the spacing between the two line segments 
of the pair labelled (3). Furthermore, the two ribbon-shaped laser beams 
of each pair are nonparallel to the ribbon-shaped laser beams of the other 
two pairs of laser beams at the focal volume. Thus, as shown in FIG. 28, 
the two line segments of the pair labelled (1) are nonparallel to the two 
line segments of the pair labelled (2), which are nonparallel to the two 
line segments of the pair labelled (3). 
As illustrated in FIG. 28, point A is a point midway between the two line 
segments of the pair labelled (1), point B is a point midway between the 
two line segments of the pair labelled (2), and point C is a point midway 
between the two line segments of the pair labelled (3), where points A, B 
and C are collinear. A coordinate system can be established to specify 
positions on the projection plane. Thus, as indicated in FIG. 28, an axis 
through the collinear midpoints A, B and C between respective pairs of 
ribbon-shaped laser beams can be designated as the X-axis, and an axis 
that intersects the X-axis (not necessarily orthogonally) at one of the 
midpoints (say, the midpoint B) can be designated as the Y-axis. 
It is convenient to define a "transmitter plane", which is a plane on a 
surface of the velocimeter that includes the three exit windows 
(corresponding to the three pairs of ribbon-shaped laser beams projected 
to the focal volume) and also the entrance window 16. The transmitter 
plane is parallel to the projection plane. In the simplified illustration 
of FIG. 2, the transmitter plane coincides with the portion of the 
anterior wall 15 of the housing structure 10 on which the exit window 14 
and the entrance window 16 are shown. In an actual velocimeter according 
to the present invention, there are three exit windows 14', 14" and 14'" 
on the transmitter plane as shown schematically in FIG. 29. In the 
simplified illustration of FIG. 3, the general direction of a single pair 
of ribbon-shaped laser beams projected from the exit window 14 to the 
projection plane at the focal volume is indicated by the broken line 19. 
In FIG. 29, the directions of the three pairs of laser beams projected 
from the exit windows 14', 14" and 14'", respectively, to the projection 
plane at the focal volume are indicated by the corresponding broken lines 
19', 19" and 19'". The general direction of the portion of the light 
scattered at the focal volume that is gathered by the transfer lens system 
at the entrance window 16 is indicated in both FIGS. 3 and 29 by the 
broken line 20. 
In FIG. 29, the two-dimensional coordinate system of FIG. 28 is extended to 
three dimensions, where the third dimension is referred to a Z-axis that 
extends to the intersection of the X- and Y-axes on the projection plane 
from a selected point on the transmitter plane (e.g., from a point on the 
optic axis of the transfer lens system at the entrance window 16). Thus, 
the Z-axis coincides with the general direction of the light gathered at 
the entrance window 16, as indicated by the broken line 20. 
The two laser beams of each pair projected from the velocimeter to the 
focal volume, which assume ribbon-shaped configurations and are spaced 
apart from each other by the predetermined distance d.sub.c at the 
projection plane, actually overlap each other at the transmitter plane. 
Nevertheless, it is convenient to designate a "midpoint" (in the sense of 
a mathematical centroid) between the two laser beams of each pair, where 
the two overlapping laser beams of each pair intersect the transmitter 
plane. Thus, as illustrated in FIG. 29, point b designates the point on 
the transmitter plane midway between the two laser beams of the pair 
projected from the exit window 14'. The midpoint of the intersections with 
the projection plane of the two laser beams projected from the exit window 
14' is point B. Similarly, point c designates the point on the transmitter 
plane midway between the two laser beams of the pair projected from the 
exit window 14"', and the midpoint of their intersections with the 
projection plane is point C, Likewise, point a designates the point on the 
transmitter plane midway between the two laser beams of the pair projected 
from the exit window 14"'. and the midpoint of their intersections with 
the projection plane is point A. 
The broken line 19'" in FIG. 29 extends from point a on the transmitter 
plane to point A on the projection plane, and indicates the general 
direction from the exit window 14'" of the pair of ribbon-shaped laser 
beams whose intersections with the projection plane produce the pair of 
line segments labelled (1) in FIG. 28. Similarly, the broken line 19' 
extends from point b on the transmitter plane to point B on the projection 
plane, and indicates the general direction from the exit window 14' of the 
pair of ribbon-shaped laser beams whose intersections with the projection 
plane produce the pair of line segments labelled (2) in FIG. 28. Likewise, 
broken line 19" extends from point c on the transmitter plane to point C 
on the projection plane, and indicates the general direction from the exit 
window 14" of the pair of ribbon-shaped laser beams whose intersections 
with the projection plane produce the pair of line segments labelled (3) 
in FIG. 28. In the perspective view of FIG. 29, it is seen that the 
directions of the three points of ribbon-shaped laser beams projected from 
the velocimeter to the focal volume (i.e., the directions of the broken 
lines 19', 19" and 19"') assume a tripod-like arrangement, where the 
broken lines 19', 19" and 19"' appear as the symmetrically disposed 
converging legs of a tripod whose base is a circle passing through the 
centers of the exit windows 14', 14" and 14"' on the transmitter plane. 
Each point in the volume between the transmitter plane and the projection 
plane can be specified in terms of coordinates (R, .theta., Z) of a 
cylindrical coordinate system, whose origin (O, O, O) is the intersection 
on the transmitter plane of the X-, Y-, and Z-axes. The Z-axis coincides 
with the general direction of the portion of the light scattered from the 
ribbon-shaped laser beams at the focal volume that is gathered at the 
entrance window 16. As illustrated in FIG. 29, R represents a radial 
distance on the transmitter plane measured from the origin, and .theta. 
represents an angular distance on the transmitter plane measured from the 
X-axis. Z represents an axial distance measured from the transmitter plane 
to the projection plane. All points on the projection plane can be mapped 
in one-to-one correspondence onto the transmitter plane. Thus, 
light-scattering events occurring in the focal volume can be precisely 
located in space as well as in time by conventional electronic means, 
because each ribbon-shaped laser beam (i.e. each source from which 
scattered light is generated) intersects the projection plane in a 
corresponding line segment that is precisely mapped onto the transmitter 
plane. 
The present invention has been described above in terms of a particular 
embodiment designed for prototype evaluation. However, other embodiments 
of the present invention for different applications would be apparent to 
practitioners skilled in the relevant art upon perusal of the foregoing 
description and the accompanying drawing. Therefore, the description and 
drawing presented herein are to be understood as being merely illustrative 
of the invention, which is defined by the following claims and their 
equivalents.