Optical monitoring system apparatus

An optical monitoring system maps high aspect ratio regions of space using an imaging or viewing device that has a field of view with a low aspect ratio and a plurality of mirrors positioned and arranged relative to the imaging device and the region of space such that the plurality of mirrors directs the image of the region of space to the imaging device in a plurality of segments. Each of the segments is smaller than the field of view of the imaging device and all segments fit together in the field of view of the imaging device such that all of the region of space is mapped and monitored by the single imaging device. The present invention eliminates the need for several imaging devices to monitor high aspect ratio regions of space.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for monitoring a high aspect 
ratio region or scene using a rectangular, low aspect ratio monitoring 
apparatus. In particular, the present invention relates to monitoring a 
highly repetitive scene for changes that can be addressed in real-time. 
2. Description of the Related Art 
The advent of charge coupled device (CCD) cameras and high speed digital 
computers has made it possible to automatically monitor objects and/or 
phenomenon, detect changes and take actions in response to detected 
changes without human intervention. Furthermore, with sufficient 
processing speed on the part of the computer, monitoring can be 
accomplished in real-time. Automated, real-time monitoring systems that 
combine cameras and computers have been used in applications ranging from 
intruder detection/security systems to the monitoring of automated 
assembly and manufacturing systems. These automated monitoring systems are 
particularly effective in applications where the reaction time of human 
monitors is too slow, where the safety of a human monitor is of concern 
and/or where the human attention span is a problem. 
In general, an automatic monitoring system includes one or more cameras 
connected to a computer system. The camera(s) observe and record a scene 
containing the objects or region of space to be monitored and the computer 
system analyzes the recorded information from the scene. The scene can be 
thought of as a region of space containing information concerning the 
monitored objects. The monitoring system camera acts to "image" or map a 
portion of the scene within the field of view (FOV) of the camera onto a 
focal plane within the camera. The imaged portion of the scene on the 
focal plane is converted into electrical signals by the camera and 
transmitted to the computer for processing and information extraction. 
Both the camera's FOV and the scene are characterized, at least in part, 
by an aspect ratio. Most commercially available cameras have a FOV with 
aspect ratios on the order of one which is considered a low aspect ratio. 
The aspect ratio of the scene depends on the application and can range 
from a low aspect ratio on the order of one to a very high aspect ratio 
that is much greater than twenty. 
Among high aspect ratio monitoring applications are monitoring a horizon 
for the appearance or disappearance of objects, monitoring objects that 
are arranged in single rows and/or oriented parallel to each other, 
monitoring objects that are moving through the scene in a parallel manner, 
and the monitoring and subsequent identification of objects moving through 
a scene by detecting markings or indicia on the objects. In each of these 
applications, the information in the scene critical for monitoring 
purposes is either necessarily confined to a narrow band as in the case of 
horizon monitoring or is completely represented by a narrow band of the 
scene. This concept is illustrated in FIG. 1 where a scene 2 consisting of 
a row of trees 3 is being monitored. The presence or absence of an object 
such as an individual tree 4 can be detected by its presence or absence in 
a narrow band 5 within the FOV of a typical camera as represented by a 
rectangular box 6. The camera maps the portion of the scene 2 in the 
camera's FOV 6 to an image 7 at the cameras focal plane. The narrow band 5 
that contains the information about the scene 2 has a corresponding narrow 
band 8 in the image 7 within the camera. The information extraction 
processing within the computer likewise can be confined to the portion of 
the image 7 contained in the image's narrow band 8. Therefore, portions of 
the image above and below the narrow band 8, i.e. portions 9 and 10, 
respectively are unnecessary for the monitoring process. 
An example of a monitoring application involving objects that move through 
a scene is non-contact monitoring of warp threads feeding into a modem 
textile loom. The warp threads feed into the loom in parallel. An 
arrangement of cameras coupled to a suitably programmed computer system 
can monitor the warp threads in real-time as they are fed into the loom 
enabling real-time detection of broken threads. An example of a monitoring 
and identification application is the reading of barcodes on train cars 
moving on a track or boxes on a moving conveyor belt. Since similar 
principles apply to other applications of real-time monitoring using 
camera/computer systems the example of loom monitoring will be used almost 
exclusively hereinbelow with the understanding that the discussion can 
readily be applied to many other endeavors by one skilled in the art. 
The monitoring of the input of a loom by camera/computer monitoring systems 
is illustrated diagrammatically in FIG. 2a. FIG. 2a shows a pair of 
cameras 11, a computer 12, and a loom 14. The computer provides automatic 
processing of the images recorded by the cameras 11. The loom 14 has an 
input region 16 where warp threads 18 enter the loom 14 in a parallel 
manner. The FOV of the camera is represented in FIG. 2a as a rectangular 
box 20. The computer 12 is programmed to detect the presence of the 
threads as they move through the camera's FOV 20. If a thread should break 
the computer 12 detects the break by the difference in the image before 
and after the break or simply by the absence the broken warp thread 18. 
The methods employed in detection within the computer are beyond the scope 
of this discussion. 
Referring once again to FIG. 2a it is evident that much of the FOV 20 of 
camera 11 and the resulting image in the camera 11 is of little or no use 
to the monitoring system. Since it is of no use, much of the image 
captured by the camera is wasted. Since the warp threads 18 pass through 
the camera's FOV 20, detection of a broken thread 18 can be accomplished 
by considering only a narrow band or box 19 perpendicular to the motion of 
the threads 18 within the FOV 20 as was illustrated in FIG. 1 for box 5. 
In fact, all of the warp threads 18 in a loom 14 with width LW could be 
monitored by considering the long narrow box 19. Also as in FIG. 1, the 
portions of the image corresponding to the portions of the FOV 20 above 
and below the intersection of the narrow box 19 and the FOV 20 are 
effectively wasted portions of the FOV 20 and consequently wasted portions 
of the image. 
The importance of the wasted of portions of the image become apparent when 
considering the number of camera's 11 necessary to monitor a given scene. 
To detect an object, an image of the object must be larger in size than 
the minimum resolution in the camera that is required to ensure detection 
of an object. In cameras typically used for monitoring purposes two 
primary factors limit the minimum resolution. These two factors are the 
diffraction resolution limit imposed by the optics and the number of 
pixels or picture elements in the focal plane of the camera. In most cases 
the optics of camera in monitoring applications are chosen such that the 
ultimate resolution limit is a function of the number of discrete pixels 
in the focal plane and not the diffraction limit of the optics. 
The pixels in the focal plane of the camera generally correspond to the 
individual sensor elements situated in the camera's focal plane. In a CCD 
camera the sensors are the charge coupled devices that give the camera its 
name. These charge coupled devices are generally arranged in a regular, 
rectangular lattice. A minimum resolution for a modem camera/computer 
monitoring system is set by the portion of the image that corresponds to a 
single pixel which in the CCD camera example is a single CCD cell. 
The minimum resolution of a camera limits the amount of the scene that can 
be captured by a single fixed camera. Therefore, it is impractical or 
impossible to monitor a textile loom with a single camera monitoring 
system. Typically, an array of cameras is required to monitor the width or 
linear extent of most modem textile looms, thereby increasing the expense 
of the monitoring system. Likewise, other high aspect ratio monitoring 
applications will require multiple camera when minimum resolution is 
considered. In the illustrated case of FIG. 2 two cameras 11 are depicted. 
In actual application many more cameras 11 would likely be required. 
Another drawback to monitoring systems known in the art is that they 
require cameras 11 with optics having very a large depth of field. The 
depth of field must be large in these systems since the distance from the 
camera to the center of the field of view or scene is much less than the 
distance from the camera to the edge of the field of view. It is difficult 
to achieve optics that simultaneously exhibit a large depth of field and a 
long focal ratio as is required for high resolution imaging. 
Many of these same principles apply to the monitoring and identification 
application. FIG. 2b illustrates a monitoring and identification 
application wherein a camera 11 is used to read barcodes 64 on objects 62 
passing through the FOV 20 of the camera 11 on a conveyor belt system 60. 
Since the FOV 20 of the camera 11 is restricted by the resolution 
considerations discussed above, multiple cameras 11 would be required in 
the case illustrated in FIG. 2b to ensure that all barcodes 64 are 
detected regardless of the object 62 size and barcode 64 position on the 
object. The single camera 11 illustrated in FIG. 2b would fail to detect 
barcodes 64 that are positioned above the FOV 20. 
The discussion herein is based on commercially available cameras having a 
rectangular image plane with a low length to width ratio hereinafter 
referred to as aspect ratio. A low aspect ratio combined with the finite 
imaging resolution afforded by the focal plane array renders the existing 
camera/computer systems inefficient for the loom monitoring purposes. 
As discussed hereinabove it is a characteristic of many monitoring 
applications such as the aforementioned loom monitoring application, that 
the information containing region of the image is confined to a narrow 
horizontal or vertical band within the field of view of the camera. Cases 
where long narrow images are to be monitored severely under-utilize the 
available imaging region of the focal plane of the camera. Therefore, 
better utilization of the available imaging region with fewer cameras in a 
given monitoring system would solve a long standing need in the art. 
SUMMARY OF THE INVENTION 
The present invention solves the problem in the related art by providing a 
system for monitoring regions of space or scenes that have high aspect 
ratios using an imaging device with a low aspect ratio. 
The system of the present invention maps a long narrow object space or 
object plane ("scene") onto a low aspect ratio image plane of the imaging 
or viewing device. The system of the present invention comprises an array 
of mirrors positioned in the field of view of the imaging device between 
the scene and the imaging device that enables a high aspect ratio scene of 
a region of space to be efficiently viewed in a low aspect ratio, 
rectangular field of view of the imaging device, for example a typical 
camera. The mirrors that make up the array of mirrors are arranged in such 
a way as to form images of individual segments of the objects in the long 
narrow scene being monitored on successive vertically stacked segments of 
a rectangular image plane of the imaging device. The arrangement of the 
mirrors in the mirror array is such that all of the individual object 
plane segments are in focus when projected onto the focal plane of the 
imaging device. 
In the preferred embodiment the imaging means is a camera such as a CCD 
camera. The system may also comprise a computer for processing the 
information received by the imaging means to provide real-time monitoring 
of the status of an object or scene. 
Examples of monitoring situations with high aspect ratio scenes include 
monitoring the horizon for the appearance or disappearance of objects, 
monitoring objects that are arranged in single rows and/or oriented 
parallel to each other, and monitoring objects that are moving through the 
scene in a parallel manner such as monitoring textile or fiber 
manufacturing looms for broken threads or other defects and the monitoring 
and subsequent identification of objects moving through a scene by 
detecting markings or indicia on the objects. In each of these 
applications, the information in the scene critical for monitoring 
purposes is either necessarily confined to a narrow band as in the case of 
horizon monitoring or is completely represented by a narrow band of the 
scene. Among the chief advantages of the present invention is that a 
single means for imaging can be used where multiple means for imaging are 
required without the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the monitoring system of the present invention 
is illustrated in block diagram form in FIG. 3. The optical monitoring 
system monitors a scene 26 such as warp threads 18 at the input of a loom, 
and comprises a mirror array 22 and a means for imaging or an imaging 
apparatus such as a camera 11. In the preferred embodiment the imaging 
apparatus is a video camera with a charge coupled device (CCD) focal plane 
array. An essentially equivalent alternative camera 11 is an analog video 
camera with an analog to digital converter that converts the video frames 
from an analog form into a digital form suitable for processing in the 
computer 12. A lens assembly 24 is used to focus an image onto the 
camera's focal plane. A typical scene 26 herein has an aspect ratio AR 
equal to 20.0+/-10.0 or more while the camera's focal plane and the 
corresponding FOV have an AR equal to 2.0+/-0.5. 
The principle element of the monitoring system of the present invention is 
the mirror array 22. The novel action of the mirror array 22 when used in 
the context of the present invention is to enable a single camera 11 with 
a low aspect ratio FOV to monitor a very high aspect ratio scene. The 
mirror array 22 accomplishes this objective by directing portions of the 
scene delineated in FIG. 3 by dashed lines 32 to different portions of the 
focal plane within the camera 11 effectively using the aforementioned 
wasted portion of the image as in the related art. 
The mirror array 22 is made up of one or more mirrors 28.sub.i, where i=1 
to N and N is a positive integer less than .infin., each mirror 28.sub.i 
having a center line and an inner and outer portion. The mirrors 28.sub.i 
are arranged at specific distances from the camera and angled in a 
specific manner relative to the camera and an object plane so as to direct 
a portion of the object plane into a portion of an image plane of the 
camera. 
Herein the object plane will be defined as a plane in the scene 26, 
perpendicular to the line of sight and the image plane will be the plane 
in the camera containing the imaging apparatus such as the CCD array. In 
the case of a loom 14 monitoring application, the object plane is the 
plane containing the warp threads 18. The resolution at the image plane is 
related to the resolution in the object plane by the magnification of the 
camera's optics 24. 
The location and orientation of the mirrors that enables the optical 
monitoring system of the present invention to work is determined by 
following a set of steps as described hereinbelow. Key dimensions and 
elements in the steps are illustrated in FIG. 4. 
A top view of the mirror array 22 is illustrated in FIG. 5. The mirror 
array 22 as viewed from behind is illustrated in FIG. 6. In the preferred 
embodiment the mirrors 28.sub.i are supported by mirror supports 50 that 
fix the height and orientation of the mirrors 28.sub.i. The mirror 
supports 50 are, in turn, rigidly connected to a base 52. The mirror array 
22 of the present invention breaks up a long, low aspect ratio region of 
the scene 26 into a number of individual segments 46 separated by boundary 
lines 32 in FIG. 4. The images of the individual segments Si are directed 
by the mirrors 28.sub.i in the mirror array 22 to successive, vertically 
stacked strip images Ii in the image plane 21 as illustrated in FIG. 7. 
The procedure for locating the mirrors in the array, orienting them 
relative to the camera and object plane as well as for determining the 
mirror widths and heights for proper operation are given in detail below. 
Hereinafter, the scene being monitored will be assumed to be the input 
section 12 of a loom 14 illustrated in FIGS. 3 and 4. It is to be 
understood that the choice of a loom 14 as the object being monitored in 
no way limits the applicability of the monitoring system to other linearly 
extended monitoring tasks. The process for arranging the mirrors in the 
mirror array begins by defining the location of the first mirror in the 
array and the width of the object, in this case a loom, that is to be 
monitored. 
Referring back to FIG. 4 the mirrors 28.sub.i of the mirror array 22 are 
arranged on a centerline C/L which is parallel to the center line of the 
scene 26. Let mgd be the distance from the camera to the centerline C/L of 
the first mirror 28.sub.i=1. The distance from the center line to a 
midpoint m of the loom 14 is R2. The width of the loom is LW and the width 
of a segment Si of the loom 14 covered by a given mirror is denoted 
SW.sub.i for segment width. The distance from the midpoint m of the loom 
14 to the point corresponding to the center of a segment 46 at which the 
center of the camera's optics 24 is to be pointed is the aim point 
(AP.sub.i). 
The arrangement of mirrors 28.sub.i of the mirror array 22 is directly 
related to the characteristics of the object being imaged and the imaging 
system being used. For the purposes of discussion the imaging system 
hereinafter is referred to as a camera 11 and lens 24. 
A modem video camera 11 is normally characterized by a number of 
parameters, including the charge coupled device (CCD) array. The CCD array 
is the means by which an image is converted to an electrical signal for 
transmission to a storage or viewing device. The CCD array is usually made 
up of a regular rectangular lattice of sensing elements known as pixels. 
The number and spacing of the pixels in the horizontal and vertical 
directions on the CCD array determines, in part, the resolution of the 
camera 11. A second set of parameters that characterize a camera are those 
associated with the camera's optics or lens system 24. The lens system 24 
characteristics include the focal length, focal ratio f#, the depth of 
field, and the effective aperture. These terms and the relationships 
between them are well known in the art. 
The focal length of the camera 11 is denoted by fl while the camera's focal 
ratio is denoted f#. The center to center spacing of the pixels in the 
camera CCD array are ES.sub.h and ES.sub.v for the horizontal and vertical 
spacing, respectively. The magnification factor is denoted by M. The 
actual resolution at the object plane is denoted RES.sub.h and RES.sub.v 
in the horizontal and vertical directions, respectively. The diameter of 
the effective aperture of the lens of the camera is A.sub.e and the 
camera's depth of field is given by FO. 
The number of elements in the camera's CCD array is nhp by nvp. The number 
of mirror strips to be used is nf which is equal to the number of segments 
S.sub.i into which the object plane image is to be broken. The height of 
the object is H.sub.o. The height of a given mirror 28.sub.i is H.sub.s. 
The height of the ith mirror 28.sub.i is HM.sub.i. The width of the mirror 
28.sub.i is WM.sub.i with IM.sub.i being the distance from the centerline 
of the mirror 28.sub.i to the edge closest to the object plane and 
OM.sub.i being the distance from the mirror 28.sub.i centerline C/L to the 
edge of the mirror 28.sub.i farthest from the mirror centerline C/L. A 
factor k.sub.s is used to avoid diffraction limitations on the optics. 
The implementation of a mirror array 22 of the present invention involves 
establishing the values of a number of variables. Some of the variable 
values may be chosen by the artisan and are independent variables and some 
are dependent on the choices made. In most cases, the distinction between 
variables that are independent and dependent is up to the artisan. 
For example, consider situation where the distance from the mirror array 22 
to the object plane residing in the input region 16 of loom 14 is 
specified as R2 and the loom width is LW. Let the number of mirrors 
28.sub.i in the mirror array 22 be arbitrarily chosen to be nf. The 
variable nf is used in the equations to avoid confusion with the subscript 
variable i. Also consider a camera with nhp pixels in the horizontal 
direction and nhv pixels in the vertical direction with a pixel to pixel 
spacing of ES.sub.h and ES.sub.v in the horizontal and vertical directions 
respectively. Let the wavelength of light be .lambda. and the diffraction 
factor be given by k.sub.s. Under these constraints, the segment width SW 
is given by equation (1) while the resolutions in the vertical and 
horizontal directions are given by equations (2) and (3) below. 
##EQU1## 
Next the magnification required can be determined from the ratio of the 
horizontal resolution at the object plane RES.sub.h, and the horizontal 
pixel spacing denoted by ES.sub.h as in equation (4). Using M from 
equation (4) and the distance R2 to the object plane the focal length fl 
can be calculated using equation (5). 
##EQU2## 
The depth of field FO must be large enough to view the image created due to 
the most tilted segment clearly but not so large that the diffraction 
limit of the optics is exceeded. Equation (6) gives the required depth of 
field, FO.sub.needed, in terms of the loom width LW and the segment width 
SW and the distance R2. The minimum required focal ratio f#.sub.min is 
then calculated using equation (7) from the depth of field from equation 
(6) and the magnification M and horizontal pixel spacing ES.sub.h. The 
maximum focal ratio f#.sub.max can be determined using equation (8). 
Generally, the maximum focal ratio f#.sub.max and minimum focal ratio 
f#.sub.min are not equal but represent a range of acceptable values for 
the focal ratio f#. While several potentially good choices exist, if the 
maximum f#.sub.max is larger than the minimum f#.sub.min of equation (7), 
an optimum focal ratio f# given by the geometric mean of the minimum focal 
ratio f#.sub.min and maximum focal ratio f#.sub.max as in equation (9). 
Having selected an appropriate focal ratio f#, the effective aperture 
A.sub.e, of the camera optics 24 can then be calculated using equation 
(10). With these values, a camera 11 and lens 24 system can be selected 
for use with the mirror array 22 for monitoring the input region 16 of a 
loom 14 or a similar scene having a high aspect ratio. 
##EQU3## 
If, upon comparison of the focal ratio f# values of equation (7) and 
equation (8) it is found that the maximum focal ratio f#.sub.max is 
smaller than the minimum focal ratio f#.sub.min, an optimum focal ratio f# 
cannot be determined using equation (9). In this circumstance, it is 
necessary that one or more of the independent variables be changed so that 
the minimum focal ratio f#.sub.min is less than the maximum focal ratio 
f#.sub.max. For instance, it may be possible to change the distance R2 and 
in order to achieve a minimum focal ratio f#.sub.min that is less than the 
maximum focal ratio f#.sub.max as calculated by equations (7) and (8) 
respectively. Alternatively, the number of mirrors, nf, in the array 22 
may be changed. Changing the number of mirrors 28.sub.i, in turn, changes 
the segment widths, SW.sub.i, enabling the desired relationship between 
the minimum focal ratio f#.sub.min and the maximum focal ratio f#.sub.max 
to be achieved. 
Having selected a camera 11 and associated optics 24, the next step in 
designing a mirror array 22 in this example of a loom monitoring 
application is to compute the mirror guard distance mgd. The mirror guard 
distance mgd is the distance from the camera lens 24 to the first mirror 
28.sub.i=1 in the mirror array 22. The mirror guard distance mgd, is 
calculated using the vertical resolution RES.sub.v desired, the number nf, 
of mirrors in the array, and the number of pixels in the vertical 
direction on the CCD array. Equation (11) gives the height of a segment 46 
in the object plane in terms of the number of vertical pixels nvp, in the 
CCD array and the desired vertical resolution RES.sub.v. The result of 
equation (11) can then be used in equation (12) to find the mirror 
28.sub.i height H.sub.s. The mirrors 28.sub.i are generally long and 
narrow due the nature of the application and are referred to hereinafter 
as strips or strip mirrors in an interchangeable manner. The term "strip" 
leads, in ram, to the subscript "s" used for the mirror or strip height 
H.sub.s. It is also understood that there are different strip heights 
H.sub.s for each of the mirrors 28.sub.i even though the subscripts on 
H.sub.s have been omitted for clarity. 
The mirror guard distance mgd is given by equation (13) in terms of 
distance R2, the effective aperture A.sub.e of equation (10), and the 
strip height, H.sub.s, of equation (12). As stated, the mirror guard 
distance mgd of equation (13) gives the distance from the camera lens 24 
and the mid-line or center line C/L of the first mirror 28.sub.i=1 of the 
mirror array 22. 
##EQU4## 
Placing the mirrors 28.sub.i closer to the camera than mgd would result in 
a loss of contrast as the images of adjacent mirrors would overlap. 
Placing mirrors farther than mgd increases the mirror size which is 
undesirable. Thus the mirrors are placed as close as possible to mgd 
consistent with the other requirements disclosed herein. 
The first mirror 28.sub.i=1 images one of the segments Si into the lowest 
portion of the image plane 21. In the specific example illustrated in FIG. 
7 the first mirror 28.sub.i for i=1 images the second segment S2 into the 
lowest image plane 21 portion labeled I2. Successive mirrors 28.sub.i 
image the other segments (S1 and S3-S5) across the width of the object 
plane to successively higher portions of the image plane 21. As 
illustrated in FIG. 7 this imaging by successive mirrors 28.sub.i results 
in the sequential stacking in the image plane 21 of the images I4, I3, I1, 
and I5 corresponding to the segments S4, S3, S1 and S5 respectively. 
Successive mirrors 28.sub.i are located at distances farther away from the 
lens 24 than the minimum guard distance mgd in such a way that the 
differences in path length from the lens 24 to the aim points AP.sub.i is 
minimized. The minimization of path length difference between aim points 
AP.sub.i within each of the segments S.sub.i minimizes the constraints on 
the lens system 24. Denoting the individual mirrors 28.sub.i in the array 
22 by a subscript i where i=1 refers to the first mirror 28.sub.i=1, the 
distances from the first mirror 28.sub.i=1 centerline to the centerlines 
of the mirrors 28.sub.i where runs from i=2 through i=nf mirror segments 
are given by DA.sub.i. DA.sub.i can be found for each of the mirrors 
28.sub.i for i=2 through nf by considering the aim points AP.sub.i of the 
segments S.sub.i and calculating the radial distance RD.sub.i for each 
segment. Equation (14) gives the aim point AP.sub.i for the ith segment 
S.sub.i in terms of its segment width SW of equation (1), the index i, and 
the loom 14 width LW. The radial distance RD.sub.i for the ith segment is 
then given by equation (15). The distance DA.sub.i from the first mirror 
28.sub.i=1 to the ith mirror 28.sub.i, is then given by equation (16). 
Next, the orientation of the individual mirrors must be determined. 
##EQU5## 
The proper angle of the ith mirror 28.sub.i must be determined such that 
the center of the ith segment S.sub.i is properly imaged onto the center 
of the image plane 21 containing the CCD array. Referring to FIG. 4, the 
angle of the ith mirror 28.sub.i relative to a line between the camera and 
the mirror is .gamma..sub.i. The angle .gamma..sub.i is given by equation 
(17) where AP.sub.i and DA.sub.i are the aim point of the ith segment 
S.sub.i and the distance from the first mirror 28.sub.i=1 to the ith 
mirror 28.sub.i respectively and the angle .psi..sub.i is given by 
equation (18). Finally, the height and width of each mirror in the array 
can be determined. All angles referred to herein are in radians. 
##EQU6## 
The height HM.sub.i of the ith mirror 28.sub.i is given by equation (19). 
The actual mirror height HM.sub.i in equation (19) is related to the 
distance R2 from the loom 14 to the mirror array 22, the segment height 
H.sub.s, of equation (12), and the total distance, TD.sub.i from the 
camera lens 24 to the mirror 28.sub.i=1 centerline C/L. The width WM.sub.i 
of the ith mirror 28.sub.i is the sum of the distance from the centerline 
C/L of the mirror 28.sub.i to the inside edge 44 of the mirror 28.sub.i 
denoted IM.sub.i, and the distance from the centerline C/L of the mirror 
28.sub.i to its outside edge 45. The inside edge 44 refers to the edge of 
the mirror 28.sub.i closest to the loom 14 and the outside edge 45 refers 
to the edge of the mirror 28.sub.i farthest from the loom 14. These 
distances, IM.sub.i and OM.sub.i, are given by equations (20), (21) 
respectively. The width WM.sub.i of the ith mirror is given by equation 
(22) as the sum of distances IM.sub.i and OM.sub.i. 
The lengths of the segments CD.sub.i and BD.sub.i referred to in equations 
(20) and (21) are given by equations (23) and (24) respectively with 
.psi., .alpha., .beta., and .delta. given by equations (18), (25), (26) 
and (27). As before, each of these intermediate results in equations (19) 
through (27) has a subscript that corresponds to the ith mirror 28.sub.i 
in the mirror array 22. 
##EQU7## 
Given the width WM.sub.i and the tilt angle .gamma..sub.i of the mirrors 
28.sub.i, their individual locations can be determined in a Cartesian 
coordinate system with the center of the lens 24 being taken as the point 
{x=0, y=0}. The two ends of the ith mirror 28.sub.i are given in such a 
coordinate system by equations (28) and (29). 
EQU P1.sub.i ={TD.sub.i -OM.sub.i .multidot.cos(.gamma..sub.i), -OM.sub.i 
.multidot.sin(.gamma..sub.i)} (28) 
EQU P2.sub.i ={TD.sub.i +IM.sub.i .multidot.cos(.gamma..sub.i), 
IM.multidot.sin(.gamma..sub.i)} (29) 
In practice, the mirror 28.sub.i heights HM are ordered so that the mirrors 
28.sub.i closest to the camera lens 24 are the lowest. This prevents 
blockage of one mirror 28.sub.i by the supports 50 of other mirrors 
28.sub.i. The mirror array 22 is built by attaching mirrors 28.sub.i of 
height HM and width WM determined above to supports 50 as illustrated in 
FIG. 5 and 6. The supports 50 hold the mirrors 28.sub.i in the locations 
and orientations determined above. 
A prototype mirror array 22 was designed and constructed. The object width 
or loom width LW=5 feet. The number of mirrors nf=6. These parameters and 
additional parameters of the prototype are summarized in Table 1. 
TABLE 1 
______________________________________ 
Input parameters for the prototype mirror array design. 
Parameter Variables Value 
______________________________________ 
Loom Width LW 1.52 meters (5.0 feet) 
# of Facets nf 6 
# of Horizontal Pixels 
nhp 733 
# of Vertical Pixels 
nvp 244 
Horizontal Element Size 
ES.sub.h 11.5 .times. 10.sup.-6 meters 
Vertical Element Size 
ES.sub.v 27.0 .times. 10.sup.-6 meters 
Distance From Loom 
R2 0.85 meters (2.8 feet) 
Diffraction Factor 
ks 2 
Wavelength of Light 
.lambda. 6 .times. 10.sup.-7 meters 
______________________________________ 
Some of the corresponding parameters values calculated using the equations 
given hereinabove for the parameters of Table 1 are given in Table 2 and 
the resulting placement of the six mirrors 28.sub.i of the prototype 
mirror array 22 are listed in Table 3. 
The placement of mirrors 28.sub.i based on the equations detailed herein 
above occasionally results in some interference between mirrors 28.sub.i. 
That is to say, some mirrors 28.sub.i might need to be located where 
others are already. This problem is overcome by slightly shifting any 
interfering mirrors. Interfering mirrors may be shifted slightly because, 
in general, there is a range of locations rather the a single location 
that result in acceptable performance. In the prototype, it was necessary 
to move several mirrors up to 2 inches from the locations calculated by 
use of the above referenced equations to avoid 
TABLE 2 
______________________________________ 
Calculated parameters for the prototype mirror array design. 
Parameter Variables Value 
______________________________________ 
Segment Width SW 0.25 meters (0.83 feet) 
Horizontal Resolution 
RES.sub.h 4 .times. 10.sup.-4 meters 
Vertical Resolution 
RES.sub.v 8 .times. 10.sup.-4 meters 
Magnification M 30.132 
Focal Length fl 2.74 .times. 10.sup.-2 meters 
Needed Depth of Field 
FO.sub.needed 
1.509 .times. 10.sup.-1 meters 
Minimum Focal Ratio 
f#.sub.min 7.0 
Maximum Focal Ratio 
f#.sub.max 9.3 
Optimum Focal Ratio 
f# 8.1 
Effective Aperture 
A.sub.e 3.4 .times. 10.sup.-3 meters 
Height of Object 
H.sub.o 1.985 .times. 10.sup.-1 meters 
Height of Strip 
H.sub.s 1.8 .times. 10.sup.-2 meters 
Mirror Guard Distance 
mgd 1.354 .times. 10.sup.-1 meters 
______________________________________ 
interference. For the prototype example, the final mirror placements are as 
listed in Table 3 where the parameters {x.sub.1, y.sub.1, z.sub.1 } are 
the locations of the lower comer of a given mirror closest to the camera 
while {x.sub.2, y.sub.2, z.sub.2 } are the locations of the upper corner 
of a given mirror farthest from the camera 11. The values are in 
millimeters and are relative to a fixed reference location at the camera 
lens in a Cartesian coordinate system with the x-dimension representing 
distance from the camera lens parallel to the line of sight of the lens. 
The z dimension represents height and is perpendicular to a plane 
containing the camera 11 and a ray extending from the mirror array 22 
center line C/L to the scene 26. 
TABLE 3 
______________________________________ 
Locations of the lower Corners of the Mirrors in the 
Prototype Array (dimensions in millimeters). 
Mirror # 
x.sub.1 
y.sub.1 x.sub.2 
y.sub.2 
z.sub.1 
z.sub.2 
______________________________________ 
1 182.9 -23.4 283.0 
27.0 12.7 20.4 
2 235.6 -35.5 340.8 
41.9 20.4 29.4 
3 309.7 -44.6 401.2 
52.5 29.4 39.9 
4 343.3 -44.4 410.8 
51.4 39.9 50.9 
5 300.2 -38.0 346.9 
43.2 50.9 60.8 
6 172.4 -27.5 201.6 
30.7 60.8 67.3 
______________________________________ 
The six facet mirror array 22 of the prototype yielded a 0.0013 inch 
resolution over the entire six foot object width of the object being 
monitored. This proved to be sufficient resolution to detect the presence 
or absence of a typical warp thread in a textile loom 14. 
As described above, the effect of the mirror array 22 is to image portions 
of the object plane onto the image plane 21. In the example application 
described hereinabove, the object plane was the input of a loom 14 and the 
image plane 21 was the CCD array inside a camera 11. FIG. 7 illustrates 
the concept for the five mirror 28.sub.i, mirror array 22 illustrated in 
FIG. 5. The high aspect ratio AR object plane containing 5 segments, S1 
through S5, is imaged onto the low aspect ratio AR image plane 21 as 
successively stacked images I1 through I5. This results in the desired 
mapping of the object plane into the image plane 21. For the loom 14 
example hereinabove, all the warp threads can be viewed with adequate 
resolution by a single, commercially available CCD camera 11. Therefore, 
the need for multiple cameras of the prior art is eliminated. 
While an example of a loom monitoring system was used in this discussion, 
it is anticipated and within the scope of the invention that there are 
many high aspect ratio monitoring situations or applications that could 
benefit from the present invention. One such application is the use of the 
mirror array 22 of the present invention in conjunction with the camera 
and a suitable computer system on board an ocean-going vessel to monitor 
the visible horizon for the appearance of other surface vessels. 
Another application for the mirror array 22 of the present invention is the 
reading of barcodes used to identify and catalog items. In this monitoring 
and identification application, the imaging device formerly identified as 
a camera 11 is replaced by a barcode reader, a device that images, detects 
and analyzes variable width parallel bars used to mark items for 
identification purposes. Cameras are often used as barcode readers. The 
mirror array 22 of the present invention is used to extend the field of 
view of the bar-code reading device thereby reducing the need for careful 
placement of the barcode relative to the FOV 20 of the barcode reader. As 
in the loom monitoring example cited above, the practice of the present 
invention has the effect of mapping an extended high aspect ratio scene 
into the low aspect ratio FOV 20 of the typical barcode reader. To better 
understand this application of the present invention, a specific example 
will now be discussed with the understanding that the same principles can 
be applied to a wide variety of similar applications. 
Consider the example of monitoring and identifying objects 62 on a conveyor 
belt system 60 using barcodes 64 on these objects 62 as illustrated in 
FIG. 8. The 5 mirror 28.sub.i mirror array 22 is positioned between the 
camera 11 and the objects 62. The mirror array 22 as described hereinabove 
maps a long, high aspect ratio scene 19 into the low aspect ratio FOV of 
the barcode reader or camera 11. As before, the lens 24 of the camera 11 
is largely responsible for defining the FOV of the camera. The effect is 
that barcodes located anywhere on the side of the objects 62 facing the 
mirror array 22 are detectable by the barcode reader or camera 11. The 
image plane 68 of the barcode reader or camera is also illustrated in FIG. 
8. The barcode image 66 of the barcode 64 passing through the scene 19 
ends up in one of the image segments I1 through I5 depending on the 
location of the barcode 64 on the side of the object 62. A single barcode 
reader or camera 11 is therefore able to detect barcodes anywhere on the 
objects 62 by virtue of the mirror array 22 of the present invention. 
Without the mirror array 22 multiple barcode readers or cameras 11 would 
be necessary. 
Thus there has been disclosed a mirror array apparatus 22 that functions in 
conjunction with a camera 11 as a means for efficiently monitoring high 
aspect ratio objects. Changes and modifications may be made to the 
invention which may be readily apparent to those skilled in the art 
without going beyond the intended scope of the invention, as defined by 
the appended claims.