One-dimensional electronic image scanner

A microwave image scanning apparatus which utilizes a dispersive waveguide wedge in front of a microwave sensor. Similar to an optical prism, the waveguide wedge resolves multi-spectral microwave energy from various directions into the same direction for detection by the microwave sensor and subsequent formation of a refined high quality video image. The waveguide wedge consists of a collection of waveguide channels having their longitudinal axes aligned substantially in parallel.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates generally to electronic imaging technology 
and, more particularly, to a one-dimensional microwave or millimeter wave 
image scanning apparatus. The apparatus of the present invention is 
especially well-suited for geographic imaging applications such as 
generating real-time video pictures of a landing strip for an airplane or 
of the surface of the ocean in search of oil contamination. 
2. Discussion 
Various types of imaging systems have been developed for use in conjunction 
with geographic imaging applications, with radar systems being one of the 
most popular. Radar and similar imaging technologies operate by 
transmitting a radiowave signal towards a target and then detecting a 
return signal reflected back from the target. The return signals and the 
rate at which they return are used to generate information about the 
target and its location. These systems, however, innately experience 
difficulties in attaining geographic images in various adverse 
environmental conditions such as snow, rain, dust or fog. 
Utilizing millimeter wave or microwave energy has provided a viable 
alternative and presents various advantages over use of other signal 
types. As microwave or millimeter wave energy is naturally present in the 
environment and is reflected to some degree off of most objects, it is 
possible to obtain a video image or picture passively, without having to 
transmit an excitation signal and without necessitating a return signal 
response. Also, microwave or millimeter wave energy is not substantially 
attenuated by atmospheric moisture such as fog, snow or rain and 
millimeter waves penetrate adverse environmental conditions such as smoke 
and dust clouds wherein suspended particles are of less than a millimeter 
in size. Due to shorter wavelengths, millimeter wave systems can be 
physically implemented with relatively small antennas, useful in 
applications where small antennas are particularly advantageous such as on 
an airplane. 
Such millimeter wave imaging devices conventionally employ a two 
dimensional focal plane array wherein a lens or other focusing element is 
used to focus millimeter wave radiation obtained from the field of view 
onto the array. Each focal plane element in the array receives a microwave 
signal and converts the radiation incident thereon into an electrical 
output signal used to drive a video display of the image. Each output 
signal generated by an array element is most often mapped to one image 
element. 
With such conventional systems, however, the resulting video picture 
produced may appear ragged or coarse due to discrete pixelization and 
image undersampling. Each pixel of the displayed image represents 
radiation from the portion of the image radiated in a direction directly 
incident on a given detector. Since there are practical physical 
limitations on the number of and spacing between focal plane elements in 
the array structure, some of the image information may be lost, especially 
that not corresponding to a central portion of a detector element. This 
not only causes greatly degraded image quality but also leads to the 
exclusion of many image enhancement techniques. More accurate information 
would be obtainable if some of the direct radiation corresponding to 
peripheral regions of the detector could be directed toward the center. 
While various mechanical scanning methods have been used to alleviate such 
difficulties, the speed at which the mechanical scanner has to operate 
limits the practical application of such techniques. 
In view of the above, there is a need for an improved imaging system which 
utilizes energy in the millimeter wave or microwave spectrum to produce a 
refined high quality video picture for which various image enhancement 
techniques are available. It is also desirable that such systems have no 
mechanical moving parts to reduce overall system complexity and to improve 
reliability. 
SUMMARY OF THE INVENTION 
The imaging system of the present invention provides unidirectional image 
oversampling in the form of an electronic scanning action by utilizing a 
dispersive waveguide wedge in front of a microwave focal plane sensor 
array. The waveguide wedge, generally formed by a collection of aligned 
waveguide channels, will operate like an optical prism to refract 
multi-spectral microwave energy from various directions into the same 
direction for detection by the sensor array and subsequent formation of a 
video image of the target. 
This unique system produces a very high quality video image while allowing 
a reduction in the number of focal plane elements in the focal plane 
array. The image produced is of a quality to allow use of sophisticated 
image enhancement techniques often precluded with more conventional 
systems. Since there are no moving parts in the system, problems inherent 
with devices utilizing mechanical scanning means are eliminated and 
increased reliability results. 
Additional objects, advantages and features of the present invention will 
become apparent from the following description and amended claims, taken 
in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
With reference to the drawings, the image scanning apparatus according to 
the teachings of the present invention is shown generally at 10 in FIG. 1. 
A dispersive waveguide wedge 12, shown detailed in FIG. 2, has a face 14, 
a base 16 and an angled surface 18. The wedge 12 is preferably formed of a 
collection of waveguide channels 20 joined together with their 
longitudinal axes (extending through the center of the channel) aligned 
substantially parallel to each other and generally perpendicular to the 
plane defined by wedge face 14. The various lengths of the waveguide 
channels 20 are staggered so that they collectively form a wedge shape. 
Preferably each channel 20 has a first end 20a cut perpendicular to the 
longitudinal axis of the channel in order to form a substantially smooth 
face 14 and a second end 20b cut at an angle .theta. to form a 
substantially smooth angled surface 18. The channels 20 collectively form 
a honeycomb pattern when the wedge 12 is viewed from the face 14 or 
surface 18. 
This dispersive wedge 12 is preferably formed with waveguide channels 20 
which are rectangular in cross-section but channels of other 
cross-sectional shapes could also be used. Also, the waveguide channels 20 
may be of a constant cross-section but are not required to be. The 
waveguide channels 20 are preferably made from a metallic material such as 
copper and may be coated with silver or gold. 
Wedge 12 may alternately be fabricated by cutting a wedge shape from a 
suitable commercially available honeycomb sheet or by forming the channels 
into a solid wedge such as by drilling or boring. These alternate methods 
of fabrication may be advantageous especially for creating channels which 
are circular in cross-section. 
A focusing element such as microwave lens 24 is placed between the wedge 12 
and a focal plane detector array 26. Lens 24 is preferably a microwave 
lens of a type commonly known by those skilled in the art and is made from 
a soft opaque polymer material such as polystyrene or Rexolite.RTM., or 
from Teflon.RTM.. The lens 24 acts like a wide angle camera lens taking 
the relatively large span of millimeter waves passing through the wedge 12 
and focusing them on a smaller focal plane detector array sensor 26. 
The focal plane detector array 26 consists of a plurality of linear 
receiver elements 28, each preferably electrically coupled to at least two 
local oscillators 30, 32 which have frequency varying capabilities. 
(However, for clarity a single linear receiver 28 is shown in FIG. 1). In 
addition to local oscillators 30 and 32, each linear receiver 28 also 
includes an antenna 34 for receiving incident millimeter wave signals, 
typically in the 94 GHz range of frequencies. Since it is difficult to 
detect at this frequency with efficiency, it is desirable to down-convert 
to an intermediate frequency (IF) using an electronic mixer 36 which 
generates sum and difference signals using the received signal from 
antenna 34 and those from the local oscillators. Preferably, for received 
signals at 94 GHz, the frequencies of local oscillators 30 and 32 would be 
in the range of 94 GHz.+-.10%. Each receiver 28 and may also include at 
least one intermediate frequency amplifier 38 to further aid detection. 
Each linear receiver 28 is preferably a printed circuit substrate having 
antenna 34, mixer 36 and intermediate frequency amplifier(s) 38 deposited 
or printed thereon by a photolithographic or other process known in the 
art. Each receiver 28 is used to detect microwave signals which are 
generated by the target, gathered through the dispersive wedge 12, and 
directed onto the receiver 28 by the microwave lens 24. Electrical signals 
output by amplifier(s) 38 in each receiver 28 can then be passed to a 
video display device 100 and/or a computer image memory for creating an 
electronic image of the target. 
This arrangement makes use of the dispersive property of the wedgeshaped 
collection of waveguides. This dispersive property is derived from the 
difference in wave velocity vs. frequency in a waveguide. Similar to an 
optical prism, this dispersive waveguide wedge can resolve multi-spectral 
microwave energy from one direction into monochromatic microwave energy 
going to different directions. By the same token, multispectral images 
from various directions can be refracted into the same direction. A sensor 
consisting of lens 24 and focal plane array 26, placed behind the 
dispersive wedge 12, is thus able to detect images from different 
directions. 
This is accomplished by varying the frequency and bandwidth selection of 
the receivers 28 which is done by varying the frequency applied to mixer 
36 by the local oscillators 30 and 32. For a heterodyning receiver of 
fixed intermediate frequency bandwidth in the microwave region, this 
spectral selection can be performed with a variation or stepping of the 
local oscillator frequency applied to the receiver front end mixer 36. As 
the local oscillator frequency is varied, signals from different parts of 
the spectrum corresponding to different angles in the field of view of the 
imaging device are detected. The frequency of each local oscillator may be 
varied or applied individually. 
The geometric shape of the tapered wedge results in one dimensional 
scanning through each waveguide channel 20, substantially along a line 
parallel to face 14 and extending in the direction of the taper. A 
scanning angle .phi. may be measured from the longitudinal axis of each 
channel 20. The magnitude of the scanning angle .phi. possible for two 
local oscillator frequencies is a function of the waveguide dimension 40 
(the dimension of the longer side for rectangular waveguides and the 
radius for circular waveguides) as well as the thickness 42 of the base 16 
of wedge 12. Table 1 below is a listing of the waveguide dimension 40 and 
corresponding wedge thickness 42 for a scan angle of 0.367.degree. and 
local oscillator frequencies f.sub.1 and f.sub.2 of 92 and 96 GHz, 
respectively. The lens dimension used was 24 inches (61 cm). 
TABLE 1 
______________________________________ 
Waveguide Dimension 
Wedge 
(cm): thickness: 
Degrees: 
______________________________________ 
Rectangular Waveguides 
.17 2 1.876282 
.18 2 .6514816 
.19 2.1 .3756104 
.2 3.199999 .375824 
.21 4.399998 .3734055 
.22 5.699996 .37117 
.23 7.099995 .3696976 
.24 8.599997 .3690033 
.25 10.2 .368988 
.26 11.90001 .3695374 
.27 13.60002 .3677826 
.28 15.40002 .3670349 
.29 17.30003 .3670731 
.3 19.30004 .3677216 
.31 21.30005 .3670425 
.32 23.40005 .3670502 
Circular Waveguide 
.102 2 1.106339 
.112 2.2 .3727951 
.122 4.099998 l.3670578 
.132 6.399996 .3689346 
.142 8.999998 .3605139 
.152 11.80001 .3697281 
.162 14.80002 .3684998 
.172 18.00003 .3674164 
.182 21.50005 .3684769 
.192 25.10006 .3678703 
______________________________________ 
The one dimensional image scanning feature of the image scanner of the 
present invention is illustrated best in FIG. 3. As shown in the figure, 
the angle .phi. changes with the local oscillator frequency applied to the 
receivers 28 in the sensor array 26. Local oscillator diplexing between 
frequencies f.sub.1 and f.sub.2 allows a first receiver 28a and a third 
receiver 28c to collect image information which would previously have been 
obtained by an additional intermediate receiver 28b with only a fixed 
local oscillator frequency of f.sub.0. Therefore, the number of receivers 
and, therefore, the number of pixels generated can be reduced by a factor 
of two or more depending on system design. Pixel reduction, however, is 
traded off against a decrease in signal integration time. 
This technique provides an improved high quality image by providing image 
scanning or oversampling in one direction. This can facilitate the 
elimination of every other linear receiver, creating a wider inner linear 
receiver array spacing. This decreases the likelihood of overheating and 
allows for the use of a thicker metallic ground plane which makes 
conduction cooling with a circulating coolant feasible. Also, since the 
linear receivers are the main cost driver in the system, reducing their 
number makes the system less expensive. This unique scanner can provide 
one-dimensional electronic image scanning to all microwave imaging devices 
that employ heterodyne detection and a focal plane that consists of an 
array of linear focal plane elements. By providing image oversampling in 
one direction, the image quality is improved and various image enhancement 
techniques can be employed to improve image quality or to reduce the size 
of the optics. 
The foregoing discussion discloses and describes merely exemplary 
embodiments of the present invention. One skilled in the art will readily 
recognize from such discussion, and from the accompanying claims, that 
various changes and modifications can be made therein without departing 
from the spirit and scope of the invention as defined in the following 
claims.