Rotating mirror optical scanning device

An optical scanning device for mechanically scanning images at a high scan ate over a wide field of view and under low light conditions. The device comprises a first rotating reflecting means such as mirrors for intersecting an input beam of light in a first path and for redirecting the beam of light into a second path. A second rotating reflecting means intersects the second path and reflects the beam of light into a third path, onto a detecting device for detecting the presence of predetermined images in the beam of light. In order to reflect the light beams over an elliptical path the mirrors are mounted on rotating shafts at a slight angle from the perpendicular to the axis of the rotating shaft. The mirrors are rotated at varying speed relationships to each other and varying phase relationships to each other in order to produce a variety of scan patterns.

BACKGROUND OF THE INVENTION 
This invention relates to an optical scanning mechanism. It has a very 
large focal plane area coverage and can accept a large f-cone from an 
external imaging system. It has a scan rate of up to 200 lines per second 
and can generate several original scan patterns by varying the settings of 
the invention. 
There are numerous mechanical scanning systems which have been built that 
provide one and two-dimensional image scan patterns for optical systems. 
These systems range from simple two-axis gimbals and rotating multifaceted 
cylinders to cam-actuated tilting mirrors or prisms and complex 
combinations of mirrors, lenses, prisms, and holographic hybrids. 
One of these systems uses a pair of counter-rotating prisms in a tube, but 
this system is strongly wavelength dependent. In all such systems, there 
is invariably, a trade-off between the optical throughput, the scan rate, 
and the Field of View (FOV). This problem is particularly acute for 
systems which must operate at low light levels under conditions which 
require high (real-time) scan rates and wide fields of view. In such cases 
the optical designer usually abandons mechanical scan generators in favor 
of arrays or other non-mechanical means of image generation. However, for 
some spectral regions, these alternate approaches do not exist. This is 
true for the very far infrared and submillimeter spectral regions. 
Furthermore, in these regions, the minimum image resolution spot size is 
large compared to the visible spectral region. In order to scan an image 
with enough resolution cells to synthesize patterns, the FOV and the 
physical area covered in the focal plane must also be large. 
SUMMARY OF THE INVENTION 
The scanning device of the invention will accommodate a large aperture 
primary mirror which is necessary in order to collect the incident 
radiation, and the large focal plane area necessary for scene resolution, 
while still providing a rapid mechanical scan rate for real-time imaging. 
In addition, the same scanning device can generate a variety of one and 
two-dimensional scan patterns simply by varying the mechanical phase and 
rotational differences between the two rotating mirrors included in the 
device. Since the device of the invention uses reflective elements to make 
the scan, it is independent of the wavelength. The focal plane scanner 
utilizes two relay lenses, but they do not produce the scanning action. 
It is an object of the invention to provide an optical scanning mechanism 
which can scan a large focal plane area and accept a large f-cone from the 
external imaging system. 
It is another object of the invention to provide an optical scanning 
mechanism which scans a large focal plane area mechanically at a rate of 
up to 200 lines per second. 
It is still a further object of the invention to provide a mechanical 
optical scanning device which is adjustable and can generate one or more 
scan patterns by varying the mechanical phase and the rotational 
velocities of a pair of rotating mirrors. 
It is yet another object of the invention to provide a mechanical optical 
scanning device which is particularly adaptable to scanning a large focal 
plane area, generated by infrared and submilimeter imaging systems. 
These and other objects, which will become apparent, are attained by the 
mechanical optical scanning device of the invention which is illustrated 
in the drawings appended hereto, and described in detail in the 
accompanying specification.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring now to FIGS. 1, 2 and 3, wherein the scanning device 10 of the 
invention comprises an imaging lens 12 for directing the beam of light to 
be scanned into the scanning device 10. Imaging lens 12 produces a focused 
image on the plane in which a field lens 14 lies. The beam of light 30 is 
directed by field lens 14, onto the surface of a first rotary reflecting 
mirror 16, which is supported for rotation on a shaft 18. Each ray of the 
light beam 30 impinges upon a different point on reflecting means 16, and, 
in turn, is reflected in a second direction or path onto the surface of a 
second rotary reflecting means 22. As seen best, in FIG. 2, the first 
rotary reflecting means 16 is a mirror mounted on shaft 18 at an angle of 
less than 90.degree. to the axis of the shaft so that each ray of light 
beam 30 which impinges upon surface 16 is reflected in an elongated 
elliptical path, as at 34. 
The second rotary reflecting means 22 is supported on shaft 24 also at an 
angle .alpha. as indicated at 20 so that the rotary reflecting means 22 
rotates in a series of planes so as to redirect the elliptical path 34 in 
a further elongated elliptical path, through a relay lens 28, and onto a 
detecting means 36. The full line and the dotted line positions in FIG. 1 
and FIG. 2 represent the path of a single point or light ray 32 which 
travels through the scanning device. In the event that the device is to be 
used to scan the entire aperture of input beam 30, field lens 14 and relay 
lens 28 may be dispensed with. 
As pointed out above, mirrors 16 and 22 are tilted on their axles or shafts 
18 and 24 at an angle .alpha. with the axis of their rotary shafts, and 
their rotational speeds are synchronized with a preset phase difference 
and sense of rotation. Point 32 of light beam 30 is reflected from mirror 
16 in a small elliptical cone as mirror 16 rotates. The axis of the cone 
is at an angle to the axis of rotation of mirror 16 and has an apex angle 
of two .alpha.. Upon reflection from the second mirror 22, a second 
conical movement is superimposed on reflected point or ray 32. When the 
rotations and phases are correctly adjusted, as illustrated in FIG. 4(b), 
the net angular movement of ray 32 can be controlled so that the vertical 
movements are substantially cancelled while the horizontal deflections are 
increased, to produce an elongated substantially flat elliptical path 
which is substantially tantamount to a straight line scanning motion. This 
results in the horizontal scan mode illustrated in FIG. 4(b). 
FIG. 4(a) illustrates the vertical scan mode and is similar to the 
horizontal scan mode illustrated in FIG. 4(b), except that it is in the 
vertical plane rather than the horizontal plane. 
A detailed mathematical analysis shows that, for a small angle .alpha., and 
the pentaprism geometry of FIG. 1, the horizontal and the vertical angular 
deflections, .theta..sub.H and .theta..sub.V are given by the following 
equations: 
EQU .theta..sub.H =2.alpha.[SIN (W.sub.1 T+.beta.)+SIN W.sub.2 T] 
EQU .theta.V+2.alpha.COS 22.5.degree. [COS(W.sub.1 T+.beta.)+COS W.sub.2 T] 
where 
.alpha.=mirror tilt angle, same for both mirrors. 
.beta.=relative phase of the direction of tilt between the two mirrors. 
w.sub.1 =angular velocity of mirror 16 
w.sub.2 =angular velocity of mirror 22 
The field of view of the focal plane scanning embodiment of the invention 
is established by consideration of the tunnel diagram of FIG. 3. The 
points (a) and (b) show the maximum coverage of the scanner for a mirror 
tilt angle and the indicated relative spacing of the mirrors 16 and 22 at 
field lens 14 and relay lens 28. 
As illustrated in FIG. 3, this is a geometrical construction. The focal 
length of lens 14 is chosen to reimage primary mirror 16 at a point 
halfway between mirrors 16 and 22. The focal length of lens 28 is chosen 
to re-image the plane of lens 14 onto the detector means 36. The scan FOV 
is then determined by specifying the desired ratio of the primary aperture 
diameter to that of lens 14 for a given f/#. The diagram in FIG. 3 shows 
that an f/2.5 imaging system can be scanned over an 11.4.degree. FOV if an 
f/1 field lens 14 and an f/2 relay lens 28 are utilized, and the focal 
plane diameter ratio to that of the f/2.5 primary mirror 16 is in the 
ratio of 1:2. 
Each revolution of the tilted mirrors yields one complete angular cycle, 
consisting of one scan line across the FOV and a second scan line back to 
the starting point, for a total of two scan lines per revolution. Thus, an 
operating frequency of 6000 rpm gives 100 rps, or 200 lines per second 
altogether. For an image with a moderate line resolution of 50-100 lines 
per frame, this gives from two to four frames per second output. 
The scanning device of the invention offers a number of advantages over 
other mechanical scanners. It has a wide FOV, a high scan rate, and a 
large f-cone acceptance capability. It can also scan very large area focal 
planes with little or only partial obscuring of the throughput. 
The device illustrated in FIGS. 1, 2, and 3 can generate either horizontal 
or vertical line scans, as described above, but can also be adjusted to 
scan a two-dimensional FOV in either a rotating radial line scan or a 
periodic spiral scan pattern, depending upon the settings of the angular 
velocities and relative phases of the two rotating mirror elements. 
As seen in FIG. 4(c), the phase and rotational speeds can be adjusted to 
provide a circular scan mode without modifying the structure of the 
scanning device itself. 
FIG. 4(d) illustrates a spiral scan mode which may be attained, again by 
adjusting the speed and phases of the mirrors in accordance with the 
illustrated formula. FIG. 4(e) illustrates a different scanning pattern 
wherein a flattened elliptical path such as that shown in FIG. 4(a) and 
4(b) is performed by the apparatus, but with the path slowly rotating 
radially above a center line to produce a rotating radial scan mode. 
Each of FIG. 4(a), 4(b), 4(c), 4(d) and 4(e) illustrates the formula for 
calculating in adjusting the speeds and phases of the rotating mirrors to 
produce the scan pattern illustrated in each.