Low vibration resonant scanning unit for miniature optical display apparatus

A resonant scanning unit utilizes a "tuning fork" design with a mirror mounted on one arm of the tuning fork and a counterbalance mass mounted on the other arm. A voice coil electromagnetic motor mounted between the arms causes the arms to move in opposite directions. Light generated by a line of LEDs is reflected from the oscillating mirror to generate a raster display. Reaction forces generated by the motions of the mirror and counterbalance mass cancel each other at the device base, reducing vibration. In addition, the inventive structure allows placement of the mirror pivot point away from the center of the mirror which allows display devices constructed with the structure to be reduced in size.

FIELD OF THE INVENTION 
This invention relates to display devices which generate a raster scan 
image by using a line of light-emitting devices displayed by means of an 
oscillating mirror. More particularly, the invention relates to means for 
reducing vibration in such devices caused by the rapid oscillation of the 
mirror. 
BACKGROUND OF THE INVENTION 
There are many different types of display devices which can visually 
display information usch as figures, numbers and video information. These 
devices include the ubiquitous cathode ray tube in which a raster is 
created by repetitively sweeping an electron beam in a rectangular 
pattern. The image is created by selectively modulating the beam to 
generate light and dark spots on the raster. 
Another display device is an electromechanical scanning system in which a 
line of light-emitting devices is modulated with the information to be 
displayed. The illuminated line is converted into a raster by means of an 
oscillating mirror thereby generating a virtual raster image. These latter 
devices have the advantage that a full "page" display can be created from 
a much smaller number of light-emitting devices than is necessary to 
generate a normal full page real image. 
In operation, an enlarged, virtual image of the illuminated devices is 
reflected from a mirror as the mirror is being physically pivoted about a 
fixed axis by means of an electromagnetic motor. Although it is possible 
to directly drive the mirror to produce oscillations, in order to reduce 
the power necessary to drive the mirror, it is possible to use a resonant 
electromechanical oscillator to move the mirror. In such an oscillator, 
the mirror is mounted on a spring attached to a frame so that the mass of 
the mirror and the spring create a mechanical resonator. An 
electromagnetic motor oscillates the mirror mass at the resonant frequency 
of the spring/mirror system. In this manner, only a small amount of power 
is needed to produce a relatively large oscillation. Such a conventional 
resonant oscillator is shown in U.S. Pat. No. 4,632,501 in which the 
mirror is attached to the base by a thin sheet of spring material. 
A problem with the conventional mirror/spring oscillator system is that the 
rapid angular oscillation of the mirror requires a large spring force to 
accelerate and decelerate the mirror. The spring force is also applied to 
the base of the device and constitutes a "reaction force". When the base 
is rigidly secured to a relatively massive object, this force is not a 
serious concern. However, when it is impossible or undesirable to attach 
the display device to a massive object, as is the case for hand-held, 
eyeglass-mounted or "heads-up" displays, the force causes vibrations which 
are, at best, annoying and, in some cases, may cause the resulting image 
to be blurred or even unintelligible. In addition, the vibration can 
disrupt the function of an accompanying instrument, such as a microscope, 
that is sensitive to vibration. Further, even if the vibration is 
acceptable, the power required to oscillate the mirror increases when the 
vibration is transmitted to an external structure. This extra power means 
a larger motor is required to insure that the motor can drive the display 
with sufficient amplitude, in turn, resulting in increased battery drain 
for portable displays. 
This problem is even more serious if the system design requires that the 
mirror pivot about a point near the end of the mirror as opposed to near 
the center of the mirror. Such a design is desirable in a hand-held 
display application because it permits use of smaller lenses and results 
in a more compact display. 
Accordingly, it is an object of the present invention to provide a resonant 
scanning unit for an optical display device in which the net reaction 
force transferred to the mounting base is reduced. 
It is a further object of the present invention to provide a resonant 
scanning unit which reduces vibration in a resonant-scanning optical 
display device. 
It is yet another object of the present invention to provide a resonant 
scanning unit which allows the electromagnetic motor which drives the 
mirror to operate in an efficient manner. 
It is yet another object of the present invention to provide a resonant 
scanner construction which uses a counter-balanced mass construction to 
reduce the net reaction force transmitted to the mounting base. 
It is still a further object of the present invention to provide a resonant 
scanning unit in which the pivot point about which the mirror oscillates 
is located at a distance from the mirror center. 
SUMMARY OF THE INVENTION 
The foregoing objects are achieved and the foregoing problems are solved in 
one illustrative embodiment of the invention in which a resonant scanning 
unit comprises a mirror and a counterbalance mass which move in opposing 
directions. As both masses are attached to the same mounting base, it is 
possible to configure the arrangement so that reaction forces caused by 
the moving masses are cancelled at the base, thereby substantially 
reducing overall vibrations. 
More specifically, the mirror support consists of a "tuning-fork" 
configuration with the mirror mounted on one arm and a counterbalance mass 
mounted on the other arm. The driving motor comprises a magnet and coil 
structure which drives one or more of the the arms so that the arms move 
in opposite directions. 
In one embodiment, the mirror is mounted to the base of the display device 
by crossed flexure springs. A counterbalance mass is also connected to the 
base of the video display device by a spring. The stiffness of both the 
mirror flexures and the counterbalance mass spring are selected so that 
the mirror and counterbalance mass have substantially the same resonant 
frequency. 
In this embodiment, a voice-coil electromagnetic motor is used to dive the 
mirror and the counterbalance mass. The motor comprises a permanent magnet 
portion and associated magnetic return path mounted on one arm of the 
tuning fork configuration and a coil mounted on the other arm. When a 
properly controlled current is applied to the coil, the permanent magnet 
is alternately attracted and repulsed from the coil. In this fashion, a 
driving force is applied to both the mirror and the counterbalance mass 
causing each to oscillate at the frequency of the driving force. 
The spring forces which accelerate and deaccelerate the mirror and 
counterbalance mass are also applied by the flexure springs to the base, 
and constitute "reaction forces". The geometry of the counterbalance mass 
and the counterbalance mass pivot point location are both selected so that 
the reaction force applied to the base by the counterbalance mass 
substantially cancels the reaction force applied to the base by the 
mirror. 
In addition, the geometry of the electromagnetic motor is selected so that 
the drive forces applied to the mirror and counterbalance mass are 
substantially equal to the air resistance forces acting on the mirror and 
counterbalance mass, with the result that little or no net force is 
applied to the base due to drive forces.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 is a schematic diagram of a typical prior art resonant 
electromechanical scanner of the type shown in U.S. Pat. No. 4,632,501. As 
this device is explained in detail in the latter patent, it will not be 
fully discussed herein. In FIG. 1, the resonant scanner is used in a 
scanned image display device of the type covered in copending U.S. patent 
application entitled Miniature Video Display System, filed on July 27, 
1987 under Ser. No. 078,295 and assigned to the same assignee as the 
present invention. As the display device is described in detail in that 
application, which is hereby incorporated by reference, only a brief 
description of the operation will be given here. 
In a scanned image type of display device, a row of light emitting devices 
10 (which may illustratively be light-emitting diodes) is electrically 
excited to selectively emit light thereby generating an illuminated line. 
In FIG. 1, the row of LEDs extends perpendicularly into the page. 
The light from LED row 10 passes through an optical system schematically 
illustrated as lens 5, which creates an enlarged virtual image of the 
LED's. The image is reflected from mirror 30 to an observer's eye 15 as 
mirror 30 is repetitively oscillated in the direction of arrow 16. By 
selectively illuminating the LEDs in row 10 as mirror 30 moves, a 
rectangular raster can be formed which can be observed by the viewer. 
The mechanism which moves the mirror is generally termed as a resonant 
scanner. It consists of a base 20 to which plane mirror 30 is attached by 
means of a flat spring 34 which extends perpendicularly into the page. 
Mirror 30 is oscillated by a drive motor consisting of two cylindrical 
permanent magnets 44 and 120 and two ring coils 46 and 125. In operation, 
one of coils 46 and 125, for example coil 125, is excited and the 
corresponding permanent magnet, 120, is either attracted into coil 125 or 
repulsed depending on the relative magnetic fields produced by coil 125 
and magnet 120. The resulting force causes mirror 30 to pivot around the 
attachment point with spring 34 so that mirror 30 oscillates in the 
direction of arrow 16. 
The remaining coil (coil 46 in the example) is used as a sensing coil to 
sense the motion of mirror 30. The electrical signals derived from the 
motion of magnet 44 relative to coil 46 are used by driving circuitry (not 
shown) to control the current provided to drive coil 125 in a conventional 
fashion and as described in the aforementioned U.S. Pat. No. 4,632,501. 
In practice, the mass and geometry of mirror 30 and the spring constant of 
spring 34 are chosen so that a resonant mass system is formed at the 
desired operating frequency. In this manner, a large excursion angle for 
mirror 30 is produced by a driving force which is much lower than would be 
required if the mirror were driven in a non-resonant fashion. 
The problem with the mirror driving system shown in FIG. 1 is that the 
spring forces which cause the mirror mass 30 to oscillate are also applied 
to base 20 and any supporting structures attached to base 20. Although 
mirror 30 is generally quite small, its motion is typically at a 
sufficiently high frequency that the forces are large in amplitude. These 
large amplitude forces are transmitted to base 20 causing it to vibrate in 
response. 
FIG. 2 shows a partial cross-sectional view of the mirror assembly of the 
present invention. The elements of FIG. 2 which correspond to those of 
FIG. 1 have been given the same numeral designations. In the FIG. 2 
structure, mirror 30 is part of a balanced assembly with two arms. One arm 
comprises mirror 30, mirror support 36 and driving coil 46. The other arm 
consists of weight 40 and permanent magnet 44. The mirror arm is attached 
to base mounting 21 by means of two flexure springs 32 and 34. Springs 32 
and 34 are both flat springs which extend into the page. As will be 
discussed hereinafter, two springs are used in a crossed arrangement to 
constrain rotation of the mirror assembly to a single axis. 
Mirror 30 is directly attached to a mirror support 36 (which may be 
comprised of a suitable plastic or other material) by means of adhesive or 
cement. One end of spring 32 is attached to mirror support 36 by means of 
a screw or rivet or other fastener 35. The other end of spring 32 is 
attached to base mounting 21 by means of another fastener 33. A second 
spring, 34, is also attached to mirror support 36 by fastener 37 and to 
base 21 by fasteners 39. Although not explicity shown, rectangular washers 
would generally be used with fasteners 39 in order to mechanically define 
the flexing point of the spring. The two flexure springs 32 and 34 act 
together so that mirror 30 and mirror support 36 effectively pivot around 
the point 48 at which springs 32 and 34 cross. Under influence of the 
driving motor, the mirror arm oscillates in the direction of arrow 16 
around point 48. 
Weight 40 is attached to the one end of spring 42 by means of fastener 43. 
Spring 42 is also a flat spring extending into the page. The other end of 
spring 42 is attached to base 21 by means of fastener 45. Weight 40 thus 
effectively pivots around an intermediate point located on spring 42 
between attachment points 43 and 45. 
Mirror 30 and weight 40 are driven by a voice-coil type electromagnetic 
motor consisting of permanent magnet 44, weight 40 and coil 46. Magnet 44 
is rigidly secured to weight 40, while the coil 46 is rigidly secured to 
mirror support 36. Weight 40 is shaped with an overhanging portion 41 
which acts to complete the magnetic flux return path and improve the 
efficiency of the motor. Circuitry is provided (not shown) to supply a 
sinusoidal current (or other periodic current, such as a square wave or 
current pulses) to the electrical coil 46. In accordance with one feature 
of the invention, the electrical connections between coil 46 and the 
driving circuitry are provided through springs 32 and 34 in order to avoid 
separate wires which are subject to fatigue from flexing. 
The sinusoidal current in coil 46 generates a fluctuating magnet field 
which causes magnet 44 and coil 46 to be alternately attracted and 
repelled at the frequency of the current. The frequency of the sinusoidal 
driving current is chosen so that mirror 30 rotates through an arc segment 
at the resonant frequency of the spring/mass system consisting of mirror 
30 and spring 32. Generally, the desired resonant frequency will depend 
upon the use of the scanner. In a scanned image display system as 
previously mentioned, the proper resonant frequency depends on the minimum 
display refresh rate to eliminate display "flicker". The resonant freqency 
can be selected in a conventional fashion by choosing the mass and 
geometry of mirror 30 and the spring constant of spring 32. 
Advantageously, the use of the illustrative structure allows a light-weight 
drive coil to be placed on the mirror assembly rather than requiring a 
heavy magnet structure to be placed on the mirror assembly. This 
arrangement, in turn, allows the electromagnetic drive motor to be 
designed for efficient operation because the permanent magnet structure on 
the counterbalance arm can be large and heavy in order to produce a high 
magnetic field strength without contributing to the mass of the mirror 
arm. 
When used in a scanned image display, the illustrative scanner has an 
additional benefit. Since the effective mirror pivot point 48 is located 
away from the center of mirror 30, a smaller mirror can be used to produce 
an optical system with a given "exit pupil". This advantage is illustrated 
in FIGS. 3A and 3B which show two display systems each utilizing an 
oscillating scanner mirror. Elements and locations in FIGS. 3A and 3B 
which are equivalent to those elements and locations shown in FIG. 2 are 
given the same numeral designations. 
Exit pupil 110 is defined as the area in which the user's eye 15 can be 
placed so as to see the entire image. In FIG. 3A, a display system is 
constructed with a mirror pivoted near the center. As shown in the figure, 
the size of the exit pupil 110 is dependent on the arc through which the 
mirror swings and the geometry of the display. In FIG. 3B, an exit pupil 
which has the same size as the exit pupil in FIG. 3A can be achieved with 
a significantly smaller lens 5 and case size when the mirror is pivoted 
near one end according to one aspect of the present invention. 
FIG. 4 shows a partial perspective view of the illustrative embodiment of 
the resonant scanning unit with an accompanying housing shown in partial 
phantom detail. FIG. 4 illustrates the connection of flexure springs 32, 
34 and 42 between mirror 30 and weight 40 and the base mounting 21. In 
FIG. 4, components which are equivalent to those shown in FIG. 2 are given 
the same numeric designations. 
Flexure spring 34 is illustratively a U shaped spring made out of a single 
layer of flat spring material. Similarly, flexure spring 32 is a single 
layer flat spring which is mounted between the legs of spring 34 at a 
right angle to the plane of spring 32. This "crossed" spring design 
constrains movement of mirror mass 30 to essentially pure rotation whereas 
the single spring design common in conventional units is subject to 
undesirable twisting movements. Spring 42 is another U-shaped spring which 
fastens weight 40 to base 21. 
The design of the mirror assembly and counterbalance mass in order to 
accomplish cancellation of the reaction forces can be carried out in a 
conventional manner. In particular, in accordance with conventional 
mechanical design theory, the reaction forces on a mass/spring system can 
be represented by a pair of force vectors which act at a conceptual "point 
of percussion". Although the actual moving part will have both mass and 
rotary moment of inertia these may be modeled as an equivalent point mass 
located at the point of percussion. 
A secondary force vector passes through the point of percussion and the 
system pivot point and represents centrifugal force. A primary force 
vector passes through the point of percussion and is perpendicular to the 
secondary force vector and represents the force needed to translate and 
rotate the mass about the pivot point. 
In operation, in the illustrative resonant scanner, the primary force 
vector has a relatively large sinusoidal magnitude which causes the mirror 
to accelerate back and forth (the magnitude of the force is greatest at 
the extreme ends of travel of the mirror). This force depends only upon 
the equivalent mass of the mirror assembly and the amplitude of mirror 
travel and, with proper pivot point placement and design, can be 
substantially cancelled by the reaction forces generated by the 
counterbalance mass. Specifically, the geometry of the mirror and 
counterbalance mass must be adjusted so that the primary force vectors are 
co-linear. In an illustrative embodiment constructed in accordance with 
the invention and operating at a scan frequency of 50 Hz., it has been 
found that this force is approximately 42 grams-force (gmf). It has been 
found that with a mirror mass of 3.36 grams, a counterbalance mass of 
10.95 grams has produced effective cancellation of this force. 
A much smaller drag force also acts upon the mirror assembly. This force is 
primarily a velocity-proportional force due to air resistance. In the 
previously mentioned illustrative embodiment constructed in accordance 
with the principles of the invention, the "Q" value for the mirror 
assembly alone is approximately 100 resulting in a drag force of about 0.4 
gmf (the "Q" value is a measure of damping and has to do with the 
sharpness of the resonant peak). In order to make up for the energy lost 
due to the drag forces, voice coil 46 supplies a force to move mirror 30. 
If the drive force is a sine wave it can be made to substantially balance 
the previously-mentioned force resulting from drag. 
Similarly, a drag force also acts on the counterbalance assembly. In the 
preferred embodiment described above, this latter force is approximately 
0.2 gmf, corresponding a to "Q" of 200 for the counterbalance mass alone. 
The "Q" of the mirror assembly alone is lower because of its larger 
surface area. To minimize any extraneous forces resulting from imperfectly 
cancelled motor forces, in the illustrative embodiment, the motor torque 
exerted on the mirror and the counterbalance mass by the voice coil motor 
must be in the ratio of 0.4/0.2. The geometry of the illustrative 
embodiment has been designed to substantially accomplish this result. 
Although a sinusoidal drive force applied in the correct ratio 
theoretically results in substantially zero net drive related forces 
applied to the base, a non-sinusoidal drive force can also be used. If the 
drive force is periodic but not sinusoidal, a small net force will be 
applied to the base, but this force may be acceptable in view of 
simplifications possible in the motor drive circuitry. 
Further, since the equivalent mass of the mirror moves in a slight arc 
rather than in a straight line, there is also secondary force vector 
(mentioned earlier) caused by the transverse motion of the equivalent 
mass. This secondary force vector represents centrifugal force. The 
magnitude of this secondary force vector is substantially sinusoidal and 
has a frequency twice the frequency of the mirror motion. The 
double-frequency force will cause a slight vibration of the device base 
or, if the base is prevented from moving, the reaction force will be 
transferred to the supporting structure. A similar force acts on the 
counterbalance mass assembly. With the mirror and counterbalance 
configuration shown in the illustrative embodiment, the double-frequency 
forces for the mirror assembly and the counterbalance mass assembly are in 
the same direction and, consequently, do not cancel. However, as the 
extent of the transverse movement of the mirror and counterbalance masses 
is very small for example, in the illustrative embodiment approximately 
.+-.42 .mu.m for the mirror and .+-.5 .mu.m for the counterbalance mass, 
the vibration which is produced is acceptable. When incorporated in a 
display of total weight 42 gm, the resulting case vibration would be .+-.7 
.mu.m. 
It is also possible to mount the scanner assembly in a compliant mount 
which can allow slight motion of the display case so that vibrations 
caused by unbalanced forces do not cause vibration of the structure to 
which the display is attached. 
FIG. 4 also shows the electrical connections of coil 46 through flexure 
springs 32 and 34. Since two separate flexure springs, 32 and 34, are used 
to support mirror 30 these springs may also be used in order to carry 
electrical current to coil 46 and, thus, eliminate the use of separate 
wires which may be subject to breakage due to repeated flexing. In 
particular, one electrical lead, 50, of coil 46 is attached to flexure 
spring 34 by means of fastener 37. Another electrical lead, 55, is 
attached to flexure spring 32 by means of fastener 35. Electrical 
connections to coil 46 can be completed by making appropriate electrical 
connections to the other ends of the flexure springs 32 and 34 at 
fasteners 33 and 39, respectively. Current is thus carried along the 
flexure springs directly to coil 46. 
FIGS. 5-8 show an alternative embodiment of a resonant scanner unit that 
uses a preferred construction of the voice coil motor. This preferred 
construction increases motor efficiency by achieving higher magnetic field 
strength in the air gap. With a motor construction as shown in FIGS. 2 and 
4, inefficiency results because the air gap between the inside diameter of 
toroidal coil 46 and magnet 44 must be sufficient to accommodate the 
varying arcs through which mirror 30 and weight 40 move. The relatively 
large tolerance which is required in order to prevent physical collisions 
results in high flux leakage and low field strength in the air gap, and, 
thus, in poor motor efficiency. 
The motor design shown in FIGS. 5-8 improves motor efficiency by optimizing 
the magnetic circuit to reduce leakage flux. Parts of the assembly which 
remain the same as the embodiments shown in FIGS. 2 and 4 are designated 
with the same numerals. In particular, the toroidal-shaped coil 46 shown 
in FIG. 4 is replaced by a rectangular coil 66 shown in FIGS. 5 and 6. 
Instead of a single cylindrical magnet 44 as shown in FIG. 4, weight 40 
has been modified to have an "E" shape with three fingers, 70, 72 and 74 
shown in the cross-sectional view of FIG. 7. Central finger 72 fits into 
the rectangular opening 73 in coil 66 as shown in FIGS. 7 and 8. The two 
side fingers 70 and 74 are provided with magnets 73 and 75 which lie on 
the outside of coil 66 as shown in FIGS. 7 and 8. 
Also shown in FIG. 5 is a slot 76 which is cut in weight 40. This slot, as 
will hereinafter be described, is used to accommodate a mechanical sensor 
that senses the position of mirror 30 and generates electrical signals 
which control the driving current to ensure that mirror 30 and weight 40 
oscillate at the desired resonant frequency. 
FIGS. 6-8 also shown an improved mechanism for attaching spring 42 to 
weight 40. As shown, spring 42 is clamped between two clamping members 130 
and 135. Clamp members 130 and 135 have slots 51 (shown in FIG. 6) which 
allow the members to be slid over fasteners 43. The ends of spring 42 
which are fastened by fasteners 43 are not slotted, thus the distance 
between the weight 40 and base 21 is mechanically fixed. However, clamping 
members 130 and 135 can be moved relative to weight 40, thus effectively 
changing the attachment point of spring 42 to weight 40. Thus, the 
effective spring length can be changed for the purposes of adjusting the 
resonant frequency without changing the basic geometry of the device. 
FIG. 9 is a partial cross-section of the resonant scanning unit of FIGS. 
5-8 fitted with a position sensor mechanism. The main components of this 
mechanism are also depicted in the partial exploded view shown in FIG. 10. 
The sensor mechanism consists of a "flag" 80 which is mounted on one end 
of rectangular coil unit 66. When the unit is assembled, flag 80 extends 
between the two arms of LED/photocell sensing unit 90. As shown in FIG. 
10, sensing unit 90 consists of a mounting bracket 92 which is affixed to 
the scanner housing as shown in FIG. 9. Two arms 94 and 96 extend from the 
bracket 92 and lie on either side of flag 80. An LED device 98 is mounted 
on arm 94 and a photodiode 100 is mounted on arm 96. 
In operation, when mirror 30 rotates through an angle which exceeds 
approximately 70% of the maximum normal operating angle, light emitted 
from LED 98 is sensed by photodiode 100 which thereupon generates an 
electrical signal. When the angle of rotation of mirror 30 is less than 
approximately 70% of the maximum normal operating angle, flag 80 is 
interposed between LED 98 and photodiode 100, in turn, preventing light 
emitted from LED 98 from reaching diode 100. The signal emitted from diode 
100 thereupon ceases. Consequently, as mirror 30 and weight 40 oscillate, 
an oscillating electrical signal is developed by photodiode 100. This 
oscillating signal is used to control the driving electronics which 
provide the current to the coil 66 of the electromagnetic driving motor. 
A schematic block diagram of the driving circuit electronics is shown in 
FIG. 11. The basic components of the circuit consists of a comparator 102, 
a phase-locked loop 104, an automatic gain control circuit 106 and a power 
amplifier 108. In FIG. 11, flag 80, LED 98 and photodiode 100 
schematically illustrate the components physically depicted in FIG. 10 and 
designated with the same numerals. The components are arranged in a 
conventional frequency control loop. More particularly, the oscillating 
output signal developed by photodiode 100 is provided to one input of 
comparator 102. Comparator 102 compares the voltage signal level to a 
reference voltage in order to standardize the waveform and sharpen the 
zero crossing points. The output of the comparator is a pulse-train signal 
which is used to drive the remainder of the circuitry. 
The output from comparator 102 is provided to a conventional phase-locked 
loop circuit 104 which is adjusted to maintain the frequency of 
oscillation at the desired value. The operation of such a phase-locked 
loop is conventional and will not be explained hereinafter in detail. 
Circuit 104 generates control signals which control the power amplifier. 
The output of comparator 102 is also provided to a conventional automatic 
gain control circuit 106 which generates a magnitude control signal. 
The control signals generated by phase-locked loop circuit 104 and AGC 
circuit 106 are provided to power amplifier which provides the driving 
current to the voice coil and completes the feedback loop. 
FIG. 12 of the drawing shows an illustrative embodiment of the resonant 
scanning unit incorporated into a miniature display device. The miniature 
display device is of the type described in detail in aforementioned 
copending U.S. patent application Ser. No. 078,295. The operation and 
construction of the display device is discussed in detail in that 
application which is hereby incorporated by reference, and will not be 
repeated in detail herein for clarity. The display device consists of a 
base 10 on which the various optical components which comprise the display 
are mounted. At one end of base 10 is mounted the header block 5 in which 
an array of light-emitting devices 15 (such as light-emitting diodes) is 
attached. Generally, such an array may be a linear array comprising two 
rows of devices which are staggered in order to compensate for gaps 
between the devices. The devices are covered by a clear cover plate 17. 
Light emitted from devices 15 is projected on mirror 30 by means of an 
optical system which consists of housing 18 in which are mounted lenses 19 
and 23. In accordance with the principles set forth in the aforementioned 
U.S. patent application Ser. No. 078295 the lens system projects the image 
of array 15 via mirror 30. 
Mirror 30 is actuated by providing a periodic current via leads 50 and 55 
(shown in FIG. 4) to coil 46, causing mirror 30 and weight 40 to 
oscillate. The oscillation of mirror 30, in turn, creates a raster image 
from linear array 15. 
Having thus described one preferred embodiment of the present invention it 
will be apparent to those skilled in the art that various modifications 
and alterations are possible without departing from the spirit and scope 
of the invention. For example, instead of the pair of flexure springs used 
to support the mirror assembly, other conventional arrangements such as 
four-bar linkages (in which each end of the mirror is attached to the base 
by means of a separate link) may be used without departing from the spirit 
and scope of the invention. Similarly, a different geometrical arrangement 
can be used in which the mirror is attached to the base at the center 
rather than at one end. The invention is not intended to be limited to the 
preferred embodiment disclosed above. It is limited in scope by the 
following claims.