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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a projection apparatus, and in particular to a
projection apparatus for effecting image formation by the use of a light
scanning optical system including an electro-mechanical transducer element
having a number of minute movable mirrors. As such a projection apparatus,
there is a recording apparatus or a display apparatus.
2. Related Background Art
For example, DMD (deformable mirror device) is known as an
electro-mechanical transducer element having the minute movable mirrors as
described above.
DMD is described in IEE Transaction on Electron Device, Vol. ED-30, No.
5544(1983), and the optical system thereof is disclosed in U.S. Pat. No.
4,454,547.
FIG. 1 of the accompanying drawings is a fragmentary plan view of DMD 7,
and reference numeral 3 designates mirrors. When each mirror is in its ON
state, the reflected light from the mirror is applied to the surface of a
photosensitive medium so that the direction X corresponds to the direction
of the rotational axis of the photosensitive medium and the direction Y
corresponds to the direction of rotation of the photosensitive medium.
That is, the direction X is a direction corresponding to the main scan
direction and the direction Y is a direction corresponding to the
subsidiary scan direction.
Now, in the conventional DMD, as shown in FIG. 1, two mirror rows have been
formed in the direction X with the same spacing as the length of each
mirror in the direction X being kept between the mirrors, and the mirrors
have been disposed in such a staggered pattern that the two mirror rows do
not overlap each other in the direction Y. The two mirror rows have been
spaced apart from each other in the direction Y while keeping therebetween
the same spacing as the length of the mirror 3 in the direction Y, and
subsidiary scanning has been effected at a pitch corresponding to this
spacing.
Accordingly, if the DMD and the light scanning optical system are
geometro-optically ideal, when all mirrors are in their ON state,
subsidiary scanning is effected at a pitch corresponding to the length of
the mirror in the direction Y, whereby light dots by the reflected light
from the mirrors ought to be formed on the surface of the photosensitive
medium without overlapping one another and without any slipping portion.
By suitably setting a signal input to a DMD driving circuit, a suitable
mirror is caused to be ON-OFF at a suitable time, whereby a desired image
ought to be formed on the surface of the photosensitive medium.
In reality, however, there occurs the divergence of reflected light by the
curvature of the mirrors of DMD (that is, ideally, only the hinge portions
of the mirrors may be curved, but actually the whole of the mirrors 3 is
curved) and there also occurs the divergence of light by the aberrations
of the other optical system than DMD and further, in DMD, the phenomenon
of diffraction occurs because of the mirrors being minute and therefore,
the intensity of light of each light dot which the reflected light by the
mirrors of DMD forms on the photosensitive medium has such a distribution
that in each dot, the central portion is large and the marginal portion is
small.
FIGS. 2A and 2B of the accompanying drawings are graphs showing the
quantity-of-light distribution in the light dot portion on the surface of
the photosensitive medium corresponding to the portion along line x.sub.1
--x.sub.1 of the first row of mirror arrays and the light dot portion
corresponding to the portion along line x.sub.2 --x.sub.2 of the second
row of mirror arrays when all the mirrors of DMD are in their ON state.
When there are only the aberrations of the other optical system than DMD,
there are provided such distributions as indicated by dotted lines in
FIGS. 2A and 2B, but in reality, as described above, on the basis of
various causes, there are provided such distributions as indicated by
solid lines in FIGS. 2A and 2B.
So, if the light dots formed by the first row of mirror arrays and the
second row of mirror arrays on the same main scan line on the
photosensitive medium at different points of time are combined together,
the quantity-of-light distribution thereof will be such as shown in FIG.
2C of the accompanying drawings. When there are only the aberrations by
the other optical system than DMD, there is provided a generally uniform
quantity of-light distribution as indicated by dotted line, but in
reality, there is provided a non-uniform distribution as indicated by
solid line.
If the uniformity of this quantity-of-light distribution is aggravated and
(I.sub.max -I.sub.min)/(I.sub.max +I.sub.min) exceeds the order of 0.05,
irregularity will appear in an image developed by a developing device.
Depending on the type of development, such irregularity will appear as a
black fringe in the subsidiary scan direction in a white solid image or a
white fringe in the subsidiary scan direction in a black solid image.
A similar phenomenon also occurs to the quantity-of-light distribution in
the light dot portion corresponding to the portion along line y.sub.1
--y.sub.1 of the mirror 3.
FIG. 3A of the accompanying drawings is a graph showing the
quantity-of-light distribution in the light dot portion on the surface of
the photosensitive medium corresponding to the portion along line y.sub.1
--y.sub.1 of the mirror when the mirrors of DMD are in their ON state.
When, as described previously, there are only the aberrations of the other
optical system than DMD, there is provided such a distribution as
indicated by dotted line in FIG. 3A, but in reality, as described above,
on the basis of various causes, there is provided such a distribution as
indicated by solid line in FIG. 3A.
FIG. 3B of the accompanying drawing shows the state of superposition of the
light dot of FIG. 3A in the portion on the surface of the photosensitive
medium corresponding to the portion along line y.sub.1 --y.sub.1 of the
mirror 3 when subsidiary scanning is effected with the mirrors of DMD
being in their ON state.
FIG. 3C of the accompanying drawings shows a quantity-of-light distribution
in which the light dot distributions shown in FIG. 3B are combined
together. When, as described previously, there are only the aberrations by
the other optical system than DMD, there is provided a generally uniform
quantity-of-light distribution as indicated by dotted line, but in
reality, there is provide a non-uniform distribution as indicated by solid
line.
If the uniformity of this quantity-of-light distribution is aggravated and
(I.sub.max -I.sub.min)/(I.sub.max +I.sub.min) exceeds the order of 0.05,
irregularity will appear in the image developed by the developing device.
Depending on the type of development, such irregularity will appear as a
black fringe in the main scan direction in a white solid image or a white
fringe in the main scan direction in a black solid image.
Thus, a recording apparatus using a light scanning optical system including
the DMD as described above has suffered from a problem that print of good
quality cannot be obtained.
Such a problem occurs not only to a recording apparatus, but also to other
projection apparatus such as a display apparatus. In the case of a display
apparatus, a suitable display screen is used instead of the photosensitive
medium in the recording apparatus, and subsidiary scanning is generally
effected with the display screen being fixed and with the position of the
light application from the optical system being moved.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a projection
apparatus which is capable of forming good images.
For the purpose of solving the above-noted problems peculiar to the prior
art, according to the present invention, there is provided a projection
apparatus characterized in that at least one of the shape and arrangement
of the movable mirrors of each mirror row and the subsidiary scan pitch is
determined so that light dots adjacent to each other in at least one of
the subsidiary scan direction and the main scan direction sufficiently
overlap each other in a pattern of light dots which the reflected light
from the movable mirrors forms on a light-receiving member with the
subsidiary scanning.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. l is a fragmentary plan view of the DMD of a recording apparatus.
FIGS. 2A, 2B and 2C are graphs showing the quantity-of-light distributions
of light dot patterns in the main scan direction.
FIGS. 3A, 3B and 3C are graphs showing the quantity-of-light distributions
of light dot patterns in the subsidiary scan direction.
FIGS. 4A, 4B and 4C illustrate the DMD.
FIG. 5 shows the construction of a recording apparatus.
FIG. 6 shows the construction of a DMD driving circuit.
FIG. 7 is a fragmentary plan view of a DMD according to the present
invention.
FIGS. 8 and 9 show a state in which mirror arrangement patterns are
superposed one upon another.
FIGS. 10A, 10B and 10C are graphs showing the quantity-of-light
distributions of light dot patterns in the main scan direction in first
and second embodiments of the present invention.
FIG. 11 is a fragmentary plan view of another DMD according to the present
invention.
FIG. 12 shows a state in which other mirror arrangement patterns are
superposed one upon another.
FIGS. 13A, 13B and 13C are graphs showing the quantity-of-light
distributions of light dot patterns in the subsidiary scan direction in
second and third embodiments of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The mechanism of DMD will first be described with reference to the
drawings. FIG. 4A is an enlarged cross-sectional view of DMD. Reference
numeral 11 designates a mirror surface which is formed of a substance such
as Al or Ag and performs the function of reflecting an incident light.
Reference numeral 12 denotes a substrate which supports the mirror surface
11 and which is formed of Au or the like. Reference numerals 13 and 14
designate supporting members for a mirror portion comprising the mirror
surface 11 and the substrate 12. The supporting member 13 is called a
mirror contact which particularly receives a hinge portion effecting an
electro-mechanical operation, and the supporting member 14 is formed of an
insulating substance of polyoxide Si. Reference numeral 15 denotes a
polysilicon gate which performs the function as the gate of MOSFET.
Reference numeral 16 designates an air gap which is a cavity of 0.6.mu. to
several .mu.. Reference numeral 17 denotes a floating field plate to which
a voltage is applied from an N.sup.+ floating source 18 by the ON-OFF
information of a transistor. Reference numeral 19 designates an N.sup.+
drain. This also performs the function of the construction of an MOS type
FET transistor. Reference numeral 20 denotes a gate oxide, and reference
numeral 21 designates a P type silicon substrate.
FIG. 4B is an enlarged front view taken from the direction of arrow A in
FIG. 4A. Reference numeral 22 designates an air clearance, reference
numeral 23 denotes a mirror oscillation portion (hereinafter simply
referred to as the mirror) which oscillates electro-mechanically, and
reference numeral 24 designates a hinge portion. Reference numeral 25
denotes the other mirror surface of the DMD surface than the mirror 23.
DMD is made by a process similar to the process of making IC or LSI.
FIG. 4C shows an electrical equivant view of DMD. Reference numeral 26
designates the equivalent portion of the mirror portions 11 and 12 to
which a voltage V.sub.M is applied. Reference numeral 27 denotes the
equivalent portion of the N.sup.+ floating source 18 to which a voltage
V.sub.F is applied. Reference numeral 28 designates a transistor
construction. The voltage V.sub.F is switched on and off to the N.sup.+
floating source 18 by ON and OFF of the D (drain) signal of the N.sup.+
drain 19 and the G (gate) signal of the gate 15. At this time, the voltage
V.sub.M is applied to the mirror portions 11 and 12, and the potential
difference is increased or decreased between the mirror portions 11, 12
and the source 18 by ON-OFF signal. In conformity with this potential
difference, there is produced between the mirror portions 11, 12 and the
plate 17 a force F corresponding to the following formula:
F.about.KV.sup..alpha.
(K: constant, V: potential difference, .alpha.: constant, F: bending force)
and the mirror portions 11 and 12 are pivotally moved about the hinge
portion 24.
The left portion of FIG. 4A shows a case where there is a great voltage
difference between the mirror portions 11, 12 and the source 18, and the
mirror 23 is bent from the hinge portion 24 and by this action, the
incident light is reflected while changing an angle twice the angle of
deviation of the mirror 23.
On the other hand, when the voltage difference is small, as shown in the
right portion of FIG. 4A, the force with which the mirror 23 of the mirror
portions 11, 12 is pulled by the plate 17 is small and thus, the mirror 23
is not curved. Accordingly, the incident light is reflected without the
mirror being deviated. DMD converts electrical ON and OFF into the ON and
OFF of the oscillation of the mirror 23 and further converts them into the
angle of deviation of light.
FIG. 5 shows a recording apparatus utilizing a light scanning optical
system including the DMD as described above.
In FIG. 5, reference numeral 29 designates the light scanning optical
system, and reference numeral 30 denotes an electrophotographic process
apparatus. These together constitute a printer.
Reference numeral 31 designates a lamp, reference numerals 32 and 34 denote
optical systems for illuminating DMD, and reference numeral 33 designates
a slit plate for the optical systems 32, 34. The slit plate 33 is designed
to illuminate only the mirror array portion of DMD. Reference numerals 35
and 36 denote bending mirrors, and reference numeral 37 designates DMD
which effects an electro-mechanical operation by the principle of FIGS.
4A-4C.
Reference numeral 38 designates a circuit for driving the DMD 37, and
reference numeral 39 denotes a lens for imaging the reflected light from
the DMD 37 on a photosensitive medium 40. Usually, only when a signal has
entered each mirror 23 of the DMD 37, the reflected light from the mirror
23 enters the pupil of the imaging lens 39.
Reference numerals 41-45 designate devices usually used in the
electrophotographic process. That is, reference numeral 41 denotes a
developing device, reference numeral 42 designates a charger for
transferring the toner on the photosensitive medium 40 onto copying paper
43, reference numeral 44 denotes a cleaner, and reference numeral 45
designate a charger for imparting charge to the photosensitive medium 40.
Reference numeral 46 denotes a light-intercepting plate for cutting the
OFF signal light of DMD.
The function as a printer is performed by a signal input to the DMD element
driving circuit 38 imparting a command input to the DMD 37. The DMD 37
electromechanically reacts in accordance with the operation principle
shown in FIGS. 4A-4C in response to a signal, and the mirror 23 is
pivotally moved. The light B of the illuminating system emitted from the
lamp 31 illuminates the mirror array portion of the DMD 37 in a slit-like
form through the illuminating optical systems 32, 33, 34, 35, 36. When the
individual mirrors 23 of the mirror array on the DMD 37 are in their OFF
state, the light B applied travels toward the reflected light D reflected
by the mirrors 23 and is intercepted by the light intercepting plate 46,
and no light reaches the photosensitive medium 40. When the mirrors 23 are
in their ON state, the light is reflected in a direction C and enters the
imaging lens 39, and a dot pattern corresponding to the mirrors 23 is
formed on the photosensitive medium 40. Accordingly, if line-like ON and
OFF signals are input to the driving circuit 38, development is effected
via the electrophotographic process, whereafter the function as a printer
in which the toner image is transferred onto the copying paper 43 takes
place.
Also, a circuit shown in FIG. 6 is usually used as the driving circuit 38
shown in FIG. 5. Reference numeral 4-1 designates an input signal
amplifier. In the case of a binary signal, the amplifier 4-l is ON or OFF,
and in the case of an analog signal, it puts out a voltage corresponding
to the amount thereof. The signal is usually input in series and therefore
is converted into parallel signals corresponding to the number of the
mirrors 23 the DMD 37 by a simipara exchanger 4-2, add the parallel
signals are stored in a register 4-3. Those signals corresponding to a row
are read out by a synchronizing signal, and via an amplifer 4-4, a
predetermined voltage signal is applied to the two rows of drains 19-1 and
19-2 of the DMD 37. On the other hand, in response to said synchronizing
signal, a gate signal is imparted to the gate 15 of the DMD 37 by a
decoder 4-5. Depending on the amount of this drain signal and the presence
or absence of such signal and depending on the presence or absence of a
gate signal in each row, the voltage of the floating source 18 of the DMD
37 is transmitted to the floating field plate 17, whereby the selection of
ON or OFF of the pivotal movement of the mirrors 23 is effected.
FIG. 7 is a schematic fragmentary plan view of DMD 37 in a first embodiment
of the projection apparatus according to the present invention.
As shown in FIG. 7, in the apparatus of the present embodiment, the length
of each mirror 23 of the DMD 37 in a direction corresponding to the main
scan direction, i.e., in the direction X, is greater than the arrangement
pitch of the mirrors in the direction X. The first row of mirror arrays
and the second row of mirror arrays are disposed in staggered relationship
while being deviated from one another by a half of the mirror arrangement
pitch in the direction X.
Also, in the apparatus of the present embodiment, the first row of mirror
arrays and the second row of mirror arrays are spaced apart from one
another by a length corresponding to the subsidiary scan pitch, i.e., the
length of each mirror 23 in the direction Y.
Accordingly, if the mirror arrangement patterns of the DMD 37 in the
apparatus of the present embodiment are moved successively in the
direction Y at a pitch corresponding to the subsidiary scan pitch and the
mirror arrangement patterns are superposed one upon another, the result
will be such as shown in FIG. 8. In FIG. 8, P.sub.1-1 and P.sub.1-2 are
the mirror arrangement patterns at the initial position, and P.sub.1-1
corresponds to the first row of mirror arrays and P.sub.1-2 corresponds to
the second row of mirror arrays. The patterns resulting from P.sub.1-1 and
P.sub.1-2 being moved in the direction Y by a distance corresponding to
the subsidiary scan pitch are P.sub.2-1 and P.sub.2-2, and the patterns
resulting from these being successively moved in the direction Y by a
distance corresponding to the subsidiary scan pitch are P.sub.3-1,
P.sub.3-2 and P.sub.4-1, P.sub.4-2.
FIGS. 10A and 10B are graphs showing the quantity-of-light distributions of
a light dot portion on the surface of the photosensitive medium 40
corresponding to the portion along the line x.sub.1 --x.sub.1 of the first
row of mirror arrays and a light dot portion corresponding to the portion
along the line x.sub.2 --x.sub.2 of the second row of mirror arrays when
all the mirrors 23 of the DMD 37 are in their ON state. As shown in these
graphs, the quantity-of-light distribution of the light dot on the surface
of the photo sensitive medium 40 in the present embodiment is such that as
in the conventional apparatus, the intensity of the central portion is
great and the intensity of the marginal portion is small. However, in the
present embodiment, the length of each mirror 23 in the direction X is
greater than in the conventional apparatus and therefore, the portion
which is great in intensity is relatively wide. A portion of about a half
of the intensity I.sub.max just lies on the portion in which a light dot
row formed by the reflection on the first row of mirror arrays overlaps a
light dot row formed by the reflection on the second row of mirror arrays.
If the light dots formed at different points of time on the same main
scanning line on the photo sensitive medium 40 by the first row of mirror
arrays and the second row of mirror arrays are combined together, the
quantity-of-light distribution thereof will be such as shown in FIG. 10C.
In this quantity-of-light distribution, the condition that (I.sub.max
-I.sub.min)/(I.sub.max +I.sub.max +I.sub.min)<0.05 is sufficiently
satisfied. As can be seen in this graph, in the solid image formed by the
apparatus of the present embodiment, the uniformity of the
quantity-of-light distribution is very good. Therefore, no white or black
fringe appears in the subsidiary scan direction.
In the above-described embodiment, an example has been shown which has such
shape and arrangement of the mirrors 23 that only the end portions of the
mirrors 23 in the direction X, i.e., the direction corresponding to the
main scan direction, overlap one another as shown in FIG. 8 when the
mirror arrangement patterns of the DMD 37 are successively moved in the
direction Y at a pitch corresponding to the subsidiary scan pitch and the
mirror arrangement patterns are superposed one upon another, but the
present invention may likewise have such shape and arrangement of the
mirrors that the end portions of the mirrors in the direction Y, i.e., the
direction corresponding to the subsidiary scan direction, also overlap one
another when the mirror arrangement patterns are superposed one upon
another. For this purpose, the subsidiary scan pitch equal to the spacing
between the rows of mirrors may be made shorter than the length of each
mirror 23 in the direction corresponding to the subsidiary scan direction.
The state in which such mirror arrangement patterns in a second embodiment
are superposed one upon another is shown in FIG. 9.
According to the second embodiment, no white or black fringe appears in
either the main scan direction or the subsidiary scan direction.
FIG. 11 is a schematic fragmentary plan view of DMD 137 in a third
embodiment of the projection apparatus according to the present invention.
The apparatus of this embodiment was also embodied as a printer which is a
recording apparatus having a construction similar to what has been
described above, and the components thereof need not be described.
As shown in FIG. 11, in the apparatus of the present embodiment, the length
of each mirror 123 of the DMD 137 in the direction corresponding in the
main scan direction, i.e., in the direction X, is the same as the mirror
arrangement pitch in the direction X. The first row of mirror arrays and
the second row of mirror arrays are disposed in staggered relationship
while being deviated from one another by a half of the mirror arrangement
pitch in the direction X.
Also, in the apparatus of the present embodiment, the first row of mirror
arrays and the second row of mirror arrays are spaced apart from one
another by a length shorter than the length corresponding to the
subsidiary scan pitch.
Accordingly, if the mirror arrangement patterns of the DMD 137 of the
apparatus of the present embodiment are successively moved in the
direction Y at a pitch corresponding to the subsidiary scan pitch and the
mirror arrangement patterns are superposed one upon another, the result
will be such as shown in FIG. 12. In FIG. 12, P.sub.1-1 and P.sub.1-2 are
the mirror arrangement patterns at the initial position, and P.sub.1-1
corresponds to the first row of mirrors arrays and P.sub.1-2 corresponds
to the second row of mirror arrays. The patterns resulting from P.sub.1-1
and P.sub.1-2 being moved in the direction Y by a distance corresponding
to the subsidiary scan pitch are P.sub.2-1 and P.sub.2-2, and the patterns
resulting from these being successively moved in the direction Y by a
distance corresponding to the subsidiary scan pitch are P.sub.3-1,
P.sub.3-2 and P.sub.4-1, P.sub.4-2.
FIG. 13A is a graph showing the quantity-of-light distribution of a light
dot portion on the surface of the photosensitive medium 40 corresponding
to the portion along the line y.sub.1 --y.sub.1 of the mirrors 123 of the
DMD 137 when these mirrors are in their ON state.
Also, FIG. 13B shows a state in which the light dot portions on the surface
of the photosensitive medium 40 corresponding to the portion along the
line y.sub.1 --y.sub.1 of the mirrors 123 of the DMD 137 when subsidiary
scanning is effected with the mirrors 123 being in their ON state are
superposed one upon another. As shown in FIG. 13B, the quantity-of-light
distribution of the light dot on the surface of the photosensitive medium
in the present embodiment is such that as in the conventional apparatus,
the intensity of the central portion is great and the intensity of the
marginal portion is small. However, in the present embodiment, the length
of each mirror in the direction Y relative to the subsidiary scan pitch is
greater than in the conventional apparatus and therefore, the portion
which is great in intensity is relatively wide. The portions of the
adjacent light dots having about a half of intensity I.sub.max just
overlap each other.
A quantity-of-light distribution in which the light dot distributions shown
in FIG. 13B are combined together is shown in FIG. 13C. As can be seen
from FIG. 13C, in a solid image formed by the apparatus of the present
embodiment, the uniformity of the quantity-of-light distribution is very
good. Therefore, no white or black fringe appears in the main scan
direction. Where the mirror arrays are in a row, the subsidiary scan pitch
may be made shorter than the length of each mirror in the direction
corresponding to the subsidiary scan direction.
The foregoing description has been made with respect to a white solid image
or a black solid image, but in the ordinary image as well, a similar
effect is obtained in the white portion or the black portion of the image,
whereby chapping of the image is eliminated.
Also, in the above-described embodiments, the mirror arrangement patterns
are disposed in two staggered rows, but alternatively, the mirror arrays
of electromechanical transducer elements in the present invention may be
suitable patterns comprising one row or three or more rows. Again in such
case, an effect similar to that described in the foregoing embodiments is
obtained.
While the foregoing embodiments have been described with respect to a case
where the projection apparatus is a recording apparatus, the present
invention also covers a case where the projection apparatus is a display
apparatus or the like.
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