Projecting optical apparatus having no trapezoidal distortion

A projecting optical apparatus has a light source which is located above a document supporting plate to illuminate a document located on the document supporting plate. A projecting lens system is located above the document supporting plate to project an image of the document on a projection screen. A reflection mirror reflects the light transmitted through the projecting lens system toward the projection screen to make an image on the projection screen. The projecting lens system and the reflection mirror are incorporated in a projecting reflection unit, so that the projecting reflection unit is movable in directions substantially parallel with the plane of the document supporting plate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a projecting optical apparatus which is 
used in an over-head projector or a similar apparatus, and more precisely 
it relates to a projecting optical apparatus in which no trapezoidal 
distortion is produced when the projecting position (height) is changed. 
2. Description of Related Art 
FIGS. 6 through 8 show a known over-head projector which will be referred 
to as OHP hereinafter. In FIGS. 6-8, a light source 12 and a projecting 
lens system 13 are arranged side by side above a document supporting plate 
11 which lies in a substantially horizontal plane, so that the optical 
axis X of the projecting lens system 13 is normal to the plane of the 
document supporting plate 11. Above the projecting lens system 13 an 
inclined reflection mirror 14 is located and has an adjustable inclination 
angle. Light emitted from the light source 12 is incident upon a document 
(paper etc.) O on the document supporting plate 11, so that the light 
reflected by the document O is converged onto a projection screen 15 which 
lies in a plane perpendicular to the document supporting plane 11 through 
the projecting lens system 13 and the reflection mirror 14 to make an 
image on the projection screen 15. 
In the OHP mentioned above, the (height position) of an image on the 
projection screen 15 can be adjusted by controlling the inclination angle 
of the reflection mirror 14. 
In an arrangement shown in FIG. 6, the reflection mirror 14 is inclined at 
an inclination angle of 45.degree. with respect to the document supporting 
plate 11 and the projection screen 15, so that the optical axis X of the 
light transmitted through the projecting lens system 13 and reflected by 
the reflection mirror 14 is normal to the projection screen 15. 
FIG. 7 shows a different position of the reflection mirror 14, in which the 
reflection mirror 14 is rotated by an angle of 10.degree. from a position 
shown in FIG. 6 in the counterclockwise direction, so that the light 
transmitted through the projecting lens system 13 along the optical axis X 
thereof and reflected by the reflection mirror 14 makes an angle (angle of 
elevation) of 20.degree. with respect to a line normal to the plane of the 
projection screen 15. 
In the conventional OHP, no trapezoidal distortion of an image O' (which is 
represented by a grid-like pattern for simplification in FIG. 8) of the 
document O, as shown by a dotted line in FIG. 8 is produced in a 
horizontal projection position shown in FIG. 6. However, at an angle of 
elevation of 20.degree. shown in FIG. 7, a trapezoidal distortion is 
produced as shown by a solid line in FIG. 8. The trapezoidal distortion 
increases as the angle of elevation increases. Furthermore, when an image 
projected on the projection screen has a trapezoidal distortion, not only 
is the image wholly distorted into a trapezoidal shape, but the light is 
focused only at one point (focal point) in the vertical direction in FIG. 
8. Namely, the image is out of focus. 
In actual use of the OHP, the projection state having an angle of elevation 
as shown in FIG. 7 often occurs rather than the horizontal projection 
state as shown in FIG. 6. Thus, an image (picture) on the projection 
screen is viewed at an angle of elevation. Consequently, a viewer looks at 
an image which is wholly out-of focus. 
SUMMARY OF THE INVENTION 
The primary object of the present invention is therefor to provide a 
projecting optical apparatus in which no trapezoidal distortion is 
produced even when the (height position) of projection is varied. 
As mentioned above the trapezoidal distortion is caused by a change in the 
angle of elevation which is, in turn caused by a change in the inclination 
angle of the reflection mirror. Accordingly, the present invention 
proposes a projecting optical apparatus in which the height position of 
the projection can be adjusted without changing the inclination angle of 
the reflection mirror, thus resulting in no trapezoidal distortion. 
The projecting optical apparatus can be applied not only to a reflection 
type projecting optical system in which a reflected image of a document is 
projected on a projection screen, but also to a transmission type 
projecting optical system in which a transmission image of a document is 
projected on a projection screen. 
To achieve the object mentioned above, according to one aspect of the 
present invention, a projecting optical apparatus is provided including a 
light source which is located above a document supporting plate to 
illuminate a document located on the document supporting plate. A 
projecting lens system is located above the document supporting plate to 
project an image of the document on a projection screen and a reflection 
mirror reflects the light transmitted through the projecting lens system 
toward the projection screen to make an image on the projection screen. 
The improvement comprises a projecting reflection unit in which the 
projecting lens system and the reflection mirror are incorporated, so that 
the projecting reflection unit is movable in directions substantially 
parallel with a plane of the document supporting plate. 
With this arrangement, the height position of projected image of the 
document on the projection screen can be adjusted by moving the projecting 
reflection unit without causing trapezoidal distortion. 
Preferably, to make it possible for light reflected from the document on 
the document supporting plate to be effectively incident upon the 
projecting lens system, the light source is movable in directions 
substantially parallel with the plane of the document supporting plate, so 
that the light source and the projecting reflection unit are symmetrically 
moved toward and away from each other with respect to an imaginary center 
of the document supporting plate. 
According to another aspect of the present invention, there is provided a 
projecting optical apparatus comprising a light source which is located 
below a transmission type of document supporting plate to illuminate a 
document located on the document supporting plate. A projecting lens 
system is located above the document supporting plate to project an image 
of document onto a projection screen. A reflection mirror reflects the 
light transmitted through the projecting lens system toward the projection 
screen to make an image on the projection screen. The improvement 
comprises a projecting reflection unit in which the projecting lens system 
and the reflection mirror are incorporated, so that the projecting 
reflection unit is movable in directions substantially parallel with a 
plane of the document supporting plate. 
A difference between the reflection type and the transmission type resides 
only in the location of the light source. Namely, the light source is 
located on the same side and on opposite sides of the projecting lens 
system with respect to the document supporting plate in the reflection 
type projector and the transmission type projector, respectively. 
Accordingly, in a transmission type projector, to make it possible for 
light reflected from the document on the document supporting plate to be 
effectively incident upon the projecting lens system, the light source is 
movable in directions substantially parallel with the plane of the 
document supporting plate. Thus, the light source and the projecting 
reflection unit are symmetrically moved towards and away from each other 
with respect to an imaginary center of the document supporting plate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIGS. 1 through 3 show a principle of a projecting optical apparatus 
according to an embodiment of the present invention. 
The internal construction of the document supporting plate 11, the light 
source 12, the projecting lens system 13, the reflection mirror 14 and the 
projection screen 15 are same as those shown in FIG. 6. In the present 
invention, the projecting lens system 13 and the reflection mirror 14 are 
incorporated together in a projecting reflection unit 20. The light source 
12 is also incorporated in an illumination unit 21. Both the projecting 
reflection unit 20 and the illumination block 21 are movable in directions 
parallel with the plane of the document supporting plate 11. The 
projecting lens system 13 is supported in the projecting reflection unit 
20 so that the optical axis X of the projecting lens system 13 is normal 
to the plane of the document supporting plate 11. The reflection mirror 14 
is supported in the projecting reflection unit 20, so that the reflection 
mirror 14 is inclined at an inclination angle of 45.degree. with respect 
to both the document supporting plate 11 and the projection screen 15. 
The projecting reflection unit 20 and the illumination unit 21 is moved 
towards and away from each other. The movement of the projecting 
reflection unit 20 and the illumination unit 21 preferably takes place 
symmetrically with respect to an imaginary center line 22 of the 
projecting reflection unit 20 and the illumination unit 21, perpendicular 
to the plane of the document supporting plate 11. 
When the projecting reflection unit 20 and the illumination unit 21 move 
towards and away from each other, the projection position (height) of an 
image on the projection screen 15 is changed. When the height is adjusted, 
no trapezoidal distortion takes place, since the inclination angle of the 
reflection mirror 14 does not change. 
In FIGS. 1 and 2, the distance between the projecting lens system 13 and 
the projection screen 15 is four times the distance between the document 
supporting plate 11 and the projecting lens system 13, so that an image 0' 
of the document 0 projected on the projection screen 15 is enlarged by 
four times. When the projecting reflection unit 20 and the illumination 
unit 21 are moved in directions parallel with the plane of the document 0 
while maintaining a symmetrical positional relationship with respect to 
the imaginary center line 22, the position of the image 0' of the document 
0 projected on the projection screen 15 is changed by four times the 
displacement of the projecting reflection unit 20 (and the illumination 
unit 21), without causing a trapezoidal distortion. Thus, if the distance 
between the imaginary center line 22 on the document supporting plate 11 
(center of the document 0) and the optical axis X of the projecting lens 
system 13 is a, the distance which is projected on the projection screen 
15 is 4a. 
The reason that the change of projection position causes no trapezoidal 
distortion in the present invention will be discussed below with reference 
to FIG. 3. 
Supposing that no reflection mirror 14 is provided (for simplicity) it can 
be considered that upon adjustment, only the projecting lens system 13 is 
moved in the directions parallel with the plane of the document 0. The 
movement of the projecting lens system 13 varies the position of the image 
0' of the document 0 on the projection screen 15. Since the image 0' of 
the document 0 on the projection screen 15 is formed in a plane including 
the projection screen 15, no trapezoidal distortion is produced. 
Consequently, if a light source (illumination system) in which the light 
reflected from the document 0 on the document supporting plate 11 is 
always uniformly incident upon the projecting lens system 13, regardless 
of the position of the projecting reflection unit 20, the illumination 
unit 21 (light source 12) can be fixedly positioned. However, it is 
practical to move both the illumination unit 21 and the projecting 
reflection unit 20 symmetrically with respect to the imaginary center line 
22 of the illumination unit 21 and the projecting reflection unit 20, 
since the light reflected from the document 0 is incident upon the 
projecting lens system 13. 
FIG. 4 shows a mechanism for moving the projecting reflection block 20 and 
the illumination unit 21 towards and away from each other, as an example. 
The projecting reflection unit 20 and the illumination unit 21 are 
supported by supports (not shown) so as to move in directions parallel 
with the plane of the document supporting plate 11. The projection 
reflecting unit 20 which has the projecting lens system 13 and the 
reflection mirror 14 has an arm 23 projecting therefrom in a direction 
parallel with the plane of the document supporting plate 11. On the other 
hand, the illumination unit 21 which has the light source 12 has a 
projection arm 24 which is opposed to and spaced from the arm 23 of the 
projecting reflection unit 20. The arms 23 and 24 have racks 23a and 24a 
which are opposed to each other and which are engaged by a single pinion 
25 which is connected to a driving motor 50. Consequently, when the pinion 
25 is rotated in the forward and reverse directions, the projecting 
reflection unit 20 and the illumination unit 21 move towards and away from 
and each other symmetrically with respect to the pinion 25 (and 
accordingly the center line 22). 
FIG. 5 shows another embodiment of a mechanism for moving the projecting 
reflection unit 20 and the illumination unit 21 to come away from and 
close to each other. 
In FIG. 5, the projecting reflection unit 20 and the illumination unit 21 
are movably supported by a horizontal guide bar 31 which extends in 
parallel with the plane of the document supporting plate 11 and which is 
located above the document supporting plate 11. 
A drive shaft 32 is provided on the document supporting plate 11 so as to 
move up and down in directions perpendicular to the horizontal guide bar 
31. The drive shaft 32 has a link supporting ring 33 secured thereto which 
has identical links 34 and 35 pivoted to the link supporting ring 33. The 
opposite ends of the links 34 and 35 are pivotedly connected to the 
projecting reflection unit 20 and the illumination unit 21, respectively. 
The drive shaft 32 has a rack 32a which is engaged by a pinion 36 
connected to a driving motor 50, so that when the pinion 36 is rotated in 
forward and reverse directions, the drive shaft 32 is moved up and down in 
the directions perpendicular to the horizontal guide bar 31 through the 
rack 32a. The movement of the drive shaft 32 causes the projecting 
reflection unit 20 and the illumination unit 21 to move toward and away 
from each other symmetrically with respect to the drive shaft 32 (and 
accordingly the imaginary center line 22) through the links 34 and 35. 
As can be understood from the foregoing, so long as the document supporting 
plate 11 is always kept to be perpendicular to the projection screen 15, 
it is theoretically possible to fix the inclination angle of the 
reflection mirror 14 to be 45.degree. with respect to the document 
supporting plate 11 and the projection screen 15. However, in actual use, 
since the perpendicularity is not always guaranteed, it is preferable to 
support the reflection mirror 14 on the projecting reflection unit 20 so 
as to adjust the inclination angle thereof. 
The above discussion has been directed to a reflection type projector in 
which the reflection image of the document is projected on the projection 
screen. 
However, the present invention can be also applied to a transmission type 
projector (FIG. 1) in which a transmission image of the document O is 
projected on the projection screen 15. In such an application of the 
invention to a transmission type projector, the document supporting plate 
11 is of transmission type, for example, made of a transparent material, 
so that the light can be transmitted through the document supporting plate 
11. The light source 12' and accordingly, the illumination unit 21' are 
provided on the opposite side of the projecting lens system 13 with 
respect to the document supporting plate 11. The light source 12' and 
illumination unit 21' are shown by imaginary lines in FIGS. 1 and 2. In 
this arrangement for the transmission type projector, it is also 
preferable to move the projecting reflection unit 20 and the illumination 
unit 21 towards and away from each other symmetrically with respect to the 
imaginary center line 22, similarly to the first mentioned embodiment. 
It is preferable to vary the direction of the illumination light (incident 
light) in accordance with the distance between the light source 12 (12') 
of the illumination block 21 (21') and the projecting reflection block 20, 
as shown in FIGS. 1 and 2. 
As can be seen from the above discussion, according to the present 
invention, the projection position of an image on the projection screen 
can be adjusted by the movement of the projecting reflection unit which 
contains the projecting lens system and the reflection mirror above the 
document supporting plate without producing a trapezoidal distortion. 
Therefore, the projecting optical apparatus according to the present 
invention can be particularly advantageously used for an over-head 
projector in which the height position of projection must be changed in 
accordance with circumstances. Thus, a clearly projected picture with no 
trapezoidal distortion can be obtained.