Patent Application
20080192208
The invention relates to the field of image projection.
More precisely, the invention relates to the folding of an imaging beam in a video projector of front projector or rear-projector type.
According to the prior art, projectors have the drawback of being relatively bulky.
To reduce their volume, a projector 1 is provided with a convex hyperbolic mirror, as illustrated in exploded form in
The objective transmits an imaging beam 14 to the convex aspherical mirror 11, which itself transmits a reflective beam to the folding mirror 15, which reflects the beam onto the screen 12 (so as to make
The rear-projector of
The object of the invention is to alleviate these drawbacks of the prior art.
More particularly, the objective of the invention is to reduce the height below the rear-projection screen (also call “chin”) or the projection distance in the case of a front projector while still maintaining a small chin (here the distance between the highest level of the projector and the highest level of the screen if the projector is located above the screen).
For this purpose, the invention proposes a projection system comprising an imaging source and an objective that are designed to produce an imaging beam and constructing a first image positioned after the objective, the first image being off-axis relative to the optical axis of the imaging beam, the system being noteworthy in that it includes a concave mirror which is positioned after the first image in the path of the imaging beam and which constructs a second image in a plane of projection from the first image.
Preferably, the concave mirror is an aspherical mirror.
According to one particular feature, the system includes at least a first plane folding mirror between the objective and the concave mirror. Advantageously, the objective comprises:
According to a preferred feature, the distance between the second lens group and the exit pupil of the first lens group is at least three times the distance between the first lens group and the imaging source.
According to an advantageous feature, the second lens group includes at least one meniscus type lens.
According to one particular feature, the second lens group is positioned between the first lens group and the first folding mirror.
According to another feature, the second lens group is positioned between the first folding mirror and the concave mirror.
Advantageously, in a representation of the system without a plane folding mirror, the optical axis of the imaging beam is perpendicular to the plane of projection, being vertical when the imaging beam strikes the concave mirror.
Preferably, the system includes a rear-projection screen in the plane of projection.
Advantageously, the system further includes at least a second plane folding mirror positioned in the path of the imaging beam between the concave mirror and the rear-projection screen.
According to another advantageous feature, the system includes means for front projection on the plane of projection.
Advantageously, the objective comprises a front group comprising three lenses, the lens located in the middle having the opposite power to the outer lenses. Thus, the front group comprises, for example, two convergent lenses flanking a divergent lens or two divergent lenses flanking a convergent lens.
Preferably, the system includes a mask comprising a black zone that absorbs the parasitic rays and a transparent zone placed in the path of the imaging beam after the concave mirror.
According to particular features, the transparent zone is full or corresponds to a cut-out in the mask.
According to a preferred feature, the transparent zone is positioned near the exit pupil of the system comprising the objective and the concave mirror.
The invention will be more clearly understood, and other features and advantages will become apparent on reading the following description, which is given with reference to the appended drawings in which:
The general principle of the invention therefore is based on the use of a concave folding mirror in a rear-projector, thereby making it possible to reduce the height below the screen.
The imager is, for example, a DMD (“Digital Micromirrors Device”) from Texas Instruments™, a transmissive LCD (Liquid Crystal Display) or an LCOS (Liquid Crystal on Silicon) device.
The objective 20 transmits an imaging beam 24 to the concave aspherical mirror 21, which itself transmits a reflective beam to a folding mirror 25, which reflects the beam onto a plane of projection where the screen 22 is positioned (to make
The concave aspherical mirror 21 has an axisymmetric (revolution) shape, the reflecting surface of which is given by the following aspherical surface equation:
where:
The objective comprises a rear group of lenses 201 to 203 and a front group of lenses 204 to 206.
The last lens 206 of the objective 20 in the path of the imaging beam is preferably an aspherical meniscus lens, the shape of which is matched to the parameters of the concave mirror 21. Its shape is therefore preferably given by an aspherical surface equation as shown above.
As an illustration, in one particular embodiment the radius R of the concave mirror 21 is 60, the parameters c and a1 to a8 are, respectively, the following: −1.59311 mm; 0; 0; −8.94×10−6; 0; 1.64×10−9; −9.74×10−13, −7.84×10−14; and 2.31×10−16. The radius R of the first surface (the imager side) of the meniscus 206 is 44.94711 mm, and the parameters c and a1 to a8 have, respectively, the following values: 0; 0; 0; −3.1×10−9; 2.88×10−5; 1.96×10−6; 7.14×10−8; 4.15×10−10; and −4.30×10−10. The radius R of the second surface (the imager side) of the meniscus 206 is 29.49554 mm and the parameters c and a1 to a8 have, respectively, the following values: 0; 0; 0; −2.7×10−4; 9.97×10−6; 6.34×10−7; —1.41×10−7; 8.98×10−9; and −1.78×10−10.
Advantageously, according to the invention, the front lens group comprises three lenses, the two lenses located in the outer positions each having an opposite power to the lens located in the middle. Thus, the front group contributes to generating curvature of the intermediate image so that the projected image is plane, while comprising a number of weak lenses. This makes it easier to manufacture it and to reduce its cost.
Thus, the front lens group illustrated in
According to an alternative embodiment of the invention, the front lens group of the objective comprises two positive (convergent) lenses surrounding a negative lens. An objective 130 comprising such a front group and able to replace the objective 20 is illustrated in
The objective 130 comprises a rear group of lenses 131 to 138 and a front group of lenses 139 to 141, these lens groups being placed on either side of the exit pupil PE of the objective.
As an illustration, the characteristics of the objective 130 are summarized in the following table (the rays, thickness and diameter being expressed in millimeters and the material of the lenses 132 to 141 corresponding to the references of the products supplied by the OHARA® company):
The rear-projector of
a and 3b illustrate a side view and a top view, respectively, of the rear-projector 3 as shown schematically in
To reduce the depth of the rear-projector 3, a folding mirror 30 is interposed between the objective 20 and the concave mirror 21. The dotted lines and the full lines represent, respectively, the elements with the beam 24 not folded and folded, respectively. The mirror 30 is vertical (that is to say parallel to the screen 22), the optical axis of the beam 24 before the mirror 21 being horizontal. The long side of the imager 23 of the rear-projector 3 is horizontal (for a vertical projection screen 22 whose long side is horizontal).
The objective 20 is placed along the side and folded preferably in a horizontal plane placed along the side, preferably with an axis parallel to the screen 22, thereby making it possible to reduce the depth of the rear-projector 3. The angle α that the mirror 30 makes with a normal screen 22 depends on the angle that the optical axis of the objective 20 makes with the screen 22. When the objective 20 is parallel to the screen, the angle α is equal to 45°. The distance between the objective 20 and the mirror 30 is such that the beam 24 does not encounter the objective 20.
In general, all the optical axes of the various elements of the unfolded projection system are perpendicular to the plane of projection, assumed to be vertical. They are therefore horizontal (for a system as shown in unfolded form, with the exception of the folding due to the concave folding mirror).
In the projector 3, the reel axis of the objective 20 remains horizontal, the screen 22 being vertical. The projector 3 has a relatively small “chin”, that is to say a relatively small value of h.
However, in alternative embodiments that allow the illumination part to be housed more easily (inclination of the optical illumination core, lamp casing, electronic card attached to the imager 23), the reel axis of the objective is inclined. This is because the axis of one element of the projection system may become non-horizontal after being folded by a folding mirror. For example, if the large mirror is inclined, all the following elements will also be inclined through twice the angle, in particular the concave mirror.
Thus,
The two folding mirrors 40 and 42 are positioned between the objective 20 and the mirror 21. The axis of the objective 20 of the rear-projector 4, when folded, is not horizontal. The imaging beam coming from the objective 20 firstly illuminates the mirror 42, which is inclined at 45° to the optical axis and perpendicular to the screen 4. The beam is thus reflected in a direction parallel to the screen 4, its optical axis being in a plane normal to the screen 4. Next, the beam reflected by the mirror 42 illuminates the mirror 40 which is inclined at 45° to the optical axis and the normal of which is perpendicular to the screen 4. The beam is thus reflected in a direction perpendicular to the screen 4 in order to illuminate the concave mirror 21.
In an alternative embodiment of the invention, the axis of the objective 20 of the rear-projector 4, when folded, is not horizontal the rear-projector 4 comprising one or more folding mirrors positioned between the objective and the concave mirror 21 in order to send the beam in a direction approximately perpendicular, and preferably perpendicular, to the screen 21.
According to other alternative embodiments of the invention, the axis of the imaging beam illuminating the concave mirror positioned after the first image in the path of the imaging beam is not horizontal. The shape of the concave mirror is then calculated in order to construct a second image in a plane of projection corresponding to the projection screen.
The long side of the imager 23 of the rear-projector 4 is vertical (for a vertical projection screen 22 with a horizontal long side).
The rear-projector 4 obviates the size constraint of the lenses of the objective 20 in order for it not to intersect the return beam 41 from the mirror 21. In the configuration of the rear-projector 4, it is also possible to use larger lenses, since the objective is below the beam 41 (there is easier separation of the fields).
Preferably, the height h′ below the screen of the rear-projector 4 is equal to one fifth (approximately) of the height of the screen. More precisely, the height h′ is less than or equal to the height of the screen divided by 5. It may also depend on the magnification of the objective 20 or of the concave mirror 21, and on the illumination system (the size of the lamp's reflector). Thus, for a projector with a 50″ screen and DMD HD3, the height h′ is, for example, less than 20 cm and typically equal to 12 cm.
The ray 242 represents the central ray of the imaging beam 24 and the rays 240 and 241 are two extreme rays.
The exit pupil PE of the objective 20 forms an image IE located in front of the mirror 21 in the path of the beam 24. The objective 20 magnifies the object image formed on the imager 23 in order to form the image IE with a magnification factor M. The magnification factor M associated with the objective 20 is preferably between 1 and 10, and even more preferably between 5 and 9.
The mirror 21 associates the exit pupil PE with a pupil PF where the rays of the imaging beam cross over in a relatively small area. The shape of the mirror 21 is calculated to create an image IM corresponding to the image IE projected on a plane of projection where the screen 22 is located. The concave mirror 21 magnifies the image IE in order to form the image IM with a magnification factor M′. The magnification factor M′ associated with the concave mirror 21 is preferably greater than the magnification factor M associated with the objective 20.
The use of a concave mirror 21 positioned after the first image in the path of said imaging beam has the advantage that the lower portion of the imaging beam, corresponding to the ray 241 is relatively high (relative to the corresponding ray in
According to an alternative embodiment, the magnification factor M′ associated with the concave mirror 21 is greater than 10.
The concave mirror 21 is preferably located below the optical axis. Preferably, the optical axis of the system in front of the concave mirror 21 is horizontal and close to the bottom of the screen 22.
The beams 62 and 61 are reflected, respectively, in non-discrete regions A″ and B′ on the mirror 21 and converge on a region corresponding to the pupil PF, the image of the pupil PE via the mirror 21.
It should be noted that the pupil PF is relatively close to the mirror 21 and that the pupil PE is further away from the mirror 21. Typically, the distance of the exit pupillary region PF from the vertex of the concave mirror 21 is between 25 mm and 60 mm. Preferably, the distance of the exit pupil 201 from the concave mirror 21 must be as large as possible.
The rear-projector 80 comprises similar components to the components of the projectors 3 and 4 (especially the components 22, 23, 25 and 30). These components bear the same references, but will not be described further.
The rear-projector 80 comprises an objective comprising a diaphragm D, a first lens group having at least one lens 70 and a second lens group having at least one lens 74. The second lens group is positioned after the diaphragm D in the path of the imaging beam and is closer to the image IE than the diaphragm D. Preferably, the distance d, between the second lens group 70 and the exit pupil PE′ of the first lens group 74 is at least three times the distance d2 between the first lens group 74 and the imaging source 23.
The exit pupil PE′ corresponds to an image of the diaphragm D formed by the first lens group 70. The second lens group 74 makes it possible to position the exit pupil PE of the objective of the projector 80 substantially at infinity. Thus, the second lens group 74 rectifies the rays of the imaging beam (for example the rays 72 and 71 corresponding to the points A and B) and makes it possible to reduce the size of the concave mirror 73 that replaces the mirror 21, its shape moreover being similar.
The second lens group 70 also applies optical corrections assumed by the mirror 21 according to the other embodiments described above. In particular, it is possible to reduce astigmatism and certain optical distortions. It also increases the aperture of the beam (aperture at 2.8). The mirror 73 has a curvature that therefore essentially makes it possible to magnify the image IE and to obtain a plane image in the plane of projection.
The concave aspherical mirror 73 has a shape the reflecting surface of which is given by the following equation:
where:
The second lens group 70 comprises one or more lenses and consists, for example, of one or more meniscus lenses (a lens group with several meniscus lenses being easier to produce). Preferably, the second lens group 70 has optical properties matched to the parameters of the concave mirror 73. Its components preferably have a shape that follows an aspherical surface equation as shown above.
As an illustration, in one particular embodiment, the radius R of the concave mirror 73 is equal to −56.202 mm, the parameters c, and a1 to a8 are, respectively, the following: −3.32197; 0; 0; −1.06×10−5; 0; −2.20×10−9; 6.68×10−11; −1.06×10−12 and 5.91×10−15.
Again as an illustration, the lens group 70 is considered to consist of a meniscus. The radius R of the first surface (the imager side) of the meniscus of the lens group 70 is assumed to be infinite, the parameters c, and a1 to a8 are, respectively, the following: 0; 0; −0.00811; 7.60×10−5; −6.43×10−6; 1.57×10−7; −1.53×10−9; 0; and 0. The radius R of the second surface (the image the meniscus of the lens group is assumed to be infinite, the parameters c, and a1 to a8 having, respectively, the following values: 0; 0; −0.01058; −1.77×10−5; −2.88×10−6; 8.41×10−8; −9.96×10−10; 0; and 0.
The first lens group 74 comprises a rear group of one or more lenses located in front of the diaphragm in the path of the imaging beam. According to alternative embodiments, it also includes a front group consisting of one or more lenses located after the diaphragm in the path of the imaging beam.
Moreover, an image IE is formed in front of the mirror 73, and the mirror 73 associates the exit pupil PE with a pupil PF where the rays of the imaging beam intersect in a relatively small area. The shape of the mirror 73 and the arrangement of the second lens group 70 are calculated to create an image IM corresponding to the image IE in a plane of projection in which the projection screen 22 lies.
In the projector 80, the second lens group 70 is positioned between the folding mirror 30 and the first lens group 74. Thus, the imager 23 transmits an imaging beam 81 through the first lens group 74 to the second lens group 70. The second lens group 70 corrects the imaging beam 81 into an imaging beam 82 which is reflected by the folding mirror 30 onto the concave mirror 73.
The optical axis of the imaging beam 81 is horizontal and parallel to the bottom of the image projected on the projection screen (as in the embodiment of the rear-projector 3). In an alternative embodiment, the optical axis of the imaging beam 81 is inclined, like the axis of the imaging beam coming from the objective of the rear-projector 4, the mirror 30 being replaced with the mirror 40 of the rear-projector 4, as illustrated in
The rear-projector 90 comprises components similar to the components of the rear-projector 80, the mirror 73, the first lens group 74 and the second lens group 70 being replaced, respectively, by a concave mirror 92, a first lens group consisting of at least one lens 95 and a second lens group consisting of at least one lens 91. These similar components bear the same references and will not be described further.
The objective of the rear-projector 90 therefore comprises a diaphragm D, the first lens group 74 and the second lens group 95. The second lens group 95 is positioned after the diaphragm D in the path of the imaging beam and is closer to the image IE than the diaphragm D. Preferably, the optical distance between the second lens group 95 (corresponding to the sum of the distances d′1 and d″1) and the exit pupil PE′ of the first lens group 74 is at least three times the distance d2 between the first lens group 74 and the imaging source 23.
The concave mirror 92 and the second lens group consisting of at least one lens 91 provides the same functions as the mirror 73 and the first lens group 74, respectively. The shape of the mirror 92 and the optical characteristics of the second lens group 91 are matched approximately to these two components.
In the projector 90, the second lens group 91 is positioned between the folding mirror 30 and the concave mirror 92. Thus, the imager 23 transmits an imaging beam 93 through the first lens group 95, the beam 93 being reflected by the folding mirror 30 onto the second lens group 91. The second lens group 91 corrects the imaging beam 93 into an imaging beam 92 which is transmitted to the concave mirror 73.
The optical axis of the imaging beam 93 is horizontal and parallel to the bottom of the image projected onto the projection screen (as in the embodiment of the rear-projector 3). In an alternative embodiment, the optical axis of the imaging beam 93 before being reflected by one of the plane or concave folding mirrors is inclined to the plane of projection.
According to an alternative embodiment of the rear-projector 90, the plane folding mirror 30 is replaced with two mirrors similar to the mirrors 40 and 42 of the rear-projector 4, these two mirrors being positioned between the first lens group 74 and the second lens group 95 so as to reduce the size of the rear-projector. Such an embodiment is compatible with a concave mirror 92 similar to the mirror 73, and a second lens group 95 consists of a single meniscus, similar to the lens group 70, precise characteristics of these components having been mentioned above by way of illustration.
The projector 100 comprises the optical components of the projector 3 except for the projection screen 22 and the folding mirror 25.
More precisely, the projector 100 comprises components similar to the components 20, 23, 30 and 21 of the projector 3, which bear the same references and will not be described further. The concave mirror 21 of the projector 100 transmits an imaging beam 102 to a screen 101 lying in a plane of projection perpendicular to the optical axis of the unfolded system, the concave mirror 21 constructing a second image in the plane of projection from a first image located between the objective 20 and the mirror 21. It should be noted that the plane of projection may be very close to the projector 100—the distance separating the optical centre of the concave mirror 21 from the plane of projection is preferably less than 1 m and even more preferably less than 50 cm.
Moreover, the chin h (the distance separating the image projected onto the screen 101 from the lowermost point of the projector 100 (assuming an image projected from the bottom up) is relatively small—it is preferably less than one fifth of the height of the projected image.
Moreover, the rear-projectors according to the various embodiments of the invention as explained above may be adapted to front projectors, a person skilled in the art replacing the rear-projection screen with a front projection screen and possibly omitting the folding mirror or mirrors located after the concave mirror in the path of the imaging beam.
According to another aspect of the invention, a mask is positioned near a small pupillary zone, the zone being placed between the objective and the screen in the path of the imaging beam. Such a mask may be employed when the exit pupil of the system is real. The mask makes it possible in particular to limit, even eliminate, the parasitic rays and/or to prevent dust from being deposited on the objective, the concave mirror and the optical elements close to the objective. As indicated above, such a pupil PF is in particular obtained when the projector employs a concave mirror for constructing an image on a plane of projection.
Advantageously, the mask 103 is a sheet of glass or plastic that includes a transparent zone 104 through which the projection beam 24 can pass. The thickness of the mask 103 is chosen to be as small as possible, preferably less than 2 mm and even more preferably equal to 1 mm or less.
As illustrated in
According to an alternative embodiment, the transparent zone of the mask is inclined to the axis of the zone imaging beam so as to reduce its angle of incidence and therefore to reduce any reflection of the imaging beam on the transparent zone.
Thus,
According to an alternative embodiment of the masks 103 and 116, the transparent zone is made of glass or plastic, and the black zone is made of any other material.
The transparent zone of the masks 103, 121 and 116 is preferably adjusted to the size of the beam 24 and located close to the pupillary zone (or pupil PF of the system comprising the objective and the concave mirror) lying between the objective and the plane of projection (typically at a distance of 5 mm or less from the pupillary zone PF corresponding to the exit pupil of the system comprising the objective and the concave mirror).
To make it easier to manufacture the masks, the transparent zone has a simple geometrical shape. Thus,
According to a variant of the mask, illustrated in
The masks illustrated in
When the transparent zone is a full zone, part of the beam may be reflected by the transparent part. According to an alternative embodiment, the parasitic rays thus obtained may be eliminated, for example by a wall approximately normal to the mask and placed behind the transparent zone in the path of these parasitic rays outside the path of the useful projection beam.
According to an alternative embodiment of the projector, the imager is off-center relative to the cases illustrated above, thereby allowing a higher projected image to be obtained. In this way, there is also greater freedom in positioning the mask and/or greater freedom in the shape of the mask.
Of course, the invention is not limited to the embodiments described above.
In particular, the invention applies to any type of projector, whether a front projector or a rear-projector.
A person skilled in the art will also be able to define the objective and the concave folding mirror which constructs a second image in a plane of projection from a first image positioned before the concave mirror, in particular in order to adapt the astigmatism and optical distortion corrections according to particular criteria and to distribute them between the various optics of the projection system.
In addition, a person skilled in the art will be able to adapt the folding of the imaging beam according to the space constraints specific to the desired projection system.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0412904 | Dec 2004 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2005/056318 | 11/29/2005 | WO | 00 | 3/10/2008 |
