MXPA05013320A - Objective for a projection or backprojection apparatus - Google Patents

Abstract

The invention relates to a projection objective comprising at least one lens (L1) and intended to transmit a divergent light beam onto a flat screen (SC). A hyperbolically shaped mirror (M1) is oriented soas to receive, on its convex face, the light emanating from the lens. The invention also relates to a corresponding projection or backprojection apparatus.

Description

Before the expiration of the time limit to amend the Pai to the codes of two letters and other abi eviations, refer to the claims and pat to be published again in the case of the 'Guidance Notes on the Codes and Abbreviations' that appear at the reception of the amendments beginning of each regular issue of the PCT Gazette OBJECTIVE FOR A PROJECTION OR RE-PROJECTION DEVICE 1. Field of the Invention The invention relates to an objective for a front projection or rear projection apparatus, which makes it possible to obtain a wide projection angle without distortion. The invention also relates to the application of such an objective to the front projection and rear projection apparatuses. 2. Previous Technique Figure 1 shows a conventional design of an overhead projector. In this design, the beam of illumination emitted by the projector is folded by one or two return mirrors Ml or M2. These mirrors make an angle of approximately 36 ° with the screen. The optical system of the overhead projector can be up to 45 centimeters thick for a screen that has dimensions of 1106 by 622 millimeters. The cone angle along the diagonal of the screen should be approximately 38 °. An acceptable distortion and a Transfer Function of Modulation (MTF for its acronym in English) acceptable can be obtained with a goal consisting of something like ten lenses for moderate cost. The thickness of the apparatus is then, for example, 50 centimeters. Another design involves folding the beam twice, as shown in Figure 2, using two mirrors Ml, M2 placed opposite each other, which are parallel to the screen, and a projection lens that works with an off-center field with respect to its optical axis. Figure 3 shows how the distance between the center of the screen and the optical axis of the lens is determined. In Figure 2, the mirror Ml lies in the plane of the screen. A ray that has to reach the top of the screen (on the left in Figure 2) must first be reflected by the top of the mirror Ml and therefore passes through a point located below the screen. The maximum angle of the field depends on the thickness d of the projector, the height H of the screen and the diameter p of the pupil of the lens according to the following formula: a = arctan [(H + p / 3) / 2d]. To produce a projector whose optical system has a thickness of 200 millimeters, with the following values: H = 622 mm and p = 4 mm, it is necessary to have an angle a with a value of 57.36 ° and the distance D between the center of the screen and the optical axis of the lens will be 591 mm. To operate correctly with these values, the system must use the side field of the objective, that is to say the source of the image to illuminate the screen is out of center with respect to the axis of the objective. 3. Brief Description of the Invention An object of the invention is to produce an objective for projecting a flat image at a distance that is even closer than in known systems. This objective also makes it possible to correct the distortions that the system could induce. In particular, the objective of the invention is to use a hyperbolic mirror in this objective. A known system, such as that described in U.S. Patent No. 5,716,118, uses a hyperbolic mirror, but the mirror used is concave and must be large in order to obtain a large image. Such a system is therefore difficult to make industrially viable due to the difficulties in producing such a large mirror. The invention relates to an objective for an industrially viable or overhead projector that makes it possible to obtain large projected images. The invention therefore relates to a projection lens comprising a combination of lenses comprising a front group (Grfront) of lenses and a subsequent group (Grrear) of lenses that are placed on either side of a diaphragm and are intended for transmit a divergent beam of light to a flat screen, and that includes at least one lens and at least one mirror of hyperbolic shape, oriented to receive, on its convex face, the light coming from the front group (Grfront) of lenses and transmit the beam to the screen. Preferably, a first focus of the hyperbolically shaped mirror is placed in the region, called in the region of the pupil, defined by the image of the diaphragm by the front group of lenses (Grfr? Nt). • According to a particular characteristic, the mirror hyperbolic is designed and placed in relation to the frontal group (Grfront) of lenses, so that the first focus of the hyperbola lies approximately in the plane of the pupil of the frontal group of lenses, whose hyperbola is located on the side opposite the hyperbolic mirror in relation to the front group of lenses, while the second focus lies approximately in the plane of the exit pupil of the front group of lenses. Advantageously, the rear group of lenses and / or the front group of lenses include / includes at least one geometric distortion correction optical element having a surface in the shape of a cone.

Preferably, this geometric distortion correction optical is located in the lens protecting group and has a surface of hyperbolic shape. In addition, this geometric distortion correction optical is preferably located in a region remote from the lens diaphragm. More precisely, the geometric distortion correction optical is preferably located in that part of the lens projector group furthest from the diaphragm. The conicity of the hyperbolic mirror (Ml) and the optical correction of the geometric distortion (L'l) can be in a proportion that is approximately proportional to the proportion of the positions of the hyperbola focuses, ie the distances P2 - hyperbola and Pl - hyperbola. In addition, a meniscus located near the pupil of the lens can be provided in order to correct the astigmatism defects induced by the hyperbolic mirror. Advantageously, the objective thus includes at least one meniscus located in that part of the frontal group or of the posterior group that is closest to the diaphragm and designed to correct the astigmatism defects induced by the hyperbolic mirror. In addition, provision can be made for the purpose according to the invention to use a peripheral field of the object plane and for the hyperbolic mirror to be located completely on one side of a plane passing through the symmetry axis of the hyperbola, to fold the beam so that the objective does not scatter a shadow over the image. Preferably, the hyperbolic mirror is located completely on one side of a plane passing through the axis of symmetry of the hyperbola; this axis of symmetry unites the hyperbola's foci. Advantageously, the optical axis of the lens is located on the axis of symmetry of the hyperbola, passing through the focus of the hyperbola. The objective lens cons in general of a combination of lenses, and therefore forms a complex lens. According to an alternative embodiment, a first, additional return mirror is positioned near the front group of lenses of the lens in a first direction corresponding to the direction of the beam transmitted by the lens, and reflects the beam in a second direction not collinear with the first address. The hyperbolic mirror is located along the second direction, and is oriented in order to receive the beam reflected by the first mirror of return. According to one embodiment, the second direction makes an angle less than 60 ° with the first direction. In addition, the objective advantageously includes two menisci located on either side of the diaphragm, the concave portions facing the diaphragm. According to a particular characteristic, the diaphragm lies in the focal plane of the posterior group of the lenses. Advantageously, the objective includes a positive lens located between one of the meniscus, belonging to the frontal group of lenses and the hyperbolic mirror. In this way, the envelope of the light rays of the field is reduced to make it easier to fold the beam of light by means of a plane mirror, in order to reduce the overall size of the lens. Such an objective is applicable to a frontal projection or rear projection apparatus. Preferably, the objective includes a screen, such as a spatial light modulator, located on one side of the optical axis of this rear group of lenses, and making it possible to transmit a modulated light beam to a region of the posterior group of lenses that is located on one side of the axis (XXA) of the posterior group of lenses. To do this, the screen, at least its optically active surface, is located completely on one side of the optical axis of the lens, i.e. of the complex objective lens. The screen is designed, in a manner known per se, to transmit a beam of modulated light to this lens, that is, to the entrance of the lens. In this way, the target is used in the displaced field so that the beam emanating from the hyperbolic mirror or, where appropriate, the additional mirror is not intercepted by the objective lens or lens. In addition, the screen is preferably flat. The invention is applicable to a rear projection apparatus in which at least a second return mirror receives the light reflected by the hyperbolic mirror, and reflects it on the rear face of the screen of the rear projection apparatus. In such an arrangement, the return mirror makes a zero angle possible with the plane of the screen. In an alternative embodiment of the invention, it makes a different angle of zero with the plane of the screen, for example 15 °, which makes it possible to reduce the total volume of the projector. According to an alternative embodiment of the invention, the second mirror lies in the same plane as the first return mirror.

Preferably, the target is then mechanically coupled to the first mirror by a support piece. 4. List of figures The various aspects and features of the invention will become more clearly apparent in the following description, and in the appended figures which show: in Figures 1 to 3, the rear-projection systems known in the art and already described above; in Figures 4a to 4c, the illustrative examples of an objective according to the invention; in Figure 5, an illustrative example of a rear projection apparatus according to the invention; - in Figure 6, another illustrative example of a rear projection apparatus according to the invention; - in Figures 7a and 7b, the diagrams that describe precisely how the light rays propagate; in Figures 8 to 10, various positions and orientations of the mirrors used within the context of the invention; in Figure 11 an example of the application of the invention to a frontal projection apparatus; in Figures 12a and 12b, an alternative embodiment of an objective according to the invention; in Figures 13a, 13b and 14, the diagrams to explain the distortion and the astigmatism corrections; and in Figure 15, an illustrative example of an objective according to the invention.

. Detailed description of the invention A basic illustrative example of an objective according to the invention will now be described with reference to Figure 4a. This objective comprises a lens L '1 which is in fact a lens formed from a combination of lenses, ie a complex lens. A mirror HY of hyperbolic shape HYP is placed on the output side of the lens and in such a way that the axis of the hyperbola passes through the foci of the hyperbola, coincides with the optical axis XX 'of the lens L'l. The light transmitted by the lens is reflected by the hyperbolic mirror and seems to come from a point p 'which is a conjugate point of the pupil of the lens (the image of the diaphragm produced by the front group of lenses).

As can be seen in Figure 4a, the hyperbolic mirror makes the beam that it reflects more divergent. Further, to prevent lens L '1 from disturbing the transmission of the reflected beam by the hyperbolic mirror, provision can be made to use only that part Ml of the hyperbolic shape lying on one side of a plane passing through the axis symmetry of the hyperbola. This axis passes through the foci of the hyperbola. The light that emanates from the L '1 lenses that can be used, is therefore that which lies on one side of a plane passing through the optical axis of the lens. An image illuminated by a light source, and intended to be projected onto the screen, will therefore be off-axis with respect to the axis of the objective. Such an arrangement can, in certain cases, induce distortions and deterioration in the Modulation Transfer Function (MTF), for example the deterioration in the spatial frequency response of the optical system. These defects are corrected by the movement of the hyperbolic mirror away from the lens, and by the interposition of an L9 lens between the lens and the hyperbolic mirror, whose lens makes it possible to balance the optical powers on either side of the lens diaphragm, and reduce the angle of incidence of the rays of the beams on the hyperbolic mirror, and especially reduce the angle of incidence of the rays farther from the axis of the hyperbola. Such an arrangement is shown in Figure 4b. In this way, the farther away the mirror-hyperbolic lens is, the narrower the field in which the lens operates. The invention is also directed to correct the astigmatism that could be induced by the hyperbolic mirror. To do this, one or more MEI and ME2 plates in the shape of a meniscus are provided, these being placed near the pupil PU of the lens, formed by the lens L'l. In the case of two menisci, these are placed on either side of the target's PU diaphragm. As shown in Figure 4c, the meniscus are placed with their concave faces opposite one another and the Clis and C2 centers of the meniscus are also located on either side of the PU diaphragm in such a way that the distance between the two faces Concave is less than the sum of the radii of the two concave faces. It will be preferred to provide two meniscuses with equivalent apertures. Figure 5 shows an illustrative example of a rear projection apparatus employing the object of the invention described in this way. A preferably flat SLM display device, such as a spatial light modulator, is used to transmit a beam that carries at least one image due to spatial modulation. This beam is transmitted by the lens L'l (the complex lens) towards the hyperbolic mirror Ml, which reflects the light on a plane mirror M2 that lies preferably in the plane of the SC screen. The beam is reflected by the mirror M2 on a second plane mirror M3, which reflects the light on the rear face of the rear projection screen SC. The SLM screen is located on one side of a plane which passes through the optical axis XX 'of the lens L'l, so as to illuminate only the hyperbolic mirror Ml which occupies only part of the HYP hyperbola lying on one side of a plane that passes through the axis of the last one. It can therefore be observed that, for image dimensions given on the screen (and therefore for screen dimensions), the thickness of the optical back-projection system can be further reduced by using the architecture of Figure 5. Figure 6 shows another embodiment of an overhead projector according to the invention. A preferably planar mirror M4 is provided between the outlet of the lens and the hyperbolic mirror. This arrangement allows the hyperbolic mirror to be moved further away from the lens to reduce the field angle of the beam. This overhead projector arrangement therefore applies the objective described in relation to Figure 4b. Figure 6 therefore shows lens L9 to reduce the field angle of the lens. Figure 7a shows in more detail the propagation of a beam in the configuration of Figure 5. Figure 7b illustrates more clearly, by "unfolding" the beam that was "" folded "by the mirror M2, the advantage in terms of the beam divergence of using a hyperbolic mirror.Folding, combined with the hyperbolic mirror, has the advantage of reducing the thickness of the optical system of the overhead projector, and double folding reduces this thickness a fortiori even more.Any different angles are possible, as long as the beams and components do not overlap each other: In the case of the large mirror M3, the angle can vary from 0 to 12 ° approximately, and In the case of the small mirror M4, the angle may vary from about 12 to about 35. Examples are given in Figures 8 and 9. Figure 8 illustrates an example in which the mirror M4 is inclined with respect to the plane of the screen Figure 9 illustrates an example in which the mirror M3 is inclined with respect to the plane of the screen.

Figure 10 shows an alternative embodiment in which the distance between the screen and the large mirror M3 is reduced, and the distance between the hyperbolic mirror Ml and the large mirror M3 is increased. A peripheral field farther from the optical axis is also used. In this way, a projector that is flatter with respect to the screen and has an acceptable base is obtained. Figure 11 shows a front projector in which the projector is located above the screen. For example, it is fixed to the ceiling in order to project on a wall of the room. The rear projection systems according to the invention are such that it is possible to obtain screens whose thicknesses can be reduced to a value of less than 20 centimeters for screens of approximately 1100 by 620 millimeters (diagonal screen of approximately 1280 millimeters). This makes it possible to have screens that can be attached to a wall. Figures 12a and 12b show an alternative embodiment of the objective according to the invention, applied to a rear-projection system. In this embodiment, the lens L'l is physically combined with the mirror Ml, and the mirror M4 lies approximately in the same plane as the mirror M3. In one embodiment, the mirrors M4 and M3 form one and the same mirror.

As can be seen in Figure 12b, the lens L'1 is mounted in an opening 01 of a mounting support part Si having an approximately hyperbolic shape.Within the opening 01, the support part SI has a surface reflective constituting the mirror Ml In one embodiment, the opening 01 is located on the axis YY 'of the hyperbolic shape of the support part SI.Figure 13a shows a detailed illustrative example of the system of the invention, without the M3 mirrors and M4, but a system that includes the M3 and M4 mirrors would have a similar configuration.The refractory part of the lens comprises a group of Grrear lenses composed of four lenses Ll to L4 and a front group of Grfront lenses composed of three lenses L5 To L7, the front group receives the light from the SLM objective, the image of which has to be projected onto the SC screen, the SLM objective being, for example, a spatial light modulator. used to illuminate the hyperbolic mirror Ml by means of the beam that it receives from the subsequent group Grrear. According to the invention, the hyperbolic mirror Ml is located relative to the group of Grfr? N lenses in such a way that one of its foci, F2, lies in the plane of the exit pupil P2 of the frontal group Grfr? Nt- The other, the virtual focus Fl lies on the plane of the pupil Pl of the virtual output of the system. It can therefore be observed that, according to the invention, the hyperbolic mirror conjugates the pupils Pl and P2 and has the advantage of increasing the field angle, and therefore of increasing the amplification of the system. In general, the pupil is not discrete and may suffer from aberrations. The exit pupil P2 of the front group Grfront therefore defines a non-discrete pupil zone Z2. By definition, this pupil zone Z2 is the image of the diaphragm produced by the front group of Grfront lenses. As indicated above, the convex hyperbolic mirror has two foci, namely a first virtual focus Fl and a real focus F2. The actual focus F2 of the hyperbolic mirror Ml is preferably placed in the pupillary exit zone Z2 of the frontal group. In this way, the focus Fl is located in the pupil area Z1 corresponding to the exit pupil Pl of the system corresponding to the combination of the frontal group Grfront and the hyperbolic mirror Ml. Since the actual focus F2 is placed in the pupil area Z2, the quality of the image projected onto an image plane corresponding to the SC screen is optimized.

In addition, a positive lens L7 located between the meniscus L5 and the hyperbolic mirror Ml, is provided in order to reduce the envelope of the light rays of the field, to make it easier to fold the light beam by means of a plane mirror , in order to reduce the overall size of the target. However, the hyperbolic mirror can introduce a geometric distortion, and an objective as shown in Figure 14a can distribute an image having a distortion as shown in Figure 14b. In order to correct this distortion, the invention provides in the later group of Grrear lenses, a lens Ll having a surface in the shape of a cone. Advantageously, this cone is a cone of the same type as the shape of the mirror Ml to thus provide an almost perfect correction of the geometric distortion. Advantageously, this cone is therefore a hyperbola. Preferably, the proportion of the cones (hyperbolic mirror Ml and posterior lens Ll) is approximately proportional to the proportion of the positions of the hyperbola focuses, that is to say the distances P2 - hyperbola and Pl - hyperbola. For example, the focal length of the equivalent posterior lens Grrear is adjusted, the pupil is placed in the focus of this lens and the hyperbola is placed at "a certain distance". This distance requires the use of a focal length, and a cone for the hyperbola, in order to obtain the given amplification (for example 64) on the screen. The shape of the conical surface of the lens or the group of Grzear lenses must have in order to correct the objective, it is such that the ratio of this cone to the cone of the hyperbolic mirror is approximately proportional to the proportion L1 / L2, representing and L2 the distances of the hyperbola focuses from the main planes of the hyperbola. These distances, particularly the distance corresponding to P2 are observed from the hyperbola through lens group equivalent distance Grfront- However, it should be noted that the lens Ll hyperbolically should be away from the lens diaphragm f, which is the case in Figure 13a, so that the pre-correction of the distortions can be performed on an extended beam. It will be noted, therefore, that the L lens designed in this way not only corrects the geometric distortions but also the curvature of the field. In addition, the astigmatism defects induced by the system do not follow the same laws as geometric distortions. These are not corrected by the previous means. For this reason, at least one meniscus such as L5 to correct the astigmatism defects induced by the system is provided. Figure 13b shows the paraxial diaphragm of the objective according to the invention, and describes the main path of the light rays emanating from the objective. In this Figure 13b, the posterior group of Grrear lenses of Figure 13a has been shown symbolically by the lens 11 and the front group of Grfron lenses has been symbolically shown by the lens 12. As can be seen in Figure 13b, the system it is telecentric. The pupil on the side opposite to the objective that is going to be projected in relation to the optical system, lies in the focal plane of the system (focal length fO): a = arctan (ho / fo) The lens 12 is designed to produce a sharp image on the screen via the surface of the hyperbolic mirror, this condition imposing on it a power f. The following equation can be written: f / 2 = l / (r "l-l + zp) if it is accepted that the power of the hyperbolic mirror in A is low The new angle of output of the ray emanating from ho is: al = a-kf / 2 = arctan (ho / fo) - -f / 2 the action of the hyperbolic mirror. In this system, the hyperbolic mirror is used to conjugate the pupils. Let fl and f2 be the positions of the foci of the h hyperbola and hm the height of incidence: hm = fl.tanal al = arctan (tana2 / í2) From this, we can deduce the equation that links the height of the object ho to that of the image Hi: Hi = Zp tan a2 ' f \ tanftarctan ('> »oH' lw fo)? > Hi = Zp. tan - / 2 Figure 15 shows an illustrative example of an objective according to the invention. The characteristics of the various elements of this objective can be summarized in the following table: Name Curvature Thickness radius Material Half Conical curvature diameter Ll 0.0000 6.8071 BK7 22.0000 -0.0114 -87.6150 0.4000 22.0000 L2 0.0396 25.2208 13.3026 FK5 22.0000 -1.1706 0.0000 0.4000 22.0000 L3 0.0186 53.6495 11.3500 SF4 19.0000 0.0431 23.2180 13.9658 12.9109 0.0532 18.8123 13.4988 BSM14 10.8059 L4 0.0000 0.0000 7.3184 PHYSICAL STOP 0.0000 2.0910 7.3184 L5 -0.0265 -37.6993 12.0000 SF4 6.3667 -0.0348 -28.7256 7.0084 10.8059 -0.0850 -11.7610 12.0000 SF4 8.1631 L6 -0.0170 -58.7766 40.0000 14.6094 -0.0188 -53.2774 15.0000 K10 34.6733 L7 -0.0214 -46.7492 0.4000 39.3627 0.0000 15.4624 BK7 49.6520 L8 -0.0088 -113.2711 233.0000 50.6673 Ml 0.0171 58.5615 -250.0000 MIRROR 105.3846 -3.5600 1007.235 0.0000 0.0000 7

Claims (16)

1 Projection lens comprising a combination of lenses comprising a front group of lenses and a rear group of lenses that are positioned on either side of a diaphragm, and are intended to transmit a divergent light beam to a flat screen, characterized in that it includes at least one hyperbolic mirror, called a hyperbolic mirror, oriented to receive, on its convex face, the light coming from the front group of lenses, and to transmit the beam towards the screen.

2. Objective according to claim 1, characterized in that a first focus of the hyperbolic mirror is placed in the region, called the pupil region, defined by the image of the diaphragm, by the front group of lenses.

3. A lens according to any of claims 1 and 2, characterized in that the rear group of lenses and / or the front group of lenses include / includes at least one optical device for correcting the geometric distortion, which has a conical shape.

4. Objective according to claim 3, characterized in that the optical correction of the geometric distortion is located in the posterior group of lenses and has a hyperbolic shape.

5. A lens according to claim 4, characterized in that the optical correction of the geometric distortion is located in a part of the posterior group of lenses furthest from the diaphragm.

6. A target according to any of claims 1 to 5, characterized in that it includes at least one meniscus located in that part of the frontal group or of the posterior group that is closest to the diaphragm, the meniscus or the meniscus are designed to correct the induced astigmatism defects. by the hyperbolic mirror.

7. A lens according to any of claims 1 to 6, characterized in that it uses a peripheral field of the plane of the object and because the hyperbolic mirror is located completely on one side of a plane passing through the axis of symmetry of the hyperbola, to fold well the beam without the object scattering a shadow over the image.

8. Objective according to any of claims 1, characterized in that it includes a first return mirror that is placed near the front group of lenses, in a first direction that corresponds to the direction of the beam transmitted by the lens, and reflects the beam in a second direction, the mirror of hyperbolic shape is located along the second direction, and is oriented in order to receive the beam reflected by the first mirror of return.

9. A lens according to claim 8, characterized in that the second direction makes an angle of less than 60 ° with the first direction.

10. A lens according to any of the preceding claims, characterized in that it includes two menisci located on either side of the diaphragm, the concave portions of which are oriented towards the diaphragm.

11. A lens according to any of claims 1 to 10, characterized in that the diaphragm lies in the frontal plane of the posterior group of lenses.

12. A target according to claim 11, characterized in that it includes a positive lens located between the meniscus belonging to the front group of lenses and the hyperbolic mirror.

13. Projection or rear projection apparatus that applies the lens according to any of claims 1 to 12, characterized in that it includes a screen located on one side of the optical axis of this rear group of lenses, which makes it possible to transmit a beam of modulated light to a region of the posterior group of lenses, which is located on one side of the axis of the posterior group of lenses.

14. Rear projection apparatus according to claim 13, characterized in that it includes at least a second return mirror that receives the light reflected by the hyperbolic mirror, and reflects it on the rear face of the screen of the rear projection apparatus.

15. Rear projection apparatus according to claim 14, characterized in that the second return mirror makes a zero angle with the plane of the screen.

16. Rear projection apparatus according to claims 14 and 15, characterized in that the second return mirror lies in the same plane as a third return mirror, placed near the front group of lenses, along a first direction corresponding to the direction of the beam transmitted by the lens and which reflects the beam in a second direction, the hyperbolic mirror is located along the second direction, and is oriented in order to receive the beam reflected by the third mirror of return.