Projection system employing multiple beam reflections

A projection system includes a projector for emitting a light beam; a screen and a first medium for converting a linear polarized form of the light beam from the projector to a quasi-circular polarized form of the light beam polarized in a first circular sense. The projector also includes a mirror for reflecting and phase shifting a quasi-circular polarized form of the light beam to a circular polarized sense substantially opposite of that exhibited by the light beam before being reflected by the mirror, and a second medium optically disposed between the mirror and the screen and for selectively converting the light beam between linear polarization and quasi-circular polarization. The projection system also includes a reflective linear polarization filter optically disposed between the second medium and the screen and for reflecting the light beam exhibiting linear polarization of a first linear sense and transmitting the light beam exhibiting linear polarization of a second linear sense substantially perpendicular to that of the first linear sense.

BACKGROUND OF THE INVENTION
 The present invention relates to a projection system, more particularly to
 a system known as a back-projector. The back-projectors currently
 available on the market are systems which use CRT-based projectors or
 liquid crystal projectors as projector. However, the present invention can
 be applied to all types of projection systems.
 A back-projector of known type comprises a projector emitting a light beam,
 a screen and means for sending the light beam to the screen. Currently,
 the means making it possible to send the light beam to the screen consist
 of plane mirrors arranged in such a way as to reflect the entire light
 beam over the whole surface of the screen while limiting the overall
 thickness of the various elements which have to be inserted into a housing
 of limited thickness. When a system of mirrors such as mentioned above is
 used, it is difficult to produce a wide screen, especially a 46-inch
 screen, since in this case the thickness of the projector remains too
 great.
 A back-projector comprising means making it possible to fold the beam
 output by the projection objective in order to reduce the depth has
 therefore been proposed in European Patent Application No. 96402512.6
 filed in the name of the Applicant.
 As represented in FIG. 1, the projection system described in this patent
 application includes a projector 1 which can, for example, be a liquid
 crystal projector emitting a light beam 2 which is already linearly
 polarised. This beam 2 is sent to a quarter-wave plate 3 in such a way as
 to produce circularly polarised light. This circularly polarised light is
 transmitted through a cholesteric filter 4. To do this, the light is
 right-circularly polarised if cholesteric filters which reflect
 left-circularly polarised light are used. The beam transmitted through the
 filter 4 is then reflected off the plane mirror 5. During this reflection,
 the beam undergoes a phase shift of .pi., and it is then left-circularly
 polarised. The beam leaving the plane mirror 5 then encounters the
 cholesteric filter 4. Since the cholesteric filter reflects all
 left-polarised light, the light issuing from the mirror is returned to the
 mirror 5 where it undergoes a new phase shift of .pi.. The reflection off
 the plane mirror therefore converts left-polarised light into
 right-polarised light, which is then transmitted in full by the filter 4
 to the screen, not represented.
 In the system described in this patent application, three reflections take
 place, two off the plane mirror and one off the cholesteric filter, as
 represented clearly in FIG. 1. In European Patent Application No.
 96402512.6, use is made of cholesteric filters or more generally
 polarisation filters which reflect light circularly polarised in a second
 sense opposite to the first sense, to obtain a projection system of small
 thickness. Now, the cholesteric filters currently available on the market
 have a number of constraints, especially as regards angle of incidence.
 Now, reflective linear polarising filters have recently appeared on the
 market. These filters transmit linearly polarised light with little loss
 and reflect light polarised perpendicularly to the transmitted direction.
 However, in this case, a straightforward reflection off a mirror is not
 sufficient to reverse the state of polarisation. It is necessary to pass
 via a quasi-circular state in order for a mirror to give the polarisation
 from one linear state to another. The purpose of the present invention is
 to propose a projection system of the type described above, according to a
 structure which avoids stray reflections.
 SUMMARY OF THE INVENTION
 Thus, the subject of the present invention is a projection system of the
 type comprising:
 a projector emitting a light beam,
 a means for converting linear polarisation into quasi-circular
 polarisation;
 a reflective linear polarising filter;
 a mirror, and
 a screen,
 the light beam arriving at the mirror being quasi-circularly polarised,
 characterized in that the means for converting linear polarisation into
 quasi-circular polarisation is associated with the reflective linear
 polarising filter.
 According to another characteristic, if the beam output by the projector is
 linearly polarised, it passes through a second means for converting linear
 polarisation into quasi-circular polarisation.
 Preferably, the means for converting linear polarisation into
 quasi-circular polarisation is cemented or laminated to the filter.
 According to another characteristic of the present invention, the system
 additionally includes an absorbing linear polarising filter positioned at
 the output of the reflective linear polarising filter, the passing axis of
 the absorbing filter being parallel to the passing axis of the reflective
 filter.
 According to yet another characteristic of the present invention, the
 system also comprises a shutter film positioned at the exit of the screen,
 the said film exhibiting a small angular acceptance and being centred on a
 mean angle of incidence which varies as a function of position on the
 screen. The principal purpose of the film is to absorb the stray images
 and hence to improve the contrast.

DESCRIPTION OF PREFERRED EMBODIMENTS
 The present invention uses the properties of reflective or non-absorbing
 linear polarising filters.
 Thus, as represented in FIG. 2, a projection system in accordance with the
 present invention comprises a projector 1, such as for example a liquid
 crystal projector which emits a linearly polarised light beam 2. This
 projector is associated with a means 7' for converting linear polarisation
 into quasi-circular polarisation, this means preferably being a
 quarter-wave plate, and a reflective linear polarising filter 9. In this
 system and as represented in FIG. 2, the linearly polarised beam passes
 firstly through the quarter-wave plate 7' in such a way as to obtain a
 quasi-circularly polarised beam, namely one exhibiting right circular
 polarisation, as referenced by d. This beam will be reflected off the
 mirror 8 where it undergoes a first phase shift of .pi. and gives a left
 circularly polarised beam g.
 The principle described above can be implemented according to various
 embodiments. A preferred embodiment of the invention is represented in
 FIG. 2. In this case, the projector 1 located inside the chassis 6 is a
 liquid crystal projector which emits a linearly polarised light beam
 through a first quarter-wave plate 7' which converts this linear
 polarisation into quasi-circular polarisation, for example right circular
 polarisation, as referenced d, so that the beam may firstly be reflected
 off a mirror 8. The mirror 8 is a plane mirror such as a mirror consisting
 of a sheet of aluminium which reflects light circularly polarised in a
 certain sense as light polarised in the reverse sense, namely exhibiting a
 phase shift of substantially .pi., as represented clearly in FIG. 2 by the
 circles d and g in accordance with the invention, a means for converting
 linear polarisation into quasi-circular polarisation, consisting for
 example of a quarter-wave plate 7, is bonded or placed parallel to a
 reflective linear polarising filter 9 placed parallel to the display
 screen 11. The left circularly polarised beam passes through the
 quarter-wave plate 7 so as to obtain a linearly polarised beam. The light
 beam thus arrives at the filter 9 where it is reflected towards the
 quarter-wave plate 7. On passing through this quarter-wave plate again,
 the beam changes from linearly polarised to left circularly polarised, as
 represented in FIG. 2. It will then be reflected off the mirror 8 where it
 undergoes a new phase shift of .pi. to give a right circularly polarised
 beam. The beam thus polarised passes through the quarter-wave plate where
 it is converted into a linear polarisation perpendicular to the first
 linear polarisation and, therefore, it can pass through the polarising
 filter 9, as represented in FIG. 2 by the arrow I. Preferably, the
 quarter-wave plate 7 is adhesively bonded or laminated to the polarising
 filter 9. This makes it possible to reduce or correct the phase shift
 introduced during the reflection off the metal mirror which is due to the
 angle of incidence and also to the embodiment of the mirrors, these being
 made from aluminium sheets provided with dielectric layers in order to
 increase the reflectivity, especially in the blue range, these layers also
 introducing some phase shift. The metal mirror 8 therefore introduces a
 variable phase shift, the effect of which is to convert circular
 polarisation into elliptical polarisation. Therefore, the polarisation on
 the linear polarising film is, also, elliptical. There is another means of
 correcting the phase shift. This means consists in positioning the first
 quarter-wave plate 7' in such a way that it makes an angle .theta. with
 the optical axis of projection different from 90.degree. so as to give a
 different phase shift for each ray.
 The projector referred to in the above description was a liquid crystal
 projector giving a linearly polarised beam as output. In the case in which
 a projector giving an unpolarised beam as output is used, an optional
 means (13) is provided for polarising the beam linearly, this means being
 positioned ahead of the quarter-wave plate (7, 7'). By way of example, a
 reflective linear polarising filter which is particularly beneficial in
 respect of the embodiments described above consists of the film sold by
 the "3M" company under the brand name DBEF. However, other means may also
 be used, such as a cholesteric filter associated with a quarter-wave
 plate, a volume holographic film operating at the Brewster conditions for
 the various layers.
 In the description, the means used to convert linear polarisation into
 quasi-circular polarisation consists of a quarter-wave plate. However, it
 is obvious to those skilled in the art that other means may be used,
 namely means which use birefringence, such as natural birefringent
 materials (e.g.: mica), artificial birefringent materials (PMMA film or
 other plastic films obtained by drawing) and means using reflection, such
 as total internal reflection (TIR) in glass (since the phase shift depends
 on the angle of incidence and is around 45.degree., two internal
 reflections are necessary to obtain circular polarisation from 45.degree.
 linear polarisation).
 In the above embodiment, the light beam passes through the quarter-wave
 plate four times. In the event that the correction of the phase shift is
 not sufficient or optimal for all wavelengths or angles of incidence, and
 according to a further characteristic of the present invention, a
 precorrection retarder plate can be added on exit from the projection
 objective. The latter will be chosen in such a way as to add or subtract a
 very particular delay value depending on the direction in space of each
 ray emitted by the exit pupil of the projection objective. This correction
 can be made inside the projector, more particularly it is possible to
 place a retarder plate, with a delay depending on position on the LCD
 valve, after the analyser.
 Various improvements can be made to the embodiment described above.
 In order to eliminate stray images due to the rays transmitted by the
 polarising filter 9, especially during the first reflection, an absorbing
 polariser 10 is positioned at the output of the reflective linear
 polarising filter 9. In this case, the passing axis of the absorbing
 polariser is parallel to the passing axis of the reflective linear
 polarising filter 9. This new polariser absorbs the light transporting the
 stray images. It makes it possible to increase the contrast of the
 projector when the latter is placed in bright surroundings such as an
 illuminated room. Thus, the light polarised linearly by the reflective
 linear polarising filter 9 is transmitted through the reflective filter,
 while the unpolarised natural light is firstly half absorbed by the
 absorbing polariser. The other half passes through this polariser and is
 partly reflected off the screen and again transmitted by the reflective
 and absorbing polarisers, thus undergoing a total attenuation of at least
 25%. This makes it possible to improve the contrast of the images by a
 factor of 3 by comparison with a projector having no absorbing polariser
 at the exit of the screen.
 According to another characteristic represented in FIG. 3, a shutter filter
 12 is positioned at the exit of the screen 11. The purpose of this shutter
 filter is to absorb the stray images such as I' and I" obtained from
 successive reflections of the light beam inside the chassis. Thus, the
 ghost images mix with the final image and disturb the appearance of the
 image, causing a loss of contrast.
 The shutter film 12 placed at the exit of the screen is adapted to the
 image ray I at each point of the image and absorbs the rays carrying the
 stray images, essentially the first stray image I'. Such a film is
 manufactured by the 3M company under the name "Light control film". The
 film used has a low angular acceptance, for example of .+-.6 degrees
 centred on a mean angle of incidence which varies as a function of the
 location at which it is placed on the projection screen. With such a film,
 the transmission in the useful band is of the order of 75% whilst the
 beams outside this band are absorbed by the structure. By a geometrical
 calculation it can be shown that it is possible, in the case described
 above, for the image beam (I) to be angularly separated from the stray
 beam (I'). This angular separation, at least equal to the angle between
 the polariser (the screen) and the mirror, is such that the shutter films
 work for this application, provided that the values of the inclination of
 the shutter films are adapted at each screen location. This variation is
 effected in x and in y.
 The use of a shutter film also makes it possible to improve the contrast of
 the image in view the external stray rays originating from the mean
 illumination of the room in which the projector is placed.