DESCRIPTION

The present invention relates to a system of illumination for videoprojector according to the preamble of claim 1.

Nowadays, small electromechanical devices such as, for instance, DMD (Digital Micromirror Device), LCD (Liquid Cristal Display) and LCoS (Liquid Cristal on Silicon) devices are widely used for manufacturing videoprojectors, because they allow to obtain small, light apparatus producing a high-quality image.

A DMD device or panel consists of a set of small square mirrors typically made of aluminium and having a side approximately 13 μm long, each of which represents an element of the image to be projected, i.e. a pixel. The mirrors can rotate about a diagonal at a certain angle, e.g. ± 12 degrees, and the rotation in either direction is produced by two electrodes located under the mirror, on opposite sides with respect to the axis of rotation. The light strikes the mirror at an angle of approximately 26 degrees relative to the perpendicular of the plane of the mirror, when the latter is "at rest", i.e. when it is not attracted by either electrode. Between the DMD device and the screen there is a projection lens, which focuses the image onto the screen.

If the mirror is turned in one direction, the light striking it will be reflected so as not to enter the projection lens, and thus it will not be sent to the screen; therefore, the pixel will be inactive, i.e. "off; if the rotation takes place in the opposite direction, the pixel will be active, i.e. "on", in that the reflection will be sent to a screen. In LCD and LCoS devices, a pixel consists of a layer of liquid crystals enclosed between two plates, or electrodes, which are arranged parallel to each other; when a light beam is sent to a plate, the quantity of light going through the liquid crystal layer can be controlled by means of the voltage applied to the electrodes and by using suitable polarizers. In the so-called "transmission-type" devices, which generally consist of LCD devices, both electrodes are transparent, therefore the light beam enters one plate, and exits from the other plate, and is then sent to the screen. In the so-called "reflection-type" devices, which generally

consist of LCoS devices, one plate is transparent and the other one is reflective, so that the light beam goes through the transparent plate, is reflected by the reflective plate, goes through ^' the transparent plate again and is finally sent to the screen.

The set of pixels represents the active surface of the panel, on which the image to be projected onto the screen is formed.

Depending on the brightness level required and on the maximum size of the screen, each system of illumination for videoprojector may use one or more devices for the formation of the image, in particular DMD, LCD or LCoS devices; preferably, each system of illumination employs devices of the same type. As to performance and reliability of a system of illumination for videoprojector , the choice of the illuminating device used is of great importance.

Normally, systems of illumination for videoprojector use systems of illumination consisting of arc lamps, which provide a high luminous intensity.

Fig. Ia shows a diagram of a system of illumination for videoprojector according to the prior art. hi the drawing of Fig. Ia, a lamp 1 emits a parallel light beam having a substantially circular section as said lamp 1 is provided with a parabolic shaped reflector.

An aspheric condenser, indicated with reference number 2, sends said light beam to an integrating bar 3, consisting of a parallelepiped made of optical-glass having an inlet face 4 and an outlet face 5,which allows to uniform the light beam exiting from the aspheric condenser 2.

More specifically, said aspheric condenser 2 sends a conical light beam, represented by the triangle f-f-f ' in the horizontal projection of Fig. Ia, to the integrating bar 3; said conical light beam is intercepted by the inlet face 4, so as to form a circle having a diameter d on the surface of said inlet face 4. Fig. Ib shows the inlet face 4 of the integrating bar 3 and the circle having a diameter d which indicates the area illuminated by the conical light beam striking said inlet face.

For simplicity's sake, unless otherwise stated, the following description will always refer to a representation of a system of illumination in a horizontal plane, thus referring to angles rather than to angular apertures of a conical light beam.

The angular aperture of the conical light beam, i.e. the angle in the vertex f of the cone, is equal to an angle 2α in the representation of Fig. Ia.

As known, due to the multiple reflections taking place within the integrating bar 3, the outlet

face 5 is evenly illuminated, as shown in the representation of Fig. Ic; in particular, each point of the surface of said outlet face 5 generates a conical light beam having an angular aperture being equal to the angular aperture of the conical light beam striking the inlet face 4. Therefore, the set of light beams generated by said outlet face 5 can be considered to be a single conical light beam whose angular aperture is represented in Fig. Ia) by the angle 2β of a triangle g-g'-g", said angular aperture being equal to the angular aperture of the conical light beam striking the inlet face 4, represented by the angle 2α; this means that 2α=2β. The light beam exiting from the integrating bar 3 is collected by a system of converging lenses 6, known as "relay lenses", and is then sent to means, e.g. a prism system not shown in Fig. Ia, adapted to break up the light beam, according to known techniques, into the three primary monochromatic components it is composed of, in particular the red, green and blue monochromatic components.

Said primary monochromatic components are then directed to the active surface of several devices for the formation of the image, e.g. three DMDs (Digital Micromirror Devices), not shown in the drawings for simplicity's sake.

The angular aperture of the conical light beam entering said integrating bar 3, and thus the angle 2α, is selected so as to use in the appropriate conditions the various components of the system of illumination which convey the light beam to the devices for the formation of the image, thereby maximizing the light transfer. In order to enhance the brightness of the system of illumination for videoprojector shown in Fig. Ia, it is necessary to increase the power of the lamp; this involves a reduction of the average life of said lamp, which is more stressed. Moreover, a higher lamp power causes the entire system of illumination to become hotter and therefore more difficult to cool, thus requiring bulky cooling solutions. An increase in the brightness of a system of illumination for videoprojector may also be obtained by using the known arrangement shown in Fig. 2, which provides the use of two lamps 1 and 1' which, for simplicity's sake, are supposed to be identical. In the representation of Fig. 2, too, both lamps 1, 1' emit a parallel light beam having a substantially circular section, since said lamps 1, 1' are provided with a parabolic shaped reflector; the use of lamps provided with a parabolic shaped reflector requires the use of collimating means, which allow to converge the parallel light beam generated by the lamps. As a result, each of said lamps 1, 1' is associated with a collimating means, in particular an aspheric condenser 2, 2', which allows to converge the light beam into the integrating bar 3.

Each lamp 1, 1' generates a light beam having a magnitude equal to 2α; the angle of incidence of the light beam when it enters the integrating bar 3 is equal to the sum of the angles of the light beams generated by both lamps 1, 1', and therefore its magnitude is equal to 4α. As already mentioned, the angular aperture of the light beam exiting from the integrating bar 3 is equal to the total angular aperture of the light beam entering the integrating bar 3; therefore, the angular aperture of the output light beam is equal to 4β=4α.

It follows that the overall angular aperture 4β of the light beam exiting from the integrating bar 3 turns out to be twice as the angular aperture of the light beam exiting from the integrating bar 3 produced by a single lamp. Fig. 2 clearly shows that, since the angular aperture 4β of the light beam exiting from the integrating bar 3 is wider, in order to intercept a portion as big as possible of the light beam it is necessary to increase the dimensions of the system of converging lenses 6 and of the other means, eg. prisms not shown in the drawings, which allow to convey the light beam to the devices for the formation of the image, e.g. DMD devices. This inevitably leads to increased costs of the components used in the system of illumination. Besides, the components normally used in the systems of illumination known in the art (e.g. devices for the formation of the image, possible polarizers for LCD devices, possible total- reflection TIR prisms, projection lenses, etc.) require the use of light beams having a limited angular aperture. Consequently, if the system of illumination employs a light beam exiting from the integrating bar 3 with a very wide angular aperture, at least a portion of said light beam will not be used. This will lead to a lower brightness level of the whole system of illumination; furthermore, the unused portion of the light beam will scatter in the system of illumination, thus causing a considerable drop in contrast. It follows that the expected brightness increase due to a doubled light source will not be reached in full.

The object of the present invention is to provide a system of illumination using a plurality of lamps which, by overcoming the above drawbacks, allows to manufacture high-brightness and high-contrast videoprojectors without jeopardizing the reliability of the videoprojector itself and at the same time avoiding any increase in the costs of the components used for its production.

In order to achieve said object, the present invention provides a system of illumination having the features described in the annexed claims, which form an integral part of the present description.

Further objects and advantages of the present invention will become apparent from the following detailed description and from the annexed drawings, which are supplied by way of non-limiting example, wherein:

- Figs. Ia ₃ Ib, Ic show a first system of illumination for videoprojector according to the prior art;

- Fig. 2 shows a second system of illumination for videoprojector according to the prior art;

- Figs. 3a, 3b and 3c show a first embodiment of a system of illumination for videoprojector according to the invention;

- Fig. 4 shows a second embodiment of a system of illumination for videoprojector according to the invention;

- Fig. 5 shows a third embodiment of a system of illumination for videoprojector according to the invention.

Note that the blocks designated with the same reference number in the various drawings perform the same function. Fig. 3a shows a first embodiment of a system of illumination for videoprojector according to the invention; said system of illumination comprises a plurality of lamps. Said plurality of lamps comprises two lamps 1, 1' which, for simplicity's sake, are supposed to be identical; it is clear that the following considerations may be easily extended to a system of illumination comprising more than two lamps, even being different from one another. Each of said lamps 1, 1' emits a parallel light beam having a substantially circular section, in that said lamps 1, 1 ' are provided with a parabolic shaped reflector.

The use of lamps 1, 1' provided with a parabolic shaped reflector requires that each of said lamps 1, I' be associated with a collimating means, in particular an aspheric condenser 2, 2', which allows to converge the parallel light beam generated by the lamps 1, 1'. According to the present invention, the lamps 1, 1' are associated with two transparent deflecting prisms, being in particular of the "wedge" type and preferably small in size, designated with reference numbers 8 and 8' in Fig. 3a; in particular, in the representation of Fig. 3a said transparent deflecting prisms 8, 8' are positioned between the aspheric condensers 2 and 2', respectively, and an integrating bar 3. The transparent deflecting prism 8 deflects the light beams coming from the lamp 1 and from the respective aspheric condenser 2, so that a first external ray 9 undergoes such a deflection as to generate a light ray 9', which strikes an inlet face 4 of the integrating bar 3 at an angle α, i.e. with an angular aperture being equal to half the angular aperture of the light beam entering

said integrating bar 3, shown in Fig. Ia ₅ when a single lamp 1 is used which lamp is adapted to generate a light beam having an angular aperture equal to 2α.

By sizing the transparent deflecting prism 8 in a proper way, a second external ray 10 of the light beam generated by the lamp 1 is deflected by the transparent deflecting prism 8 so as to undergo a minimum deflection.

Likewise, the transparent deflecting prism 8' deflects the light beam coming from the lamp I ¹ and from the respective aspheric condenser 2' so that a first external ray 11 undergoes such a deflection as to generate a light ray 11 ' striking said inlet face 4 of the integrating bar 3 at the same aforementioned angle α, whereas a second external ray 12 undergoes a minimum deflection.

The use of the transparent deflecting prisms 8 and 8' allows the light beam produced by the lamps 1 and 1' to strike the inlet face 4 of the integrating bar 3 with an angular aperture being substantially equal to 2α, i.e. equal to that obtained in the representations of Figs. Ia, Ib, Ic when the system of illumination employs just one lamp 1. Fig. 3b shows the inlet face 4 of the integrating bar 3, on which of course two illuminated areas appear, each having a diameter d, instead of a single area as in the representation of Fig. Ib.

Fig. 3c shows an outlet face 5 of the integrating bar 3, which face, due to the multiple reflections taking place within the integrating bar 3, is evenly illuminated. As stated with reference to the representation of the prior-art system of illumination of Fig. 1 a, which uses a single lamp 1, the conical light beam exiting from the integrating bar 3 has an angular aperture being equal to that of the conical light beam entering the integrating bar 3; it follows that, also with reference to the representation of Fig. 3a, the angle 2β turns out to be equal to the angle 2α. The light beam exiting from the integrating bar 3 is collected by a system of converging lenses 6, known in particular as "relay lenses", and is then sent to decompose means, e.g. a prism system not shown in Fig. 3a, adapted to divide the light beam, according to known techniques, into the three red, green and blue primary monochromatic components. Said primary monochromatic components are further directed toward the active surface of devices for the formation of the image, e.g. three DMDs (Digital Micromirror Devices), not shown in the illustrations.

The association between the transparent deflecting prisms 8, 8' and the lamps 1, 1' of the system of illumination for videoprojector thus allows to obtain a light beam produced by said

lamps 1 and 1' whose angular aperture, both at the inlet and at the outlet of the integrating bar 3, is substantially equal to that obtained in systems of illumination using a single lamp 1, as in the example of Fig. Ia.

It follows that, in order to direct the light beam exiting from the integrating bar 3 to the devices for the formation of the image, which for instance may comprise three DMD, LCD or LCoS devices, one may use the same components, in particular "relay lenses" 6 and prisms not shown in the drawings, as those used in systems of illumination for videoprojector using just one lamp 1, thus with no increase in the costs of the components used in the system of illumination. Furthermore,the use of the transparent deflecting prisms 8, 8' also allows to obtain a light beam exiting from the integrating bar 3 having a narrow angular aperture, so as to allow for a full utilization of the whole light beam in the system of illumination for videoprojector according to the present invention, thereby preventing any brightness and contrast reduction phenomena from arising in the entire system of illumination. Furthermore, in the system of illumination according to the present invention it is possible to turn on the lamps 1, 1' at a power being lower than the nominal power, thereby obtaining a longer life of said lamps 1, 1'.

It is clear that the system of illumination for videoprojector according to the present invention may be subject to many variations without departing from the novelty spirit of the inventive idea.

In the embodiment of Fig. 3a, the light beam produced by the lamps 1, 1' is divided, according to a known technique, into the three red, green and blue primary monochromatic components; these are then sent to a plurality of devices for the formation of the image to be projected, e.g. to three different DMD devices. In case the system of illumination for vodeoprojector uses a device for the formation of the image comprising just one DMD, LCD or LCoS device, said system of illumination may advantageously comprise a rotating wheel, also known as colour wheel, designated 13 in Fig. 4 and positioned between the transparent deflecting prisms 8, 8' and the inlet face 4 of the integrating bar 3. As known, said colour wheel 13 is subdivided into at least three sectors, each consisting of a dichroic filter associated with one of the three primary monochromatic components; the rotation of the colour wheel 13 allows the light beam sent by the device for the formation of the image to take on the colour of the three different primary monochromatic components in succession.

Fig. 5 shows a further embodiment of the system of illumination for videoprojector according to the present invention, which comprises a plurality of lamps 14, 14'. Said lamps 14, 14' are provided with an elliptic shaped reflector adapted to generate a conical light beam; as a result, this solution does not require the use of collimating means associated with each of said lamps 14, 14' in order to converge the light beam. Referring to the representation of Fig. 3a, the transparent deflecting prisms 8, 8' are positioned between the lamps 14, 14' and the inlet face 4 of the integrating bar 3.

It is clear that many other variations and applications of the system of illumination described herein by way of example are possible for those skilled in the art, as well as that in the practical realization of the invention the components may have shapes and dimensions being different from those described or be replaced with other technically equivalent elements.

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