CA2203266A1 - Projector with multiple lamp light source - Google Patents

Abstract

A projection system (18) is described which efficiently combines the output from multiple lamps (19A, 19B) images of which are focused to a common point. The projected screen brightness is multiplied over that of a conventional single lamp of equivalent wattage. The screen brightness can be increased or diminished by switching individual lamps (19A, 19B) on or off, adjusting the light level for conventional overhead transparencies, or low transmission liquid crystal display projection panels.

Description

W O 96/16351 PCTrUS95/12896 1 PROJECTOR WITH MULTIPLE LAMP LIGHT SO~RCE
3 Field of the Invention 4 The present invention relates to projection systems or projectors, particularly transmissive 6 overhead projectors.

8 Backqround of the Invention 9 With the increasing use of full-color, computer-generated and photographic transparencies, and liquid 11 crystal display (LCD) projection panels, there is a 12 need for projection systems of increased brightness.

13 This has been recently addressed by the use of higher 14 wattage tungsten-halogen lamps, metal-halide lamp technology, and high-efficiency anti-reflection 16 coatings on the optical components. The use of higher 17 wattage tungsten filament lamps increases the 18 difficulty of cooling, and arc discharge lamps, such as 19 metal halide, are relatively expensive.

In the past, several attempts to increase the 21 illumination level of projection systems were 22 characterized by the use of multiple lower wattage 23 lamps. In the case of episcopic projection, commonly 24 practiced in opaque projectors, it is simply necessary to illuminate the opaque copy with light from several 26 sources. The scattered light from the copy which 27 enters a projection lens is then directed to the 28 screen. This type of projection system is described, 29 for example, in U.S. patent no. 4,979,813.

In diascopic projection, light from a lamp is 31 collected by single or multiple condensing lenses, 32 passes through a projection transparency, and is 33 focused to the projection lens. This mode of 34 projection is commonly used in 35 millimeter and CA 02203266 l997-04-2l W O96/16351 PCTrUS95/12896 1 overhead projectors, and gives a brighter projected 2 image than episcopic projection. See, for example, 3 U.S. patent nos. 3,547,530 and 3,979,160. However, 4 since the condensing lens system can only efficiently focus light from one single point (lamp position) to 6 another single point (projection lens position), 7 diascopic projection is usually limited to the use of a 8 single lamp. This inherently limits the brightness g which may be achieved on the screen.
Several attempts have been made to efficiently 11 combine the output of multiple lamps and bring them to 12 a common focus. U.S. patent no. 1, 887,650 describes a 13 system combining eight lamps, U.S. patent no. 3,770,344 14 describes an overhead projector combining four lamps, and Japanese patent nos. 4-179046 and 5-199485 describe 16 projectors combining two lamps. All of these devices 17 suffer from a common deficiency in that the combined 18 output from the lamps is not truly integrated. The 19 combined lamp output beams are contiguous, yet 20 spatially separated. The result is that should one 21 lamp fail, the screen image brightness does not 22 diminish uniformly, but rather individual sections of 23 the screen image become completely dark, making part of 24 the screen image unreadable.
True integration of the light from multiple 26 sources, allowing them to be focused to a common point, 27 iS a more demanding task. U.S. patent no. 4,952,053 28 describes an overhead projector combining two lamps, 29 U.S. patent no. 5,231,433 describes a method for 30 integrating two collimated light beams by means of a 31 linear grooved reflector or a linear grooved refracting 32 element. Japanese patent no. 5-232399 describes a 33 method for combining and integrating the output of two 34 lamps by means of a beam splitter and multiple-pass 35 reflections. The efficiency of these systems is limited 36 by the achievable reflectance of the reflector W 096/16351 PCTAUSg5/128g6 1 coatings, geometric shading losses, and by the high 2 chromatic dispersion of the refracting elements.
4 Summarv of the Invention The current invention avoids the deficiencies of 6 the prior art, providing a projection system with 7 complete and efficient integration of the output of 8 multiple light sources, to increase screen brightness.
9 This is accomplished by a series of Fresnel collecting and focusing lenses, and a l~near beam combining 11 prismatic film that utilizes total internal reflection 12 (TIR). The projection system can be in the form of an 13 integrated projector designed to project electronically 14 generated or stored information or in the form of an overhead projector adapted for full-size overhead 16 transparencies, or the reduced format of LCD projection 17 panels. The present invention further relates to a low 18 profile overhead projector.

srief Description of the Drawinqs 21 Figure 1 is a cross-sectional view of a linear 22 prismatic film of the prior art that deviates a beam of 23 light by refraction and total internal reflection.
24 Figure 2 is a cross-sectional view of a 60 apex angle linear prismatic film of the current invention.
26 Figure 3 is a cross-sectional view of the 27 prismatic film of the present invention and illustrates 28 schematically the integration of two collimated light 29 beams.
Figure 4 is a schematic illustration of a 31 transmissive overhead projector system that combines 32 and integrates the output of two lamps.
33 Figure 5 is a schematic illustration of an 34 alternate arrangement of a projection system that combines and integrates the output of two lamps.

CA 02203266 l997-04-2l W O96/16351 PCTrUS95/12896 1 Figure 6 is a schematic illustration of a 2 projection system that combines and integrates the 3 output of four lamps.
4 Figure 7 iS a perspective view showing desirable shapes of lenses for use in the present invention.
6 Figure 8 is.a perspective schematic illustration 7 showing condensing optics used with the present 8 invention to form an elliptical shaped beam at the 9 rectangular Fresnel lens collimator.
Figure 9 is a schematic illustration of an 11 integrated liquid crystal projection system that 12 combines and integrates the output of two lamps.

14 Detailed DescriPtion of the Invention U.S. patent no. 4,984,144 describes a dispersive 16 linear prismatic film 1 that deviates a beam of light 17 by refraction and total internal reflection, as shown 18 in Figure 1. The isosceles triangle micro prisms 2 19 have an apex angle cY of 69. An incident ray 3 enters 20 facet 4 at an entrance angle e of about 75, where it 21 is refracted. It is then totally internally reflected 22 at facet 5, and ray 7 exits perpendicular to the planar 23 face 8. The large entrance angle ~ of about 75 was a 24 requirement for this prismatic lens in its intended use 25 in a high aspect ratio light fixture.
26 Figure 2 shows the linear prismatic film 9 of the 27 CUrreIlt invention, consisting of a series of isosceles 28 triangle shaped micro prisms 10 with an apex angle ~x of 29 60. When an extended area collimated light beam 11 30 enters the film perpendicular to facet 12 at the 31 specific entrance angle O of 60, then the light rays 32 13 exit perpendicular to plano surface 14 with the 33 deviation occurring entirely by total internal 34 refraction at facet 15. Since there is no refraction 35 at surfaces 12 or 14, there is no dispersion and the 36 ray deviation is independent of the refractive index of W O96116351 PCT~S95/12896 1 the material. Moreover, since ray 11 of the entrance 2 beam touches both the peak and valley of adjacent 3 microgrooves, the entrance beam 11 completely fills the 4 TIR facet 15, and there are no geometric losses or spurious ray deviations.
6 For the 60 apex angle film, Figure 3 illustrates 7 that two collimated incident beams 16a and 16b can be 8 e~ficiently and spatially combined. The individual 9 exiting ray bundles 17a and 17b from the left and right incident beams 16a and 16b are interlaced on a micro 11 scale, such that the intensity of light over the total 12 area of the film 9 is effectively doubled.
13 For the linear prismatic beam-combining film 9 of 14 the current invention, the following conditions need to be satisfied for most efficient operation:
16 1) The incident light 16a and 16b must be 17 collimated so that light rays from each lamp 18 enter the entire prismatic film 9 at the same 19 entrance angle ~;
2) The preferred vertex angle ~ of the linear 21 prismatic beam-combining film 9 is 60 plus 22 or minus 2; and 23 3) The preferred entrance angle 0 of the 24 collimated light entering the linear beam combining film is 60 plus or minus 3.5.
26 If the prism vertex angle ~ is greater than 62, 27 less than so~ of each reflecting facet is utilized.
28 For example, for an acrylic plastic prismatic film with 29 a refractive index n = 1.492, a vertex angle ~ of 62O, and an entrance angle 0 of 63.5, only about 9o~ of 31 each reflecting facet is utilized. Excessive 32 underfilling of the reflecting facets causes the 33 collimated exit beams 17a and 17b produced by each 34 adjacent microprism to be spatially separated, and dark banding begins to appear on the illuminated projection 36 screen.

W O9611635~ PCTrUS95/12896 1 On the other hand, when the prism vertex angle 2 is less than 58; less than 90~ of the incident light 3 rays 16a and 16b exit perpendicular to the film. For 4 example, for an acrylic plastic prismatic film with a refractive index n = 1.492, a vertex angle ~ of 58, 6 and an entrance angle 0 of 56.5, about 10~ of the 7 incident rays miss each reflecting facet. The light 8 missing the reflecting facet exits the film in an 9 uncollimated and uncontrolled direction, and does not contribute to the illumination on the projection 11 screen.
12 Table 1 below illustrates the fraction of beam 13 filling of each facet (BFF) for various vertex angles 14 of the film 9. Values of BFF less than unity represent underfilling of the reflecting facet, while values of 16 BFF greater than unity represent overspilling of the 17 reflecting facet: As explained above, more than 10~ of 18 the incident light is spatially separated or wasted at 19 vertex angles ~ above 62 or below 58, upon exiting the film 9.

WO g6/16351 PCT/US9Sn2896 2 Table 1 4 ~ 0 BFF
42.3 1.55 7 58 56.5 1.11 8 59 58.3 1.05 9 60 60.0 1.0 61 61.7 0.947 11 62 63.5 0.895 13 70 77.7 0.446 16 It will be seen from Table 1 that the entrance 17 angle 0 of the collimated light 16a and 16b generally 18 changes in a similar fashion as the vertex angle a 19 changes, but the relationship is not linear. This is because at entrance angles 0 other than 60 the 21 refractive index n of the material of the film has an 22 effect. The exact relationship for the entrance angle 23 0 necessary to produce maximum screen illumination at a 24 given vertex angle ~ is given by the equation:

~3 = 90 - ( 2 + asin ( n cos ( 3 ~ ))) 7 wherein:
8 0 = said angle of inclination of said collimated light 9 a = said included angle of said prism sides n = index of refraction of the material of said film.

CA 02203266 l997-04-2l WO96l16351 PCT~S95/12896 1 Figure 4 shows a projection system 18 that 2 efficiently integrates the output from two lamps l9a 3 and l9b, providing uniform screen brightness when each 4 lamp l9a or l9b is individually on, and doubling the screen brightness when both lamps l9a and l9b are on.
6 Light sources l9a and l9b are positioned at the focal 7 point of rectangular-shaped Fresnel lenses 20a and 20b, 8 respectively, which collimate the light beams. Each g collimated beam fills the stage aperture 21 at an entrance angle 0 of 60. The stage aperture 21 can be 11 square, to accommodate full-size overhead 12 transparencies, or a reduced size rectangular format, 13 to accommodate LCD projection panels. Near the stage 14 aperture 21 is the 60 linear prismatic film 9. The integrated and collimated light exiting from the 60O
16 linear prismatic film 9 enters a circular Fresnel lens 17 22 which focuses the light to the projection lens 23.
18 A glass platen 24 is usually placed above the Fresnel l9 lens 22 to supports the overhead transparency or LCD
projection panel. If the projected facet widths of the 21 linear prismatic ~ilm 9 are less than the resolving 22 power o~ the eye at normal screen viewing distances, 23 then each light source l9a and l9b appears to fully 24 illuminated the entire screen. with both lamps l9a and l9b on, the screen brightness is effectively doubled 26 over the brightness produced by a single lamp l9a or 27 l9b.
28 Figure 5 shows an alternate arrangement using 29 folding mirrors 25a, 25b, and 26, to combine the output o~ the two light sources l9a and l9b. An additional 31 60 beam combining linear prism 9a is required.
32 Figure 6 shows a configuration that combines the 33 output of four light sources l9a, l9b, l9c, and l9d.
34 In this arrangement, the additional linear prismatic film element 9b, additional Fresnel lens collimators 36 20c and 20d, and additional folding mirrors 25c and 25d 1 are shown. Thi`s cascading process can be further 2 extended to integrate the output of additional light 3 sources.
4 The beam shaping requirements to fill rectangular optical elements in this multiple lamp projection 6 system are most easily achieved when these optical 7 elements have an aspect ratio L/W that is close to 8 unity, e.g. a square perimeter. To achieve this 9 condition, it is preferable that the each additional optical element be oriented along the shorter side of 11 the rectangular element preceding it. For the two lamp 12 system shown in Figure 4 and illustrated again in more 13 detail in Figure 7, with a rectangular linear prismatic 14 element 9 having a length L1 = 8 units, and a width Wl =
6 units (L1/W1 = 1.33), if the Fresnel lens collimators 16 20a and 20b (only 20a is shown) are oriented along the 17 W1 dimension, then the length L2 of the collimator is L2 18 = W1 = 6 units, and the width W2 of the collimator =
19 Ll/2 = 4 units, giving an aspect ratio L2/W2 = 6/4 =
1.5. If the Fresnel lens collimators were oriented 21 along the L1 dimension, then the collimator aspect ratio 22 = L2/W2 = 8/3 = 2.67, and beam shaping is more difficult 23 to achieve.
24 Similarly, for the four lamp system shown in Figure 6, with a square prismatic element 9 having a 26 length L1 = 12 units, and a width W1 = 12 units, then 27 the linear prismatic element 9a must have an aspect 28 ratio L2/W2 = 12/6 = 2. If the Fresnel lens collimators 29 20a and 20b are oriented along the W2 dimension, then the aspect ratio of the collimators = L3/W3 = 6/6 = 1, 31 which is the ideal beam shaping requirement.
32 It is also important to note that for each 33 additional level of multiple light sources, e.g. two 34 lamps, four lamps, eight lamps, etc., that the area of each additional linear prism element or Fresnel 36 collimator, is halved. This limits the achievable W O~6/16351 PCTrUS95/12896 1 light collection and sets a practical limit on the 2 number of lamps that can be integrated.
3 Figure 9 shows a projection system that 4 efficiently integrates the output of two lamps, as is also illustrated in Figure 4, again providing uniform 6 screen brightness when each lamp is individually on, 7 and doubling the screen brightness when both lamps are 8 on. A polarization-modulating display 32, such as a 9 liquid crystal display, is positioned between Fresnel lens 22 and the 60 linear prismatic film 9 to define 11 an optical window through which light from the lamps is 12 directed. This liquid crystal display panel 32 13 comprises a layer of liquid crystalline material which 14 can be of a twisted nematic or a supertwisted nematic enclosed between two transparent substrates or plates.
16 Each of these plates may comprise a transparent control 17 electrode which can be divided into a large number of 18 columns and rows, thus defining a large number of image 19 elements in the display panel. These image elements are controlled by driving the electrodes, and the image 21 display panel is referred to as passively controlled.
22 Alternatively, one of the substrates can be provided 23 with an electrode while the other is provided with 24 semi-conductor drive electronics. A device employing this type of control is referred to as an actively 26 controlled image display panel.

28 Exam~le 29 Described below with respect to Figure 7, and further illustrated in Figure 8, is a specific 31 arrangement that has been constructed for use as an LCD
32 overhead projector. The rectangular stage aperture 33 size has a length Ll = 228.6 millimeters (9.0 inches) 34 and a width W1 of 171.5 millimeters (6.75 inches), giving an aspect ratio of Ll/Wl = 4/3, a common ratio 36 for many LCD projection panels. The light source 1 filaments 26 (only one is shown) are positioned about 2 11 millimeters behind a pair of glass condensers 27 3 which collect and direct light to the Fresnel lens 4 collimator a. The light sources 26 are 400 watt, 36 volt flat mandrel capsule types, ANSI Code designation 6 EVD. A spherical reflector 28 having a diameter of 60 7 millimeters and a radius of curvature of 32.5 8 millimeters focuses the back rays in the forward 9 direction. An aspheric symmetric Pyrex condenser 29, having an approximate focal length of 55 millimeters 11 and a diameter of 60 millimeters, focuses the light 12 from the lamps 26 into a light cone of circular cross-13 section. A square optical crown glass cylinder lens 30 14 having an approximate focal length of 175 millimeters, and dimensions of 82 by 82 millimeters, is placed in 16 close proximity to the Pyrex condenser 29. This 17 cylinder lens 30 further compresses this circular cone 18 of light in one direction to form an elliptical shaped 19 beam 31. The single element Fresnel lens collimators 20a have a focal length of 178 millimeters, operate 21 between f/.75 and f/1.0, and are oriented symmetrically 22 at 30 from the vertical. The height W2 of the Fresnel 23 collimator 20a is half the length L1 of the stage 24 aperture, W2 = 114.3 millimeters (4.5 inches), and the width L2 of the Fresnel collimator 20a is the width of 26 the stage aperture, L2= 171.5 millimeters (6.75 inches).
27 The aspect ratio L2/W2 of the collimating Fresnel lens 28 is 1.5. To efficiently fill the rectangular aperture 29 of the collimating Fresnel lens, the following relationship should be approximated: L2/W2 = A/B, where 31 A and B are the major and minor axes of the elliptic 32 cross-section 31 of the light beam at the plane of the 33 Fresnel collimator as shown in Figure 8.
34 As illustrated in Figure 4, the 60 linear prismatic film 9 was fabricated in 2 millimeter thick 36 acrylic plastic, with dimensions slightly larger than CA 02203266 l997-04-2l W O96/16351 PCTrUS95/12896 l the stage apert~ure, and the width of each individual 2 prismatic groove 10 was about 0. 5 millimeters. A
3 rectangular acrylic Fresnel lens 22, having a focal 4 length of about 325 millimeters, groove widths between 0.5 and 0.125 millimeters, and approximately the same 6 size as the linear prismatic film 9, was placed between 7 the linear prismatic film 9 and a glass platen 21 8 defining the stage aperture. A triplet projection lens 9 23 of 330 millimeter focal length projected an image of the stage to fill a 60 inch wide screen at 11 approximately 6. 7X magnification.
12 With this configuration, an average screen 13 illumination of 140 foot-candles was measured from each 14 individual lamp, and an average screen illumination of 280 foot-candles with both lamps operating. This is 16 equivalent to the brightness of a 7000 lumen square 17 aperture overhead projector projecting a 60 inch square 18 image.
19 It can be appreciated by those skilled in the art, that by the use of additional folding mirrors in the 21 configurations described, modifications of the 22 arrangement of optical components can be achieved.
23 These modifications can reduce the base height of the 24 projector by variation of width and length of the projector base containing these components. It will 26 also be appreciated that although specific examples of 27 the invention have been illustrated, the invention is 28 more generally applicable to any device which requires 29 collimated light in the optical path.

Claims (9)

Claims:

1. A projection system for projecting an image onto a screen comprising :
an enclosure having an optical window;
a linear beam-combining film having a flat first surface defining a plane and a structured second surface opposite said first surface, said structured surface having a plurality of linear triangular prisms each having two sides having an included angle ranging from about 58° to about 62°; and at least one light source disposed at least one end of said film and emitting collimated light in a direction which is perpendicular to the axes of said linear prisms and inclined at an angle ranging from about 56.5° to about 63.5° with respect to a perpendicular to said plane of said film such that said light is directed to substantially totally illuminate one of said two sides of said linear prisms and is totally internally reflected within said film to exit said flat first surface and be directed toward the optical window as a collimated beam.

2. A projection system according to claim 1 wherein said angle of inclination of said collimated light from said light source is related to said included angle of said prism sides by the equation:

wherein:
.THETA. = said angle of inclination of said collimated light .alpha. = said included angle of said prism sides n = index of refraction of the material of said film.

3. A projection system according to claim 1 further including at least a second light source emitting collimated light in a direction which is perpendicular to the axes of said linear prisms and inclined at an angle ranging from about 56.5° to about 63.5° with respect to a perpendicular to the plane of said film such that said light is directed to substantially totally illuminate the other of said two sided linear prisms and is totally internally reflected within said film to exit said flat first surface and be directed toward the optical window as a collimated beam.

4. A projection system according to claim 3 wherein said angle of inclination of said collimated light from said light source is related to said included angle of said prism sides by the equation:

wherein:
.THETA. = said angle of inclination of said collimated light .alpha. = said included angle of said prism sides n = index of refraction of the material of said film.

5. A projection system according to claim 3 wherein said light emitting from each light source illuminates all portions of the screen.

6. A projection system according to claim 1 wherein said optical window is rectangular.

7. A projection system according to claim 6 further comprising a rectangular-shaped collimator for collimating said light from said light source.

8. A projection system according to claim 7 wherein said rectangular-shaped collimator is a Fresnel lens.

9. A projection system according to claim 7 wherein said rectangular collimator has a length to width aspect ratio of not greater than 2.