GB2388486A - Projection unit including light absorbing drape and extraction fan - Google Patents

Abstract

A rear projection unit (22) includes a screen (12) and a mount for a projector (20). A cavity is created by arranging a light absorbing material over the frame structure holding both. Any light (58) entering the cavity is partly absorbed by the material and partly directed to the rear of the cavity. A second light absorbing material (fig. 8) assists in the absorption of light within the cavity, while an extraction fan eliminates the optical effects of airborne dust within the cavity, giving a clearer and brighter projected image. A new material including holes for absorbing light is also disclosed.

Description

( 1 2388486

PROJECTION UNIT

F I ELD OF THE I EVENT I ON

The invention relates to a projection unit, and in particular to a rear projection unit.

5 BACKGROUND OF THE INNi-ENTION

For many years comp.nie., have required back/rear projection booths/ rooms to shown their Cr.'rporate Films, Slides and Videos to a Styler audiences. Trc;diionally these projection booths have been purpose built, 10 constructed from a selection of building materials, and these back project iC.T1 booths have been used at exhibitions, Station Pre;entaticus and "in house" in Training rooms, Boardrooms, etc. The requir_mer!t for the installai:io-i of B-ck,/rcar 15 projection booths/rooms are depenc. :lt on the application and the constraints on the space available. They are therefore usually designed and built or a "one off" basis and generally comprise of ?. number of disparate eler_nts combined together to form the booth. Ilese disparate so elements would normally comprise 4 walls, a ceiling and a floor, with a large hole cut in ore wall, to allow the insertion Or a rear projection screen, thrQug which the projected image is seen, a projector and lens, and some electronics including an electrical power feed and a :5 playback and sound system.

Located somewhere within the booth/room is a film, slide or Video Projector, usually mounted in a separate projector stand, sometimes directly pointing at the screen, sometimes indirectly via a mirror. Mirrors are -: sual'y..s-:lc, to -so -e the ccrect p-e,ec On

f 2 - distance, when the depth of the room is insufficient to allow for direct projection onto the screen. The projection distance being determined by the size of the image required, together with the projection lens being 5 used. These booths/rooms are generally square or rectangular. The projection screen surfaces are made from various translucent materials with varying tints in an attempt to improve contrast ratios sacrificing projected light as a result.

10 It is usual for film, slides anc7 video to be viewed in a darkened room just like the Cinema, thereby removing th possibility of any ambient light interfering with the projected image. The darkening of the room also has the effect of increasing the perceived brightnesEi and the IS contrast of the projected image becaus_ the pupil inside the eye Of the viewer, enlarges, to allow more light to fall on to the retina of th- eye, which increases the perceived brightness of the projected image to the viewer.

Perceived brightness to the fluman eye is very dependent on 20 the overall contrast between Black and White. As a general rule the blacker the blacks, the brighter the image to the eye.

Other than the purpose built rooms described above, the market of single integrated rear projection systems is 25 entirely confined to the sms.ll display market with vertical/upright scan In diagonals up to 1400mm. The mc,t: common vertical/upright sizes are in the 1000mm-1150mm screen diagonal range. However, and without: exception, all these integrated projection systems are indirect So projection systems and all feature vertical/upright screens, i.e. they all use varying types of optical mirros/reflection surfaces between the projector and the vertical/upright sore n surface

There are no direct projection systems within the integrated projection display markets. All other larger, direct or indirect, rear projection systems are found in very high cost, purpose built installations only, and 5 where proper viewing of the vertical/upright screens can only be properly viewed in low ambient light levels.

We have appreciated a number of problems with existing rear projection units.

First, inside buildings, the amount of ambient light from JO fixed position lighting being reflected off the front-

scrface of the screen into the e es of the assembled viewers, makes the projected image difficult, uncomfortable and in some instances impossible to view.

Outdoors, the s^.,,e problems exist:. to a much greater i, den ee due to the high daylight levels of ambient light.

Second, the amount of ambient from fixed position lighting and the general ingress of external natural a Client light.-, ii, the viewing area travailing through the translucent s rein into the "projection area" and which then travels around the projection ares. and back out with the projected image, results in "washing out" the proj ct.d image. T-

ne_ result is that the image is difficult, if not impossible to view. Inside a projection unit, a certain amount of the light from the projected im.gc is reflected 25 off the internal side of the translucent screen back into the internal "projection area". This additional, randomlv-

reflected, light mixes with the incoming ambient light and travels around the projection area passing out through the screen with the image, thus increasing the 'washing out,, 30 effect.

Third, vertical/upright screens, which are not centred at head height and in line with the assembled viewers eyes, create c_:,2_ie auu2tio..al prvUl_,.,3 du.c to -he "A

centreH viewing angles. These "off centre,' viewing angles create additional bright and dark screen areas (various known as hot and cold spots etc.) because the viewers eyes are not directly in alignment with the projector and the 5 projected image, being always offset due to the fact that the screen is in the vertical/upright plane.

Fourth, rear direct/indirect projection systems suffe-

from air borne atmospheric dus-./pollution which causes the optics to become coated with air borne grime which causes lo light loss, and further, the projected light that is emitted from the optics is caused to retract in the air between the point of exit from the projectio;: cpti.cs a-:d the rear of the screen material. This intern air refraction downgrades the clarity of the projected image-.

Is I.a_tly, in general, we have appreciated the need for a rear projection unit which does not require a purpose hu;1t room.

PRIMARY G F THE IEN1'I ON

The invention is defined in the independent claims to 20 which reference is directed. Preferred features are set-.

out in the dependent claim_.

in embodiment of the intention provid_- a rear projection unit comprising a screen for viewing from a front side and a fra,:e attached at the rear side of the screen, the frame 25 having a front portion proximate the screen and a rear portion away from the screen, the rear portion including a mounting for mounting a projector at a desired position in relation to the screen wherein the unit includes a light absorbing material arranged to define a main cavity within lo the frame bordered by the screen at one side and the light absorbing material on other sides and further comprising t-' eX't_':ir1 fell G'-1-G;-_=U C' U;-C- 44' '-ly a-_ c-v-e- a

- 5 - projector within the main cavity and to expel air from the cavity thereby drawing air into the main cavity through the light absorbing material, whereby the air passing over the projector is filtered through the light absorbing 5 material. This provides particular benefit in using the light absorbing material for light absorption and se' filtration. A cavity which absorbs light and is substantially free of dust particles is thereby produced.

These and other features are set out in the claims.

lo The embodiment provides a convenient projection unit in contrast to known purpose built booths/rooms. The use of a frame and a mounting is generally more convenient that providing a separate podium or mirror arrangement within such a purpose bui booth/room. The invention could be fitted to an existing back projection facility or be made as a new unit.

An embodiment of the invention also resides in a light absorbing material arranged to d.fine a main cc,.vit: within the frame bordered by the screen a'- one s de and the light so absorbing material Cal other sides. The light absorbing material further e.;sres extraneous light within the cavity is not emitted through t.!e screen. second light absorbing material defines an inner cavity w thin the main cavity. Within this embodiment, preferred features include the use of baffles and Additional light absorbing cavities. In particular, an idler cone of light absorbing cloth from the projector to the screen enhances light absorption. In an embodiment, the frame comprises a plurality of so configurable arms, the arms being moveable to change the position of the mounting for the projector with respect to the screen. This embodiment of the invention provides a

r - 6 - significant advantage in allowing the unit to be used with a variety of different projectors and screen sizes/shapes.

The arms can simply be configured as required to change the position of the projector. This embodiment includes s various further preferred features including pivotably mounting the arms, providing expendable arms, and particularly arranging the connector points at the rear of the arms to define a shape of the same aspect ratio as the connection points at the front. This ensures that the lo mounting attached at the rear c-tn keep the projector centrally with respect to the screen as the arms are varied in length, provided that the arms retrain Cf equal lunch. The unit can thus he enlarged or reduced without requiring recalibration.

15 An embodiment also includes a new light absorption arrangement. This arrs:,gement can be used in conjunction with any of the proceeding embodiments, or independently, for example in other projection systems. This eood2ment comprises a light absorption arrangement comprising a so generally light at orbing material having a plurality o first openings on a first surface each communicating with a second smaller opening in.. second surface, and having an internal surface between each first and second op ning, whereby at least a portion of light impinging on the first Is surface passes one or more first openings, is partially absorbed and partially reflected on the internal surface and directed by the internal surface out of the second smaller opening.

In essence, the material forms a light trap to absorb as lo much light as possible, and direct any unabsorbed light away from the first surface. The material can be arranged to form a cavity with the first surface on the inner side, thereby absorbing light within the cavity and directing unabsorbed light out of the cavity. Preferably, each

- 7 first opening, second opening and internal surface forms a generally conical shape.

In an embodiment, the frame tapers from the front portion to the rear portion. This shape in itself provides an 5 advantage that extraneous light within the frame is directed generally towards the rear portion away from the screen by reflection within the cavity defined by the frame. In addition, as light is directed in this way, there will be multiple reflections=, at each of which some ]ig.t will be- absorhec-l thereby reduci:;g the extcr?.neous lig;t within the cc-iiy of the frame. laths light absorbing charac:Leristic of Ah.- shape is also enhanced t!y further features.

Be, EF DLCRIPI-'ON OF THE FIGURES 5 An embodiTnent of the invention will now be described, by way of example only, in which: Figure 1: is a perspec.i-;e view of a rear projection from_; Figure la: is a cross section of a ball joins To as used in the frame of Figure 1; Figure lb: is a cross section of the ball joint of Figure la received in its housing; figure lc: is a plan view of a shelf support; 2s Figure Id: is a cross section view of a shelf support; Figure 2: is a schematic view of an expendable frame member as used in the frame of Figure 1; 30 Figure 2a: is a cross section of an adjustable arm as used in Figure 2; Figure 2b: is a cross section of a slide used in Figure 2; yue 2.: -'nets a-. i. sc-. as uses. -A igu-e 2;

- 8 Figure 3: is a cross section of the projection frame with cross braces; Figure 4: shows a series of side elevations with the frame in different 5 configurations; Figure 5: shows a further series of side elevations showing adjustment of the length of the frame; Figure 6: shows the light absorption 10 arrangement.; Figure 7: shows additional light absorption baffles; Figure 8: shows a further light absorption arrangement; :5 Figure 9: shows the.- air filtration system embodying the invention; Figure 10: shows a projection frame in use; and Figure 11: snows a further light absorption arrangement. 20 DESCRI PTION OF AN EMBODIMENT

The main components of a rear projection unit embodying the invention are shown in Figure 1. This comprises a screen 12 which can be any generally translucent back-

proje_tion screen which can be a double glazing stile with s ant.irelective surface acrylic tinted mounted alone, or in conjunction with a soft PVC type mounted behind. Other possibilities include using the screen as a flat panel style loudspeaker. These are all within the scope of the invention. 30 The screen 12 is fitted within a surround 10 which comprises a light weight modular, constructional section aluminium anodised structure which can be made/adjusted to create any shape or size of screen such as, triangular, S"=, reck_:-_ Or, hef1 eta.; Atari it -her.

r ratio. The surround 10 is completely modular having quick release corners which are mitred and clamped. The sides of the screen surround 10 have provision for supporting hanging eyes to be inserted to allow for the suspension of s the unit above/below an assembled viewing audience, which when used in conjunction with a rear hanging eye allow screen surface to be angled dcwn/up towards the viewing audience. This feature allows the unit to be mounted to achieve "on centre" viewing angles, and the offset angle lo of the screen surface to all sources of localized ambient light eliminates/minimise screen reflection as would have otherwise been seen by the Viewer. Such ambient Light reflections would have had the effect of washing out the image, so that it would not have been see;: by any viewer 5 of the screen.

Supporting the screen surround 10 is a structural foundation 11. This is a light weight modular, rear screen constructional aluminium foundation supporting framework which is attached all the wary round the rear of JO the screen surround. This supporting framework for,ms the constructional foundation of the unit and the remaining rear framework of the unit is attached at the corners thereof. A rectangular screen has 4 corners and attachments, a triangular 3, a hexagon 6 and so on. The Us design is also self supporting to ensure that components of the system cannot become dislodged and dangerous. The sides of the supporting framework screen have provision for the attachment of supporting legs (not shown'. Thes.-

attachments provide supporting leg swivels to allow the so screen to be angled down/up towards the viewing audience, when also supported by a rear leg. The screen can also be angled via a combination of either 2 front legs with the rear hanging, or with the front hanging and the rear supported by a single supporting leg.

- 10 The projection unit has a frame 26 having a plurality of arms, in this case a pair of upper arms 24 and a pair of lower arms 22. The frame could be arranged with planer sections, but arms are preferred for the advantage of 5 configurability. The frame 26 has a front portion 27 nearer the screen and a rear portion 28. The front portion 27 is attached to the screen foundation 11 itself supporting the screen 12.

Ur.2.ikc any other systems available today the frame 26 is a lo cone/pyramid shape, with the rear portion 28 tape-- no from th front portion 27. This is created by the 4 framework arms 22, 2 required, (4 on a rectangular screen, 3 c: a triangular screen and so on), from the 4 corners of the screen sue round 10, and has been specifically designed to ensure that at all tiffed, all the incoming ambient light/d_.ylight entering through the front projection screen 12, becomes completely directional by being constantly reflected (in one direction only) down "h. -

inside of the framework system towards the rear of the JO framework and thereby abso-rced. The cone/pyraiid shape of the real- framework has also been specif.i.cally designed to en,.re that at all ti,._s, and and all of the Projected image/iight being reflected off the inside surface of the projection s re-n also becomes completely directional by s being constantly refl.--cted (in one direction only) down the inside of the framework system towards the rear of the framework and absorbed.

The shape in itself thus provides the advantage of reducing the problem of light from within the unit So emitting from the front screen other than the intended picture. A variety of shapes in which the rear portion 28 is tapered from the front portion 27 are possible, but a square based truncated pyramid is preferred.

f - 11 -

The frame includes ball and socket joints 16 at front 14 and rear 18 corners attaching the frame 26 at the front to the screen foundation 11, and at the rear to a rear section 20. The quick release ball and socket joints are 5 further shown in Figures l(a) and (b).

The modular quick release, Remountable, ball joints are positioned at each corner of the light weight modular, rear screen constructional foundation, framework. It is these ball joints that allow the rear framework to 10 function as an adjustable and very flexible cone. This feature allows complete varia ion n the three dimensional position of the all arms' arlales to th* front screen, as any change in projection distance (length) or in the size of the r a- projection platform necessitates that th 15 three dimensional position of all the framework arms is changed. This feature effectively facilitates the us* of any type of projector with any type focal length lens, or configuration of lenses, or the I. -?. of multiple projector set ups, multi-media, etc. so The modular adjustable arms 23, 24 of rear screen framework run back from the screen surround foundation 11, to a rear projection platform. These adjustable supporting framework arms are attached at rear projection platform by quick release ball joints in the case of 25 screen centre projection systems, or by a combination of ball joints or ball hiryes for offset projection systems.

The rear section 20 is shown in more detail in Figures lo and Id. The rear section 20 comprises a vertical panel to which a shelf 21 is attached protruding into the cavity of So the unit. The shelf 21 is supported at the front by a cross support 19 which itself is attached to studding 17 supported at each corner by attachment to the ball joints 1B. The cross support 19 is sliAably attached to the

- 12 studding 17 to allow the shelf to be positioned as necessary to support a projector.

The ball joints 16 shown in Figures l(a) and (b) comprise a ball head 30 and ball stem 32. The ball head is 5 received in a ball housing 34 such that the ball stem 32 is displaceable through an angle, such as 30 from one side to another in any direction. The ball coupling includes a slidable shield 36 mounted on a section 38 of the foundation 11 (at the front) or rear section 20 (at 10 the rear).

The arms themselves of the frame are further shown in Figure 2. Tie modular adjustable arms 22, 24 of the framework 26 cater for different projection distances as dicto.-ed by the ph-y-sical characteristics of s.:-een sine and a-:iect ratio, together with the lees type and focal length. The adjustable arms work on the principle of releasing a locked internal aluminium slide 42 which is locked within the special rectangular aluminium arm section 40, which in moving out/in expands/reduces the A. overall length of thin adjustable arm. Insert- 44 ire used to extend the arms.

To maintain the overall strength and clean lines of the unit, Sections of the special rectangular aluminium arm 46 are re-inserted/renoved from in th- gap from a f_xe:: ens' 48 created by the movement of the sliding section. Onc--

the desired length of the adjustable arm has been fixed the slide together with the re-inserted/removed sections are re-locked using a handle 50, thereby creating a solid structural component of the correct length.

So It should be noted that the keeping of all modular arms to the same length wi11 automatically ensure that any projector mounted on an appropriate rear projection Platform Cal section 20, which r.as the same aspect ratio oL

- 13 the projection screen, will effectively place the projectors lens at the horizontal and vertical "screen dead centre". The ability to extend/retract the modular arms facilities the use of all types of projectors, whether of the "screen dead centre" variety or those projection systems that have been designed to be offset, but require a vertical screen centre. The frame structure of upper 24 and lower 22 arms could be sufficient to support the screen and projector. However, further lo bracing is preferred, as shown in Figure 3.

A supporting, and locking framework is required fo' c.intaining the strength, stability and particularly the correct position of all the adjustable arms, as this ensures that all supportinc3 components are kept precisely s in the same plane, and in square to the front screen.

Even if the adjustable arm- are of the same length they will only guarantee to place: the projector le:ls at the screen d-ad contra and to provide true alignment if their are -n square with the front projection screen, i.e. the to rear projecticn platform mounting is perpendicular to the projection screen, and the rear face or vertical section 20 of the projection platform is upright and parallel to the projection screen. Out of square arms, whilst being of the same length, can place the projector lens out of 25 true alignment. Thus stabilising brace arms 52, 54 are provided. Each positioning and stabilizing arm 52, 54 creates an adjustable and locking "V" shaped cross brace, which joins 2 adjacent adjustable arms 22, 24. Usually Lucre is 1 So supporting "V" brace for each adjustable arm forming the pyramid type shape frame 26. Each adjustable positioning and stabilizing arm comprises a centre plate 56 which can be locked via a Kipp handle 50/or other type of positionable handle, in any appropriate position on any 3: adjustable ar.,. c'. each adJus'ablc cc,.te Plate a-e 2

- 14 swivel arms, attached by screws, and located at the ends of which are attached further smaller positioning plates 56 with locking handles 50.

One such adjustable and lock V" shaped arm is fitted to 5 each side of the pyramid with the centre plates and 2 positioning plates being locked onto the 2 side arms via a Kipp handle/or other type of positionable handle. This action has the effect of creating parallel arms to the sides of the pyramid. Once all sides of the pyramid have lo been locked the pyramid becomes a solid structure. Th* adjustment flexibility of the "V" sh-:?ed positioning and stabilizing arms will always fit the pyramid framework 26 irresp dive of how much the modular adjustable arms 22, 24 of the framework have beer adjusted. It should be 15 noted that every adjustment in the length of the adjustable arm creates a different rear screen angle for each adjustable arm, and this positioning and stabilisiry arm Solves this problem.

The V'' shaped locking arms will also fit all units sit up so for offset Irojectors or those systems housing 2 or more projectors. Additional arm support could also be provided (not shown) located tow rds the very rear of the unit are 2 or more vertical platform supports which are attached to the :s adjustable arms. These supports comprise adjustable length studding to cater for the different angles and variances in distances caused when an adjustable arms is extended or reduced. This vertical studding is attached to the arms via quick release ball joints. The vertical so studding allows for the use of screen centre projectors as well as those projectors that are offset. The purpose of this studding is to provide additional vertical stability at this point of the pyramid whilst also providing the rOIlt TT;i_ _ s-..crts ' _-: t.c p'-^j CtiO,1 Fat tfm'^m; A,

( - 15 addition the studding also provides for a left and right levelling adjustment of the projection platform, so that the projection platform remains parallel with the screen surround. 5 Additional mounting points of studding could also be built into the system to allow for the use of additional projectors within the same system. Also the mounting points for the studd-ng/shelf support for projection platform, allow for the svetem to be used on it's side or lo upside down. When the screen is in a standard normal rectangular landscape format and when it is subsequently turned on it's side it crf.--.t::- a portrait or poster type a format projection.

A projector may be mounted on a projection platform (not -id shown) which itself is fixed to the rear section 20. The -

rer section 20 is a pc.lel which has the same aspect ratio a the front screen, but other configurations and sizes could be used. To obt.-in the benefit Of automatic centering of the projects_, it is simply required that then so rear section 20 maintains the b.-.11 joints of the rear portions of the arms 28 in the same aspect ratio as the ball joints of the front portions off the arms 27.

The projection platform thus forms the rear of the unit and comprises a rear vertical section 20 arid projection -

:5 platform on to which projectors) are mounted. The rear unit is attached to the adjustable arms via quick release ball joints. The shape, aspect ratio of the rear vertical section is usually in the same aspect ratio as the front -

projection screen albeit being much smaller in size, as so this helps to ensure that the projector's lens is always situated at the "screen dead centre", with those projectors that require this feature, or is the vertical 5 centre for offset projectors. The front of the projection -

platform is attached to the framework via a slotted h1 r

- 16 support which attaches to the vertical studding and which provides the necessary levelling requirement for every projector. A projection platform may have a number of projection shelves, either the normal horizontal shelf or shelves, and/or vertical shelves to allow for the installation of any number of projectors 2, 3, 4 or more projectors can be housed within the rear system.

The projector(s) are bolted to the projection platforms via c swivel adjusts: It. Once the projectors are bolted lo in position it may be necessary, once they have been switched on, to move the projected image left or right to fit the screen. This is no; a problem asrioci..ted with focus but a problem cssoci.ted witty optics mounted within the projector, as even small misalignments will appeal out 15 true with the screen.

The 3 dimensional adjustmeri!-s of the projected image Al.: achieved by: l) the swivel adjustment which allows led- and right adjustment, so 2) the shelf studding adjustment which allows top to bottom image shift, 3) the shelf studding adjustment also Allows parallel image sh it to precis, ely match the image to the screen.

I, Examples of the configuration of the frame are shown in Figures 4 and 5.

The benefits of the generally tapered pyramid shape of the frame in terms of light collection are shown in Figure 6.

Any light 58 entering the front of the screen 12 is 30 directed towards the back of the unit towards the rear vertical section 20. This light direction is by a covering 60 which is supported by the frame 26 hic2

( - 17 thereby defines an enclosure. The covering is preferably a stretchy material which is stretched over the frame as -

this assists in changing the size and angles of the frame arms. The material can be stretched over the outside, or 5 supported on the inside of the frame. As shown in Figure 7, both an exterior covering 22 and interior 62 may be used. A significant portion of all incoming ambient light is absorbed by the internal light absorbing system. This lo achieved through the pyramid shape of the unit in conjunction With light absorbing materials including the covering. The shape of the unit ensures thatany/a31 reflected light entering through the translucent screen, is automatically 15 passed to the back of the unit. To assist- in the trapping and absorption of ail the incoming light a number of light -

absorbing baffles 64 are placed in front of, and along the length of the internal black fabric, along a;1 four sided of the pyramid. Those baffles, which Ease att:ached via so Velcro comprise similar strips of black soft material which are placed randomly or regularly apart, along and all around the entire internal length of the unit. Thus all light being entering the unit is either reflected off the internal wall of the unit into a baffle, or is 25 reflected back off the baffle towards th. back of th unit, striking another wall and thus becoming trapped inside that baffle. At each surface, a portion of the light is absorbed so that the shape and baffles increase -

the amount of light absorbed.

30 As shown in Figure 8, an additional light absorbing cone 66 is also used to encourage and ensure further light absorption. This additional light absorbing cone comprises a black perforated light weight black material -nick. is so __hd avid lung along a'1 tic p-a,-d,;.-2ls,

to form a further smaller projection cone, which runs from the front of the lens of the projector to the back of the screen. This perforated cone is supported by elasticated wire, with all perforated walls terminating close to the 5 lens cowling of the projector(s). This perforated material allows light to pass out of the direct projection area into the outer layer of the light absorbing material.

The translucent nature of the black perforated material allows light to pass into the light absQrhina waste area.

lo Tile black material creates a shadow reflection which helps to increase the overall rate of light absorption within the baffle area. Thus the increase in licit absorption within the unit improves the overall contrast ratio and picture quality.

s According to the inventor, I.. further significant advantage is provided by combining the fabric covered unit with an air cooling fan arranged to draw air over the projector and out of the unit as stow. in Figure 9. Since the entire unit is covered in a special black material which JO acts as fine air filter, all incoming air entering the unit from the outside is clean of all particles. The exhaust creates a low pressure area within the unit, and therefore air is drawn naturally into the unit via its large surface.

25 This clean air within the internal projection area not only ensure that there is no refraction of the projected light during its internal travel through the air to the screen by dust particles, but also ensures that perfectly clean air is always drawn into the projector(s) and this so clean air more efficiently cools the optical system and projector lamps without depositing any detrimental material.

t - 19 Although the material on the inside of the cover is always a soft dense black, the outside may be white or any other selected colour. The outside of the material used on outdoor systems is waterproof.

5 Lastly, a unit in use is shown in Figure lo which shows how the unit may be suspended from hanging points 68, or-

supported by leg supporting points 70.

A further light absorbing arrangement is shown in Figure ll. This could be used in addition to, or in place of, lo the inner light absorbing cone 66. This comprises a black material 72 such as polystyrene or foam clot with a -pries of entry apertures 74, each leading to a generally cortical light trap with an exit aperture 76, which is smaller t;-ia the entry aperture.

15 For purposes of identification I have named this new material the Lighteraser. Each Lighteraser is cons ruc-^-d from thick sections of either polystyrene boards (75; to 305mm)jor similar light weight materials such as foam À c.

All boards are black.

A These Lighteraser boards can be.^-d with, or can replace the black perforated light weight fabric their. formed the additional internal light cone, the boards being placed in the same position, running from the screen, at the front of the unit! back to the projection lens.

:s Like the light weight perforated material, the 4 light weight Lighteraser boards form the same basic internal cone/shaped many sided internal pyramid.

Each Lighteraser is constructed from a thick black polystyrene sheet of a chosen thickness, within which are So constructed a series of one-way Lightblots, each LightUlot being i..ternally, am. off axis cone shaped tube, he lea a

- 20 light entry point 74 and a light exit point 76 at the LighUgate. Each LighLblot effectively provides a one way Black Hole, any unabsorbed light exiting from the LighLblot at the Lightgate. Each LighLblot comprises a circular round opening, or ellipse on the front edge of the Lighteraser, and is of a chosen diameter or cross section. As the "off axis'' LighLblot passes through the board the diameter decreases at a chosen rate, to form an off axis cone which lo will absorb light. Any unabsorted light exiting at the "Lightgate", a hole located the other end of the LighLblot. The Lightcgate is also of a chosen reduced diameter or ellipse.

The Lighth,lots are not cut at 90 angle to the surface but 15 are cut at any "off axis,, diagonal angle of between 10 and DO , (perpendicular) to the surface.

The Lighteraser is placed with the LighLblot Lightgates facing outwards towards the light absorbing zone and backwards towards the Projector.

: Thus any incoming light will be reflected off the inside surface of one of the LighLblots, being reflected all the way down the cone shape of the LighUblot, with any unabsorbed light exiting at the small LighEgate (hole) , pass ng directly into the light absorbing area.

25 Alternatively, any incoming light will be reflected off the Lighteraser to the opposite Lighteraser, and will either pass into a Lighthlot or will again be reflected across, eventually passing into a Lighthlot.

Because all the exit points at the LighEgates are small so round and elliptical holes, all facing towards the rear of the unit, none of the focussed light exiting a LighUgate If!! Lie 2:ec'eu vac'- - arc, c- A? ano..er In-_.- exit

( - 21 hole. Thus all light is completely trapped and absorbed in the light absorbing zone.

To further ensure that no light can be reflected into a LighUgate there is a small raised mound to the front of 5 the LighUgate, with a minimum equivalent height equal to the width of the Lightgate. This raised mound acts as a type of awning protecting the gate from any reflected light, and ensuring that no reflected light can enter back through the gate.

Claims (5)

( - 22 CLAIMS

1. A rear projection unit comprising a screen for viewing from a front side and a frame attached at the rear side of the screen, the frame having a front 5 portion proximate the screen and a rear portion away from the screen, the rear portion including a mounting for mounting a projector at a desired position in relation to the screen, wherein the unit includes a light absorbing Material arranged to lo define a main cavity with.n the f ame bvrd=red by the screen at one side and the light absorbing material on other sidles and further cc.mpricing an e:trac'.ion fan arranged to draw cooling air over a projector within the main cavity Id to expel air from the 15 cavity thereby drawing air into t-* main cavity through the light acc;orhing material, whereby the air passing over the projector is filtered through the light absorbing material

2. A projection: unit acco: cling to claim 1, wherein the so light absorbing material comprises a cloth.

3. A projection unit according to claim 2, where),! the cloth comprises a dark inner side.

4. A projection unit a cording to any of claims 1, 2 or 3, further comprising one or more baffles arranged within the cavity for absorbing extraneous l ght and arranged away from the cone of light produced by a projector so as not to impede projection.

5. A projection unit according to claim 4, wherein the one or more baffles are attached to the inside of the 30 frame.

- 23 5. A projection unit substantially as herein described and with reference to the figures.