GB2451895A

GB2451895A - Treatment of rear projection screens to avoid reflections

Info

Publication number: GB2451895A
Application number: GB0716083A
Authority: GB
Inventors: Kevin Michael Jarvis
Original assignee: EQUIPE ELECTRONICS Ltd
Current assignee: EQUIPE ELECTRONICS Ltd
Priority date: 2007-08-16
Filing date: 2007-08-16
Publication date: 2009-02-18
Anticipated expiration: 2027-08-16
Also published as: GB0716083D0; GB2451895B

Abstract

A method of treatment of a rear projection screen 1 in the construction of a collimating image display apparatus suitable for flight simulation employing digital projectors, in which the positions of the projectors are not constrained by the presence of reflected light from the inner surface of the rear projection screen. The treatment involves applying a surface covering in at least some areas with a paint or any amorphous liquid which once applied dries to a suitable coat scattering light in all directions. The coating may be applied as a layer of soft gel with a removable backing sheet, or the internal, or concave face may be treated using liquid abrasive paste or mechanical action.

Description

METHOD FOR USING DIGITAL PROJECTORS IN COLLIMATING

iMAGE DISPLAY APPARATUS [01] This invention relates to a method for the application of digital projectors, unrestricted in location, within a collimating image display apparatus commonly used for flight simulators.

The method overcomes problems with unwanted reflections that are particular to modern digital projectors.

[02] Projection of images by means of Cathode Ray Tube (CRT) projectors onto a curved rear projection screen, which is in turn viewed via a large approximately spherical mirror, is a well known and commonly used method of creating highly realistic scenes in flight simulators.

The rear projection screen is normally, but not restricted to an acrylic material, coated on the outside with a diffusing or other type of rear projection surface, but with a shiny and hence reflective surface on the inside or concave side. In a typical multi-projector system of 3 or more projectors, light reflected from the outer parts of the screen can illuminate other parts of the screen and cast an interference image on top of the desired image. With CRT projectors this is not a problem because the exit pupil of lenses used to focus the CRT image onto the rear projection screen is very large resulting in a small depth of focus for the image, which in turn leads to a very defocused reflection. This resulting soft focus reflected image is not then easily visible by the trainee pilot of the flight simulator although is does cause a reduction in contrast.

[03] Since CRT projectors are becoming increasingly replaced by the modern replacement digital projectors in many applications, a problem arises in the collimated display mentioned above in that the comparatively small exit lens pupil of the digital projector provides a corresponding significantly larger depth of field, further resulting in a sharply focussed reflected image where it strikes the inside of the curving rear projection screen. The distracting nature of this reflected image is further exacerbated by small perturbations in the inner screen surface that can focus the reflected images into bright spots and lines. This can give the appearance of "tree" like structures on top of the simulated image that can also cause significant distraction.

[041 The position of the reflections is dependent on the position of the projectors in relation to the screen. A typical system comprising three projectors and 180 degrees of horizontal viewing angle (figure 3.) will have reflections only resulting from the two side projectors, as reflections from the centre projector will be reflected back to the rear of the simulator where there is no screen. In the 180 degree viewing angle collimating display it is the outer regions of the two outer projectors that will cause reflected images. Early collimating systems were limited in horizontal field of view to 150 degrees (Figure 4.). In these it is possible to position projectors such that there is no reflection falling on the viewing area. Modern collimated displays extend horizontal viewing angles to 220 degrees or more and use 5 or more projectors (Figure 5.). In such displays there is no position to place a projector so as to avoid the reflections other than near the front of the rear projection screen (Figure 6.). Locating the projectors in positions as shown in figure 6. causes further problems. First, the light is not falling on the screen at or near to 90 degrees known as on-axis projection. This causes an attenuation in the direction that the observer is viewing that not only dims the image but also results in a poor blending interface with the adjacent projector that is sensitive to head movement and pilot position due to the gain characteristics of the rear projection screen. In a typical commercial airline cockpit with side-by-side pilot positions, this means that the balance in brightness between the two projectors seen by one pilot may be quite different to the other.

Second, The acute off-axis projection angle causes significant distortions, loss of resolution, brightness and possibly focus, and third, the location of projectors away from the rear edge of the screen is highly inconvenient for maintenance access in a typical flight simulator. A Fourth problem that can prevent the use of positioning projectors forward of the screen is. the effect on moving the centre of gravity has on some simulator motion systems. Some motion control.

configurations may not be able to provide full performance with projectors moved forward into this position.

[05] The aim of this invention is to define a method by which to overcome the reflections resulting from digital projectors with small exit lens pupils when arranged in conventional and convenient locations for maximum performance as well as convenience, in relation to a typical reflective acrylic rear projection screen within a wide viewing angle flight simulator image display system. The invention is not limited to flight simulators as it applies to a wide range of training devices for land vehicles, trains, ships, submarines and any other vehicle or craft.

[06] An embodiment of the invention is now described with reference to the accompanying drawing showing the main functional blocks in which: -[07] Figure 1. Illustrates a localised area 2 of a typical rear projection screen 1 in which surface treatment is applied and includes a border area 4 where this treatment is feathered into the untreated area.

[08] Figure 2. illustrates the main components of a typical collimating display system, namely Projector 10, Rear Projection Screen 1 and Collimating Mirror 11. They are shown in relation to a flight simulator cockpit 12 [09] Figure 3. illustrates the light paths in the boundaries of each projector beam for a typical 3 projector, 180 degree viewing angle collimating image display system within the confines of the rear projection screen 1. Projectors 10 are also shown.

1101 Figure 4. illustrates the light paths in the boundaries of each projector beam for a typical 3 projector, 150 degree viewing angle collimating image display system within the confines of the rear projection screen.

[11] Figure 5. illustrates the light paths in the boundaries of each projector beam for a typical 5 projector, 220 degree viewing angle collimating image display system within the confines of the rear projection screen. Lf

(12] Figure 6. illustrates the ligbt paths in the boundaries of each projector beam for a 5 projector, 220 degree viewing angle collimating image display system within the confines of the rear projection screen 1, with two of the projectors 15 relocated forward to avoid reflections.

LI3J Figure íA & 113. Illustrate rays ot light 21) alter (A) retlection trom au untreated acrylic surface 30 and (B) a treated acrylic surface 31.

[14] Referring to Figure 1, the method of applying this invention to the rear projection screen (1) in the collimating image display system of a typical flight simulator is described in the following steps: 1. An area 2 of the screen 1 from which reflections emanate is calculated and the area boundary 3 transferred onto the inside of the screen using non-permanent marker pen.

2. A border area 4 subtending typically 3-5 degrees of angle from the pilots viewpoint is then also marked onto the screen in non-permanent ink by the outer boundary (5) 3. This first defined area 2 is then treated in a way that changes the inner surface so as to scatter the light uniformly in a wide range of directions. The effect of this is illustrated in Figure 7B, by the ray 20, after striking a treated surface 31 is scattered quite uniformly into many different directions 23, thus eliminating direct reflections from the shiny surface depicted in Figure 7A, were ray 20 is reflected in single direction 21.

This treatment may take various forms, including but not limited to, an amorphous liquid application, laminates such as film gel applied from a sheet, chemical etching, mechanical abrasion using liquid paste and mechanical abrasion using solid materials.

The level of treatment is quite critical as too much of any of the above methods will lead to reduction in focus of the image and even light reduction if applied in excess.

Only enough treatment sufficient to remove the visible cross reflections is necessary, any more will create the detrimental effect described above.

S

4. The border area 4 is treated in a similar way to 2 above but the treatment is feathered between maximum treatment at boundary 3 down to minimum treatment at boundary 5.

The requirement for the feathering area is dependent on the quality of the treatment and the skill with which it is applied. If the quality of treatment in area 2 is of the highest order then the need for the border area 4 is practically none. If however the quality of treatment is such that the boundary of area 2 has a visible effect on the final display then the width of the boundary area 4 must be chosen accordingly. (2

Claims

1. A method of preventing unwanted interference of reflected light within a curved rear projection screen, when using multiple digital projection devices as projection light sources, by means of treatment of the internal concave surface of the said screen to scatter light uniformly in all directions, so allowing the projecting light sources to be placed in optimum favourable positions relative to the projection screen.

2. A method of construction as in claim 1, where a calculated area alone is treated for its reflective properties.

3. A method of construction as in claim I and 2, where an additional border area is used with feathered or graded treatment in order to provide a smooth transition between treated and untreated areas.

4. A method of construction as in claim 1, where the whole internal or concave face of a rear projection screen is treated for its reflective properties.

5. A method of construction as in claim 1, whereby treatment of the internal or concave face of a curved rear projection screen is applied by a formulated paint or any amorphous liquid that once applied dries to a suitable coat scattering incident light in directions.

6. A method of construction as in claim 1, whereby treatment of the internal or concave face of a curved rear projection screen is in the form of application of a layer of soft gel applied by means of a preparation attached to a backing sheet that is then removed after application.

7. A method of construction as in claim 1, whereby treatment of the internal or concave face of a curved rear projection screen is applied by a chemical means of etching, erosion or by any other reaction with the surface material.

8. A method of construction as in claim 1, whereby treatment of the internal or concave face of a curved rear projection screen is applied by liquid abrasive paste and mechanical action.

9. A method of construction as in claim 1, whereby treatment of the internal or concave face of a curved rear projection screen is applied mechanical action using solid abrasive materials.

10. A method of construction as in claim 1, whereby the projectors used are of a type including but not exclusive to DLP, DMD, LCD and LCOS.

11. A method of preventing reflected light substantially as described herein with reference to figures 1-7 of the accompanying drawing.