Acoustically transparent projection screen

Abstract

An acoustically transparent projection screen material consisting in a warp knit (such as tricot or simplex) textile fabric, made of synthetic yarn characterised in that it is dyed in a light grey colour having a value between 230 and 170. The knit gauge of the fabric is preferably superior to 24. The screen structure facilitates ambient light rejection (ALR) and enhances reflection of light striking it at an incidence of 30 to 60 degrees for use with Ultra-Short Throw (UST) projectors.

Description

Background of the invention Projection screens are widely used in Home Cinema installations, in combination with video projectors. In a typical installation, there is a video projector, connected to one or more source(s) of video signal, located at one end of the room in which the installation is made, whereas the projection screen is facing the projector at the opposite end. It is desirable that the projection screen is transparent to sound waves. The reason is that, for a good coherence between the sound and the image, important parts of the soundtrack should appear to come from the image projected onto the screen. To achieve this, some of the loudspeakers allotted to the soundtrack playback need to be located behind the screen. Such a layout is mandatory in commercial cinemas and is widely used for home cinema installations as well. To enable such installations, the screen is designed to let the sound waves pass through with as little attenuation as possible. Typically, such screens are designated as acoustically transparent and are either perforated plastic sheets or textile sheets, either woven or knitted. Another desirable characteristic of projection screens is called Ambient Light Rejection, or ALR. A recurrent problem with installing a projection system in an ordinary room is the presence of ambient light that interferes with the projected light and reduces its contrast to a level that seriously degrades the overall image quality. More precisely, the ambient light being reflected together with the light coming from the projector reduces the contrast by reducing the black level -which is the absence of reflected light. ALR screens are typically directive, better reflecting incident light from a definite direction than light coming from any other direction. The favourited direction is the one where the axis of projection from the projector is, generally perpendicular to the screen plane. Such ALR screens are generally made of a plastic sheet provided with an optical coating that creates the directivity. They can be made acoustically transparent with multiple perforations. To facilitate home cinema installations in ordinary living rooms, a new type of projector has been developed recently and provide the convenience of being installed very near to the projection screen, either above or below it. Instead of having a projection axis comprised between 0 degrees and 15 degrees, like classic projectors, these have a projection axis comprised between 30 and 60 degrees. Such projectors are called Ultra-Short Throw projectors, or UST. As UST projectors are mainly used in ordinary living rooms with no or limited control over the ambient light condition, a good contrast performance can only be obtained with the use of ALR screens. However, due to their specific projection axis, ordinary ALR screens cannot be used because they are designed to reject light coming with an incident angle superior to the typical projection angle of classic projectors. Specific screens have been designed to address this issue, with a dissymmetric directivity, reflecting mainly light with an incident angle comprised between 30 and 60 degrees from one direction only. Generally, such screens have a structure of a Fresnel prism. This structure is carved in the screen material, that needs to be significantly thicker than the classic screens. The nature and thickness of a material that can be carved to provide a Fresnel prism structure prohibits the required multiple tiny perforations for making it acoustically transparent. The object of the present invention is a projection screen material that overcomes this limitation. More precisely, the object of the present invention is a projection screen material that is acoustically transparent, enhances reflection of the light coming with a 30 to 60 degrees incidence with respect to the screen plane. Further, a projection screen material according to the present invention reduces the reflection of the ambient light coming from other angles of incidence. According to a preferred embodiment of the invention, the projection screen material is a knitted fabric made of polyester. The fabric is dyed in a light grey colour. The value of the grey on a grey scale which range is from 0 (black) to 250 (white) is comprised between 170 and 230. Preferably, the structure of the fabric is a warp knit. The features and advantages of the present invention will be better understood on reading the following non-limitative detailed description, with reference to the accompanying drawings in which: Figure 1 represents a classic configuration of a projection installation Figure 2 represents an ultra-short throw (UST) projection installation Figure 3 represents an enlarged cut view of a typical projection screen dedicated to UST projection Figure 4 represents the structure of a projection screen material according to a first embodiment of the present invention Figure 5 represents the structure of a projection screen material according to a second embodiment of the present invention Figure 6 represents an installation comprising a projection screen according to the invention with a loudspeaker placed behind it Referring to Fig. 1, it is schematically represented a video projection installation comprising a video projector 12 placed above and behind a spectator 10. The video projector 12 is directing its light output in a beam at a projection screen 11, which is substantially vertical and facing said beam. The main axis of the projected light beam is approximately normal to the plane of the projection screen 11, or with an angle within + / - 20 degrees with respect to the direction normal to the screen plane. Such an installation is of the most common type and is often referred to as long throw projection. Fig.2 represents another type of installation comprising another type of projector 13 designated as Ultra-Short Throw, or UST. It differs from the installation illustrated in Fig.l as the projector 13 is of a type dedicated to said UST application, having a light beam projection axis approximately comprised between 30 to 60 degrees with respect to the screen 11. Such a projector design makes it suited to sit very near to the screen 11, either at the bottom (as represented) or at the top of it. This type of installation is easier to install than the one illustrated in Fig.l and is generally more suited for living rooms or non-dedicated spaces. Quite often, such spaces do not provide controlled lightning, and the ambient light can interfere with the image on the screen. To alleviate this problem, the screens used in said UST projection are often of a specific type represented at Fig.3. Referring to Fig 3., the screen 11 comprises at least a front surface made of a Fresnel prism 15. This Fresnel prism exhibits a sawtooth-like profile comprising a plurality of prisms joined and parallel to each other, arranged either as a straight horizontal array, double cross arrays or in semi-circular arrays, according to the design choice. The prisms are reflective on the side which is angled approximately 45 degrees from the general screen plane, whereas they exhibit a side which is approximately normal to the general screen plane and that is coated with a black light-absorbent material 16. The step of the Fresnel prism 15 is made smaller than one pixel, so that its structure is not visible during projection. Generally, the Fresnel prism 15 is carved in the screen material, or in the said at least front surface. This favours the reflection of the light coming from below the screen at an angle comprised between approximately 30 and 60 degrees, whereas the light coming from other directions, especially when the Fresnel prism is arranged in a semi-circular pattern, is mainly absorbed. Such screens have been so far considered as the state-of-the-art in UST projection screens. However, they have the limitation of not being acoustically transparent, as they are carved in a solid material, typically a sheet of plastic. This prohibits the correct placement of loudspeakers behind it. The present invention is an acoustically transparent screen material that is also enhancing the light reflection when the light incidence direction is in a range comprised between 30 degrees and 60 degrees with respect to the screen plane. Further, the light reflection is attenuated when its incident direction is outside of said range. A preferred embodiment of the invention is a stretched fabric screen (Figs. 4 and 5), preferably of a knitted structure. It is preferably made of polyester or another synthetic fibre, like viscose for example. In a first embodiment of the invention, the structure of the knit is known as warp-knit. Two examples of warp-knit structures are represented in Figs. 4 and 5. The Fig.4 represents a warp-knit structure known as Tricot. The Fig. 5 represents a warp-knit structure known as Simplex. Other knitted structures are also within the scope of the present invention. The knit gauge, defining the fineness of the said fabric is preferably superior to 24. In the preferred embodiment of the invention, the knit gauge is 32. The advantage of a screen made of a knitted fabric is its permeability to air, hence allowing sound wave to pass through with little or negligible attenuation. This allows placing loudspeakers behind the screen, facilitating the coherence between the sound and the projected image. The inventors have discovered that, unexpectedly, a screen made of a knitted fabric reflects more light in the direction normal to the screen plane when dyed in light grey, provided the incident angle is approximately within the range of 30 degrees to 60 degrees. This is the typical incident angle of the projection axis of UST projectors. On the contrary, the light grey colour of the dyed fabric attenuates the light reflection by partially absorbing it when the incident light comes from directions that are outside the 30 to 60 degrees angle range. The limit, however, is progressive. The dye used in the preferred embodiment of the present invention is a disperse dye. The unexpected increase in light reflexion from a typical UST projector combines with the attenuation of light reflection when the light is coming from other incident angles, this being due to the grey colour which absorbs part of the light. In turn, this combination improves the ratio between the brightness of the projected image and the attenuated brightness of the unwanted reflected light from the environment. The overall contrast of the reflected image is directly proportional to this ratio. It is therefore improved with a projection screen according to the invention. Fig. 6 represents an installation of a video projection system using a projection screen 18 according to the present invention. A projector 13 is of the UST type, projecting an image onto the screen 18 with a projection axis comprised between 30 degrees and 60 degrees. Ideally, the projection axis should be approximately 45 degrees. The screen 18 according to the present invention is made of a warp-knit fabric dyed to a grey colour that is about 210 on a grey scale of a range from 0 to 250, 0 being black and 250 being white. This value of the dyed grey colour is non-limitative, and a grey dye of a value comprised between 230 and 170 is within the scope of the present invention. The screen 18 being permeable to air is acoustically transparent, allowing to place at least one loudspeaker 17 behind the screen aiming toward said spectator 10.

Claims

1. A projection screen material consisting in a textile fabric, acoustically transparent, made of synthetic yarn characterised in that it is dyed in a light grey colour having a value comprised between 230 and 170.

2. A projection screen material according to claim 1 characterised in that the dye is of the disperse type.

3. A projection screen material according to claim 1 characterised in that the yarn is polyester.

4. A projection screen material according to any one of the preceding claims, characterised in that it is knitted.

5. A projection screen material according to claim 4, characterised in that the knit gauge is superior to 24.

6. A projection screen material according to claims 4 to 5 characterised in that the knit is a warp-knit.

7. A projection screen material according to claims 4 to 5, characterised in that the knit is of a tricot structure.

8. A projection screen material according to any one of the claims 1 to 6, characterised in that the knit is of a simplex structure.

Images