Dark-coloured or black projection screen

Abstract

An article used as black or dark-colored front-projection screen includes two distinct diffusely reflecting scattering elements, taking the form of parallel sheets or planes, qualified first and second scattering elements, wherein the first scattering element is translucent or transparent and the second scattering element is of dark color.

Claims

1. An article used as front-projection screen comprising first and second scattering elements, which are two distinct diffusely reflecting scattering elements, taking the form of parallel sheets or planes, wherein: the first scattering element is translucent or transparent and comprises at least one textured surface and is chosen from a textured substrate or a specularly transmitting and diffusely reflecting transparent layered element comprising: two transparent external layers having substantially the same refractive index and each having a smooth external main surface, and a central layer intermediate between the external layers, the central layer comprising at least one transparent layer of refractive index different from that of the external layers or a metal layer, all contact surfaces between two adjacent layers of the layered element, one of the two adjacent layers being a metal layer or the two adjacent layers being two transparent layers of different refractive indices, being textured and parallel to one another, a root-mean-square slope Rdq of the profile of the textured surface is strictly higher than 0.2°, and the second scattering element is of dark color.

2. The article as claimed in claim 1, wherein the article has a black luminance lower than 30 cd/m.sup.2 measured in an environment illuminated to 350 LUX in the privileged scattering direction, the specular being excluded, on the side of the first scattering element.

3. The article as claimed in claim 1, wherein the article has a white luminance higher than 100 cd/m.sup.2 measured in an environment illuminated to 350 LUX in the privileged scattering direction, the specular being excluded, on the side of the first scattering element.

4. The article as claimed in claim 1, wherein the second scattering element has a uniform diffuse reflection.

5. The article as claimed in claim 1, wherein the first scattering element has a nonuniform diffuse reflection.

6. The article as claimed in claim 5, wherein the diffuse luminous reflection of the article, measured on the side of the first scattering element, has at least one maximum in a direction distinct from the direction of specular reflection.

7. The article as claimed in claim 1, wherein the article has maximum variations in normalized gain as a function of angle of observation, measured on the side of the first scattering element, between 0° and 60° with respect to the normal to the screen, that are: in absolute value, larger than 0.2, and in percent, larger than 30%.

8. The article as claimed in claim 1, wherein an external layer of the layered element is formed by a textured substrate chosen from polymers, glasses or ceramics comprising at least one textured surface.

9. The article as claimed in claim 1, wherein the central layer of the layered element is obtained by depositing, by cathode sputtering, a single layer or a stack of layers.

10. The article as claimed in claim 1, wherein the second scattering element is a scattering coating.

11. The article as claimed in claim 10, wherein the scattering coating is an opaque and absorbent coating chosen from mineral or organic paints and enamels.

12. The article as claimed in claim 10, wherein the scattering coating is deposited on an additional substrate, and the additional substrate and the first scattering element are securely fastened by lamination using a sheet made of plastic material.

13. The article as claimed in claim 12, wherein the sheet made of plastic material is an external layer of the layered element.

14. The article as claimed in claim 10, wherein the scattering coating is deposited on the face of the additional substrate that is: furthest from the transparent layered element or closest to the transparent layered element.

15. A front-projection system comprising an article used as front-projection screen as claimed in claim 1 and a projector, the article comprising a front face onto which is projected the image and a back face wherein the second scattering element is separated from the front face at least by the first scattering element.

16. A reflective front-projection method wherein a front-projection screen operating in reflection and a projector are provided, said method comprising projecting by virtue of the projector images onto the front-projection screen, said front-projection screen comprising an article as claimed in claim 1.

17. The article as claimed in claim 1, wherein the root-mean-square slope Rdq of the profile of the textured surface is between 0.5° and 40°.

18. The article as claimed in claim 1, wherein the root-mean-square slope Rdq of the profile of the textured surface is between 1.0° and 30°.

Description

EXAMPLES

(1) The light transmission TL and light reflection RL in the visible in % are measured according to standard ISO 9050:2003 (illuminant D65; 2° observer) with radiation at normal incidence.

(2) I. Materials Used

(3) 1. First Scattering Element: Layered Element

(4) These trials were carried out with layered elements such as defined in table 1.

(5) TABLE-US-00001 TABLE 1 Layered Layered Layered element 1 element 2 element 3 Materials First external layer Satin glass 1 Satin glass 2 Satin glass 1 Central layer Layer Layer Layer of TiO2 of TiO2 of TiO2 Second external layer PVB sheet PVB sheet EVA sheet Texture Rdq °   5.6 .sup. 17.5   5.6 Scattering in reflection The maximum normalized   8.8   1.4   8.8 gain DRDM* 0° 0° 0° Angle of measurement of 3° 3° 3° luminance in the DRDM** Viewing angle 125°  >130°   125°  *Direction of maximum diffuse reflection with respect to the normal to the screen **With respect to the direction of maximum diffuse reflection

(6) The satin glasses correspond to textured glass substrates also qualified satin substrates of transparent rough glass. These textured substrates of 4 mm thickness comprise a textured main surface obtained by acid etching.

(7) The central layers are layers of titanium oxide of 60 nm deposited by magnetron cathode sputtering (“magnetron deposition”) so as to conform to the textured surface of the textured substrates.

(8) The first scattering elements used in these examples are all scattering elements the diffuse reflection of which is nonuniform and that have a privileged scattering direction in the specular direction, i.e. an angle of maximum scattering corresponding to the specular direction. Therefore, the maximum luminance is in each case measured at an angle of 3° with respect to the direction of specular reflection corresponding to the privileged scattering direction excluding the specular.

(9) 2. Second Scattering Element: Black Scattering Coating

(10) The black or dark-colored scattering coating consists of a black lacquer. This black coating is an opaque and absorbent coating. It is deposited on a planar SGG PLANICLEAR® glass substrate of 4 mm thickness. The lacquer and substrate together form a sheet of the black lacquered glass sold by Saint-Gobain under the trade name Planilaque®.

(11) This black coating has the following properties: a light transmission TL lower than 5%, a light reflection RL of about 5%, L* about equal to 27, and a*, b* are comprised between 0 and −1, measured in reflection on the side of the substrate opposite to the side on which the coating has been deposited.

(12) 3. Articles

(13) The articles according to the invention are obtained by laminating the layered elements with a black lacquered glass. The second external layer made of PVB or EVA of the layered element is used as lamination interlayer allowing the lacquered glass and the layered element to be assembled.

(14) For the examples Inv.1 and Inv.2, the black or dark-colored scattering coating formed by the lacquer is located on the face of the substrate not in contact with the plastic sheet made of PVB.

(15) In the case of a lamination scattering-coating side, a plastic sheet made of EVA or a plastic sheet made of PVB associated with a tie layer is preferably used for reasons of mechanical strength. For the examples Inv.3, Inv.4 and Inv.5, the black or dark-colored coating formed by the lacquer is located on the face of the substrate in contact with the PVB or EVA sheet.

(16) TABLE-US-00002 Article*: Comp. 1 Comp. 2 Inv. 1 Inv. 2 Layered element No 1 1 2 Glass substrate Yes Yes Yes Yes Black coating Yes No Yes Yes Article*: Inv. 3 Inv. 4 Inv. 5 Layered element 1 1 3 Tie layer — Yes — Black coating Yes Yes Yes Glass substrate Yes Yes Yes *The various elements are positioned in the article in the order indicated in the above tables. For example, for the article Inv. 1, the layered element makes direct contact with a glass substrate that itself makes direct contact with a black coating.
II. Visual Assessment and Optical Properties

(17) FIG. 3 corresponds to photographs of the comparative article 1 and of the article according to the invention being used as front-projection screen.

(18) The measurement of the 350 Lux is carried out with a light meter placed in front of the screen facing the turned-off video projector. The main plane of the screen (plane of the image) is vertical. Only ceiling lights are used. There is no source of direct lighting directed toward the surface of the screen.

(19) The brightness of the image projected onto the article according to the invention and its contrast (Inv.1, photograph 3D) are highly improved with respect to an image projected onto a black lacquered glass alone (Comp.1; photograph 3G).

(20) Visual observations in order to compare the intensity of the black side of the layered element were carried out. A score of 1 to 3 was attributed with “1” qualifying the least intense black and “3” the most intense black.

(21) TABLE-US-00003 Example Comp. 1 Inv. 1 Inv. 2 FIG. Visual assessment 3G 3D — Intensity of the black Face-on, 350 LUX 3 1 1 At 45°, 350 LUX 3 2 1 Face-on, 0.5 LUX 3 3 3 At 45°, 0.5 LUX 3 3 3 Metallic appearance No Yes Yes

(22) The transmitted rays are mainly absorbed by the black lacquer. However, in projection, a not very bright ghost may be visible but only at large angles of observation, between 75° and grazing angles. Thus, this effect is not discomforting for the spectator.

(23) In addition, it is possible to decrease the severity of this effect by placing the black or dark-colored scattering coating closer to the scattering texture. To this end, article Inv.3 differs from Inv.1 in that the lacquered glass is laminated not on the glass side but on the lacquer side. In this variant, and in variants Inv. 4 and Inv. 5, the lacquer is located in the interior of the article right next to the scattering texture. Therefore, ghost images are minimized and become almost invisible under the usual conditions of observation of a screen.

(24) From an aesthetic point of view without protection, the visual appearance of the article is partially modified with respect to the visual appearance of a black or dark-colored coating not superposed on a layered element

(25) When direct illumination of the article is limited or indeed if the light scattered by the texture is not scattered in the direction of the observer, then the article appears of color close to that of the black or dark-colored coating, in particular dark black in the case of black Planilaque® sheets.

(26) When a bright light illuminates the surface of the screen, the latter appears dark grey with a metallic appearance or metallic effect. All of these properties endow the article with a look that may be likened to the effects developed for the paints of motor-vehicle bodywork.

(27) III. Properties of the Screen

(28) 1. Analysis of the Contrast

(29) Contrast measurements were carried out in order to test the front-projection screens of the invention. These measurements were carried out with artificial lighting the average illuminance of which was 350 lux. Contrast is defined as the ratio of the luminance measured when the projector is displaying a white image (white luminance w. I.) to the luminance measured when the projector is displaying a black image (black luminance, b. I.).

(30) The table below gives the luminance and contrast measurements for black lacquered glasses alone (Comp. 1), for diffusely reflecting transparent elements alone (Comp. 2) and for the article Inv.1.

(31) TABLE-US-00004 Measure- ment Con- Article angle LUX w.l. b.l. trast Observations Comp. 1  3° 350 21.3 2.5 8.5 w.l. too low Comp. 2  3° 350 1440 11 137 b.l. too high Inv. 1  3° 350 1380 7.7 179 w.l. and b.l.: ok Comp. 1 45° 350 16.2 0.63 25.7 w.l. too low Comp. 2 45° 350 41 4.7 8.7 b.l. too high Inv. 1 45° 350 42 1.7 24.7 w.l. and b.l.: ok

(32) Two characteristics must be present for the quality of the projected images to be good in terms of brightness, a white luminance, in the privileged scattering direction, higher than 50 cd/m.sup.2 and a high contrast.

(33) The invention allows projected images of very good contrast to be obtained, in particular when it is used in an illuminated environment (daylight and/or artificial light). The white luminance is very high whereas the black luminance remains low. In comparison, a white Planilaque® sheet under illumination of 350 LUX has a white luminance of 1300 cd/m.sup.2 and a black luminance of about 55 cd/m.sup.2 at 0°. The article according to the invention has a white luminance comparable to that obtained with a white lacquered glass of Planilaque® type and a black luminance seven times lower.

(34) Observed at 45 degrees, the contrast is of the same order of magnitude for the article according to the invention and for black lacquered glasses alone, with a dark black and a white luminance close to 50 cd/m.sup.2. Such a contrast is sufficient for information to be easily read.

(35) 2. Viewing Angle

(36) The viewing angle corresponds to the angle at which the contrast reaches 4.5:1, which is the minimum contrast required to comfortably read information. Concretely, when the angle of observation increases, the brightness of the image decreases. The article Inv.1 has a screen viewing angle of 125° (±62.5°/normal to the screen).

(37) 3. Normalized Gain of the Screen

(38) The variation in normalized gain depending on angle of observation is calculated in the following way: in absolute value (AV): ΔG=|G.sub.3°−G.sub.45°| in percentage (P): ΔG=|G.sub.3°−G.sub.45°|/G.sub.3°*100 with G.sub.3° and G.sub.45° the gain at 3° and 45°, respectively.

(39) TABLE-US-00005 Uniform diffuse Gain at 3° Gain at 45° ΔG reflection Comp1 0.12 0.10 AV: 0.02 Yes Comp2 9.1 0.2 AV: 8.9 No P: 98% Inv1 8.8 0.3 AV: 8.5 No P: 97%

(40) The gain of the article Inv.1 is 8.8 and the gain of Inv.2 is 1.4.

(41) A gain higher than 1 at 0° means an increased brightness at small angles of observation. A high gain level, such as that measured for article Inv.1, allows the article to be used as a screen in an illuminated environment while preserving a good image quality, the images appearing bright and contrasted.

(42) IV. Visibility of Marks

(43) The visibility of finger marks on the surface of a material or article was measured using the following protocol.

(44) The mark was deposited on the surface to be measured with a device comprising a rubberstamp and the pattern to be transferred was a fingerprint. An artificial sebum solution (body fat+aqueous solution) supplied for example by Essilor® was deposited on the stamp.

(45) The application of the sebum to the stamp then the transfer of the print were carried out using a rigorous protocol so as to obtain a maximum repeatability and a completely identical mark.

(46) The surface with or without print was illuminated with a strip light of white LEDs (directional lighting). A camera filmed the scene from normal to the surface to be measured. The relative positions of the “surface to be measured”/“light”/“camera” remained the same.

(47) Visibility was quantified in the following way: Acquisition of an image of the surface without print, Transfer of the print, Acquisition of an image of the same surface with print, Extraction of the outline of the print, Measurement of the brightness L1 in the extracted area, Extraction of the same zone in the image without mark and measurement of the luminosity L0, Expression for the contrast=mark visibility %=100×(L1−L0)/(L1+L0).

(48) A finger mark is transferred to the surface to be evaluated. Images are then taken and analyzed in order to evaluate the visibility of the finger marks. A contrast value is then determined. A high contrast corresponds to a high visibility.

(49) The contrast measured on a finger mark is 9 times lower with the article Inv. 1 (contrast=5) then with the article Comp. 1 corresponding to the black lacquered substrate alone (contrast of 45). The article according to the invention indeed allows the visibility of finger marks to be considerably decreased.

(50) V. Other functionalities

(51) The photographs in FIGS. 4B, 4C and 4D illustrate the “blackboard” functionality that can be combined with the projection-screen functionality: Figure B: kitchen worktop, Figure C: school marker boards, Figure D: meeting room.

(52) The article renders colors very well and has a good resolution. The resolution is mainly related to the properties of the texture used and in particular to the typical period of the “grains” at this texture (RSm=132 μm+/−50 μm).

(53) VI. Conclusion

(54) The article according to the invention indeed has the following advantages: the white luminance of projected images is sufficiently high to allow a good contrast and therefore a good viewing quality to be obtained whatever the level of illumination of the room, the brightness, contrast (180:1) and resolution of the image allow an excellent readability of projected information, the high gain allows bright images to be obtained including in an illuminated environment, viewing angle remains compatible with a high number of existing projection situations, the rendering of colors is very good, the visibility of smudges is decreased.

(55) The black or dark-colored article according to the invention used as a front-projection screen allows image projection to be combined with use as a marker board.