EP1540385A2

EP1540385A2 - Diffusing substrate

Info

Publication number: EP1540385A2
Application number: EP03769558A
Authority: EP
Inventors: Laurent Teyssedre; Thomas Bertin-Mourot; Aurelia Prat
Original assignee: Saint Gobain Glass France SAS; Compagnie de Saint Gobain SA
Current assignee: Saint Gobain Glass France SAS; Compagnie de Saint Gobain SA
Priority date: 2002-09-11
Filing date: 2003-09-03
Publication date: 2005-06-15
Also published as: WO2004025334A2; TW200407630A; CN1695074A; FR2844364A1; AU2003278248A1; CN100397104C; US20060099441A1; WO2004025334A3; AU2003278248A8; JP2006512596A; FR2844364B1; PL374658A1; KR20050046756A

Abstract

The invention concerns a diffusing substrate (20) comprising a glass substrate (21) and a diffusing coating (22) deposited on said glass substrate, characterized in that the glass substrate (20) has light transmission at least equal to 91 % on the wavelength range between 380 and 780 nm.

Description

DIFFUSING SUBSTRATE

The present invention relates to a diffusing substrate intended to homogenize a light source. The invention will be more particularly described with reference to a diffusing substrate used to homogenize the light emitted from a backlighting system.

A backlighting system which consists of a light source or “back-light” is for example used as a backlighting source for liquid crystal screens, also called LCD screens. It appears that the light thus emitted by the backlight system is not sufficiently homogeneous and presents too great contrasts. Diffusing means associated with the backlight system are therefore necessary to homogenize the light. Among the liquid crystal screens, a distinction is made between screens incorporating a structure called "Direct Light" for which the light sources are located inside an enclosure and the diffusing means are located in front of the light sources, and screens incorporating a so-called "Edge Light" structure for which the light sources are positioned on the side of the enclosure, the light being conveyed to the diffusing means on the front face by a waveguide. The invention relates more particularly to LCD screens with a "Direct light" structure.

The invention can also be used when it comes to homogenizing the light coming from architectural flat lamps used for example on ceilings, floors, or walls. They may also be flat lamps for urban use such as lamps for advertising panels or lamps that can constitute shelves or bottoms of display cases.

A satisfactory solution from the point of view of uniformity consists in covering the front face of the backlight system with a plastic plate such as than a polycarbonate or an acrylic polymer containing mineral fillers in the mass, the plate having for example a thickness of 2 mm. However, since this material is sensitive to heat, the plastic does not age well and the release of heat generally leads to a structural deformation of the plastic diffusing means, which results in a heterogeneity of the luminance of the image projected at the level of the LCD screen. for example.

It may then be preferred as diffusing means a diffusing layer such as that described in the French patent application published under the number 2 809 496. This diffusing layer composed of particles agglomerated in a binder is deposited on a substrate, for example in glass.

The inventors have shown that the use of such diffusing means causes numerous reflections of the light generated by the backlight system at the interfaces of the glass substrate. And although the backlight system has reflectors to reflect the light reflected by the glass substrate that could not be transmitted, the light returned by the reflectors to the glass substrate is however only partially transmitted , a part being reflected again and returned again by the reflectors and so on. Also, all of the light is not transmitted as soon as the backlighting system is put into operation, but undergoes several back-and-forth movements before passing through the diffusing substrate with some losses. The inventors have chosen to name this phenomenon, the "recycling" phenomenon.

Having highlighted this phenomenon of recycling, a problem which until now had never been raised, the inventors established that it was necessary to study the quality of light transmission through the diffusing substrate in order to obtain a suitable luminance. of light coming out of the substrate.

Furthermore, the inventors have shown that a too thick glass substrate could generate an excessive absorption and consequently generate an insufficient luminance resulting in the weakening of the luminance of the image on an LCD screen for example. The object of the invention is therefore to provide a diffusing substrate which comprises a glass substrate coated with a diffusing layer and which makes it possible to optimize the luminance of the lighting generated via such a substrate.

According to the invention, in order to optimize the luminance of the lighting generated via the diffusing substrate which comprises a glass substrate and a diffusing layer deposited on said glass substrate, the diffusing substrate is characterized in that the glass substrate has a light transmission at least equal to 91% over the wavelength range 380 to 780 nm, and preferably at least equal to 91 , 50%, for a glass with an index of 1.52 ± 0.04. The inventors have been able to demonstrate that the luminance dependent on the quality of the light transmission of the substrate is a function of the parameters that are the linear absorption coefficient and the thickness of the glass substrate, the linear absorption coefficient being linked to the glass composition of the substrate.

Also, according to one characteristic, the glass substrate has a total iron content such that:

7110

[Fe ₂ O ₃ ] _t <

(1.52xe + 0.015) + (17.24xe + 0.37) xRedox

with [Fe ₂ 0 ₃ ] t expressed in ppm and corresponding to the total iron in the composition, e being the thickness of the glass in mm, and the Redox being defined by Redox = [FeO] / [Fe2O3] t, the Redox being between 0 and 0.9.

According to another characteristic, the iron content must be even more limited if the light transmission is at least equal to 91.50%. This rate is then such that

2110

[Fe ₂ O ₃ ] _t ≤

(1.52xe + 0.015) + (17.24xe + 0.37) xRedox

with [Fe ₂ 0 ₃ ] t expressed in ppm and corresponding to the total iron in the composition, e being the thickness of the glass in mm, and the Redox being defined by Redox = [FeO] / [Fe ₂ 0 ₃ ] t, the Redox being between 0 and 0.9.

Also, according to a first embodiment, the glass substrate has a minimum light transmission of 91.50% for a thickness e of 4.0 mm at most, with a total iron content of 200 ppm and a Redox of less than 0 .05.

According to a second embodiment, the glass substrate has a minimum light transmission of 91% for a thickness e of 4.0 mm at most, with a total iron content of 160 ppm and a Redox equal to 0.31. For this same rate of iron and Redox, the thickness e will be 1.5 mm at most to ensure the property of minimum light transmission of 91.50%.

According to yet a third embodiment, the glass substrate has a minimum light transmission of 91% for a thickness e of at most 1.2 mm, with a total iron content of 800 ppm and a Redox equal to 0.33. According to yet another embodiment, the glass substrate has a minimum light transmission of 91% for a thickness e of at most 1.2 mm, with a total iron content of 1050 ppm and a Redox equal to 0.23.

According to one characteristic, the glass composition of the glass substrate of the invention comprises at least the following constituents:

According to another characteristic, the diffusing layer of the substrate of the invention is composed of particles agglomerated in a binder, said particles having an average diameter between 0.3 and 2 microns, said binder being in a proportion between 10 and 40% by volume and the particles forming aggregates whose size is between 0.5 and 5 microns. The particles are semi-transparent particles and preferably mineral particles such as oxides, nitrides, carbides. The particles are preferably chosen from oxides of silica, alumina, zirconia, titanium, cerium, or a mixture of at least two of these oxides. For more details, refer to published application FR 2 809 496.

Finally according to the invention, this diffusing substrate will in particular be used in a backlighting system which can be arranged in an LCD screen or in a flat lamp.

Other advantages and characteristics of the invention will appear in the following description with reference to the appended drawings in which:

• Figure 1 illustrates a backlight system;

FIG. 2 illustrates curves giving for a light transmission of 91% the content of the global iron Fe ₂ 0 ₃ as a function of the Redox with respect to several thicknesses of glass; FIG. 3 illustrates curves giving for a light transmission of 91.5% the content of the global iron Fe ₂ 0 ₃ as a function of the Redox with respect to several thicknesses of glass. For the sake of clarity, the dimensions are not respected between the different elements.

FIG. 1 illustrates a backlighting system 1 intended for example to be used in an LCD screen of dimension 17 "for example. The system 1 comprises an enclosure 10 comprising an illuminant or light sources 11, and a diffusing substrate in glass 20 which is associated with enclosure 10.

The enclosure 10, about 10 mm thick, has a lower part 12 in which the light sources 11 are arranged and an opposite upper part 13 which is open and from which the light emitted from the sources 11 propagates. The lower part 12 has a bottom 14 against which are arranged reflectors 15 intended to reflect on the one hand, part of the light emitted by the sources 11 which was directed towards the lower part 12, and on the other hand, part of the light which has not been transmitted through the diffusing substrate but reflected by the glass substrate and back-scattered by the diffusing layer. The arrows shown schematically illustrate the paths of the light emitted from the sources 11 and recycled in the enclosure.

The light sources 11 are for example discharge lamps or tubes commonly called CCFL for "Cold Fluorescent Cathode

Lamp ”, HCFL“ Hot Cathode Fluorescent Lamp ”, DBDFL for“ Dielectric Barrier Discharge Fluorescent Lamp ”, or LED type lamps for

"Light Emitting Diodes".

The diffusing substrate 20 is attached to the upper part 13 and held integral by mechanical fixing means not illustrated such as clipping cooperating with the enclosure and the substrate, or else kept posed by mutual engagement means not shown such that 'A groove provided on the periphery of the surface of the substrate cooperating with a peripheral rib of the enclosure.

The diffusing substrate 20 comprises a glass substrate 21 and a diffusing layer 22, of thickness between 1 and 20 μm, arranged on one face of the glass substrate, facing or opposite the upper part 13 of the enclosure. . For the composition of the layer and its deposition on the glass substrate, reference is made to the published French patent application 2 809 496.

The substrate 21 for supporting the layer is made of transparent or semi-transparent glass for the visible wavelength range. It is characterized according to the invention by its low absorption of light, and has a light transmission T _L at least equal to 91% over the wavelength range 380 to 780 nm. The light transmission is calculated under a D65 illuminant, in accordance with standard EN410. Examples of embodiment of the glass substrate 21 are given below in the form of a table, indicating for each of them the glass composition whose contents are expressed in% by weight, the overall iron content, the ferrous iron content. , the Rédox as well as the TL SOUS light transmission illuminant D65. The light transmission TL is calculated for a given thickness e of the glass substrate. Examples 1a, 1b, 2 and 3 are glass substrates which respond to the property of light transmission at least equal to 91% while Example 4 is not suitable. These examples are commercial glass substrates sold under the following names: Example 1a: B270 from the company SCHOTT with e = 0.9 mm,

Example 1b: B270 from the company SCHOTT with e = 2.0 mm, for examples 1a and 1b only the thicknesses are different but the glass composition is identical;

Example 2: OPTIWHITE from the company PILKINGTON with e = 1.8 mm; Example 3: CS77 from the company SAINT-GOBAIN GLASS with e = 1.1 mm;

Example 4: PLANILUX from SAINT-GOBAIN GLASS with e = 2.1 mm

Note that these compositions have impurities whose nature and proportions are, for some of them, summarized below: Cr ₂ O ₃ <10 ppm MnO <300 ppm V ₂ O <30 ppm TiO2 <1000 ppm.

The light transmission T _L is calculated over the wavelength range 380-780 nm according to standard EN 410 from the transmission τ which is defined in a known manner by Beer-Lambert law: τμ) ≈ (l - R (Λ)) ² xe- ^{α (/ l) xe}

with R, the reflection factor, α, the linear absorption coefficient, oc and R being a function of the wavelength of the light emitted, and e, the thickness of the substrate.

The light transmission T _L is therefore linked to the linear absorption coefficient α and to the thickness e of the substrate 21.

The inventors have therefore demonstrated that the glass composition of the substrate as well as its thickness influence the light transmission of the substrate. More particularly, the overall iron content (expressed as Fe ₂ 0 ₃ ) and the redox of the composition play a major role in the linear absorption coefficient. In the invention, Redox is defined as being the iron content in reduced form (expressed in FeO form) contained in the overall iron content (expressed in Fe ₂ O _{3 form} ) (FeO / Fe ₂ θ ₃ ratio).

Also the thickness of the substrate can be selected according to the glass composition used. The inventors have established a relationship between the parameters that are, the thickness of the glass, the total iron and the redox of the glass composition, leading to the required light transmission property. This stress relationship can be written in the following mathematical form, the total iron content in the composition is such that for a light transmission T _L greater than or equal to 91%:

7110

[Fe ₂ O ₃ ] _t <

(1.52xe + 0.015) + (17.24xe + 0.37) x Redox

with [Fe ₂ 0 ₃ ] t corresponding to the total iron in the composition expressed in ppm, e the thickness of the glass in mm, and Redox = [FeO] / [Fe2O3] t, the Redox being between 0 and 0.9

As a variant, the stress can be given on the thickness for a given glass composition and is such that for a light transmission T _L greater than or equal to 91%:

7110 / [Fe ₂ O ₃ ] t -0.015 -0.37 x Redox e <1.52 + 17.24 x Redox

For a light transmission TL of 91.5% which is a minimum preferred value according to the invention, the total iron content in the composition must be even lower than that expressed above for a lower transmission limit equal to 91% , and is such that:

2110

[Fe ₂ O ₃ ] _t <

³ (1.52xe + 0.015) + (17.24xe + 0.37) xRedox

or the thickness should be such that

2110 / [Fe ₂ O ₃ ] t - 0.015 - 0.37 x Redox e ≤ 1.52 + 17.24 x Redox

The inequalities given above connecting the contents of the couple

(Fe ₂ 0 ₃ , Redox) and the thickness of the substrate can be translated in the form of curves for characteristic thicknesses of glass. Also, FIG. 2 illustrates curves giving, respectively for several given thicknesses, the content of the global iron Fe ₂ 0 ₃ as a function of the Redox for a light transmission TL of 91%. Substrates of a determined thickness, the iron and redox values of the glass composition of which are situated on or below the reference curve for the same thickness chosen are suitable for meeting the property of light transmission which must be at least minus 91%.

In this figure have been positioned the points EX1, EX2, EX3, EX4 of the couple (Fe ₂ θ ₃ , Redox) of the glass composition corresponding to examples 1 a and 1 b for the point EX1, and to examples 2, 3, 4 for the other points, respectively, EX2, EX3, EX4.

Note that point EX1 is well below the 2.1 mm curve, and even below the 4 mm curve. Consequently, the glass substrate of Examples 1a and 1b is suitable with a thickness of 0.9 mm and 2.0 mm respectively, and the glass composition could even be suitable with a higher thickness, up to at least 4 mm, to present a minimum light transmission of 91%. However, it is not in the interest of the realization of the backlight system to increase the thickness of the elements because the current will tends to reduce the size of the LCD screens in terms of thickness. Also, we will not consider a thickness greater than 4 mm.

The same remark applies to the point EX2 which is well below the curve corresponding to the thickness of 1.8 mm of the substrate of example 2. The glass composition of example 2 would be suitable for a substrate of a thickness not exceeding 4.0 mm to present a minimum light transmission of 91%.

It is also noted that the point EX3 is below the curve of 1.1 mm which corresponds to the thickness of example 3. However, with a thickness greater than 1.2 mm (curves below this point ), the glass composition of Example 3 would no longer be suitable for satisfying a minimum transmission of 91%.

On the other hand, the point EX4 is well above the 2.1 mm thick curve corresponding to Example 4 which is not suitable. We can nevertheless deduce that by reducing the thickness of this type of glass so that it is thickness less than at least 1.2 mm (curves above this point), this glass composition would be suitable for obtaining the property of a light transmission of 91%.

FIG. 3 illustrates curves giving, respectively for several given thicknesses, the content of the global iron Fe ₂ O ₃ as a function of the Redox for a minimum light transmission TL equal to 91.50%.

It can be seen that for a light transmission of 91.50% which constitutes a preferred minimum value of the invention, only examples 1a and 1b are suitable, the point EX1 of which is situated well below the curve corresponding to the thickness 2, 1 mm. The other examples are not suitable for ensuring a light transmission of at least 91.50% because the points EX2, EX3, EX4 are located above the curves corresponding to the respective thicknesses of Examples 2, 3 and 4. It can be noted that the point EX2 is appreciably above the curve corresponding to the thickness of 1.8 mm, and that it would be appropriate for the glass composition of Example 2 to produce a less thick substrate of 1.5 mm for example (this which corresponds to the first curve located above the point) in order to ensure the minimum light transmission property of 91.50%.

The glass substrate 21 is therefore used as a support for the diffusing layer 22 in order to constitute the diffusing substrate 20 which is associated with the enclosure 10 to constitute the backlight system 1. It is then possible to measure in a known manner the luminance of the light coming from the enclosure and passing through the diffusing substrate. The table below summarizes for examples 1a, 1b and 2 to 4 the luminance associated with the light transmission. The reported values of the luminance correspond to a measurement made perpendicular to the surface of the diffusing substrate and for a diffusing substrate (glass substrate and diffusing layer) of diffuse transmission of 60%, that is to say that the diffusing substrate generates 40% light back-scattering which is recycled inside the enclosure.

Furthermore, the glass substrate also has the advantage of serving as a support for depositing coatings with functional layers such as an electromagnetic isolation coating which, moreover, can constitute the diffusing layer 22 as described in the patent application. French FR 02/08289, a coating with a low-emissivity function, an anti-static, anti-fog, anti-fouling function, or even a function of increasing the luminance. This latter function may actually be desired for an application of the diffusing substrate to an LCD screen.

A coating having the function of further increasing the luminance by tightening the scattering indicator is for example known in the form of an optical film sold under the name CH27 by the company SKC.

The table below indicates, in addition to the light transmission for the glass substrate 21, the luminances of the lighting obtained without coating CH27 and with the coating CH27 on the diffusing substrate 20, as well as a result of comparison of these two luminances expressed in %. The reported values of the luminance correspond to a measurement made perpendicular to the surface of the diffusing substrate and for a diffusing substrate (glass substrate and diffusing layer) of diffuse transmission of 60%.

It is noted that of course the luminance increases with the coating CH27 which it is the function, but also that the increase in luminance is much higher when the light transmission is greater. These results show the advantage of using the least absorbent glass substrate 21 possible to optimize the luminance of a backlight system. As such, the substrate of Example 1a or 1b will be preferred.

Claims

1. Diffusing substrate (20) comprising a glass substrate (21) and a diffusing layer (22) deposited on said glass substrate, characterized in that the glass substrate (20) has a light transmission at least equal to 91% over the wavelength range 380 to 780 nm.

2. Diffusing substrate according to claim 1, characterized in that the glass substrate (20) has a light transmission at least equal to 91.50% over the wavelength range 380 to 780 nm.

3. Diffusing substrate according to claim 1, characterized in that the glass substrate (20) has a total iron content such that:

7110

[Fe ₂ O ₃ ] _t <

(1.52xe + 0.015) + (17.24xe + 0.37) xRedox

4. Diffusing substrate according to claim 2, characterized in that the glass substrate (20) has a total iron content such that:

2110

[Fe ₂ O ₃ ] _t

(1.52xe + 0.015) + (17.24xe + 0.37) x Redox

with [Fe ₂ θ ₃ ] t expressed in ppm and corresponding to the total iron in the composition, e being the thickness of the glass in mm, and the Redox being defined by Redox = [FeO] / [Fe203] t, the Redox being between 0 and 0.9.

5. Diffusing substrate according to any one of the preceding claims, characterized in that the diffusing layer (22) is composed of particles agglomerated in a binder, said particles having an average diameter of between 0.3 and 2 microns, said binder in a proportion of between 10 and 40% by volume and the particles forming aggregates whose size is between 0.5 and 5 microns.

6. Diffusing substrate according to claim 5, characterized in that the particles are semi-transparent particles and preferably mineral particles such as oxides, nitrides, carbides.

7. Substrate diffusing any one of the preceding claims, characterized in that the glass substrate (20) has a glass composition based on at least the following constituents:

8. Diffusing substrate according to claim 1 or 2, characterized in that the glass substrate (20) has a minimum light transmission of 91.50% for a thickness e of 4.0 mm at most, with a total iron content of 200 ppm and a Redox less than 0.05.

9. Diffusing substrate according to claim 1, characterized in that the glass substrate (20) has a minimum light transmission of 91% for a thickness e of 4.0 mm at most, with a total iron content of 160 ppm and a Redox equal to 0.31.

10. Diffusing substrate according to claim 2, characterized in that the glass substrate (20) has a minimum light transmission of 91.50% for a thickness e of 1.5 mm at most, with a total iron content of 160 ppm and a Redox equal to 0.31.

11. Diffusing substrate according to claim 1, characterized in that the glass substrate (20) has a minimum light transmission of 91% for a thickness e of at most 1.2 mm, with a total iron content of 800 ppm and a Redox equal to 0.33.

12. Diffusing substrate according to claim 1, characterized in that the glass substrate (20) has a minimum light transmission of 91% for a thickness e of 1.2 mm at most, with a total iron content of 1050 ppm and a Redox equal to 0.23.

13. Use of a diffusing substrate as described according to one of claims 1 to 12 to produce a backlighting system.

14. Use according to claim 13 for which the backlight system is arranged in an LCD screen.

15. Use according to claim 13 for which the backlighting system is arranged in a flat lamp.