Patent Application
20050218810
The present invention relates to the field of light sources, and in particular relates to flat or planar light sources based on the excitation of a phosphor by a low pressure gas discharge under the action of an alternating current.
Flat or planar light sources are useful for many applications such as in general lighting and backlights for liquid crystal displays (LCD). A flat light source is traditionally made of a linear light source such as cold cathode fluorescence light (CCFL) tube together with a flat (two-dimensional) light guide/diffusion layer to disperse the light in a plane from which light can be scattered out. A two-dimensional array of point sources such as tiny lamps and other sources such as light emitting diodes (LED) can also be regarded as a flat light source to a certain extent. U.S. Pat. No. 6,212,213, for example, describes a two dimensional LED array for the purpose of projection displays.
Field emission devices (FED) making use of electron beams inside a vacuum to excite a layer of phosphor can also be regarded as an alternate form of a flat light source. Even though FEDs were invented originally for display applications, they can also be used as an intense flat light sources for many applications. However, in a FED, good electrodes with high electron emitting efficiency and a high vacuum are needed. It should also be noted that the phosphor in a FED needs to be highly efficient in converting the energy of electron bombardment into visible light.
True planar light sources consisting of a pseudo-two-dimensional gas discharge and a phosphor layer are also known in the art. In order to maintain a uniform two-dimensional gas discharge, techniques such as barrier ribs, linear arrays, are used. U.S. Pat. No. 4,945,281 teaches a light source with an array of electrodes to excite a flat discharge. U.S. Pat. No. 6,628,066 describes a flat light source with spacer elements to separate individual discharges. In all cases, the resultant pseudo-two-dimensional gas discharge is then allowed to excite a phosphor as in an ordinary fluorescent lamp (FL). None of the inventions has optical elements to optimize the output of the light source in terms of polarization or recycling of rejected light or in collimation.
According to the present invention there is provided a planar light source comprising: a glass cell comprising first and second glass walls, a low pressure gaseous mixture inside said glass cell, means for striking a gas discharge inside said gas cell, said gas discharge being capable of producing ultraviolet photons, optically reflecting coating on said first glass wall adapted to reflect visible light, a phosphor layer the first glass wall which is capable of converting ultraviolet photons into visible light, and an optical coating on the inside of the second glass wall which reflects substantially all ultraviolet light and transmits substantially all visible light.
In a preferred embodiment of the invention a sheet type reflecting polarizer may be placed on the outside of the second glass wall, this reflecting polarizer being adapted to reflect linearly polarized light of one polarization and to transmit linearly polarized light of a perpendicular polarization. More preferably still a quarter wave retardation plate may then be placed on an exterior surface of the second glass wall between the glass wall and the polarizer.
In order to increase the brightness of the planar light source, in a preferred embodiment of the invention a light scattering film may be provided on top of the reflecting polarizer that can limit the angle of emission of the light to be predominately in the forward direction.
The means for striking a gas discharge may comprise a pair of electrodes through the side of the said glass cell, or may comprise a radio frequency source located outside the said glass cell.
The gaseous mixture may comprise a mixture of inert gases and mercury or compounds of mercury, and preferably the light source may then further comprise a heating device for raising the cell temperature to above 30° C.
The phosphor layer may be continuous over the surface of the first glass wall, or may be patterned.
Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which:
In a traditional cold cathode fluorescent lamp, the low pressure plasma discharge is confined in a cylindrical glass tube 1 (
In a cold cathode discharge, it is important to choose the electrode materials appropriately so that electron emission is maximized for the voltages applied. In addition, it is also important to optimize the composition as well as the pressure of the gas inside the glass tube 1 so that a large amount of ultraviolet light can be generated with high efficiency. The literature has plenty of discussions of these issues and the art is well known. In a type of lamp where there are no electrodes, the plasma discharge is excited via a radio frequency source just outside the lamp.
In the present invention, at least in preferred forms, there is provided a plnar or flat light source based on a two-dimensional gas discharge and that is highly suitable for a range of applications such as in projection displays. Moreover, preferred embodiments of the invention provide means of increasing greatly the efficiency of the photoluminescence emission process from such gas discharges on the phosphor. Such light sources are truly planar and the light is emitted from one side of the light source only. This light source can then be collimated, as well as converted into linear polarization by various means for projection applications.
In all projectors, the image forming light valve is planar. The light source is traditionally a pseudo-point source such as an arc lamp. A planar light source of the type to be described below can be imaged directly onto such imagers with high efficiency, and the light on the imager can be imaged onto the projection screen by a projection lens. A flat light source is therefore ideal for projection applications.
This flat geometry has many advantages. Firstly, since light is to be emitted from one side only, the opaque side can be coated with a totally reflecting mirror 8 (
Secondly, an important advantage of this CCFL flat light source is that the emitting phosphor faces the emitting side. This is very different from conventional tube type CCFL. As shown in
Thirdly, the flat light source can have enhanced efficiency by recycling of the UV light from the plasma discharge. Since UV light is emitted from the plasma discharge 3 in all directions, some UV light will go in the opposite direction of the phosphor layer 9. An optical coating 4 can be fabricated on the inside of the light source to reflect the UV light back into the phosphor layer 9. Hence all the UV light is utilized.
Fourthly, an optional polarization conversion film may also be included. The flat geometry of this light source makes it very simple in converting the polarization. The conversion is carried out by using a transmittive/reflective polarizing film 10 (
The angular distribution of the present planar light source is essentially Lambertian with a formula of
I(θ)=Io cos θ
where θ is the angle form the normal. This distribution is shown as the dotted line in
E=AΩ
where A is the area of the light source and Ω is the solid angle of the emission. Thus it is important to reduce Ω for projector applications. This can be accomplished by the use of diffuser films as indicated in this invention.
In the first preferred embodiment of this invention, the flat light source is composed of a rectangular cell made of glass with a shape as shown in
In this first preferred embodiment, the plasma discharge can either be excited using a pair of electrodes 2, or by means of electrodeless discharge using an external circuit. The plasma discharge induced either by the electrodes or the external circuitry will produce ultraviolet photons. Very often the ultraviolet photons are produced by having mercury in the gas mixture, although this is not a requirement of the present invention. If mercury is used, however, the glass cell may have to be heated slightly (to above about 30° C.) to increase the concentration of mercury vapour and so an additional heating device may preferably be provided. The ultraviolet photons produced will impinge on the phosphor to produce visible light. Some ultraviolet photons may impinge on the phosphor directly, while some may first be reflected back by optical coating 4. Visible light generated from the phosphor layer in the direction of the upper glass wall is allowed to escape from the light source from the top surface, while visible light that leaves the phosphor layer in the opposite direction will be reflected by the reflecting coating 8 towards the upper surface where it may then leave the cell.
In the second preferred embodiment of the present invention, the construction of the flat light source is substantially the same as the first preferred embodiment except that a layer of reflective polarizer 11 is provided on top of the light source, as shown in
In the third preferred embodiment of the present invention, an additional film 12 is placed on top of the light source. The purpose of the film is to limit the emission angle of the light so that it is predominately in the forward direction. There are several such films available in the market as “brightness enhancement films”. They can be structured surfaces such as the Vikuti™ film from 3M Company, or Light Diffuser Film™ holographic scattering films from Physical Optics Company.
In all preferred embodiments, the placement of the light reflecting layer 8 can be either inside the glass cell or outside of it, as shown in
