GB2247536A

GB2247536A - Stepped wedge shaped plate light spreading device

Info

Publication number: GB2247536A
Application number: GB9018759A
Authority: GB
Inventors: Richard Paul Russell
Original assignee: Brookes and Gatehouse Ltd
Current assignee: Navico UK Ltd
Priority date: 1990-08-28
Filing date: 1990-08-28
Publication date: 1992-03-04
Anticipated expiration: 2010-08-28
Also published as: GB9018759D0; GB2247536B

Abstract

An optical device for spreading light that consists of a thin transparent wedge shaped plate which has face 4 fashioned with a number of small steps 5 in such a manner that light entering the device from the thick end 1 is reflected and emitted from face 3. The angled faces of the steps 5 may be curved, multi-facetted or finely roughened so the light reflected, by each step is spread out. The other face of each step may be parallel to face 3. The device may be incorporated in a liquid crystal display and may be used with the light emitting face 3 towards or away from the area to be illuminated. With the light emitting face 3 away from the area to be illuminated a reflector must be incorporated to reflect the light back through the device. <IMAGE>

Description

LIGHT SPREADING DEVICE This invention relates to a device for the illumination of display devices which require illuminating over a relatively large area. It is practically employed when there is limited space behind the area being illuminated as is often the case with liquid crystal display devices.

Liquid crystal display devices are passive in nature, that is they require to be illuminated if they are to be seen.

It is common practice to illuminate them from the back particulary in conditions of poor ambient lighting.

Miniaturization of the illumination device is often desirable.

Two primary methods that are widely employed are the use of a diffuse light source like an electro-luminescent lamp, which has an area large enough to illuminate all the display and the use of discrete lamps with some optical device to spread the illumination over the required area. Although electro-luminescent lamps provide very good uniformity of brightness a number of factors depending on the application mean that methods using discrete lamps may be preferable.

However the provision of sufficient uniformity of brightness over the entire display efficiently is a problem.

The invention is an optical device that includes a thin transparent wedge shaped plate that can be placed behind the area to be illuminated to spread the light from one edge over the required area. The front face from which light is emitted has a flat smooth surface and the back face of the plate is fashioned with a plurality of small steps. One face of each step is angled in such a manner that light entering the device from the thick end is reflected through the front face. The angled faces of the steps may be curved or multi-facetted so the light reflected, by each step is spread out to improve the uniformity of illumination. In one embodiment of the invention, the angled faces have a finely roughened surface so the light is spread by the irregular reflections and the other face of each step has a polished finish and is approximately parallel to the light emitting face.The steps may be arranged close together so that the illumination over the required area appears sufficiently uniform. The light spreading device may be incorporated in various display and indicating devices in particular liquid crystal displays devices. It may be used with the stepped face or the flat light emitting face towards the area to be illuminated. With the stepped face towards the area to be illuminated a reflector should be placed behind the light spreading device to reflect the light back through the device to the area to be illuminated. When used in this manner the increased optical path length allows further spreading of the light from the individual steps. Also the reflector can be an irregular light reflector to help spread the light from the individual steps.With the light emitting face towards the area to be illuminated a diffusing layer may be included between the area to be illuminated and the light emitting face to improve the uniformity of illumination. A reflecting layer may also be included behind the light spreading device to reflect any light emitted from the stepped face and improve the efficiency.

The invention is described by way of example with reference to the accompanying drawings, wherein: Figure 1 is a perspective view of a light spreading device according to the present invention.

Figure 2 is a cross-sectional view taken along line A-A in Figure 1.

Figure 3 is an enlarged small section of Figure 2 showing a ray of light being reflected at a step.

Figure 4 is an enlarged cross-sectional view showing an alternative embodiment of the small reflecting step.

Figure 5 is a cross-sectional view showing the light spreading device of the present invention used for the illumination of a liquid crystal device.

Figure 6 is a cross-sectional view showing a second example of the light spreading device of the present invention used for the illumination of a liquid crystal device.

Figure 7 and 8 are respectively a plan view and a side view of a light spreading device as shown in Figure 1 giving dimensions.

Figure 1 and 2 show an embodiment of the light spreading device of the present invention. It consists of a thin transparent wedge shaped plate with its thick end 1 being located towards the light source 2. So as to receive light into the device end 1 is preferably smooth. The face 3 from which light is emitted has a flat smooth surface. Surface 4 is fashioned with a number of small steps running parallel with end 1. Figure 3 shows an enlarged cross-section of one of the small steps. Face 5 of the step is arranged at such an angle that light from the source is reflected up through face 3. For light travelling parallel to the top face 3 the acute angle 6 of the step face 5 and face 3 should be between (900 - C) and (900 - C)/2 for the light to be emitted from face 3.Where C is the critical angle given by: C = sin~1 ( n2 / nl nl is the refractive index of the device n2 is the refractive index of the surrounding medium.

The other face of the step, 7 is arranged approximately parallel to face 3 and has a smooth surface so light with an angle of incidence greater than the critical angle is internally reflected along the device. Figures 7 and 8 give exemplary dimensions of this embodiment.

In another embodiment of the device the reflecting face of the step 5 is curved or multi-facetted hence reflecting the light over an arc. If the curve is parabolic as shown in Figure 4 then it can be arranged such that light travelling parallel to face 3 appears to be radiating from a point at the focus of the parabola 8. The parabolic curve is given by the equation: r ( 1 - cos e ) = 2 a where: r is the distance from the focus 6 is the angle of a line from the focus a is a constant that may be determined from:

where: h is the height of the step and 61 and 62 define the arc over which light is reflected.

For the light travelling parallel to face 3 to be reflected by the step and emitted from face 3 the following conditions should apply: 61 > 62 61 < 1800 - 2 C 62 > 900 - C For maximum efficiency the reflecting face 5 of the steps should be smooth, however to help spread the reflected light they may have a finely roughened surface.

While the light spreading device is usually rectangular as shown in Figure 1, it is not necessarily restricted to that shape. The steps are usually parallel to the thick end 1, but they are not necessarily restricted to that configuration and may be curved, for example; concentric arcs with their centre near the light source. Further, the thick end may be extended in order to form other parts of an optical system for illuminating the wedge.

Figure 5 is a cross-sectional view showing the light spreading device 9 of the present invention used for the illumination of a liquid crystal device 10. The light emitting face 3 is towards the liquid crystal 10. To improve the uniformity a light diffusing layer 11 may be incorporated between the light emitting face 3 and the liquid crystal 10. A reflecting layer 12 may be incorporated behind the light spreading device 9 to improve the efficiency by reflecting any light escaping from the stepped face 4 to the area being illuminated. The reflector 12 is preferably a diffuse reflector. The use of a reflector 12 can also enhance the display when the liquid crystal display device 10 is being illuminated from the front.

Figure 6 shows another example of the light spreading device 9 used for the illumination of a liquid crystal device 10.

The light emitting face 3 is away from the liquid crystal 10 and a reflector 12 is used to reflect the light back through the device 9 to the liquid crystal device 10. This has the advantage of increasing the optical path length from the stepped face 4 to the area being illuminated.

Claims

Claims:

1. An optical device for spreading light includes a thin transparent wedge shaped plate which can be placed behind the area to be illuminated and one face of which is fashioned with a plurality of small steps in such a manner that light entering the device from the thick end of the plate is reflected through the other face to the area to be illuminated.

2. An optical device as claimed in claim 1 wherein the angle of the small reflecting steps to light emitting face is such that light entering the device from the thick end is reflected due to the difference in refractive index in the device and that of the surrounding medium, that is between (900 - C) and (900 - C)/2. Where C is the critical angle given by: C = sin~1 ( n2 / nl nl is the refractive index of the device n2 is the refractive index of the surrounding medium.

3. An optical device as claimed in Claim 2 wherein the faces of the small reflecting steps are provided with a polished surface whereby the light is regularly reflected.

4. An optical device as claimed in Claim 2 wherein the faces of small reflecting steps are provided with a polished curved surface whereby the reflected light is spread out.

5. An optical device as claimed in Claim 2 wherein the faces of small reflecting steps are provided with a polished multi-facetted surface whereby the reflected light is spread out.

6. An optical device as claimed in Claims 1 to 5 wherein the faces of small reflecting steps are provided with an irregular surface whereby the light is irregularly reflected.

7. An optical device as claimed in Claim 1 to 6 wherein a diffusing layer is included between the wedge shaped plate and the area to be illuminated.

8. An optical device as claimed in Claim 1 to 7 wherein a diffusing reflector is included behind the wedged shaped plate.

9. An optical device as claimed in Claim 8 wherein the face of the wedge shaped plate with the small steps is towards the area to be illuminated.

10. An optical device as claimed in Claims 1 to 9 for illuminating liquid crystal displays.

11. An optical device substantially as described herein with reference to Figures 1 to 8 of the accompanying drawings.