WO2002048754A1

WO2002048754A1 - Collimator lenses

Info

Publication number: WO2002048754A1
Application number: PCT/GB2001/005377
Authority: WO
Inventors: William Crossland; Andres Dames; Benn Miller
Original assignee: Screen Technology Limited
Priority date: 2000-12-13
Filing date: 2001-12-05
Publication date: 2002-06-20
Also published as: JP2004515816A; GB0030410D0; KR20020086522A; CN1401087A; AU2002220891A1

Abstract

A collimator comprises a screen (1) defining an array of apertures (2). A convex lens is associated with each aperture, and a transparent region (5) is positioned between the screen and the lens to provide a spacing therebetween. The surface (4) of the screen which faces the lenses is light absorbing.

Description

COLLIMATOR LENSES

The present invention relates to the field of collimators . There are many applications in which it is necessary to employ a collimated light source, and many different collimation systems have been proposed in order to provide such a source. As will be appreciated, there are some applications for which it is necessary to have a particularly good degree of collimation in order for the overall system to operate effectively.

An example of such a system is a photo-luminescent LCD system. In such a system it is desirable to provide illumination in the blue and/or ultraviolet range to a modulating liquid crystal device, the illuminating light having a high degree of collimation, perhaps plus or minus 15° or less (as defined below) . It will be appreciated that such requirements have therefore meant that such a display system has required a relatively costly and complex collimation component that is also relatively inefficient. This has resulted in such systems having a higher power consumption than is perhaps desirable, as well as increasing the overall cost of such systems.

The present invention is directed toward the provision of a low cost collimation device that provides a relatively high efficiency but which also enables the provision of a good degree of collimation such that it can be used in systems of ' the type referred to above .

According to the present invention there is provided a collimator comprising: a screen defining an array of apertures; a convex lens associated with each aperture; and a transparent region positioned between the screen and the lenses to provide a spacing therebetween, wherein the surface of the screen which faces the lenses is light absorbing.

The lens may be plano-convex. The transparent region may be formed from a single screen of transparent polymeric or glass material, and may have the screen attached or formed thereon.

The surface of the screen facing away from the lenses may be reflective. The transparent region may be formed so that it is integral with the lens.

The lenses may be hexagonal or rectangular in order to provide a closely packed array of lenses. A light absorbing baffle may be provided between each lens, and may extend both into the transparent region, possibly contacting the screen, and away from the transparent region.

The present invention also provides a system for producing collimated light comprising a collimator of the type described above in combination with a light source comprising at least one ' light generating element and surrounding reflective box with one surface of the box possibly being formed by the surface of the screen facing away from the lenses. In such a system the light generating source may produce light in the range of 350-

420nm.

The present invention also provides a photo- luminescent LCD system including a light generating source of the type described above . The present invention can provide a system for producing collimated light where the intensity of the light is constant across an area at some distance from the source. This is achieved by relaxing the collimation and allowing a level of divergence from each collimating unit. This averages the intensity across the entire area. The benefit of producing a uniform light intensity for use in display apparatus, such as a photo luminescent LCD, is to achieve an image, when evenly modulated, that has a constant brightness to the eye . One example of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a side cross-sectional schematic view of an example of the present invention;

Figure 2 is a plan and side cross-sectional schematic view of an illumination system that may be used in combination with the example in figure 1;

Figure 3 is a diagram showing the operation of the key components of the example of figure 1 in use;

Figure 4 is a graph which illustrates the level of collimation achieved in one example of the invention; Figure 5 is a diagram showing alternative lens configurations and illustration of overlap;

Figure 6 is a cross-sectional view of an alternative embodiment of the invention; and

Figure 7 is a cross-sectional view of a photo luminescent LCD that may use the present invention.

Referring to Figure 1, a collimator according to the present invention has a screen 1 which defines an array of apertures 2. The screen 1 has on one surface a reflective coating 3, and on the other surface a light absorbing material 4. The benefits of this particular configuration will be discussed below.

Positioned adjacent to the screen 1 is a transparent layer 5, which has a refractive index significantly greater than that of air, and which is transparent and formed from glass or a polymeric material that is transparent at the wavelength at which the device is operating. If the collimator 15 is, for example, employed in a photo- luminescent LCD system 20, typically the region 5 should be transparent for light of a wavelength of 350-420nm. Positioned adjacent to the transparent region 5 is a lens 6 associated with each aperture 2. Both the lens 6 and transparent region 5 should have as high a refractive index as possible. In some cases due to commercial considerations the refractive index of the lens 6 being less than that of medium 5. This ensures that an appropriately narrow cone of light is transmitted through medium 5 to the lens 6, maximising lens unit packing and hence efficiency. The lens 6 is a plano-convex arrangement that has a shape optimised to collimate light from the aperture 2 to as great an extent as possible for a given operating wavelength. The transparent region 5 may be formed, from a single sheet which passes over and through all the apertures 2 in the screen 1. In a case where the screen 1 is thicker than the order of lOO m the surface of the transparent region 5 that is adjacent to the aperture 2 may protrude through the aperture such that its surface may be flush with the reflecting surface of the screen 1. The section of the transparent region 5 which is exposed through the aperture 2 may be treated with a dielectric coating such that high angle light incident at the aperture is reflected back into the light box 12. As the collimating unit is now accepting a more narrow cone of light it can be placed closer to its adjacent units, thereby increasing the efficiency. This increase must be balanced against the reduced first pass of light through the aperture. The array of lenses 6 may also be formed from a single sheet which passes over all the apertures 2 and the transparent region 5. Whether these units 5, 6 are formed individually or from a single sheet, an external baffle 9 may be provided between each lens 6 in order to prevent high angle light from exiting the collimation system 15 or passing into adjacent lenses and then exiting. In the case where the transparent region 5 is formed from individual units, internal baffles 8 may be provided between adjacent units in order to reduce the effects of high angle light coming either directly from the aperture 2, reflecting back towards the surface 1 from the curved surface of the lens 6, or refracting into a lens after leaving an adjacent lens.

Figure 2 shows how the collimator 15 of figure 1 may be employed as part of a light illuminating unit 12. In the light illuminating unit 12 there is provided one or more light sources 10 that provide light at the desired operating wavelength. These light sources are surrounded by a box 13, which is coated with reflective material on its inner surface so that light from the light source 10 is reflected until it leaves the light box 13 at apertures 2 in screen 1. The light then passes through the transparent region 5 and lenses 6 and is collimated to provide a collimated light source. In order to increase the efficiency of such a light source it will be apparent that there are particular benefits in providing the screen 1 with a reflective coating 3 on its light receiving surface, so that light which does not pass immediately through apertures 2 is reflected back into the box 13 for subsequent passage through the apertures 2.

Figure 3 illustrates many of the design parameters (in two dimensions) that can be considered to be relevant to the overall performance of the invention. It will be appreciated that the design of the collimator is optimised for each particular application. This optimisation process will be significantly affected by the following parameters:

Unit Area The unit area is the area of each of the collimating hole/lens components that make-up the array. This unit will be hexagonal in shape for circular lenses as they do not tessellate exactly. Across such arrays there is a variation in the intensity of the light. The units can be made small enough to ensure that the human eye cannot perceive variations in the brightness. Coupled with this, within the PL-LCD arrangement, the separation of the collimator from the phosphor screen can be increased until the variation in the brightness for a given unit size cannot be perceived by the human eye. Thus the unit size chosen is dependent on the depth of the display and the distance from which the display will be viewed

(e.g. an acceptable range may be 1 to 10mm) . Aperture Unit Area /Unit Area Ratio

This is a good first order measure of the efficiency of the system as it shows what percentage of the light incident on the underside of the collimator is transmitted. Some of this light is subsequently lost due to the absorbing topside of the aperture plate and some of the reflected light is recovered. A required efficiency is now defined, which allows the aperture area to be calculated from the chosen unit area and the Aperture Area/Unit Area ratio (alternatively a required collimation can be defined and the aperture area can be modelled to achieve this) . An example would be in the range 5% to 20%. Aperture/Lens Radii Ratio a/1

The radius of the aperture can be calculated from its area and, for example, the radius of the base of a circular lens is defined by the maximum radius that will fit in the unit area for a design where the lenses do not overlap. In this example the area of the lens will be 91% of the unit area (i.e. that of the hexagon) . Hence the radius of lens

0.91xUnitsize π

Hence the aperture/lens radii ratio can be calculated.

E.g. a typical aperture/lens radii ratio will be 30% to 40%.

Vertical Depth of Transparent Medium 5, d

The separation of the "base" of the lenses from the plane of the apertures, d, is now defined for a maximised refractive index of medium 5. The primary consideration is the majority collection of the solid angle of light from the aperture. In the case where the base of the lens is just large enough to collect all the light, the vertical depth of transparent medium 5 is defined as t, where: t=(l-a)/2tanθ_t However it may be preferably to increase d to greater than t and not collect all the light, the remaining light will be incident on the vertical baffles 8. This will improve the collimation, but decrease the efficiency. Conversely reducing d to less than t has limited effect on the collimation whilst decreasing the efficiency. In this case the unit area can be reduced with a corresponding efficiency gain. In the example hardware d=2τnm and is slightly less than t. Refractive Index of Medium 5 and Lenses 6, n_s and n₆ The refractive index will always be maximised for both materials subject to manufacturing requirements and cost considerations. This is simply to minimise the solid angle of light and decrease the unit size, thereby increasing the efficiency. There will be a commercial benefit (cheaper and manufacturing ease) to having a reduced refractive index and this must be balanced with the efficiency gain to having a high refractive index. There will be less loss to the efficiency by reducing the refractive index of the lens as opposed to that of medium 5, as this will not increase the solid angle of light leaving the aperture. Typical example values are 1.49 to 1.522. Definition of curved Lens Surface, S

An aspherical curvature can be better optimised and is preferred to a spherical lens, although spherical lenses may be used in certain cases . This design is optimised by avoiding, where possible, light incident on the curved lens surface at high (near-glancing) angles, as this will reduce the percentage of reflected light at this dense medium/air interface. Degree of Lens Overlap, O Where lenses are allowed to overlap, the unit size is reduced and more light passes through the greater density of apertures. Additionally, the redundant area at the intersection of the lenses will be reduced increasing the efficiency. This will be at the cost of increasing the overlap at each lens intersection, reducing the efficiency as some of the light incident on the circular base of one lens will enter the adjacent lens, or be lost if baffles are deployed. Baffles

These are to absorb the following light: 1. Light incident outside the base of a lens originating from its corresponding aperture ^• is absorbed by internal baffles .

2. Scattered light due to imperfections in the transparent medium 5 is absorbed by internal baffles, external baffles and the top side of the aperture plate.

3. Light incident on the lens surface which is not transmitted but reflected is absorbed by internal and external baffles and the top side of the aperture plate.

Deep external baffles can be used to increase the collimation but this is a very inefficient method.

Optimisation of the collimator is preferably performed to produce the desired level of collimation (e.g. ±8° to

40° for PL-LCD applications) . There is a requirement to emit a higher proportion of the light thus improving the efficiency of the system rather than concentrating solely on producing normal or near normal light which will result in non-uniform intensity across the subsequent display.

In one example, apparatus has been produced which uses circular apertures 2, of diameter 1.71mm, etched into 0.25μm stainless steel sheet to form screen 1. This sheet had one reflective surface (facing a light source) and one blackened surface. The circular lenses 6 are of 5mm diameter resulting in the open area of the aperture 2 representing 11.7% of the total area of the lens 6, or 10.6% of the area of the underside of the collimator (made up of a number of unit areas) . The vertical depth, t, of the transparent region 5 is specified at 2mm. The lenses 6 and transparent region 5 were both made from glass and have a refractive index of 1.522. There were no baffles 8, 9 included in this design. With this configuration the critical angle θ_s is exceeded resulting in some light being incident outside the outer edge of the lens 6.

The level of collimation achieved is shown in the graph in Figure 4. The collimation angle is defined in these examples as the level where 50% relative intensity is achieved.

Figure 5 shows alternative lens 6 configurations^'. Each of the lens 6 and transparent medium 5 units may have, in plan view, a hexagonal, square or rectangular shape in order to provide optimum packing in the collimator array. The circular lenses 6 in Figure 5a were used in the example configuration. Hexagonal lenses (Figure 5b) can be selected to achieve 100% coverage and hence the light is allowed to be incident outside the extremity of a circular lens in the example given.

The use of a pin hole and associated convex lens 6 to produce collimated light is well known. However, the employment of a pin hole results in a collimator 15 which is extremely inefficient as most of the illuminating light will be blocked by the screen 1 forming the pin hole. Accordingly, the present invention employs apertures 2 which are much larger than what would be considered to be a pin hole, the apertures 2, for example, forming 11.7% of the total surface area of the screen forming them. This improvement in ^"efficiency has, however, significant drawbacks in terms of the quality of the collimated light. In the case where an aperture 2 is employed, any position on the lens 6 will not receive light solely from a single angle, but from a range of angles. The most divergent ray from the normal originates at the edge of the aperture 2, passing through the centre axis of the lens and being incident on the surface of the lens 6 at a point . This point also receives more near normal light from the other extremity of the aperture 2. These two extreme rays represent the bounds of the angle of incidence of light while within the plane, and the power of the lens 6 surface at the point must be compromised to best' refract the range of rays to its normal. This will result in the aforementioned compromise of the optical power of the lens 6 across its surface. This compromise results in a reduced level of collimation, with light diverging out to higher angles of incidence, which can make the collimated configuration unusable for some applications where excessive divergent light can cause problems.

Figure 6 illustrates a further embodiment of the present invention. The lenses 6' are formed from glass spheres and located in a preformed unit designed to act as light absorbing internal baffles. This unit may be made from metal (e.g. aluminium) and may have a blackened finish either by direct surface treatment such as anodising or by painting. The reflective surface may be achieved by attaching a perforated sheet with a white coating to the underside of the preformed unit.

Figure 7 illustrates how a collimator 15 may be used in a system with a photo luminescent LCD 20. The figure shows . an alternative illuminating unit 12' being used in conjunction with the collimator 15 and a display system 20. The display system comprises two polarisers 21, 23, an LCD 22, a visible reflector stack 24, a phosphor screen 25 and an anti-glare filter 26. Such a system is described in patent publication no. WO95/27920.

In a photo luminescent LCD application 20 of this type a significant amount of divergent light can affect the contrast ratio achieved in the LCD 22, making the display unworkable. However, with the present invention, the provision of an absorbing surface 4 on the screen 1 quite unexpectedly overcomes many of the problems associated with generation of light at higher angles of incidence. The employment of an aperture 2, rather than a pin hole results in a significant degree of high angle of incidence light by reflection of light from the surface of the lens 6 back towards the screen 1, this light is then reflected from the screen 1 and back out at high angles of incidence through the lens 6. In the case of. the present invention, a provision of light absorbing material 4 on the screen 1 prevents such diverging reflection from being passed back out through the lens 6. This reduces the amount of high angle of incidence light from being emitted- by the collimator 15, increasing the average degree of collimation whilst still maintaining high efficiency. The provision of additional baffles 8,9 can improve this effect even further.. However, as discussed above, it may be desirable to utilise some level of divergent light to ensure that a constant light intensity is achieved across the display.

Claims

1. A collimator comprising: a screen defining an array of apertures; a convex lens associated with each aperture; and a transparent region positioned between the screen and the lens to provide a spacing therebetween, wherein the surface of the screen which faces the lenses is light absorbing.

2. A collimator according to claim 1, wherein the transparent region is formed from a single sheet of glass or polymeric material .

3. A collimator according to claim 2, wherein the transparent region has the screen attached or formed thereon.

4. A collimator according to any preceding claim, wherein the surface of the screen facing away from the lenses is reflective.

5. A collimator according to any preceding claim, wherein the transparent region is formed so that it is integral with at least one lens.

6. A collimator according to any preceding claim, wherein the lenses have a hexagonal or square or rectangular planar surface in order to provide a closely packed array of lenses.

7. A collimator according to any preceding claim, further comprising a light absorbing baffle provided between adjacent lenses.

8. A collimator according to claim 7, wherein the baffle extends into the transparent region, away from the transparent regio -, or both.

9. A collimator according to any preceding claim, wherein the apertures have a surface area of at. least 8% of the screen.

10. A collimator according to any preceding claim, wherein the apertures have a dielectric ^• stack formed thereon.

11. . A .system for producing collimated light comprising a collimator according to any preceding claim in combination with a light source comprising at least one light generating element and- surrounding, reflective box.

12. A system according to claim il, wherein the light generating source produces light in the ultraviolet or visible blue wavelength 365nm to 400nm.

13. A photo-luminescent LCD system including a system according to claim 11 or 12.