WO2013076641A1

WO2013076641A1 - An optical redirection layer for a luminaire

Info

Publication number: WO2013076641A1
Application number: PCT/IB2012/056547
Authority: WO
Inventors: Lieven Raf Roger Desmet
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2011-11-22
Filing date: 2012-11-19
Publication date: 2013-05-30

Abstract

An optical redirection layer for a luminaire that is configured to redirect a portion of the light transmitted through it in a predetermined manner. The optical redirection layer comprises an optical axis and a substrate that extends in a radial direction away from the optical axis, configured so that light passes from a luminaire into the optical redirection layer. Another aspect of the invention relates to a luminaire that comprises a light source, a light guide and an optical redirection layer. The light guide is configured to direct light towards the optical redirection layer. Another aspect of the invention relates to a method of manufacturing an optical redirection layer for a luminaire. A continuous light redirecting element is formed in a surface of the substrate that extends in a radial direction away from the optical axis of the luminaire.

Description

An optical redirection layer for a luminaire

FIELD OF THE INVENTION

The present invention relates to an optical re-direction layer that is arranged to cover the light-exit surface of a light guide of a luminaire, a luminaire comprising the optical redirection layer and a method of manufacturing the optical redirection layer of the invention.

BACKGROUND OF THE INVENTION

As light emitting diode (LED) based illumination and LED luminaires become increasingly attractive, due in part to improvements in efficiency and decreasing prices, they are beginning to become a popular alternative to the traditional tube luminescent (TL) based luminaires.

A luminaire of the prior art is disclosed in US 2010/0053959, which describes a light with an optical redirection layer comprising multiple prism like structures, arranged in concentric circles around the optical axis. The optical redirection layer is configured to redirect light received from the source in a predefined direction resulting in a relatively uniform beam of light.

The quality of the beam that is redirected by the optical redirection layer is very dependant on the prism top angle quality, with any deformation of this edge causing stray light and increased glare.

Deformations in the prism top angle can occur as a result of low manufacturing tolerances that are inherent with the compression moulding process required to mass produce the optical redirection layer in significant quantities. For example if the manufacture cycle time is too short, or the compression pressure too low during moulding, the plastic will not fill the complete prism cavity in the master metal mould, resulting in a prism with rounded top angles.

The relationship between prism top angle and beam quality has been confirmed using computer simulation, the results of which are illustrated in Figure 1. It has been found that, to achieve an acceptable beam, the deformation of the prism top angle should stay within 10 μιη to meet the glare norm (shown as a dashed line in Figure 1) at a polar angle of 60°. Figure 1 shows the effect of the deformation of the redirection foil on the optical beam quality of the luminaire. It can be observed that for no deformation, shown as a solid light line, the beam quality is high, whereas with a 25 μιη diameter rounded prism top, shown as solid dark line, the beam quality is substantially degraded.

SUMMARY OF THE INVENTION

The present invention seeks to provide an optical redirection layer, a luminaire incorporating said layer and, a method of manufacturing said optical redirection layer that overcomes or at least substantially alleviates the problems with the optical re-direction layers and luminaires known from the prior art.

The invention particularly seeks to provide a redirection layer that is particularly suitable for use in a LED based luminaire that forms a light beam with a good angular distribution and low glare whilst being suitable for mass production.

According to the invention, there is provided an optical redirection layer for a luminaire comprising a substrate having an optical axis, wherein a continuous light redirecting element is formed in a surface of the substrate that extends in a radial direction away from the optical axis and through which light passes from a luminaire into the optical redirection layer.

In a preferred embodiment, the continuous light redirecting element is in the form of a spiral extending away from the optical axis.

Preferably, the continuous light redirecting element is visible as a series of triangular elements extending from the optical axis when viewed in cross section through the substrate taken along a plane extending from the optical axis. These may be formed from a spiral shaped groove cut into the surface.

The triangular elements may comprise a first surface facing towards the optical axis, arranged at a first angle normal to the surface of the layer, and a second surface facing away from the optical axis, arranged at a second angle normal to the surface of the layer, wherein the first and second surfaces combine at a tip of the triangular element.

The present invention also provides a luminaire comprising a light source, a light guide and an optical redirection layer for a luminaire, wherein the light guide is configured to direct light towards the optical redirection layer.

The light guide may have a light-entry portion with a light entry plane, a tapering portion with a light reflecting surface and a light-exit surface, wherein the light entry portion is arranged to guide the light from the light-entry plane in a first direction towards the light reflecting surface, the light reflecting surface being arranged in relation to the first direction so that incident light from the light-entry portion is reflected towards the light-exit plane, towards the optical redirection layer.

The present invention also provides a method of manufacturing an optical redirection layer for a luminaire comprising a substrate having an optical axis, comprising the step of forming a continuous light redirecting element in a surface of the substrate that extends in a radial direction away from the optical axis.

The step of forming a continuous light redirecting element in a surface of the substrate may comprise the step of cutting a spiral shaped groove in said surface. The spiral shaped groove may be cut using a precision diamond turning process.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as other aspects, objects, features and advantages of the present invention, will be better understood through the following illustrative and non- limiting detailed description, with reference to Figures 2 to 4 of the appended schematic drawings showing currently preferred embodiments of the invention.

Figure 1 shows a graph to show the effect of deformation of the optical redirection layer on the optical beam quality of a luminaire;

Figure 2a shows a cross-sectional side view of an optical transmissive redirection layer according to a first embodiment;

Figure 2b shows a top view of the lens in Figure 2a.

Figure 3 a shows a cross-sectional side view of a luminaire arrangement according to a second embodiment; and

Figure 3b shows a top view of the luminaire arrangement in Figure 3 a.

Figure 4 shows the results of a raytracing simulation that compares the optical performance of an optical redirection layer arranged in spiral and concentric circle configurations.

DETAILED DESCRIPTION

Figures 2a-2b show a cross-sectional side view and a top view, respectively, of a light transmissive optical redirection layer according to a first embodiment which is configured to adjust and tune the light distribution emitted from a luminaire.

The optical redirection layer 121 comprises a light-entrance surface 109 and a light-exit surface 130. An integral, triangular prism-like component 123 is formed in the lens light entrance surface 109. The triangular component 123 is in the form of a protrusion, or ridge, in the x-y surface that encircles the centre of the optical axis 140 in a continuous spiral formation. As the redirection layer comprises a single continuous prism-like groove in the substrate material surface, rather than a discontinuous collection of individual concentric prism rings such as known from the prior art, the redirection layer can be machined by a continuous precision diamond turning process meaning that unprecise and discrete replication may be avoided.

The diamond turning of the proposed spiralled prism structure has a cycle time that is on par with the standard compression moulding technique, and does not compromise the optical function of the redirection layer.

The triangular component 123 presents a first surface 125 facing in the direction of the optical axis, and a second surface 127 facing away from the optical axis. The first surface 125 is arranged at a first angle <¾ in relation to the normal to the surface of the layer and the second surface 127 at a second angle <¾. The surfaces 125, 127 meet and form the tip of the triangular component 123.

A light ray entering the optical arrangement at the lens light-entrance surface 109 will thus first be refracted in the first surface 125 of the triangular component 123 at an air to optical redirection layer interface, and then be reflected by total internal reflection (TIR) in the second surface 125 of the triangular component 123 at an optical redirection layer to air interface. The last reflection directs the light ray towards the lens light-exit surface 130 of the optical redirection layer 121, which it passes by refraction at an optical redirection layer to air interface. The optical redirection layer may thus have a collimating and/or focusing effect on any light that enters through the lens light-entrance surface 109.

In Figure 2a trace 143 shows the path of an exemplary light ray.

In a first detailed example based on the first embodiment, the optical redirection layer has a refractive index of about 1.6.

The material of the optical redirection layer 121 may generally and

advantageously have an optical absorption of less than 4/m, provide low haze and scattering, contain particles smaller than 200 nm, be able to sustain an operational temperature higher than 75° C.

In the foregoing the refractive index of the optical redirection layer has been about 1.6. Other refractive indices may be used, preferably in the range of 1.4-1.8. However, as will be recognized by the skilled person, the hitherto discussed dimensions, angles, etc. may need to be adapted accordingly, which the skilled person will be able to do based on the information disclosed herein. The optical redirection layer of the modelled in using the 'LightTools' computer software package, and an optical raytracing simulation was performed, the results of this simulation are shown graphically in Figure 4, with the polar angle plotted against the illuminance on a logarithmic scale for the both the spiral configuration of the prism-like groove in the optical redirection layer, as in the present invention, and, an 'ideal' prism- like groove (that has no deformations or rounding in the prism top-edge), arranged in the concentric circle configuration disclosed in US 2010/0053958. It can be observed in Figure 4 that the two different groove configurations have very similar optical properties.

The groove in the optical redirection layer is arranged in an Archimedean spiral that can be described in polar coordinates by Equation 1. r = aB

Equation 1

Where r is the radius to the centre and Θ is the polar angle. Equation 2 shows the relationship defining the spiral pitch.

Pitch = 2 T\r

Equation 2

The cycle time required to cut the spiral can be found from the spiral length, L, defined by Equation 3.

Equation 3

It is possible to calculate the length of time required to cut a spiral, using Equations 1 to 3, based on the spiral radius and pitch. For example, in the above described embodiment the redirection foil has a triangular component filling the surface between a radius of 35 mm and 83 mm, with respect to the surface of the foil, and a pitch of 0.5 mm, therefore the calculated Archimedean spiral length is approximately 36 m. A study performed by G. Gubbels in 'Diamond turning of glassy polymers' suggests that realistic diamond cutting speeds in PMMA or PC polymer, which could comprise the optical redirection layer, are between 3 to 10 m/s with a cutting chip of 10 um thickness. Therefore the spiral can be cut in 12 seconds, which compares favourably to the time required for manufacture using traditional compression moulding techniques.

It should be noted that it is possible to alter time by modifying the cutting depth or the pitch of the spiral groove in the optical redirection layer. Figures 3a-3b show a second embodiment of the invention, wherein the lens is combined with a light guide of a luminaire. The light guide 101, here circularly symmetric in a surface y-x, has a cylindrical through-hole 102, which inner side is a light-entry plane 105 covered by a light emitting layer 113, here a layer that emits light upon illumination, preferably a phosphor layer. The light emitting layer 113 is not in direct contact with the light-entry plane 105, instead there is a small, equidistant air gap between the light-entry- plane 105 and the light emitting layer 113. The gap is preferably as small as possible without there being any optical contact between the plane 105 and the layer 113, preferably the gap is less than 1 mm. The layer 113 may even be in mechanical contact with the plane 105, as long as there is no optical contact. Note that in Figure 3 a the shown gap between the layer 113 and the plane 105 is exaggerated. In most implementations the light emitting layer may be considered to be located at the same distance from the centre as the light-entry plane 105.

In the shown embodiment there is a second light guide 157 shaped as a tube, or cylinder with a cylindrical through-hole 132 in the centre, concentrically located in the cylindrical through hole 102. The second light guide 157 has a light input surface 158 facing the centre of the through-hole 132 and a light output surface 168 facing the light emitting layer. The second light guide further has lateral surfaces 159, i.e. the end surfaces of the cylinder which are perpendicular to the light input and output surfaces 158, 168. These surfaces are preferably not in optical contact with neighboring objects, but instead interfacing an optically less dense medium, preferably air, i.e. are in optical contact with a medium of lower refractive index than the second light guide 157. The light emitting layer 113 is shown at a distance from the light output surface 168 i.e. in non-optical contact with the second light guide, but may in alternative embodiments be in optical contact.

The second light guide 157 provides a collimating effect which increases efficiency. However, it can be noted that the second light guide is not required for the function as such of the luminaire arrangement in Figures 3a-3b. Hence, in alternative embodiments, the second light guide may be omitted.

At the lower part of the cylindrical through hole 132 there is a light source 117, preferably a light emitting diode (LED), which may be omnidirectional. The light source may be attached to a substrate (not shown), such as a PCB. In other embodiments there may be one or many light sources also at other positions, such as at various positions in the mixing cavity 132. For example, to produce white light one or more blue LED(s) 117 can be used in combination with a yellow or orange phosphor layer 113. Opposite to the light source 117, at the other end of the cylindrical hole 102, there is a mirror 115 covering the opening of the cylinder. The mirror 115 presents an inclined surface for reflecting light from the light source 117 towards the light emitting layer 113, light which else would escape via the cylinder opening. Since the light source is arranged so that it also illuminates the light emitting layer directly, the mirror 115 is not necessary, although it increases efficiency. Alternatively the mirror may be flat (not inclined) and/or may have diffusely reflective properties for light spreading.

In Figure 3 a, when the light source 117 directly or indirectly provides light to the light input surface 158 of the second light guide 157, the light is passing an air interface owing to the through hole 132 and will by this be refracted into an optically denser medium being the second light guide. As a result there will be a collimating effect of the light entering the second light guide 157 and the amount of light that can be guided to the light output surface by TIR in the lateral surfaces 159 increases. Preferably the refractive index of the second light guide is at least about 1.4 since that allows for TIR in the lateral surfaces 159 for light incident on the light input surface 158 virtually independent on an angle of incidence, provided that the lateral surfaces are also interfacing air or other medium with similar or lower refractive index. It should be understood that the second light guide 157 also is helpful and efficient for guiding back-scattered light from the light emitting layer entering via the light output surface 168 so that the light, at lower loss, can be incident on the light emitting layer 113 at another location, e.g. at an opposite side of the through hole 132.

In an example implementation it was found that with a second light guide 157 present in the centre of the luminaire there was a significant increase of light passing the light emitting layer. Since efficiency drops when the thickness of a luminaire of this kind decreases, due to that more reflections causing losses are required in a thin structure, adding a second light guide 157 can be used to reduce thickness at maintained efficiency.

When the light emitting layer 113 emits light as a response from illumination by the light source 117, it emits light towards the outer side of the light-entry plane 105 of the light guide 101. Owing to that the light emitting layer 113 covers the light-entry plane 105 and is arranged very close to it, light will, via the small air gap, be incident on the light-entry plane 105 at virtually all possible angles of incidence, i.e. from about +90° to -90° in relation to the normal of the light-entry surface 105. The air gap means there will be an interface of lower refractive index to higher refractive index and Snell's law will determine a largest entry angle (< 90°) of the light entering the light guide 101, i.e. the situation is similar as for the light entering the second light guide. This provides some control of the light entering the light guide 101 and will, for example, make it easier to fulfill requirements related to angular distribution of the light, which will be explained in some detail below.

The light entering the light guide 101 via the light-entry surface 105 is first guided in a light-entry portion 103 of constant thickness, here equal to the thickness t_lg of the light guide 101. Light that fulfills the conditions of TIR in inner planes 106, 110 of the light guide 101 will be guided towards a tapering portion 107 of the light guide 101, which portion 107 presents a reflecting surface 111 that is inclined and facing in the direction of the light- entry plane 105. The reflecting surface 111 is arranged with an angle β in relation to the normal of the light-exit plane 105 and the surface x-y of the light guide.

The reflecting surface 111 reflects light incident from the light-entry portion 103, i.e. from the x-direction in Figure 3a, towards a light-exit plane 106, which is in a perpendicular relationship to the light-entry plane 105. In other words, owing to the enclosing light-entry plane 105, light entering via the light-entry plane 105 and travelling in the surface x-y of the light guide 101 is being redirected by the reflecting surface 111 and thus escapes the light guide 101 "out-of-surface" (in the z-direction in Figure 3a) via the light-exit plane 106.

Owing to the "refractive" collimating effect when the light enters the light guide 101 via the light-entry plane 105 and/or the "reflective" collimating effect when the light is guided in the first portion 103 of constant thickness, the reflecting surface 111 can be designed to only handle incident light in a limited angular range, i.e. with a predetermined degree of collimation. The angle β is selected so that a uniform light beam with a desirable beam width (at full-width-at-half-maximum, FWHM) can be achieved. In most practical applications the angle β will be relatively small, such as in the range of 1°-15°.

To ensure that light does not leave the reflecting surface 111 via refraction, a mirror layer 119 may be provided to cover the outside of the reflecting surface 111.

Preferably the mirror layer 119 is arranged at a small distance from the light guide surface so that there is no optical contact.

In the surface (x-y) of the light guide 101 there is an angular distribution of the light. Owing to that the light emitting layer 113 will emit light into the light guide via the light-entry plane 105 at a distance of about Rl from the centre, not all light will be incident on the reflecting surface 111 at 90° in the x-y surface as would have been the case without the cylindrical hole and instead a "point like" light source in the centre of the light guide. Note that this applies in the shown x-y surface and not when light is incident on the reflecting surface from directions that are not in this surface. When light from the light emitting layer is entering the light guide at the distance Rl from the centre, a largest angle φ of light incident on the reflecting surface in the surface of the light guide occurs where the tapering portion 107 and the reflecting surface 111 begin, i.e. at a distance R2 from the centre. It can be noted that non-optical contact between the light emitting layer 113 and the light-entry plane 105 typically will make the largest angle smaller than the angle φ indicated in the Figure 3a when light is refracted into the light guide 101 via the light-entry plane 105.

Still referring to Figures 3a-3b, a lens comprising an optical redirection layer 121, a lens light-entrance surface 109 and a lens light-exit surface 130, is arranged so that the lens light entrance surface 130 covers the light-exit plane 106 of the light guide 101. The redirection layer 121 may take care of the final adjusting and tuning of the light distribution.

The optical redirection layer 121 further comprises a lens light-exit surface 130. A Triangular component 123 is formed in the light entrance surface 109. The triangular component 123 is in the form of a protrusion, or ridge, in the x-y surface that encircles the optical axis 140 in a continuous spiral formation.

The triangular component 123 presents a first surface 125 facing in the direction of the optical axis, and a second surface 127 facing away from the optical axis. The first surface 125 is arranged at a first angle <¾ in relation to the normal to the surface of the layer and the second surface 127 at a second angle <¾. The surfaces 125, 127 meet and form the tip of the triangular component 123, which tip may be in contact, but preferably not in optical contact, with the light-exit plane 106. It should be noted that mechanical contact not necessary results in optical contact, as will be recognized by the skilled person.

It is mainly "air-pockets" in the form of the valleys between adjacent turns of the triangular component 123 that are directly facing the light guide.

A light ray leaving the light-exit plane 106 of the light guide 101 will first be refracted at a light guide to air interface, pass the air filled "valley" between adjacent turns of the triangular component 123, reach the lens light-entrance surface 109 and be refracted in the first surface 125 of the triangular component 123 at an air to re-direction layer interface, and then be reflected by TIR in the second surface 127 of the triangular component 123 at a re-direction layer to air interface. The last reflection directs the light ray towards the opposite surface of the redirection layer 121, which it passes by refraction at a re-direction layer to air interface. The re-direction layer may thus have a collimating and/or focusing effect on the light from the light guide. It may be noted that the redirection layer 121 shown in Figure 3 has a cavity formed above the mirror 115. However, the exact design of the redirection layer in that area is typically of less significance since it is not participating in the re-direction of light.

Moreover, in Figure 3 a trace 143 shows the path of an exemplary light ray emitted by the light emitting layer 113 in response to illumination by the light source 117.

The material used to manufacture the optical redirection layer of the second embodiment may have similar properties to those disclosed in the description of the first embodiment of the invention.

It will be appreciated that the term "comprising" does not exclude other elements or steps and that the indefinite article "a" or "an" does not exclude a plurality. A single processor may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to an advantage. Any reference signs in the claims should not be construed as limiting the scope of the claims.

Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combinations of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the parent invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of features during the prosecution of the present application or of any further application derived there from.

Other modifications and variations falling within the scope of the claims hereinafter will be evident to those skilled in the art.

Claims

CLAIMS:

1. An optical redirection layer for a luminaire comprising a substrate having an optical axis, wherein a continuous light redirecting element is formed in a surface of the substrate that extends in a radial direction away from the optical axis and through which light passes from a luminaire into the optical redirection layer.

2. An optical redirection layer according to claim 1, wherein the continuous light redirecting element is in the form of a spiral extending away from the optical axis.

3. An optical redirection layer according to claim 2, wherein the continuous light redirecting element is visible as a series of triangular elements extending from the optical axis when viewed in cross section through the substrate taken along a plane extending from the optical axis.

4. An optical redirection layer according to claim 3, wherein the triangular elements comprise a first surface facing towards the optical axis, arranged at a first angle normal to the surface of the layer, and a second surface facing away from the optical axis, arranged at a second angle normal to the surface of the layer, wherein the first and second surfaces combine at a tip of the triangular element.

5. An optical redirection layer according to any preceding claim, wherein the continuous light redirecting element is formed from a single groove cut into the surface.

6. A luminaire comprising a light source, a light guide and an optical redirection layer for a luminaire according to any preceding claim, the light guide being configured to direct light towards the optical redirection layer.

7. A luminaire according to claim 6 wherein the light guide has a light-entry portion with a light entry plane, a tapering portion with a light reflecting surface and a light- exit surface, wherein the light entry portion is arranged to guide the light from the light-entry plane in a first direction towards the light reflecting surface, the light reflecting surface being arranged in relation to the first direction so that incident light from the light-entry portion is reflected towards the light-exit plane, towards the optical redirection layer.

8. A method of manufacturing an optical redirection layer for a luminaire comprising a substrate having an optical axis, comprising the step of forming a continuous light redirecting element in a surface of the substrate that extends in a radial direction away from the optical axis.

9. A method according to claim 8, wherein the step of forming a continuous light redirecting element in a surface of the substrate comprises the step of cutting a spiral shaped groove in said surface.

10. A method according to claim 9, including the step of cutting said spiral shaped groove using a precision diamond turning process.