WO2013093741A1

WO2013093741A1 - Optical diffuser

Info

Publication number: WO2013093741A1
Application number: PCT/IB2012/057323
Authority: WO
Inventors: Maarten Marinus Johannes Wilhelmus Van Herpen; Hugo Johan Cornelissen; Liesbeth Van Pieterson
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2011-12-21
Filing date: 2012-12-14
Publication date: 2013-06-27

Abstract

An optical diffuser (100) comprising: a diffusing member (102) comprising scattering particles dispersed in an optically transparent material, the diffusing member (102) further comprising a plurality of portions (104) being arranged according to a predetermined pattern and having a predetermined size distribution, wherein a concentration of the scattering particles between the portions (104) is substantially larger than a concentration of scattering particles in the portions (104).

Description

Optical diffuser

FIELD OF THE INVENTION

The current invention relates to a light modulating element. In particular, the current invention relates to a diffusing element.

BACKGROUND OF THE INVENTION

In lighting applications, the light emitted by a light source is often passed through a diffusing material in order to achieve the desired lighting effect. Diffused light is seen as "softer" and the homogeneity of the light distribution can be improved by proper use of diffusing elements.

Diffusion of light may be explained as light being scattered in a material so that light exiting a diffusing element is distributed over a larger area and/or angle compared to light entering the diffusing element. A typical diffusing material may comprise a transparent material incorporating scattering particles. The diffusion of light in a diffuser made from such a material is limited by the thickness of the diffuser. Thus, certain desirable diffusing properties are given by the thickness of the particular diffuser.

However, in many lighting applications it is desirable to reduce the size of the luminaire. Many lighting applications therefore utilize small size light sources such as light emitting diodes (LEDs), both due to the small size and due the energy efficiency of LEDs. Diffusers are particularly important when point sources such as light emitting diodes are used in order to provide a desirable lighting effect from a light source which may otherwise be seen as unpleasantly sharp, particularly so for "mood-lighting" and domestic lighting applications.

Consequently, as the performance of conventional diffusers is dictated by the thickness of the diffuser, such diffusers may not be suitable for applications where a small size luminaire having good diffusive properties is desirable.

Accordingly, there is a need for an improved diffuser accommodating the need for smaller diffusers having similar or improved properties compared to conventional diffusers. SUMMARY OF THE INVENTION

In view of the above-mentioned desired properties of a diffuser, and the above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide an improved diffusing element providing improved diffusing properties over conventional diffusers.

According to a first aspect of the invention, this and other objects are achieved by an optical diffuser comprising: a diffusing member comprising scattering particles dispersed in an optically transparent material, the diffusing member further comprising a plurality of portions being arranged according to a predetermined pattern and having a predetermined size distribution, wherein a concentration of the scattering particles between the portions is substantially larger than a concentration of scattering particles in the portions.

Scattering particles should in the present context be understood as objects comprised in the optically transparent material having a refractive index higher than that of the transparent material so that light traveling in the material changes direction when interacting with a scattering particle. Scattering particles may have varying sizes and geometries and may also be provided, for example, in the form of fibers. Moreover, the scattering particles may also be reflective particles. For example, scattering particles may comprise Ti0₂. Furthermore, the scattering particles may preferably be essentially uniformly distributed in the optically transparent material outside the portions.

The optically transparent material is a material in which light of a selected wavelength range may travel essentially without absorption or scattering such as glass, silica or quartz or it may be a transparent plastic or composite material. However, the material may equally well be transparent for all wavelengths in the visible spectra.

Portions, here defined as distinct regions in the diffusing member, are distributed in the diffusing member according to a predetermined, preferably repeatable, pattern and having a predetermined, preferably repeatable, size distribution. The portions can be understood as volumes in the diffusing member where the boundary between a portion and the diffusing member is defined by the change in concentration of scattering particles, where a

concentration of scattering particles in the portions is substantially lower than a concentration in the diffusing member. A repeatable pattern should here be understood as a pattern which may be formed repeatedly in the same manner in a manufacturing process, thereby repeatedly yielding optical diffusers having the same properties.

The present invention is based on the realization that the diffusing properties of a conventional diffusing material comprising a uniform distribution of scattering particles may be improved by introducing portions in the material where the concentration of scattering particles is substantially lower. In a conventional diffuser comprising scattering particles, each step contributes equally to the average transport properties of light. A step is here defined as the travel of light between two consecutive scattering events. By

incorporating portions, in which substantially no scattering takes place, in a conventional diffuser, the scattering of light is not uniform as it is propagating in the material. Instead, occasionally, exceptionally long propagation steps free from any scattering events take place. The long steps are enabled by the portions having substantially lower concentration of scattering particles, and by controlling the size and distribution of the portions, the diffusing properties of the material may be tailored. The performance of a diffuser may be gauged by comparing the diameter of a beam of light entering a diffuser with the diameter of the same beam leaving the diffuser, for a given thickness of the diffuser. In a diffuser where long steps takes place, diffusive properties are improved as the propagation of light through the material is governed more by the long steps free of scattering than by the propagation limited by scattering events. Transport of light where long steps are more frequent and make the dominant contribution to the transport may be referred to as Levy flight, Levy transport or Levy propagation of light.

Furthermore, propagation of light according to Levy flight may be referred to as super-diffusion, and a diffusing element in which Levy flight takes place may accordingly be called a super-diffuser.

The theoretical basis for Levy flight for light can be found in "A Levy flight or light", Nature Letters, Vol. 453, pp. 495-498 (2008), where it is described how Levy flight of light can be achieved by incorporating glass spheres having a particular size distribution in a conventional diffusing material comprising scattering particles. The specific size distribution required to achieve Levi transport of light may be derived from the theoretical models in the aforementioned reference. However, in said reference, the glass spheres are randomly distributed in the diffusing material and therefore the diffusive properties may vary greatly from one sample to another. An advantage of the present invention is that the light propagation properties, and thereby the amount of diffusion, is controllable as the portions responsible for the long steps are arranged in a predetermined and controllable pattern.

Furthermore, the configuration of the portions may be tailored to suit the particular requirements at hand, for example, it is possible to provide a thinner diffuser having comparable performance compared to conventional diffusers. Alternatively, for a given thickness, improved diffusing performance may be achieved and it is also possible to tailor to a diffusing element having different diffusing properties in different regions.

In various embodiments of the present invention, the plurality of portions are arranged such that at least a portion of light propagating in the optical diffuser is propagating according to a Levi- flight mechanism. For a spatial distribution and size distribution of portions where light propagates according to a Levy flight mechanism, the diffusion properties of an optical diffuser may be further improved.

According to one embodiment of the invention, the concentration of said scattering particles in the portions may preferably be substantially zero. By ensuring that no or at least very few scattering events take place within the portions free of scattering particles, the transport properties in the diffusing member may be more easily determined. However, should a low concentration of scattering particles be present in the portions, Levy flight of light may still be achievable for the same or another configuration of the portions.

Furthermore, according to various embodiments of the invention, the diffusing member may advantageously be provided in the form of a sheet. By providing the diffusing member as a planar structure such as a sheet, it is straightforward to form a desired distribution of portions in the sheet. Furthermore, the portions may be distributed so that different areas of the sheet have different diffusing properties in order to create, for example, patterns or various lighting effects. However, the diffusing member may equally well be provided in the form of basically any geometric structure such as a cylinder, a cube, a sphere or the like as long as portions having a predetermined configuration may be formed.

In one embodiment of the invention, the portions may preferably be essentially cylindrical. An advantage of using cylindrical portions is that they are relatively simple to form in a diffusing member in the form of a sheet or layer and that they are easy to manufacture with repeatability. Furthermore, a cylindrical portion having a circular cross- section resembles the glass spheres previously referred to. Thereby, modeling and

simulations predicting the performance of the diffusing member may be more easily performed. However, portions having a different cross section, such as elliptical, rectangular or any other polygonal shape may also be used. For example, different diffusion properties in different directions may be achieved by introducing portions with non-circular cross sections. Furthermore, the portions may also be spherical portions arranged in a predetermined and controllable pattern.

In one embodiment of the invention, the portions may have a refractive index essentially the same as a refractive index of the optically transparent material. By forming the portions such that the refractive index is the same in the portions as in the surrounding material, refraction as light passes from the optically transparent material to the portion, and vice versa, may be avoided. Thereby, the portions may be seen simply as portions of the diffusing member free from scattering particles.

In one embodiment of the invention, the plurality of portions comprises portions of at least 10 different sizes. Ten or more different sizes of the portions may be used, the number of different sizes of portions used will depend on a range of parameters such as the pattern of the portions, the cross section, the thickness of the diffusing member, the desired diffusing properties etc. Typically portions of approximately 10 to 20 different sizes may be used. Furthermore, the cross sections of the portions may be in the range of 5 micrometer to 500 micrometer.

According to one embodiment of the invention, the portions may advantageously be extending through the diffusing member. In the case where the portions are extending through the diffusing member, transport of light following Levy flight may be achieved in lateral directions, but not in the axial direction of the cylindrical portions, thereby forming what may be seen as a 2-dimensional super diffuser. However, in many applications the lateral direction of the diffuser is the most important direction for diffusion of light.

Furthermore, the diffusing element may be arranged so that the preferred direction of light transport is perpendicular to the axial direction of such cylindrical portions.

According to various embodiments, the optically transparent material may advantageously be a flexible material. A transparent flexible material may for example be a silicone rubber or a flexible plastic material. Forming the diffusing member in a flexible material allows the incorporation of an optical diffuser according to embodiments of the invention in areas such as textile applications.

In one embodiment of the invention, the optical diffuser may further comprise a diffusing element arranged adjacently to the diffusing member. For example, a

conventional diffusing element may be arranged adjacently to a diffusing element according to embodiments of the invention in order to further tailor the properties of the resulting light being emitted from the optical diffuser. A conventional diffusing element arranged adjacently to the diffusing member is particularly advantageous in that it makes it possible use the aforementioned 2-dimensional super diffuser to achieve diffusion in all three dimensions by adding a conventional diffusing element.

In one embodiment of the invention, the optical diffuser may further comprise a reflective element arranged adjacently to the diffusing member. Arranging a reflective or partially reflective element on one or more sides of a diffusing member may direct the light so that several passes are made through the super-diffusing member before it is emitted from the diffuser. Thereby, the diffusing properties of the optical diffuser may be further improved. Furthermore, the diffusing member may be sandwiched between a diffusing element and a reflective element. By arranging the diffusing member between a diffusing and a reflective element, a combination of reflection and additional diffusion may be achieved thus offering additional flexibility in the design of an optical diffuser.

In one embodiment of the invention, a luminaire is provided, which comprises at least one light source, and an optical diffuser according to any of the aforementioned embodiments, wherein the optical diffuser is arranged adjacently to said light source so that light emitted from said light source is diffused by said optical diffuser.

According to a second aspect of the invention, it is provided a method for manufacturing an optical diffuser, comprising the steps of: providing a diffusing member comprising scattering particles dispersed in a first optically transparent material; forming a plurality of cavities in the diffusing member, said cavities having a predetermined pattern and a predetermined size distribution. The cavities may advantageously be distributed over substantially the entire surface area of the diffusing member.

The optically transparent material may be any material suitable for the transmission of light. It may for example be a rigid material such as glass, quartz or silica. Furthermore, the optically transparent material may be a flexible material such as a flexible plastic material or silicone rubber or it may be a pliable material such as a fabric or a porous foam material. Similarly, the scattering particles may be any particles having a refractive index different from the refractive index of the material in which they are dispersed.

Furthermore, the portions in the optically transparent material having a substantially lower concentration of scattering particles may for example be air- filled cavities in which case a super diffuser may be made for example in a fabric or a foam-like material. Any suitable method for controllably forming cavities or holes in a material may be employed. Examples of such methods include, but are not limited to, etching, drilling, sandblasting, molding, stamping, imprinting etc. The cavities may be recessions, depressions or the like formed in the diffusing member, or the cavities may extend through the diffusing member.

According to one embodiment of the invention, the method for manufacturing an optical diffuser may further comprise the step of filling the cavities with a second optically transparent material, wherein a concentration of scattering particles in the first optically transparent material is substantially larger than a concentration of scattering particles in the second optically transparent material.

Effects and features of this second aspect of the present invention are largely analogous to those described above in connection with the first aspect of the invention.

It should be noted that the invention relates to all possible combinations of features recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiments of the invention, wherein:

Fig. 1 schematically illustrates an optical diffuser according to an embodiment of the invention;

Fig. 2 schematically illustrates an alternative embodiment of the optical diffuser according to the present invention;

Fig. 3 is a schematic cross-section view of a luminaire comprising an optical diffuser according to embodiments of the invention;

Fig. 4 schematically illustrates a method for manufacturing an optical diffuser according to the present invention; and

Fig. 5 is a flow chart outlining the general steps of a manufacturing method according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the present detailed description, various embodiments of an optical diffuser according to the present invention are mainly discussed with reference to an optical diffuser wherein the diffusing member is a glass element comprising Ti0₂ scattering particles. It should be noted that this by no means limits the scope of the present invention which is equally applicable to a wide range of material combinations while still being in line with the main inventive concept.

Fig. 1 schematically illustrates an optical diffuser 100 according to an embodiment of the invention. The diffusing member 102 is provided in the form of a sheet or a layer made from an optically transparent material such as glass comprising scattering particles dispersed in the transparent material. In the present example, Ti0₂ scattering particles having a refractive index which is higher than that of the surrounding glass are used. The average size of the scattering particles may be in the range of 1 - 1000 nm. Typically the scattering particles may have an average size in the range of 1 - 10 nm. The diffusing member further comprises portions 104 in the form of cylindrical regions substantially free from scattering particles extending through the diffusing member 102. Here the portions are filled with an optically transparent material having the same properties as the surrounding optically transparent material. The portions may advantageously be formed in the same glass material from which the diffusing member is formed. The size distribution of the portions may be tailored according to the desired diffusing properties of the optical diffuser. The diameter of the cylindrical portions is typically in the range of 5 micrometer to 500 micrometer. A particularly favorable size distribution and pattern may be found for example by limited trial-and-error starting from the size distribution given in the aforementioned scientific article "A Levy flight for light" or by performing optical ray-tracing simulations. The number of different sizes of the cylinders may for example be 10 to 20, again depending on the desired diffusing properties.

Fig. 2 schematically illustrates an optical diffuser 200 where conventional diffusing layers 202 and 204 are arranged on both sides of the diffusing member 102. The layers may also be partially reflective so that light makes several passes through the diffusing member 102 before being emitted, thereby further improving the diffusing properties of the optical element.

Fig. 3 illustrates a luminaire 300 wherein light emitted by a light emitting diode (LED) 302 arranged on a LED carrier 304 is diffused by an optical diffuser 100 according to various embodiments of the invention. Fig. 3 schematically illustrates the diameter of a light beam is increasing as it is passing through the optical diffuser (100).

Fig. 4 schematically illustrates the general steps of a method for manufacturing an optical diffuser according to an embodiment of the invention and Fig. 5 is a flow chart outlining the general steps of the manufacturing method. First, in step 501 a mask 400 is arranged on a conventional diffusing member 402 in the form of a sheet made from an optically transparent material comprising scattering particles.

In step 502, cylindrical openings 404 are made in the diffusing member 402 according to the openings in the mask 400. The openings 404 may be holes reaching through the diffusing member as illustrated or they may be cavities which only reach partly through the diffusing member. The holes may be done in many different ways such as by drilling, sandblasting, etching or by various other methods known in the art. Finally, in step 503, the holes are filled with an optically transparent material essentially free from scattering particles. Thereby, an optical diffuser 100 in which light may propagate according to Levy flight is formed having improved diffusing properties compared to conventional diffusers.

In an alternative method for manufacturing an optical diffuser, the mask may be used for introducing spherical portions free from scattering particles in the diffusing member.

Moreover, an optical diffuser as illustrated in Fig. 2 may for example be formed by first forming cavities in the diffusing member followed by the addition of a conventional diffusing layer, thereby forming a diffuser having three distinct diffusing regions from just two elements.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, additional optical elements such as reflectors, lenses and filters may be incorporated in the optical diffuser in order to achieve desirable lighting effects.

Claims

CLAIMS:

1. An optical diffuser (100) comprising a diffusing member (102), the diffusing member (102) comprising

scattering particles dispersed in an optically transparent material, and a plurality of portions (104) being arranged according to a predetermined pattern and having a predetermined size distribution,

wherein a concentration of the scattering particles between the portions (104) is substantially larger than a concentration of scattering particles in the portions (104).

2. Optical diffuser (100) according to claim 1, wherein the plurality of portions (104) are arranged such that at least a portion of light propagating in the optical diffuser is propagating according to a Levy flight mechanism.

3. Optical diffuser (100) according to claim 1 or 2, wherein the concentration of the scattering particles in the plurality of portions is substantially zero.

4. Optical diffuser (100) according to any one of the preceding claims, wherein the diffusing member (102) is provided in the form of a sheet.

5. Optical diffuser (100) according to any one of the preceding claims, wherein the portions (104) are essentially cylindrical.

6. Optical diffuser (100) according to any one of the preceding claims, wherein the portions (104) have a refractive index essentially the same as a refractive index of the optically transparent material.

7. Optical diffuser (100) according to any one of the preceding claims, wherein the plurality of portions (104) comprise portions of at least ten different sizes.

8. Optical diffuser (100) according to any one of the preceding claims, wherein a cross section of the portions (104) is in the range of 5 micrometer to 500 micrometer.

9. Optical diffuser (100) according to any one of the preceding claims, wherein the portions (104) are extending through the diffusing member.

10. Optical diffuser (100) according to any one of the preceding claims, wherein the optically transparent material is a flexible material.

11. Optical diffuser (100) according to any one of the preceding claims, further comprising a diffusing element (202) arranged adjacently to the diffusing member (102).

12. Optical diffuser (100) according to any one of the preceding claims, further comprising a reflective element (204) arranged adjacently to the diffusing member (102).

13. A luminaire (300) comprising;

a light source (302); and

the optical diffuser (100) according to any one of the preceding claims, wherein the optical diffuser (100) is arranged adjacently to the light source

(302) so that light emitted from the light source (302) is diffused by the optical diffuser

(100).

14. Method for manufacturing an optical diffuser (100), comprising the steps of:

providing (501) a diffusing member (402) comprising scattering particles dispersed in a first optically transparent material;

forming (502) a plurality of cavities (404) in the diffusing member (402), the cavities (404) having a predetermined pattern and a predetermined size distribution.

15. Method according to claim 14, further comprising the step of filling the cavities (503) with a second optically transparent material, wherein a concentration of the scattering particles in the first optically transparent material is substantially larger than a concentration of scattering particles in the second optically transparent material.