WO2007005456A2

WO2007005456A2 - Illumination system with a condensing sphere

Info

Publication number: WO2007005456A2
Application number: PCT/US2006/025145
Authority: WO
Inventors: Wanli Chi; Nicholas George
Original assignee: University Of Rochester
Priority date: 2005-06-30
Filing date: 2006-06-28
Publication date: 2007-01-11
Also published as: WO2007005456A3

Abstract

A highly efficient LED based illumination device includes an optically transparent, dielectric, light condensing enclosure having a generally spherical illumination light reflecting/light refracting surface portion having an index of refraction and a designated back surface portion having an index of refraction and a designated back surface portion, wherein a region of the light reflecting/light refracting surface portion is a reflecting illumination light exit portion. The device includes at least one LED illumination source disposed adjacent the back surface portion, wherein the light emitted from the at least one LED illumination source is at least totally internally reflected, and refracted out of the enclosure having a controlled angular spread illumination pattern.

Description

ILLUMINATION DEVICE WITH A CONDENSING SPHERE

Cross Reference to Related Applications

This application claims priority to U.S. provisional application Serial No. 60/695,340 filed June 30, 2005 entitled Illumination Device With A Condensing Sphere. This application is incorporated herein by reference.

Federally Sponsored Research

This invention was made with U.S. Government Support under Contract Number DAAD 19-03-1-0185, awarded by the US Army Research Office. The United States government has certain rights in the invention.

Background of the Invention

Field of the Invention

Embodiments of the invention generally relate to an optical illumination device (or system) and, more particularly, to an illumination device including a condenser enclosure and at least one associated LED illumination source, which is useful for compact illumination applications. Embodiments of the invention are also directed to an integrally diffused LED illumination source.

Background of the Invention

Conventional light emitting diode (LED) illumination systems utilize an LED with a lens or mirror to collect and direct the light over an intended illumination path. Figure 6 A shows an exemplary conventional design of such a system in which a lens 2 collects and directs light from an LED 1. Figure 6B shows an exemplary conventional design of an LED illuminator having a parabolic mirror 3 that collects light emitted from the LED Ia. Other examples are disclosed in U.S. Patent Nos. 6,246,170 and 6,227,685, incorporated by reference herein. In typical LED illuminator design, device size must be compromised with light collecting efficiency to obtain the desired output illumination. Since an LED emits light from a number of die surfaces, some of the light will be emitted in directions that cannot be collected by the lens 2 shown in Figure 6A. In the case of a mirror-based illuminator as shown in Figure 6B, the mirror has to be made quite deep to have good collection efficiency. The LED and its support structure also acts to block the outgoing light. Thus, control of the broad angular distribution of light from an LED illuminator is important for cosmetic purposes as one observes the object being illuminated. Improvements are needed and desirable in this regard based on the performance and design of conventional LED illuminators referred to above.

Another disadvantage of conventional LED illumination systems can be attributed to the effects of their partial or weak coherence. These may be described as fine-scale spatial features as compared to the coarse illumination path control outlined above. Even with improved path control, we have observed a five to 20% variation in LED illumination that has a narrow angular extent. While this fine-scale variation in intensity is not particularly troublesome to an observer, the inventors have found that it limits the performance of an electro-optical readout system that has been designed to resolve very fine-scale patterns. For example, in a handheld scanner that is designed to read fine-scale barcodes, this variation in the intensity of the illumination causes a deleterious signal or noise in the readout, for instance, by a photodiode array. The inventors' experiments demonstrated an average 10% variation in intensity from an LED with an optical linewidth of 30nm at 617nm. The irregular angular variation in the intensity of the illumination is on the order of magnitude of 10mm at Im distance, or one-hundredth of a radian. This fine scale variation also is rich in harmonic content. The speckle-like fluctuations from this type of source are approximately in the range of λ/w where λ is the average wavelength and w is the effective aperture length of the source. As is well- known from the published literature on speckle, the speckle variations themselves would be on the order of 2E- 3 radians. Low contrast is observed because the LED typically is a multi-tone source that causes some averaging over that observed from a monochromatic source.

In view of the above, the inventors have recognized the advantages of an LED illuminator that is free of (or significantly improves upon) these fine-scale intensity variations. One particular, non-limiting benefit of, e.g., a reduction of a 10% spatial noise to a level below three percent would be a direct improvement in resolution of a handheld (barcode) scanner.

Accordingly, there is a recognized need for an LED illumination source and an LED illumination device or system whose coarse and fine features as discussed above are improved upon and can be controlled. Improved light collection efficiency, compactness, low cost, excellent performance parameters and other valuable attributes are benefits provided by embodiments according to the instant invention. Useful applications of the invention embodiments may include, but are not limited to, barcode scanners, fiber optic communication systems, automotive/aircraft/transport vehicle illumination, and architectural lighting.

Summary of the Invention

An embodiment of the invention is directed to an LED-based illumination device. The device includes a first, optically transparent, dielectric, light condensing enclosure. The enclosure has a sized, generally spherical surface portion having an index of refraction ni_e that will either totally internally reflect the LED illumination light, directly forwardly refract the LED illumination light out of the enclosure to form an illumination path or, to a much lesser extent than either above phenomena, non-totally internally reflect the LED illumination light within the enclosure as lost energy. A region of this light reflecting/light refracting surface portion over which the LED illumination light is refracted out of the enclosure is referred to as an illumination-light exit portion of the first enclosure. The enclosure also has a designated back surface portion and at least one LED illumination source that is attached to the inside or outside surface of the back surface portion, or is disposed adjacent the back surface portion either within or without the first enclosure. The index of refraction, n_le, and the surface shape of the illumination light reflecting/light refracting portion of the first enclosure provide a totally internally reflecting surface for a significant portion of the LED illumination light, while a significant portion of the LED illumination light is also refracted out of the enclosure to form the illumination path. The back surface portion provides a specular reflection or diffuse scattering angle to the totally internally reflected light that breaks the total internal reflection. That light is then refracted out through the exit portion of the enclosure, and so on. The light refracted out of the enclosure has a controlled angular spread illumination pattern at least partially controlled by the exit portion shape and condenser enclosure index of refraction. In an exemplary aspect, the at least one LED illumination source is disposed within the enclosure. In an aspect, an LED illumination source is disposed on a center optical axis of the device. When the LED source comprises multiple LED dies, such LED sources may be provided in a selected pattern inside of or outside of the enclosure. Exemplary patterns include, but are not limited to, one or two-dimensional arrays, a ring or rings, or other shape configurations depending on the desired illumination application.

For ease of discussion and simplification of the summary description herein, the following aspects of the above described embodiment are presented in relation to a spherical enclosure and a single LED die illumination source located within the enclosure adjacent the back surface portion along a center axis of the enclosure. In various aspects, the back surface portion may be in the shape of a planar circular disc connected along its circumferential periphery to the reflecting/refraction portion of the enclosure, or it may, for example, have a convex or concave spherical or other arcuate cross sectional shape with a circular perimeter, or it may have a staircase or ramped cross sectional profile with a circular perimeter. Without exception, every surface shape will act to break the cycle of total internal reflection (TIR) inside the enclosure to ultimately refract the light out through the exit portion. The inner surface of the back surface portion may be mirror coated for specular reflection of the illumination light. Alternatively, a thin diffuser may be applied as a mild diffuser to the inner surface of the back surface portion to scatter the light and thus break the cycle of TIR. The planar disc shaped back surface portion is a special case in that it need not be reflectively or diffusively coated at all. If left uncoated, light that is totally internally reflected from the spherical enclosure surfaces will also be totally internally reflected from the flat back surface, as a person skilled in the art will understand. In a particular aspect, the planar back surface portion is coated with a mild diffuser. As used herein, the term 'mild diffuser' refers to a peak-to-valley surface roughness much less than lOμ, and generally on the order of one micron. As used herein, the term 'thin diffuser' will also be used. An exemplary 'thin diffuser' may be formed of polystyrene spheres having diameters in the range of about 0.1 to 0.5μ. Two or more layers of these polystyrene spheres can be carried in a clear lacquer or a clear UV curable optical cement, for example, or an alkyd resin material known in the art as clear glyptal, which can then be painted on the surface. The thickness of this mixture can be in the range of about 0.1 to lmm and will be referred to as a 'mild, thick diffuser'.

According to a particular aspect of the above described embodiment, the size of the illumination device may be determined by a size ratio between the LED and the enclosure, which satisfies a condition that a light ray from an edge of the LED and a point on the circumferential edge of the back surface portion where it connects to the reflecting/ refracting surface portion of the enclosure undergoes total internal reflection. For certain enclosure configurations, this sets the lower limit on the size of the condensing sphere with respect to the size of the LED die.

According to an embodiment, the LED-based illumination device described above may further include one or more lens elements and/or a mild diffuser disposed externally adjacent the light exit portion of the enclosure along the center optical axis of the device. According to an embodiment, the LED-based illumination device described above may further include a second, optically transparent, dielectric, light condensing enclosure having an index of refraction n_2e that is less than n_e, that partially or fully surrounds the first enclosure. A region of the second enclosure will similarly have a refracting illumination-light exit portion.

Another embodiment of the invention is directed to an illumination device comprising two or more of the illumination device embodiments as set forth above. Multiple illumination devices can be disposed in a housing, wherein each of the illumination devices has a center optical axis, and wherein each of the illumination devices are oriented such that all of the center optical axes project within an overall illumination pattern having an angular spread less than about +50 degrees. In a particular aspect, the overall illumination pattern has an angular spread less than about +25 degrees.

Another embodiment of the invention is directed to an individual LED light source having an integral diffuser. The light source includes an LED die having a plurality of light emitting surfaces and a mild, thick diffuser (as that term is defined herein above) integrally coupled to at least one of the plurality of light emitting surfaces. Alternatively, the at least one light emitting surface may contain an applied matte Finish as is known in the art, which would act as a thin diffuser (as that term is defined herein above).

The objects, advantages and benefits provided by the various embodiments of the invention will become more apparent to the reader in view of the detailed description of a preferred embodiment, the attached drawing figures and the appended claims, which solely define the invention. Brief Description of the Drawings

FIG. 1 is a schematic view of an illumination device showing a single LED source located within a dielectric spherical shaped condenser enclosure having a flat back surface according to an exemplary embodiment of the invention;

FIG, 2 is a schematic view of an illumination device showing a single LED source located within a dielectric spherical shaped condenser enclosure having a convex spherical back portion according to an exemplary aspect of the invention;

FIG. 3 is an end view from the right side of the illumination device of FIG. 1/

FIG. 4 shows a schematic ray-tracing diagram of an illustrative LED-based illumination device;

FIG. 5 is a simplified optical model of the device shown in FIG. 4;

FIGS. 6 A and 6B are diagrammatic illustrations of two conventional style LED- based illumination devices;

FIG. 7 is a diagrammatic illustration of an LED die having an integral diffuser on a surface thereof according to an embodiment of the invention;

FIG. 8 is a diagrammatic cross sectional view of an alternative back surface portion shape of an LED-based illumination device according to an aspect of the invention;

FIG. 9 is a diagrammatic cross sectional view of an alternative back surface portion shape of an LED-based illumination device according to an aspect of the invention; FIG. 10 is a diagrammatic cross sectional view of an alternative back surface portion shape of an LED-based illumination device according to an aspect of the invention;

FIG. 11 is a schematic illustration of an LED die according to an embodiment of the invention;

FIG. 12 is a graph of the far- field angle θ versus the angle θ_\ of the corresponding ray inside the condensing sphere for four different refractive indices according to an aspect of the invention;

FIG. 13 is a graph of the illumination half angle versus the refractive index of the condenser enclosure according to an exemplary embodiment of the invention;

FIG. 14 is a graph of relative intensity versus angles of the far- field illumination pattern for an exemplary LED-based illumination device according to an embodiment of the invention;

FIG. 15 is a graph of the light collection efficiency verses the refractive index of the condensing sphere according to an illustrative embodiment of the invention;

FIG. 16 is a graph showing the results of an exemplary calculation according to an illustrative embodiment of the invention;

FIG. 17 schematically shows an LED-based illumination device including a refracting optical component having a diffuse coating located external to the condensing enclosure for additional illumination control according to an exemplary embodiment of the invention;

FIG. 18 schematically shows an LED-based illumination device including several separate optical refractive and diffusive components located external to the condensing enclosure for additional illumination control according to an exemplary embodiment of the invention;

FIG. 19 schematically shows an LED-based illumination device including an array of multiple LED sources disposed within the condensing enclosure according to an exemplary embodiment of the invention;

FIG. 20 schematically shows an LED-based illumination device including a linear array of multiple LED-based illumination devices disposed in a housing according to an exemplary embodiment of the invention;

FIG. 21 schematically shows an LED-based illumination device having a second dielectric enclosure according to an exemplary embodiment of the invention; and

FIG. 22 illustrated two exemplary refractive index profiles obtainable from the device shown in FIG. 21.

Detailed Description of the Invention

An embodiment of the invention is directed to an LED-based illumination device 10. FIG. 1 is a cross sectional schematic view of an exemplary device 10-1 used to illustrate the structural aspects of an embodiment of the invention. The device 10-1 includes a first, optically transparent, dielectric, light condensing enclosure 14-1. The enclosure 14-1 is a dielectric material such as, but not limited to, glass, polycarbonate, acrylic, plastic or other suitable material known in the art. The material is characterized by an index of refraction ni_e. The enclosure has a designated (described further below), generally spherical illumination light reflecting/light refracting surface portion 15 and a designated back surface portion 16-1. As shown in FIG. 1, back surface portion 16-1 is planar. The back surface portion has a circular periphery 32 as shown more clearly in the end-on view of device 10 shjown in FIG. 3 where it joins an edge of the light reflecting/light refracting surface portion 15 of the enclosure. The device also includes at least one LED illumination source 12 disposed adjacent the back surface portion 16-1. As illustrated in FIG. 1, the LED 12 is located immediately adjacent the back surface portion 16-1 within the enclosure 14-1 along a center optical axis 35 of the device. As will be described further below, one or more LEDs may be provided in a given location or pattern inside or outside the enclosure. The LED(s) may be positioned immediately adjacent the back surface portion of the enclosure or, they may be located a finite distance from the back surface. The one or more LEDs can be embedded in the enclosure or otherwise tightly attached to the enclosure using, e.g., an index matching material as known in the art.

A schematic drawing of an illustrative LED die 12 is shown in FIG. 7. The LED die 12 has a cubic shape having side and front surfaces 1, 2, 3, 4, 5 (surfaces 4 and 5 are not shown) that emits light (illustrated by wavy arrows) in all directions from all surfaces. Wires 12a supply power to the LED. A Lumileds P4 Series LED Chip manufactured by Lumileds Lighting, LLC of San Jose, California is an example of an LED of the type described; however, various others may be obtained and used.

Although the exemplary device as illustrated in FIG. 1 has a spherical light reflecting/light refracting surface portion 15, it will be appreciated by a person skilled in the art in view of the description that follows that the surface portion 15 need not necessarily be smooth and continuous; rather, a faceted design having a generally spherical shape, or other particular variations of surface design may be used as long as the functional attributes of the enclosure are maintained, as further described below.

For ease of description and understanding the principle of the embodiment, the device 10-1 will be assumed to have a smooth and continuous, spherical light reflecting/light refracting surface portion 15 and a single LED 12 located immediately adjacent the planar back surface portion 16-1 inside of the enclosure 14-1, as shown in FIG. 1. The device has a center axis 35. The device 10-1 will emit a controlled illumination path 40 to the right of the figure as illustrated.

Light emitted by the front and four side surfaces of LED 12 will primarily either be refracted out of the condensing enclosure 14-1 or totally internally reflected at the surface 15 of the condensing sphere. A relatively small percentage of the light emitted by LED 12 will be non-totally internally reflected by surface 15 and lost. As is known, whether a light ray is totally internally reflected depends upon the angle of incidence of the light ray to the surface 15. Light rays whose angles are greater than the critical angle equal to arcsin(l/n), where n is the index of refraction of the enclosure 14-3 material, will be totally internally reflected. Light rays whose angles are less than the critical angle will be refracted by the enclosure surface 15 and exit the device in the form of the illumination path 40 (rays 18', 19'). The region of the surface 15 through which the light rays refract out of the enclosure is referred to herein as the light exit portion 21. For example, the light rays represented by the dashed lines 18 are directly refracted by the surface 15 through the light exit portion 21 and are emitted from the device as rays 18' in the forward direction as part of the illumination path 40. The rays 18 are directly refracted because their angle with respect to surface 15 is at or less than the critical angle. The light rays represented by the solid lines 19, 20 are totally internally reflected by the surface 15 of the enclosure without energy loss. If back surface portion 16-1 was a continuation of surface 15 as shown by the dotted line (i.e., surface 15 then being a perfect sphere), then the light rays 19, 20 would be trapped within the enclosure, hi fact, if surface 15 is spherical, the TER. rays (e.g., 19, 20) striking planar back surface portion 16-1 will continue to be totally internally reflected from that surface because their incidence angles are greater than the critical angle. However, when they again encounter surface 15, they will refract out through the exit portion 21 as rays 19', 20' as part of illumination path 40 (it may take more than one reflection from surface 16-1 for these light rays to be refracted out of the spherical enclosure 14-1, but since there is no energy loss in total internal reflection, the condensing enclosure is an extremely efficient illuminator). Thus, planar back surface 16-1 does not require a reflective mirror coating to function as a reflective surface, however, the power supply wires 12a may appropriately be coated with light reflective material to avoid light absorption loss. They can also be mechanically set very close to LED 12.

As will be further described below, the illumination path angle, i.e., the angular spread of light 40 exiting enclosure 14-1, is determined by the refractive index of the dielectric material of the enclosure, the position of LED 12 with respect to the enclosure, and the size of the back surface portion 16-1.

FIG. 2 shows a related device embodiment 10-2 of the invention. The differences between illumination device 10-2 and device 10-1 as shown in FIG. 1 reside in the shape of back surface portion 16-2 and the exemplary position of LED 12. Unlike the planar back surface portion 16-1, illumination device 10-2 has a back surface portion 16-2 that is spherically convex and essentially continuous with spherical light reflecting/light refracting surface portion 15. The periphery 32 of back surface portion 16-2 is connected to the edge of enclosure surface 15 similarly to that as described above for device 10-1. The interior surface of back surface portion 16-2 is made to be an optically diffuse surface as indicated by the wiggly line 16-2a. The diffuse surface 16-2a acts to break the total internal reflection of light rays totally internally reflected from surface 15. The light rays represented by the thick line 18a are directly refracted out of the enclosure 14-2 through exit portion 21 as rays 18a' to form part of the illumination path 40. The light rays represented by the lighter line 19a, after being totally internally reflected a number of times are scattered by diffuse surface 16-2. As before, part of the light energy will be refracted out through the exit portion as rays 19a', while some amount of the light remains trapped and will be scattered again by surface 16-2. As further shown in FIG. 2, the LED 12 is positioned within the enclosure 14 along the center axis 35 of device 10-2 a distance d from the back surface portion.

Before describing the scattering surface characteristics of the diffuse back surface portion 16-2, it should be appreciated that the embodiments of an illumination device 10 are not limited to the specific shapes of the back surface portions 16-1, 16-2 as described above. Other shapes including, but not limited to, concave staircase 16-3 (FIG. 8), concave ramp 16-4 (FIG. 9), conical or concave meniscus 16-5 (FIG. 10), and others, may be used. A particular back surface portion 16 may be cut, deposited, or coated onto an appropriate region of the enclosure 14 surface.

The following discussion of the diffuse surface of the back surface portion 16 of condensing enclosure 14 applies equally to the planar surface 16-1 of device 10-1, the convex spherical surface 16-2 of device 10-2 and any other appropriately shaped back surface portion. It is of no consequence that planar back surface 16-1 does not require any surface enhancement or modification (e.g., reflective coating or roughening), whereas, due to its shape, surface 16-2 does require some form of optical diffusivity.

In general, the back surface portion 16 may be made into a specularly reflecting surface through the application of a reflective material, such as a metallic paint or other reflective coating as is well known in the art. The surface may likewise be made to diffusely reflect (scatter) light by known means including, e.g:, doping the back surface portion or by embedding flakes or small particles with a different index of refraction than the enclosure material.

Despite providing control over the maximum angular spreading of the illumination beam 40 by the refracting attributes of the condensing enclosure 14, the device 10 still may not provide an optimum illumination beam for a particular application. For example, we have observed a 5% to 20% intensity variation in an illuminating beam that is otherwise narrow in angular extent. The observed fine-scale spatial features of the illumination beam are suspected to be due to the partial coherence of typical LEDs resulting in the well known phenomenon of speckle. For a more complete discussion of speckle, the reader is directed to US Pat. No. 6,259,561, the disclosure of which is herein incorporated by reference to the fullest allowable extent. This fine-scale intensity variation has been demonstrated to limit the performance of an electro-optical readout system that has been designed to resolve very fine-scale patterns. For example, in a handheld scanner that is designed to read fine-scale barcodes, this variation in the intensity of the illumination can cause a deleterious signal or noise in the readout, e.g., by a photodiode array. In our experiments, we observed an average 10% variation in intensity from a conventional LED illumination source with an optical linewidth of 30nm at a wavelength of 617nm. The irregular angular variation in the intensity of the illumination is on the order of magnitude of 10mm at Im distance, or one- hundredth of a radian. This fine scale variation also is rich in harmonic content. We further observed that the speckle-like fluctuations from this type of source are in the approximate range of λ/w where λ is the average wavelength and w is the effective aperture length of the source. As is well-known from the published literature on speckle, the speckle variations themselves would be on the order of 2E-3 radians. Low contrast may result from the fact that the LED is a multi-tone source and this causes some averaging over that observed from a monochromatic source.

According to a particular aspect of the LED-based illumination device embodiments described herein, a mild diffuser (i.e., peak-valley surface roughness much less than lOμ, and on the order of about lμ) is incorporated into the condenser structure. The spherical condensing lens structure is ideal for this, since it makes it possible to have a cascade of "thin" or mild diffusers which are separated along the optical path of the output beam. (An example of a 'thin' diffuser is the matte finish on anti-reflective picture glass). In an exemplary aspect, the diffuser includes a cascade of thin, mild diffusers, acting independently. For example, generally spherical polystyrene particles having diameters of between about 0.1 to 0.5μ are commercially available. Diffusing particles in this size range represent thin diffusers. Nine such diffusers, for example, spaced by a few coherence lengths will, on average, reduce the 10% variation in intensity to less than 3%. The reduction of a 10% spatial noise to a level below 3% translates directly to an improvement in resolution. It is possible to package the diffuser into a length as thin as 0.1 to lmm. For instance, the tiny polystyrene spheres can be embedded in a transparent lacquer or transparent epoxy (e.g., UV curable optical cement or glyptol). This mixture can then be painted on the back surface portion of the condensing enclosure, creating a coating that is approximately lmm thick. We describe this diffuser as a mild, thick diffuser. Another very effective diffuser may be obtained by embedding the tiny particulates in the condensing optics of the spherical enclosure itself. Another effective means is to locate the particulates close to the LED. One can use tiny polystyrene or quartz particulates suspended in a polycarbonate host or carrier. Another similar diffusing structure may be obtained by entraining tiny particulates, including air bubbles, in a polystyrene host. These various diffusing structures can be obtained in a variety of ways that are evident to an optical designer of ordinary skill.

Overall efficiency of the LED-based illumination device is important. As such, it is advantageous to have as lossless a diffuser as possible. Therefore, according to another exemplary embodiment as shown in FIG. 11, an LED die 12-1 as shown in FIG. 11 (similar to LED die 12 in FIG. 7) has at least one face (e.g., 1) of the die coated with a mixture 52 of tiny polystyrene spheres embedded in a transparent lacquer or transparent epoxy as described above. The selected output face of the LED die can be painted with such a mixture to provide an integral mild diffuser. This integrally diffused LED source could be used in the illumination device embodiments described herein, or in any application that would benefit from such a LED source of illumination.

The optical design of an illumination device according to an embodiment of the invention will now be described. FIG. 4 shows a schematic ray-tracing diagram of an illustrative LED-based illumination device 10-3. The device includes a condensing enclosure 14-3 having a spherical light reflecting/light refracting surface region 15 and a circular, convex spherical back surface portion 16-3. The back surface portion 16-3 has a mild diffusing surface as described above. An LED 12 is disposed inside of the enclosure immediately adjacent the back surface portion 16-3 along a center axis 35 of the device. The arrow AB denotes a ray from the front face edge (1) of the LED 12 to the peripheral edge 32 of the back surface portion 16-3 where it connects to surface 15. As shown, dashed line 18-3 represents a light ray that is directly refracted out of the enclosure through exit portion 21, while solid line 19-3 represents a light ray that is totally internally reflected until it is eventually refracted our of the enclosure through exit portion 21.

FIG. 5 shows a simplified optical model 10' of the illustrative device 10-3 shown in FIG. 4. Assume the LED 12 is point source located at the center of the back surface portion 16-3 inside the condensing sphere 14. One can further assume that the scattering mirror is circular, as this will generate a circularly symmetric illumination pattern. The back surface subtends a half-angle a as shown.

Angular Distribution of Illumination Pattern

First we consider the rays 18-3 (FIG. 4) emitted by the LED 12 that are directly refracted out of the condensing sphere as rays 18-3'. For a light ray emitted at the angle θ i , the ray 18-3 ' will refract to the far field with a direction denoted by the angle θ. Using Snell's law, one can compute the angle θ as

0 = 26>i - arcsin(n sin 0i), (1) where n is the ratio of refractive index inside and outside the condensing sphere, and θ\ must satisfy

0i < 0i_ma_x = arcsin(l/n) (2) to avoid total internal reflection so that the ray will refract at the surface of the sphere.

With further reference to FIG. 5, FIG AA shows a graph of the far-field angle θ versus the angle θ_\ of the corresponding ray inside the sphere for four different refractive indices: n = 1.33, 1.5, [6(3 ^m) - 8]^1/2, and 1.7, based on Eq. (1). FIG AA shows how the maximum value of θ, denoted by θ^, varies with the index of refraction as follows. It is straightforward to compute the maximum angle of the illumination pattern for the directly refracted rays. One can solve dθ/dθ_\ = 0 to find the angle θ_\, and then substitute it into Eq. (1). The result is

Θ_L = 2 arcsin [(4 - n²)/3n²] ^m - arcsin [(4 - n²)/3] ^m (3) where θ_\_ is the maximum angle of illumination pattern for rays from the LED that are directly refracted by the sphere. FIG. 13 shows the relationship between the angle Θ_L and the refractive index, n, of the condensing sphere material. Note that θ^ only depends on the refractive index of the condensing sphere (n < 1.81).

When n = [6(3^1/2) - 8]^1/2 = 1.547, the rays from the LED with angle 0, very close to θ_{\ max} will be refracted by the sphere in a far- field direction outside the pattern defined by 0_L. An example is shown in the lower right of FIG. 12 where n = 1.7. However, the contributions from these are negligible compared to the total energy. Now consider the rays that are totally internally reflected and scattered by the back surface portion. Let point P be positioned at the circumferential edge of the scattering back surface portion where it connects with the surface 15; thus P subtends the angle a as shown in FIG. 5. A fraction of the scattered rays from point P are refracted by the condensing sphere and radiated to the far field. By symmetry, the maximum angle of the far-field illumination pattern θ$ can be written as θ_s = θ^' _L + a, (4) where θ i = θι as expressed in Eq. (3).

At this point, one can calculate the angular distribution pattern of the illumination field. This calculation is complicated and we have used the nonsequential ray tracing function of the ZEMAX^® optics system design software (Zemax Development Corporation, http://www.zemax.com) to provide quantitative results. FIG. 14 shows the relative illumination intensity for the spherical condenser lens. In this example, the refractive index of the sphere is chosen to be 1.5. The back surface portion extends over a half angle of a = 10°. The far- field intensity is calculated with an imaginary planar detector located at a distance of 10m to the right of the sphere with pixel size of 150mm x 150mm. The curve in FIG. 14 represents the point source radiating to the right half-plane only, i.e., the radiation angle is 180°. The difference between the far-field intensity distribution for an isotropically radiating source (not shown) was observed to be small, implying that the far-field illumination pattern depends only slightly on the intensity distributions of the source. It was further observed that there are two regions in the illumination pattern, the center bright region with half angle #_L and the extended taper region between #_L and θs, where 0_L and θs are determined by Eqs. (3) and (4). In the current example, 0_L = 11 -4° and Θ_L = 21.4°. Equations (3) and (4) are thus sufficient for the initial choices of the refractive index and scattering mirror size in the illumination design. The curve in FIG. 14 shows a circular ring peak and a center peak in the illumination pattern. The main energy contribution to the peak of the circular ring is from the rays refracted from a narrow ring defined by θ_\ (θ = #_L) shown as point M in FIG. 12; the main contribution to the center peak is from the rays refracted from a narrow ring defined by ^₁ (0 = 0) shown as point N in FIG. 12. For a simulation with n = 1.33, in the illumination pattern (not shown) there still exists a circular ring peak, but there is no center peak. The center peak intensity value is actually the average intensity over a region of 150mm x 150mm. The arrow at the top of FIG. 14 means the on-axis intensity value is much higher. The average axial intensity in a 1.5mm x 1.5mm region, calculated using ray optics, is over 20 times larger than that in the 150mm x 150mm region as shown in FIG. 14. The exact axial intensity is irrelevant for two reasons: (1) diffraction is not taken into account in the simulation, and (2) the central peak decreases and even disappears when the ideal point source is replaced by a source with finite size, as will be shown below.

Light Collection Efficiency

In the LED-based illumination device embodiments described herein, the light loss mainly comes from the reflection (not total internal reflection) of the rays from the enclosure surface 15-3, shown as the short-dashed ray F in FIG. 4. We have assumed that the back surface portion 16-3 is small enough that these partially reflected rays cannot reach the scattering surface without significant loss of energy due to more partial reflections at the sphere surface. Also, we have assumed the rays are not polarized. From Fresnel reflection formulas, the energy transmission coefficient can be written as T(0i) = 1 - Vi[[n cos θ_λ - (1 - n² sin² ^₁)¹'²/ n cos θ_x + (1 - n^l sin^z θι)^ιuγ +

[cos 0, - n(l - n² sin² ^₁)¹⁷²/ cos θ_x + n(l - n² sin² 0,)^1/2]²] . (5)

The theoretical light collecting efficiency can be expressed as η

άθ{\ . (6)

The substitution of Eqs. (2) and (5) into Eq. (6) yields the efficiency of the condensing sphere. In this derivation of efficiency, we have assumed that the rays refracted through the sphere exit portion defined by θ_\ < θ_\maκ are uniformly distributed over the sphere surface. In FIG. 15, the efficiency calculated from Eq. (6) is compared to the efficiency using the ZEMAX simulation. In the ZEMAX simulation, we assumed the point source radiated 360° isotropically, and the back surface scattering portion had a size defined by α=10°. Since there is no absorption loss in the idealized condensing sphere, an imaginary large spherical detector array is placed around the sphere. The efficiency is calculated by finding the ratio of power in the illumination direction defined by θ < θ$ and the total power on the detector array. There are two reasons for slightly higher values from the ZEMAX results: (i) the partially reflected rays that can be scattered by the scattering back surface portion and sent in the illumination direction are included in the ZEMAX simulation; (ii) in the instant example, the scattered rays immediately before refraction by the sphere are more likely to have a smaller θ_\ value, which means a higher transmission coefficient as shown by Eq. (5). However, the small difference between the two curves in FIG. 15 implies that these two effects are not significant. Therefore, one can use Eq. (6) to make a first-order estimation of the light collecting efficiency. The curves in FIG. 15 show that the condensing sphere is capable of collecting most of the energy emitted by LED. For an index of refraction of 1.5, the central beam of radiation contains slightly more than 90% of the optical light emitted.

Example

With reference to FIG. 4, a cube-shaped LED 12 die with a size of t = 0.3mm was buried inside the edge of the sphere 14-3 that had a refractive index n = 1.5. The size of the back surface portion 16-3 is defined by α =10°. The refractive index, n, and a are the same as those used in the description of FIG. 14 for ease of comparison. Four different cases of condensing spheres 14-3 with diameters of 6, 10, 20, and 40mm are calculated. Forty percent of light energy is emitted from the front surface and 15% from each of the four side surfaces of LED 12. The light-emitting pattern is Lambertian for all five surfaces. We believe that for a condensing system it is advantageous to effectively couple the 60% energy emitted from the side surfaces of the LED. The intensity of the illumination pattern was calculated with an imaginary planar detector located at a distance of 10m away from the sphere 14-3. The pixel size of the detector was 150mm x 150mm. The results for the 6, 10 and 20mm spheres are shown in FIG. 16. Since the illumination patterns are rotationally symmetric, only radial data is plotted. The symmetry of the illumination pattern with a cubic die implies that condensing spheres with different die shapes, e.g. a cylindrical die, will generate similar far-field illumination patterns. We have observed that that the shape and radiation pattern of the LED die have little effect on the far-field illumination pattern. These results fit well with the simplified model for the angular spread of the illumination pattern. The maximum angular spread of illumination pattern is very close to θs ⁼ 21.4°. The angle of the central bright region is close to Θ_L = 11,4°, as calculated from the simplified model of FIG. 5.

The exemplary illumination device 10-3 may be manufactured by encapsulating (or molding) the LED 12 within a material having a desired index of refraction with respect to the medium expected outside the sphere surface 15-3 into a sphere 14-3 of desired size. Also, the sphere 14-3 may be fabricated by embedding an LED in a block of material of the desired index of refraction, and then machining the block by diamond turning (e.g., lathe) into a sphere 14-3 of desired size. In the current example, the medium is air, and thus n in the above equations is the index of refraction of the condenser enclosure material. If, however, the medium is not air (e.g., water, or optical lens material), it will have a different index of refraction , thus n in the above equations must be divided by the index of refraction of the medium. A reflection or scattering coating as desired can be formed as known in the art and as described herein above. As stated earlier,^" electrical wires (or chip) of LED 12 may be coated with reflective material if such are located within the sphere.

We have further observed that the size of the LED die has a significant effect on the far- field intensity pattern. As the diameter of the condensing sphere decreases, the intensity distribution deviates from the simplified model. As shown in FIG. 16, the illumination patterns begin to change and the central and ring peaks disappear for small sphere diameters. More generally, in the geometrical optics domain, the far-field pattern depends on the size ratio between the LED die and the condensing sphere. The ray reflection and refraction pattern will remain identical if, e.g., a 6mm condensing sphere with 0.3mm die is changed to a smaller package of 3mm sphere with 0.15mm die; thus the far field also has the same intensity distributions.

For the LED-based illumination device configuration 10-3 shown in FIG. 4, the size ratio between the LED die 12 and the sphere 14-3 needs to satisfy a condition that the ray from the edge of the LED to the edge 32 of the scattering mirror, denoted by AB in the figure must undergo total internal reflection. This sets the lower limit on the size of the condensing sphere for this type of setup. In the above example of a 0.3mm cubic die, α=10°, and n = 1.5, the diameter of the sphere can be as small as 6mm. The efficiency of the four different exemplary condensing spheres were calculated with ZEMAX. The light collecting efficiencies for 6, 10, 20, and 40mm spheres were 90.2, 90.7, 90.8, and 91.0%, respectively. These results coincide with those of the simplified model, which gives an efficiency of 91.13% according to the ZEMAX calculation and 90.09% from Eq. (6). We observed that the efficiency remains about the same for different sizes of condensing spheres as long as the rays from the LED to the edge of the scattering back surface portion undergo total internal reflection.

It should be appreciated that other embodied configurations of condensing spheres as described herein above are not constrained by the size limitation discussed immediately above. One case, or instance, would be where the LED die is attached to the outside of the condensing sphere enclosure.

FIGS. 17, 18, 19 and 20 illustrate alternative LED-based illumination device embodiments of the invention. FIG. 17 illustrates an illumination device 10-4 including an illustrative condensing sphere 10' as modeled in FIG. 5 and a lens element 102 disposed external to the condensing sphere 10' adjacent the exit portion 21 of the sphere. The exemplary lens element 102 includes a refracting portion 105 having a surface 115 that is coated with a mild diffuser 116 as described herein above. The lens element 102 provides additional coarse illumination path control as well as fine-scale intensity variation control.

FIG. 18 shows a conceptual embodiment of an LED-based illumination control device 10-5 similar to the device 10-4 shown in FIG. 17. In device 10-5, separate refracting elements 121, 125 and a diffuser 129 are disposed outside of the enclosure adjacent the exit portion 21 of the sphere.

FIG. 19 schematically shows a device 10-6 having a plurality of LEDs 12a, 12b, 12c...arranged in an array pattern inside of the condensing enclosure. These variations and others may provide noncircular symmetric illumination patterns or other desired illumination patterns.

FIG. 20 schematically shows a device 200 having a linear array of condensing spheres 10 disposed in a housing 203. This type of device can be used to produce a line illumination pattern, for example. Two dimensional array configurations are also possible.

It may be advantageous to construct the reflecting or diffusing back surface portion of the illumination device embodiments out of metal, which may function as a heat sink for LED 12 in high power applications.

FIG. 21 schematically shows an aspect 10-7 of an LED-based illumination device embodiment according to the invention. An illumination device such as device 10-3 represented in FIG. 4 is enclosed by a second, optically transparent, dielectric, light condensing enclosure 14-4 having an index of refraction n_2ethat is less than n_e. The second enclosure 14-4 has a light exit portion Il similar to exit portion L i ot enclosure 14-3. Second enclosure 14-4, which may partially enclose the first enclosure, can be used to further modify the output illumination pattern. FIG. 22 illustrates two exemplary refractive index profiles obtained by device 10-7. The solid line indicates a step index profile. The dotted line illustrates a gradient index profile.

Having thus described the various embodiments of the invention, it will be apparent to those skilled in the art that the foregoing detailed disclosure is presented by way of example only and thus is not limiting. Various alterations, improvements and modifications recognized by those skilled in the art, though not expressly stated herein, may be made and are intended to be within the spirit and scope of the claimed invention. Additionally, the recited order of processing elements or sequences, or the use of numbers, letters, or other designations, is not intended to limit the claimed processes to any order except as may be specified in the claims. Accordingly, embodiments of the invention are limited only by the following claims and equivalents thereto.

Claims

We claim:

1. An illumination device comprising: a first optically transparent, dielectric, light condensing enclosure having a designated, generally spherical illumination light reflecting/light refracting surface portion having an index of refraction ni_e and a designated back surface portion, wherein a region of the light reflecting/light refracting surface portion is a refracting illumination light exit portion; and at least one LED illumination source disposed adjacent the back surface portion, wherein the light emitted from the at least one LED illumination source is at least totally internally reflected from the light reflecting/light refracting surface portion and refracted out of the enclosure through the refracting illumination light exit portion, further wherein the light refracted out of the enclosure has a controlled angular spread illumination pattern.

2. The illumination device of claim 1, wherein the at least one LED illumination source is disposed within the enclosure.

3. The illumination device of claim 2, wherein the at least one LED illumination source is disposed on a center optical axis of the device.

4. The illumination device of claim 1, wherein the back surface portion is generally planar and has a generally circular circumferential region where it connects to the light reflecting/light refracting surface portion of the enclosure.

5. The illumination device of claim 1, wherein the back surface portion is a portion of a sphere having an arcuate, concave cross sectional profile and a generally circular circumferential region where it connects to the light reflecting/light refracting surface portion of the enclosure.

6. The illumination device of claim 1, wherein the back surface portion a portion of a sphere having an arcuate, convex cross sectional profile and a generally circular circumferential region where it connects to the light reflecting/light refracting surface portion of the enclosure.

7. The illumination device of claim 1, wherein the back surface portion has a ramped cross sectional profile and a generally circular circumferential region where it connects to the light reflecting/light refracting surface portion of the enclosure.

8. The illumination device of claim 4, wherein the back surface portion is an uncoated surface having an index nj_e.

9. The illumination device of claim 2, wherein the back surface portion has a reflective mirror coating.

10. The illumination device of claim 2, wherein the back surface portion has an optically diffuse coating.

11. The illumination device of claim 10, wherein the diffuse coating is a mild diffuser comprising a plurality of particles adhered to the back surface.

12. The illumination device of claim 11, wherein the plurality of particles are one of polystyrene and quartz, further wherein each of the particles has a diameter in the range between about 0.1 to 0.5 microns.

13. The illumination device of claim 10, wherein the diffuse coating is a mixture of a thin diffuser and a carrier that form a mild, thick diffuser having a thickness of less than or equal to about 1.0mm.

14. The illumination device of claim 13, wherein the carrier is one of a clear lacquer and a clear UV curable optical cement.

15. The illumination device of claim 2, wherein a size ratio between the at least one LED and the enclosure satisfies a condition that a light ray from an edge of the at least one LED and a point on the circumferential edge of the back surface portion where it connects to the generally spherical illumination light reflecting/light refracting surface portion undergoes total internal reflection.

16. The illumination device of claim I₃ further comprising at least one of at least one lens element and a mild diffuser disposed externally adjacent the light exit portion of the enclosure along the center optical axis of the device.

17. The illumination device of claim 1, further comprising a second optically transparent, dielectric, light condensing enclosure having an index of refraction n_2ethat is less than n_e, at least partially enclosing the first enclosure, wherein a region of the second enclosure is a refracting illumination light exit portion.

18. A light source, comprising: an LED die having a plurality of light emitting surfaces; and a mild light diffuser integrally coupled to at least one of the plurality of light emitting surfaces.

19. The light source of claim 18, wherein the mild diffuser is a mixture comprising a plurality of one of polystyrene particles and quartz particles.

20. The light source of claim 19, wherein each of the polystyrene or quartz particles has a diameter in the range between about 0.1 to 0.5 microns.

21. The light source of claim 19, wherein the mild diffuser mixture has an applied thickness of less than or equal to about 1.0mm.

22. The light source of claim 19, wherein the mixture further comprises one of a clear lacquer and a clear UV curable optical cement.

23. An illumination device, comprising a plurality of illumination devices as set forth in claim 1 disposed in a housing, wherein each of the plurality of illumination devices has a center optical axis, further wherein each of the plurality of illumination devices are oriented such that all of the center optical axes project within an overall illumination pattern having an angular spread less than about +50 degrees.

24. The illumination device of claim 23, wherein the overall illumination pattern has an angular spread less than about +25 degrees.