Patent Grant
8662716
This invention generally relates to optical elements and, more particularly, to side-emitting, optical elements and methods thereof.
Typically, a light emitting diode (LED) die emits light into a full hemisphere. For some applications, such as for general room lighting, such an output can be desirable. However, for other applications, such as for display-panel backlighting, a side-to-side output emission can be required. Typically, in such applications it is desirable that the light is emitted into only a few degrees of angle relative to a display-panel viewing plane, so the light can become homogenized before exiting the backlight.
A variety of devices have been developed to condense an LEDs hemispherical emission and redirect it into sideways direction. Unfortunately, none of these prior devices perform efficiently and generally emit light into a broad exit angle, or into other unmanageable directions.
One such well-known side-emitting optical 1, element is illustrated in
Other rays, such as ray 6, that are emitted from the source 14 in a direction greater than about 30 degrees from the optical axis 3 enter the lower refractive section 2, whereupon they are incident on output surface 9. These rays refract through the output surface 9 into a direction that is somewhat perpendicular to the optical axis 3.
Yet other rays, such as ray 10, that are incident on the upper surface 5 of the upper TIRing section 4 at or near where the optical axis 3 intersects the upper surface 5, will refract through upper surface 5 into a non-side-emitting, or non-radial, direction. These rays intersect the upper surface 5 at or near the apex of the upper surface. Ideally the apex is dead-sharp, but because of non-ideal manufacturing processes the apex will have a non-zero radius of curvature, and will also be non-zero in size. Light rays such as ray 10 will refract through this section into a non-controlled, non-radial, and undesirable direction.
It is difficult to block rays such as ray 10 by installing a plug into the recess 11, because the plug will need to be in optical contact with the upper surface 5, which will destroy the TIRing property of the upper surface 5, and compromise the performance of the optical element 1. Furthermore, the plug cannot be placed into the recess and be expected to stay in place due to friction, which necessitates the use of an adhesive along upper surface 5, which will further compromise the TIRing performance of the upper surface 5 and degrade the performance optical element 1.
Because of the stray light rays, such as ray 10, the inability to block them at the upper surface 5, and the generally poor side-emission characteristics of the optical element 1, the optical element 1 is undesirable to use in many applications.
An optical element in accordance with embodiments of the present invention includes an outer TIRing section having a circumferential curved sidewall between a light input surface at one end and a light output section at the upper end that provides substantially total internal reflection of light emitted from the input surface and incident upon the sidewall of the lower section. A conical TIRing upper surface in conjunction with a cylindrical output surface constitutes a light output section, that redirects light input to it from the lower TIRing section into a substantially radial direction. Lastly, a section that is recessed within the outer TIRing section and the upper light output section intercepts light from the source and prevents it from directly reaching the output section, and instead by TIR redirects the light so that it passes through a lower portion of the outer TIRing section such that the light exits into a substantially radial direction.
An opaque plug can be provided to fit in the recess of the present invention that serves to block stray light from exiting in an axial direction, wherein the plug is only in optical contact with the sidewalls of the inner recess so that the TIR capabilities of the conical TIRing upper surface and the lower TIRing surface are not compromised.
Accordingly, the present invention provides an optical element that may be optically coupled to one or more LED sources to provide low-loss light concentration in a radial direction. Additionally, the present invention provides a condensing element that is easy and inexpensive to manufacture and which has a compact design.
The present invention relates to a compact and highly efficient optical element that is well suited to collecting the light emitted from a source, and redirecting the light so that it exits the optical element in a sideways direction. An optical element 20 in accordance with embodiments of the present invention is illustrated in cross-section in
Referring to
The upper output section 24 consists of a conical-shaped (in non-cross-section) TIRing surface 28 and a cylindrical-shaped (in non-cross-section) output surface 30. The inner recessed section 26 consists of a recess 32 at the bottom of which is located a TIRing surface 44. The inner recessed section 26 is located below the conical void 34 bounded by the TIRing surface 28. Finally, an input surface 36 is located at the bottom of the present invention 20, and is bounded by the inner recessed section 26 and the outer TIRing section 22.
Typically the outer TIRing section 22, the upper output section 24, the inner recessed section 26, the conical-shaped TIRing surface 28, the cylindrical-shaped output surface 30, the recess 32, the conical void 34, and the input surface 36 are circularly symmetric about an optical axis 3, although they can be asymmetric as well in order to obtain special non-radially symmetric light output distributions.
A light source 14 is normally situated at the input surface 36, at the optical axis 3. Typically, the input surface 36 is a few millimeters wide, and is plano, as shown in
The light source 14 may be a colored light source, such as red, green or blue or contain phosphor to emit a white colored light. Another way in which to generate white light may include providing red, green, and blue colored LEDs in combination. The present invention may utilize a single source 14, although other numbers and/or types of light sources could be used with the optical element 20. However, when using more than one light source, the sources should be close to one another and near the optical axis 3. By way of example only, multiple light sources 14 about the optical axis are illustrated in
Light from the source 14 is typically emitted into a full Lambertian hemisphere into the optical element 20, although other distributions may be utilized, and the input distribution may be less than a full hemisphere.
Referring more specifically to
The TIRing section 22 has a one-sided, rotationally symmetric configuration, although the TIRing section 22 may have other types and numbers of sides, shapes, and configurations, such as four-sided, six-sided, eight-sided, triangular, square, and rectangular and could have an asymmetric configuration. The TIRing section 22 has a circumferential curved sidewall 38 between a light input surface 36 at one end and a light output surface 30 at the other end that provides a substantially total internal reflection of light emitted from the source 14 and incident upon the sidewall 38, although the TIRing section 22 may have other numbers and types of top, bottom and side walls.
The light input surface 36 has a plano configuration to facilitate the attachment of a light source 14, such as an LED, although the light input surface 36 may have other configurations, such as convex or concave, or have a recess or protrusion to facilitate placement, installation, or attachment of the source 14.
The sidewall 38 is formed to have a curvature that provides substantially total internal reflection (TIR) of light entering at the light input surface 36 although the sidewall 38 could have other properties and configurations. In particular, the slope angle of the sidewall 38 is selected so that light from the source 14 will be substantially totally internal reflected at all locations on the sidewall 38.
Ideally after light rays from the source 14 are TIRed by the sidewall 38, they are directed into a direction that is substantially parallel to the optical axis 3, as illustrated by ray-segment 42B in
To facilitate molding, in particular the mold-release operation of an injection molding process, the cylindrical output surface 30 can be made non-cylindrical, and have a slight inward angle as illustrated by θ1 in
The inner surface 44 of the inner recessed section 26 has a rotationally symmetric configuration about and tapers to an end point at an optical axis 3, although the inner surface 44 may have other configurations and other types and numbers of sides, such as four-sided, six-sided, eight-sided, triangular, square, and rectangular and could have an asymmetric configuration.
The inner recessed section 26 also has a sidewall 46 which is formed to be substantially linear or non-curved in cross-section, whose sides may be parallel to an optical axis 3. However, the sides of sidewall 46 are generally not parallel to the optical axis 3, such that the diameter of the recess 32 is greater at the top (or opening end) than at the bottom of the recess 32, to facilitate removal of the optical element 20 from the mold during manufacturing. The sidewall surface 46 may also be symmetric about an optical axis 3, but may be asymmetric, or may have other numbers and types of walls in other shapes and configurations, such as concave, convex, parabolic, elliptical, or otherwise characterized by a polynomial. By way of example only,
The purpose of the inner surface 44 is to TIR all light rays that are incident upon it, such as ray-segment 40A, from the source 14 into a somewhat radial direction as illustrated by ray-segment 40B. These rays are then incident on the outer TIRing surface 38, and refract through it into a radial output direction as illustrated by ray 40. Clearly the design of the inner surface 44 must take into account the surface profile of the TIRing outer surface 38 so that the emitted rays 40 are directed into a sideways or radial direction.
An isometric view of the optical element 20 is presented in
In
The operation of the condensing element 20 will now be described with reference to
A light ray 40B that exits the source 14 non-obliquely, within a cone subtending about ±30° relative to the optical axis 3, becomes incident on the inner TIRing surface 44 of the inner recessed section 26, whose prescription is designed to allow the light ray 40B to TIR, as well as be directed into a substantially sideways direction relative to the optical axis 3. The reflected light ray 40B is then incident on the TIRing outer surface 38 of the outer TIRing section 22, whereupon the ray 40B undergoes refraction and exits through the outer surface 38 in a direction substantially perpendicular to the optical axis 3.
Note that the angle of incidence of ray-segment 40B on the outer TIRing surface 38 does not exceed the critical angle, and therefore does not TIR, but instead refracts through the outer TIRing surface 38. On the other hand ray-segment 42A is obliquely incident on the outer TIRing outer surface 38, and since its angle of incidence exceeds the critical angle, ray 42A will TIR instead of refract through the outer TIRing surface 38.
A diagram illustrating an example of the geometrical calculations for determining the curvature of the TIRing outer surface 38 is illustrated in
In these calculations, the variables are:
⊖i: The light exit angle from the source 14 with respect to the light input surface 36;
⊖S: The instantaneous angle of a differential TIRing surface element of the TIRing outer surface 38 with respect to the light input surface 36;
⊖R: The angle of incidence that the light makes with the differential surface element of the TIRing outer surface 38;
h1: The vertical distance from the base of the input surface 36 to the rays point of incidence on the TIRing outer surface 38;
h2: The vertical distance from the source 14 to the rays point of incidence on the TIRing outer surface 38;
ρ: The radial distance from the light source 14 to the point of incidence on the TIRing outer surface 38.
Additionally, in these calculations tan ⊖i=h2/ρ which can be restated as
⊖i=tan−1(h2/ρ). (Equation 1)
Also, by inspection
180=⊖R+⊖R+(90−⊖R)+(90−⊖S) (Equation 2)
or equivalently
⊖S=⊖R. (Equation 3)
Also
180=⊖i+(180−⊖S)+(90−⊖R) (Equation 4)
which simplifies to
⊖S=⊖i−⊖R+90 (Equation 5)
and combining equations 1, 3, and 5 we have:
⊖S=tan−1(h2/ρ)/2+45°. (Equation 6)
A spreadsheet can be prepared from these calculations with the coordinates of the profile, ρ and h2, as input variables, and the instantaneous slope ⊖S of the TIRing outer surface 38 as the output variable. For the optical element 20 of the present invention, such a spreadsheet is presented in
The design of the output section 24 will now be discussed with reference to
The goal of the design effort is to determine values of ⊖1 and ⊖5 so that ⊖4=⊖1. By inspection we have:
⊖3=a sin [sin(⊖1)/n] Equation 7
⊖6=⊖3+⊖2 Equation 8
90=⊖6+⊖5−⊖1 Equation 9
⊖5=⊖2 Equation 10
where n is the refractive index of the material constituting the output section 24. This set of four equations has six variables, which can be constrained by picking a value for any two of the six variables. A good choice is to select ⊖1 to be 3° (a nominal draft angle) and n=1.50, in which case ⊖3=2°, ⊖5=45.5°, and ⊖6=47.5°. Under these conditions, the exiting ray 42 propagates out of the optical element 10 in a direction perpendicular to the optical axis 3.
The process for designing the inner recessed surface 44 will be described with the illustration presented in
Before the design of the inner TIRing surface 44 can commence, the prescription of the outer TIRing surface 38 must be ascertained as described previously. The design of the inner TIRing surface 44 is done in several steps, two of which are iterative, as illustrated in the flowchart in
One such piece-wise linear definition of the inner TIRing surface 44 is presented in the table of
One key feature of the optical element 20 is that light emitted from a source 14 is not allowed to directly enter the upper output section 24. Recall that the upper output section 24 is optimized to accept input light that is substantially parallel to the optical axis 3. Should light from the source 14 enter the upper output section 24, it would necessarily enter from a direction that is not parallel to the optical axis 3 in which case it would not be properly redirected by the upper output section 24, and would exit from the upper output section 24 in an uncontrolled and undesirable direction.
Light from the source 14 is prevented from entering the upper output section by the location and diameter of the inner TIRing surface 44. Any light from the source 14 that would propagate to the upper output section 24 is instead always incident on the inner TIRing surface 44. That is, the upper output section 24 resides in the shadow of the inner TIRing surface 44. Care must be taken to ensure that inner TIRing surface 44 is wide enough and low enough in the inner recessed section 26 to prevent all rays from the source 14 from directly entering the upper output section 24. Of course oblique rays, such as rays 42A, 42B, 42, that indirectly enter the upper output section 24 by way of the outer TIRing section 22 are allowed.
There are two conditions that are considered for determining the relative sizes of the inner TIRing surface 44 of inner recessed section 26, the input surface 36, and the conical TIRing surface 28 of the upper section 24. The first condition is illustrated by a ray which represents light emitted form the source nearly perpendicular to the optical axis 3. The ray exits from the source 14 in a direction nearly perpendicular to the optical axis 3, and is incident upon the outer TIRing surface 38 very near the input surface 36. The ray is then TIRed into a direction substantially parallel to the optical axis 3. This ray misses the outer perimeter of the inner TIRing surface 44 of inner recessed section 26 as it propagates towards the conical TIRing surface 28 of the upper output section 24. That is, the position of the start of the surface of the outer TIRing section surface 38 at the input surface 36 must be large enough so that the ray misses the outer perimeter of the inner TIRing surface 44. Thus, the diameter of the input surface 36 is greater than or equal to the diameter of the inner TIRing surface 44 of inner recessed section 26.
The recess sidewall 46 is typically drafted to facilitate molding of the optical element 20 in accordance with manufacturing techniques known to those skilled in the art. Additional clearance can be provided to ensure that rays exiting from the source 14 in a direction nearly perpendicular to the optical axis 3 also miss the sidewall 46 of the recess 32. Thus, the diameter of the input surface 36 should be greater than or equal to the diameter of the opening of the recess 32 where it intersects the conical TIRing surface 28 of the upper section 24.
The second condition to be considered in determining the relative sizes of the TIRing surface of inner section 44, the input surface 36, the conical TIRing surface 28 is illustrated by a marginal ray. The marginal ray is representative of rays that miss the periphery of the inner TIRing surface 44 of inner recessed section 26 by a small amount, and are incident on the outer TIRing surface 38 of the outer TIRing section 22. To ensure that a ray from the source 14 is not directly incident on the output surface 30, the height of the outer TIRing section 22 can be increased, so that upper output surface 30 begins well above the intersection point of the ray on the outer surface.
The effectiveness of the present invention is demonstrated by the output of a ray trace shown in
For comparison purposes,
One defect of the present invention is the appearance of a central lobe of output light that peaks at approximately ±5° as can be seen in the graph of
The percentage of light flux contained in ray-segment 96 relative to incident ray-segment 92 depends upon the angle of incidence ⊖C and the refractive index of the material comprising the optical element 20, but is generally less than 10%. Ray-segment 96 is then incident on the recess sidewall 46, and refracts through it into ray-segment 98, which directly contributes to the objection lobe of stray light. This stray light, accumulated from the Fresnel reflections of many rays incident on the outer TIRing surface 38 produces the central lobe of light apparent in the central part of the graph of
In a preferred embodiment, the stray light 98 of the optical element 10 can be removed by the addition of an opaque plug 100, as shown in
The plug 100 is somewhat funnel-shaped, and consists of an upper conical section 102, an upper stem section 104, a seal 106, and a lower stem section 108. The upper conical section 102, the upper stem section 104, the seal 106, and the lower stem section 108 can be fabricated as one monolithic structure using molding processes well-known to those versed in these techniques.
The upper conical section 102 is designed to be placed into the conical recess 34 of the upper output section 24, but the conical surface 102 of the plug must not be in optical contact with the upper TIRing surface 28 in order to ensure that the TIRing characteristics of upper TIRing surface 28 are not compromised. The upper conical section 102 of the plug 100 is designed so that it is in mechanical contact with the output section 24 at just location 110.
The seal 106 is situated between the upper stem 104 and lower stem 108, and is a ring-shaped protuberance about the stem that serves two purposes. Firstly, the seal intercepts and blocks stray light rays 98, which in turn precludes the development of the objectionable lobe of central stray light seen in
The lower stem section 108 is optional, but can be provided as a gating location required during the molding operation. If it is provided, it must not extend deep into the recess 32 to the point where it is in optical contact with the inner TIRing surface 44, as that would compromise the TIRing capability of the inner TIRing surface 44 which is needed for proper operation of the optical element 20.
As mentioned earlier, the effect of the plug 100 is to remove the central lobe of light that is output substantially on-axis as seen in
While it has been implicit in the description of the optical element 20 that it is desirable to limit the angular width of the radial emissions, there are some cases where a broader angular emission is desirable. In
One way to widen the angular output emission profile is to make the upper TIRing surface curved, as illustrated in
One challenging aspect of the optical element 10 is its moldability. Specifically, since all of the exterior surfaces need to be optical quality with good scratch and dig characteristics, a location for a gate needed for an injection molding fabrication process is not available. To remedy this, a region of additional material 240, as shown in
An additional challenging aspect of the optical element concerns the attachment of the source 14 or a film layer 16 to the input surface 36 of the optical element 10. Generally an adhesive will be used for the attachment, but if a film layer 16 is being bonded to the input surface 36 then the adhesive is prone to wicking or being pressed to an area at the edge of the outer TIRing surface 38, which will compromise the TIRing performance of the outer TIRing surface 38. To mitigate this, a special reservoir or recess 138 as shown in
Alternately, if no film layer 16 is used and the source 14 is to be bonded directly onto the optical element, then a protuberance 136 can be provided within the input surface 36 as shown in
An additional benefit of the present invention is to improve the efficiency of the light source. Referring to
In operation, when power is supplied to the LED die 14 by the electrical conductor 202, light rays are emitted by the LED die 14 into the transparent layer 200. One of these rays is a non-obliquely emitted ray 210, which is emitted by the LED die 14 into the transparent layer 200, at angle θe with respect to the center-line CL. Ray 210 propagates through the transparent layer 200 and exits into the surrounding medium, such as air, at angle θout in accordance with Snell's Law.
Another ray is an obliquely emitted ray 212 which is emitted by the LED die 14 into the transparent layer 200 at an oblique angle. When the emitted ray 212 reaches surface 201 of the transparent layer 200, TIR occurs at the uppermost surface 201 of the transparent layer, and the ray 212 is reflected back on the substantially non-reflective conductor 202 where it is substantially absorbed. Accordingly, a portion of the rays that are emitted obliquely by the LED 14 are lost.
Referring to
In operation, when the same ray 212 described earlier with reference to
When ray 212 is emitted at an extremely oblique angle, such that it reaches surface 201 at a location beyond the input surface 36 of the TIRing condensing element 20, it will not propagate into the TIRing condensing element 20 but instead TIR at the surface 201, and reflect back onto the substantially non-reflective conductor 202 and be substantially absorbed. In other words, while the TIRing condensing element 20 does act to improve the efficiency of the system, it does not achieve a collection efficiency of 100% because some rays, the highly oblique rays, are still being lost.
By way of example only, a numerical example to illustrate a typical efficiency improvement with the input surface 36 of the TIRing condensing element 20 on the surface 201 of the transparent layer 200 will now be described. If the refractive index of the transparent layer 200 is 1.556, then its critical angle is 40.0°. To facilitate the calculations, a table of emissions, in percent, as a function of θe, in degrees, is presented in
Again, assuming in this particular example, the critical angle is 40.0° then from the rightmost column of the table in
Next, if the input surface 36 of the condensing element 20 is now in optical contact with the surface 201 of the transparent layer 200 and in this particular example the radius of the input surface 36, ρo, is 1.0 mm and the width w of the transparent layer 200 is 0.1 mm, the collection angle of the emitted light θe is then tan−1(1/0.1)=84.3°. From the rightmost column of the table in
Having thus described the basic concept of the invention, it will be rather apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, and is not limiting. Various alterations, improvements, and modifications will occur and are intended to those skilled in the art, though not expressly stated herein. These alterations, improvements, and modifications are intended to be suggested hereby, and are within the spirit and scope of the invention. Additionally, the recited order of processing elements or sequences, or the use of numbers, letters, or other designations therefore, is not intended to limit the claimed processes to any order except as may be specified in the claims. Accordingly, the invention is limited only by the following claims and equivalents thereto.
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