Abstract:
A positive-powered lens for collecting and organizing the light output from a plurality of light sources into a single secondary source that has an anterior surface, upon which are disposed light-collecting tessellates that are arranged in an ordered geometrical (e.g., a square, rectangular, circular, or oval) pattern surface, and a posterior surface that is convex. The tessellates can have a common surface equation a different surface equation but equivalent focal lengths. The tessellates are associated with a plurality of light sources, each having a proximal face that is coplanar with the focal planes of the tessellates.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    a) Field of the Invention  
           [0002]    The present invention relates generally to fiber optics lighting systems and, specifically, to a fiber optics illumination device employing a plurality of light-emitting diodes (LEDs), said LEDs arranged in a spaced and ordered geometrical organization, and a single optical element, for example, a lens, for collecting light from said LEDs, and distributing said light to a common exit pupil. Said exit pupil may serve as the secondary source of illumination for a variety of lighting needs including but not limited to fiber optics lighting and digital projection system applications.  
           [0003]    b) Description of the Prior Art  
           [0004]    The LED comprises a semiconductor chip, or die, inside a cylindrically shaped plastic envelope, the light-emitting end portion of which is an epoxy dome lens having a roughly elliptical or spherical surface. The die is roughly a cube 0.25 millimeters on a side that is positioned within a silver-plated reflector cup. The reflector cup, which is located at the focal plane of the dome lens, is shaped as a truncated frustum of a cone. The reflector cup and dome lens cooperate to shape the emergent luminous intensity distribution of the LED.  
           [0005]    Single and multi-chip discrete LEDs are typically used in instrumentation applications. Frequently, optical fibers are used in conjunction with these LEDs for this purpose. FIG. 1 is a schematic that illustrates such an integrated LED driven fiber optics system  10 . LED  11  is disposed within an emitter condenser housing  14 , wherein its luminous intensity is focused by condenser lens  13  onto the proximal face of optical fiber  16 . Condenser lens  13  may be either a one-quarter pitch gradient index lens, as manufactured by NSG America, Inc., Somerset, N.J., or a molded aspherical lens, as manufactured by Geltech, Inc., Alachua, Fla. Optical fiber  16  is typically a plastic fiber consisting of an inner acrylic (polymethyl methacrylate) core coated with an evaporated thin film cladding of fluorinated polymer.  
           [0006]    The proximal end portion of fiber  15  is supported by end connector means  16 , positioning the fiber-input face at the focus of lens  13 . The distal end portion of the said fiber is similarly supported by end connector means  18 , which is, in turn, attached to panel  20 . The distal output face of fiber  16  cooperates with end-fitting lens  17  to favorably distribute the emerging light to suit its intended application.  
           [0007]    LEDs are also employed in industrial and automotive applications as packaged products, which directly replace incandescent lamps for brighter, longer and less expensive operation. Packaged products are typically arranged in clusters wherein a plurality of LEDs is arranged in an ordered, geometrical pattern, as schematically illustrated in FIG. 2 a  and FIG. 2 b . Referring to FIG. 2 a , packaged product  32  consists of a light emitting end portion and, disposed at it opposite end, an electrical socket connector  34 . The light emitting end portion consists of a plurality of LEDs  11  disposed so that their respective longitudinal axes are co-parallel to each other and to the longitudinal axis of package  32 . Each LED  11  emits light into a cone having an interior angle  2   a , the central axis of said radiation being coincident with the longitudinal axis of said LED.  
           [0008]    The plurality of LEDs  11  may be arranged in an ordered, geometrical pattern on proximal surface  30  of package  32 , such as shown in FIG. 2 a . Whereas these packaged products may be used as stand alone devices, such as, for example, in vehicular tail, turn indicator and stop light applications, it would be desirable to have a means in which to collect the respective luminous intensity of each LED and distribute same into a single aperture, or exit pupil, thus forming a low cost, high intensity light source.  
           [0009]    There is known an application of a packaged LED product having a composition as illustrated in FIG. 3. Said known packaged LED light source  40  is composed of a plurality of light transmitting optical systems, each optical system consisting of an LED disposed on mounting board  31 , a condenser lens housing assembly  14 , including condenser lens  13  and fiber coupler  16 , and optical fiber  15 . The proximal end portions of each optical fiber  15  is disposed to receive the focused radiant energy transferred by the condenser lens  13  from conjugate LED light source  11 .  
           [0010]    Said light transmitting optical systems are arranged in an ordered, geometrical pattern conforming to the ordered, geometrical arrangement of the LEDs as laid out on the mounting board of the packaged product. The distal end portions of the plurality of fibers  15  are combined to form a closely packed incoherent fiber bundle having an application-specific cross-section  58 , such as, but not limited to, a circle or a rectangle.  
           [0011]    The aforementioned known method of collecting and organizing the radiant energy output of a packaged LED product, or a cluster of LEDs, through the use of a plurality of light transmitting optical systems, the exact number of which may correspond to the number of LEDs in said cluster, has significant drawbacks. First, the throughput luminance in each light transmitting system can be expressed as  
             I=I   0 τ c τ f (1− R ) 5    
           [0012]    where I 0  is the source illuminance, τ c  and τ f  are the selective absorptions of the condenser lens and fiber material media respectively, and R is the reflection losses at the optical surface of the system. Substituting conservative values for τ and R in the expression above, it is seen that the output illuminance of each light transmitting optical system may be reduced by as much as 45 percent of the initial source illuminance. Second, the plurality of light transmitting optical systems is considered to be cost-ineffective given the plurality of parts and the cost of labor for assembly of said parts.  
           [0013]    Thus, there is a need for a light collecting and organizing means for packaged LED products, such as LED clusters, that is inexpensive and more energetically efficient relative to known optical systems.  
         SUMMARY OF THE INVENTION  
         [0014]    It is the object of this invention to provide a collecting and organizing means for light emitted from light emitting diodes, particularly light emitting diodes that are arranged in clusters or in packaged products. A further object of this invention is to provide an improved light collecting and organizing means having luminosity characteristics that is substantially superior to those of the convention prior art systems.  
           [0015]    The above and other objects of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0016]    [0016]FIG. 1 is a sectional view of a light transmission optical system for a light emitting diode source;  
         [0017]    [0017]FIG. 2 a  is a side view of a packaged light emitting diode product;  
         [0018]    [0018]FIG. 2 b  is a plan view showing the plurality of light emitting diodes arranged in an ordered, geometrical pattern;  
         [0019]    [0019]FIG. 3 is a sectional view of a prior art light collecting and organizing means employing a plurality of light transmitting optical systems;  
         [0020]    [0020]FIG. 4 is a plot showing the angular light distribution characteristic, or directivity, of a typical light emitting diode;  
         [0021]    [0021]FIG. 5 is a plot of the spectral irradiance function of a typical white light emitting diode over the visible portion of the electromagnetic spectrum;  
         [0022]    [0022]FIG. 6 is a plot of the CIE 1931 chromaticity diagram showing the location of white light emitting diode and its color temperature;  
         [0023]    [0023]FIG. 7 is a sectional view of the present invention showing, for purposes of explanation, the paths of principal and aperture rays, in the meridional plane, traced from two light emitting diodes disposed on opposite sides of the optical axis;  
         [0024]    [0024]FIG. 8 a  is a sectional view of the collecting and organizing means of the present invention showing the light transmitting surface tesellations on the anterior surface and the convex posterior surface;  
         [0025]    [0025]FIG. 8 b  is a plan view of the light transmitting surface tessellations on the anterior surface of the light collecting and organizing means of the present invention;  
         [0026]    [0026]FIG. 9 is a sectional view of the light collecting and organizing means of the present invention showing ray tracings originating from a plurality of light emitting diodes and terminating at a common exit pupil plane. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]    Prior to describing the present invention, pertinent optical characteristics of the light emitting diode will be discussed. It is believed that this discussion will aid in an understanding of the utility of the present invention.  
         [0028]    The direction of light propagation of the LED is along the longitudinal axis of the device. The angular light distribution, or its directivity, is defined in terms of beam-spread propagation. The angular measures of numerical aperture (NA) and angle of field characterize beam-spread propagation. NA is defined as  
           NA =sin α 
         [0029]    where α is one-half the included angle between the 50 percent power points as illustrated in FIG. 4. The angle of field β is defined as the included angle between the 20 percent power points. When &lt;β≈&lt;2α, the luminous intensity distribution (candela/steradian) will be uniformly distributed and the edges of the field will be sharply defined. Typically, the numerical apertures of LEDs range in value from 0.17 to 0.90. Angles of field range from 40° to 160°. In the preferred embodiment of the present invention, the LED will have a NA falling in the range of 0.50 to 0.67.  
         [0030]    Color temperature of a light source is an important consideration, particularly when that light source is used to drive optical fibers. Selective absorption in the core of an optical fiber is by far the major contributor to overall loss in transmission efficiency. In particular, absorption losses in the blue region of the visual spectrum are much greater than in the associated yellow, green or red portions. In the case of a given length of low numerical aperture fiber, the transmission of the blue end of the spectrum is typically 34 percent as great as the transmission in the yellow middle of the spectrum.  
         [0031]    For good color rendition, therefore, it is essential to use a light source having a high color temperature. The concept of color temperature arises from the apparent color of an object as it is heated to various temperatures. When the object is hot enough to glow it is said to be incandescent. Special classes of incandescent objects that emit radiation with 100 percent efficiency are called blackbody radiators. Specifically, an ideal blackbody glows with a color that depends only on temperature, making it an ideal color standard. Thus, by adjusting the temperature, a wide range of color sensations is produced. Color sensations are specified as blackbody temperatures in degrees Kelvin. The peak wavelength λ, in microns, of a blackbody at color temperature T may be calculated from the expression  
         λ max =2897.8 T   −1    
         [0032]    Substituting for values of T, it is seen that the higher the color temperature the lower the wavelength. At a color temperature of 7000°K, the peak wavelength is 414 nanometers, which is near the blue end of the visible spectrum.  
         [0033]    Referring to FIG. 5, a plot of the spectral irradiance function of a white LED, it is seen that there is a strong emission in the blue region of the visible spectrum, making it an ideal light source for fiber optics applications. The exitance of the white LED has sufficient blue color bias to compensate for the blue absorption of the optical fiber. As is seen in the CIE 1931 chromaticity diagram (FIG. 8), the white LED has a color temperature in the range of 6000° to 8000° K.  
         [0034]    It will be appreciated that the unique optical characteristics of the LED hereinbefore described are implemented in the preferred embodiment of the present invention for the purposes of providing an inexpensive and efficient light collecting and organizing means.  
         [0035]    The light beam collecting and organizing lens means of the present invention consists of a positive power optical element  50  schematically illustrated in FIG. 7 a . Optical element  50  is composed of an anterior surface  54  and a posterior surface  52 . The material medium of optical element  50  is a borosilicate crown glass having properties suitable for thermo-plastic compression molding such as, for example, glass type B270, manufactured by Schoft Glaswerks, Mainz, Germany.  
         [0036]    In the preferred embodiment, anterior surface  54  is a rotationally symmetric aspheric surface described by the polynomial expression  
           Z=cy   2 {1+[1−(1+ k ) c   2   y   2 ] ½ } −1   +dy   4   +ey   6   +fy   8   +gy   10    
         [0037]    where Z is the z-coordinate of the surface, c is curvature (reciprocal of the radius), y is the radial coordinate, k is the conic constant and aspheric deformation coefficients d, e, f, and g.  
         [0038]    Posterior surface  52  is the construct of a plurality of sub-aperture refractive elements, or light transmitting tessellates, embossing said surface in a prescribed two-dimensional geometrical pattern, such as, for example, a square (FIG. 7 b ). Each tessellate is centered on a local optical axis, said axis being co-parallel to the global optical axis of lens  50 . Said tessellates all have positive optical power and are juxtaposed in rows and columns wherein the pitch, or separation along both X- and Y-axis, may be constant. Said arrangement forms a system aperture function.  
         [0039]    In one embodiment of the present invention, the tessellated surface consists of tessellates that are all optically identical. The optical surface of each sub-aperture tessellate is a rotationally symmetric (about its local optical axis) aspheric defined by the same polynomial expression given hereinbefore.  
         [0040]    In another embodiment of the present invention, each tessellate need not be identical to one another, rather each tessellate may be defined by its own unique polynomial expression. Given that the aperture function of a centered optical system is bi-symmetrical about its Y-axis, the tessellation process may be simplified in that individual uniquely defined tessellates need only populate one-half of the aperture function. For example, any uniquely defined tessellate centered at an aperture coordinate (x 1 , y 1 ) will have a corresponding, identically defined tessellate centered on the opposite aperture coordinate (−x 1 , y 1 ).  
         [0041]    This invention is concerned with a light collecting and organizing lens means illustrated schematically in FIG. 8 wherein, for purposes of clarity, only two tessellates are shown, each disposed on opposite sides of the Y-axis of said lens means  50  with aperture coordinates of (y I ,0) and (−y I ,0) respectively. Each LED  11  is longitudinally separated from surface  53  being disposed at the front focal distance of tessellate  52 . The angular light distribution, or directivity, emitted from each LED  11  is collected by lens means  50  and is propagated to system focal plane  58 . Surfaces  52  and  54  of lens means  50  cooperate to correct spherical aberration at surface  58 .  
         [0042]    Further, a virtual aperture stop plane  56  is formed longitudinally within the material medium of lens means  50 , between surfaces  52  and  54 , and positioned at a focal plane of anterior surface  52 . In the said arrangement, two important benefits are derived. First, the object space between LED source  11  and light transmitting tessellate  52  is telecentric, thereby cooperating with the directivity of LED source  11  without potential loss of luminous intensity. Second, the LED  11  and exit pupil  58  are seen to be conjugates of one other. Accordingly, the luminous intensities I 1  and I 2  respectively collected by tessellates  52  disposed at coordinates (y i ,0) and (−y I ,0) are propagated by lens  50  as narrow beams of radiation traveling along symmetrically separate paths about the global axis to exit pupil  58  with exactly the same magnification. These beams of radiation coincide in pupil  58  without the effects of parallax. As a result, the respective illuminance of each beam of radiation, being free of vignetting, is combined within the boundaries of exit pupil  58 , essentially doubling the available luminous flux in said pupil aperture.  
         [0043]    [0043]FIG. 9 is a cross-sectional view of the preferred embodiment of the present invention showing the ray paths originating from multiple LED sources  11 , propagating through the light collecting and organizing means  50  and terminating at exit pupil plane  58 . It will be appreciated that the luminous intensity of an LED and the number of LEDs in a given cluster determine the illuminance at exit pupil  58 . In the case of a collecting and organizing means utilizing a packaged cluster of  36  LEDs, the illuminance in exit pupil  58  would be thirty-six times the contribution of a single LED. Exit pupil  58  represent a portal through which the combined irradiance of a plurality of LEDs can be transformed into a single secondary light source useful for a variety of applications, such as but not limited to, digital projection, fiber optics lighting, displays, signage, etc.  
         [0044]    In summary, it can be seen that the present invention provides a means for collecting and organizing the light output from plurality of light sources, such as packaged LED products, to form a single secondary light source having favorable luminosity, longevity and efficiency characteristics that are not readily attainable with conventional light sources. In the preferred embodiment, the said means is accomplished by a single positive-power lens element. The salient features of the present invention include:  
         [0045]    1. Optimum transfer of illuminance and luminous intensity;  
         [0046]    2. Cost effectiveness in manufacture and assembly.  
         [0047]    Apart from the luminous losses associated with Fresnel surface reflection and selective absorption of the material media of the present invention, the only appreciable loss in luminous intensity would be due to residual manufacturing errors—losses which are common to any potential lighting solution.  
         [0048]    While the above description contains many specificities, they should not be construed as limitations on the scope of the invention, but rather as exemplifications of the preferred embodiments thereof. Many other variations are possible, for example, the geometrical pattern of tessellation, choice of refractive material, type of aspheric surface, etc. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims or their equivalents.