Abstract:
A light source is provided which comprises an LED array containing a plurality of LEDs, and a lens array containing a plurality of lenslets. The lens array is aligned with the LED array such that one lenslet is disposed over each LED, wherein each of said plurality of lenslets comprises at least first and second sublenslets having first and second respective optical centers, and wherein at least one of said first and second optical centers deviates from the geometric center of the lenslet.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional application No. 61/661,279, filed Jun. 18, 2012, having the same title, and having the same inventors, and which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure pertains to methods for arranging lenses so as to spread light optimally from an array of LEDs to an area to be illuminated, and to systems made in accordance with such methods. 
       BACKGROUND OF THE DISCLOSURE 
       [0003]    Various light sources based on LED arrays are currently known to the art. At present, it is common to provide each LED in the array with non-imaging optics based on total internal reflection. Such optics, which are commonly referred to as compound parabolic collectors (CPCs), are typically either placed individually over each LED or are molded into the array with the same spacing as the LEDs. Although they are optically efficient, these non-imaging optics are thick and expensive to produce, and do not necessarily direct light to the proper locations for a desirable illumination pattern. 
         [0004]    Alternatively, some LED luminaries use arrays of lenses. Typically, each lens in the array is ahead of each LED, and has its optical center centered on the LED. Such luminaries produce a pattern in the illuminated area which is strongest directly in front of the luminaire, and which is not uniform over the illuminated area. 
       SUMMARY OF THE INVENTION 
       [0005]    In one aspect, a light source is provided which comprises an LED array containing a plurality of LEDs, and a lens array containing a plurality of lenslets. The lens array is aligned with the LED array such that one lenslet is disposed over each LED, wherein each of said plurality of lenslets comprises at least first and second sublenslets having first and second respective optical centers, and wherein at least one of said first and second optical centers deviates from the geometric center of the lenslet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is an illustration of a prior art LED illumination device containing an LED array and an optical element, and depicting the optical footprint of the device. 
           [0007]      FIG. 2  is an illustration of a prior art LED illumination device containing an LED array and an optical element, and depicting some of the details of the optical element. 
           [0008]      FIGS. 3-6  depict some possible configurations for the illuminated area of each lenslet in the lens array of an LED illumination device and with the LEDs partially illuminated. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]      FIG. 1  is an illustration of a prior art LED illumination device containing an LED array and an optical element, and depicting the optical footprint of the device. With reference thereto, the manufacturing and application constraints for such a device often require that the luminaire  101  be designed with an LED array  103  consisting of multiple LEDs  105  attached to a flat circuit board  107 . These constraints also require that the associated optics (which, in the case depicted, is a lens array  111 ) spreads light reasonably uniformly across the illuminated area  115  over angles as large as 70° to 80° from an axis which is perpendicular to the circuit board  107 . 
         [0010]    As shown in  FIG. 2 , this light spreading may be most economically achieved with a lens array  111  consisting of multiple lenslets  113  which are molded or otherwise formed in a sheet of plastic or glass, and which are placed parallel to, and in the proximity of, the plane of the LEDs  105 . Typically, there will be as many lenslets  113  in the lens array  111  as there are LEDs  105  in the LED array  103 , and the dimensions of each lenslet  113  in the plane of the lens array  111  will be equal to the spacings between LEDs  105 . 
         [0011]    It is generally preferred that a smooth, or substantially smooth, surface be presented to the illuminated area. In order to avoid the complexity, expense, and light loss of an additional layer of plastic or glass, the refractive surfaces  117  of the lenslets  113  must face the LEDs  105 , and the lenslets  113  must therefore be plano-convex, or substantially plano-convex. The smooth, or substantially smooth, outer surface  119  of the sheet of plastic or glass may be planar or prismatic, or may be curved as a pillow, a saddle, a portion of a cylinder or cone, or in various other ways. In some embodiments, the outer surface  119  may be equipped with a diffuser to spread the image of each LED  103  a few degrees so as to improve the evenness of the illumination in the illuminated area  115 . 
         [0012]    LEDs generally emit over angles as large as 120° to 180°. In order to capture all, or most, of the emitted light, the lenslets  113  must be large in extent compared to their focal length (that is, they must be “fast” lenslets). The LED-to-LED spacing on the circuit board  107  dictates the area available for the optics associated with each LED  103 . The spacing d between the plane of the lens array  111  and the plane of the LEDs  105  is typically less than the LED-to-LED spacing, and may be half of it. Stylistic considerations usually further reduce the spacing between the lens array  111  and the plane of the LED array  105 . 
         [0013]    To date, some success has been achieved in this type of configuration with LED light sources which use aspheric lenses (and in particular, aspheric Fresnel lenses which are made correct for conjugates of the focal length and infinity) with the focal length on the plano side, but positioned with the LED on the grooved side. Such a configuration permits the use of a master which is made for, or is useful in, other applications. Unfortunately, total internal reflection in this type of configuration limits the aperture to approximately f/1, which is inadequate to capture and redirect a desirable portion of the light. 
         [0014]    This situation may be visualized by observing the illuminated area of each lenslet in the lens array with the LEDs partially illuminated.  FIG. 3  shows several possibilities for such a configuration. In the first configuration  201 , the optical center  203  of the lenslet is centered on the emitting area  205  of the lenslet, the illuminated area  207  of the lenslet, and the area available  209  for the lenslet. In the second configuration  221 , the optical center  223  of the lenslet is outside of the emitting area  225  of the lenslet but within (though off-center of) the illuminated area  227  of the lenslet and the area available  229  for the lenslet. In the third configuration  241 , the optical center  243  of the lenslet is outside of the emitting area  245  of the lenslet, and is also outside of the illuminated area  247  of the lenslet and the area available  249  for the lenslet. 
         [0015]      FIG. 4  illustrates the situation obtained when light is sent to a far corner of the pattern. As seen therein, in such a configuration  301 , the optical center  303  of the lenslet is outside of the emitting area  305  of the lenslet, and is also outside of the illuminated area  307  of the lenslet and the area available  309  for the lenslet. Of course, and in contrast to the configuration  241  of  FIG. 3 , the illuminated area  307  in the configuration  301  of  FIG. 4  is shifted to the corner of the area available  309  for the lenslet. 
         [0016]    The configuration  301  of  FIG. 4  typically produces an illuminated area  307  or bright area on the lenslet. This illuminated area  307  can be visualized by intercepting the rays headed for the area to be illuminated with the eye, and noting the size and shape of the bright area. This illuminated area  307  covers only about one quarter of the area of a lenslet in the lens array. Consequently, the remaining dark area can be used to illuminate other areas of the pattern, such as the center, but these areas are usually well enough illuminated by scatter from the vertical surfaces of the Fresnel lenses and cross illumination by adjacent LEDs. Moreover, sending light to the far corners of the area to be illuminated may require that the lenses forming the lenslets be as fast as f/0.3, because they will need to be used very far off axis. Such a lens will have very steep groove angles (sharp grooves with angular extent of 75° or so). 
         [0017]    More profitably, configurations may be utilized in which the dark areas are used to send light to the remaining three far corners of the pattern.  FIG. 5  illustrates a particular, non-limiting embodiment of a configuration in accordance with the teachings herein which has been constructed using such an approach. This approach typically results in a configuration  401  which typically has double mirror symmetry, with each lenslet in the array comprising four sublenslets. The emitting area  405  in this embodiment is centered on the lenslet, and the optical centers  403  of the sublenslets are outside of the area available  409  for the lenslet. The configuration  401  of  FIG. 5  will produce a bilaterally or quadralaterally symmetric pattern in the illuminated area, assuming that the plane of the LEDs is parallel to the plane of the illuminated area (as well as to the plane of the lens array, of course). 
         [0018]    It will be appreciated that various luminaires may be produced which have a configuration the same as, or similar to, the configuration of  FIG. 5 . The specific design of these configurations will depend, for example, on the desired pattern in the illuminated area. However, the general method of producing these configurations remains the same. In particular, the useful area of the lenslet is determined by noting the bright or illuminated area as seen from the direction being illuminated, and the dark area is utilized to send light in other directions in which it is needed by dividing the lenslet into two or more sublenslets. 
         [0019]      FIG. 6  shows another particular, non-limiting example of a configuration for an LED illumination device made in accordance with the foregoing approach. In the particular embodiment depicted, the configuration  501  features a plurality of lenslets, each of which comprises a plurality of sublenslets. The optical centers  503  of the sublenslets are outside of the emitting areas  505  of the lenslets. The limit of the area available  509  for the lenslets is indicated by dashed lines. 
         [0020]    The embodiment of  FIG. 6  demonstrates that the area of one lenslet associated with one LED can be intruded upon by an extended portion of a lenslet associated with an adjacent LED. By this method, the illumination level in the illuminated area can be increased, typically between a factor of two and a factor of four. 
         [0021]    The above description of the present invention is illustrative, and is not intended to be limiting. It will thus be appreciated that various additions, substitutions and modifications may be made to the above described embodiments without departing from the scope of the present invention. Accordingly, the scope of the present invention should be construed in reference to the appended claims.