Abstract:
Disclosed is an LED arrangement for a microscopy instrument ( 200  FIG.  2 ) comprising a light emitting area ( 112 ), and a part-spherical solid and light transmissive cap ( 120 ), in light communication with the light emitting area, the cap having a hemispherical surface ( 126 ) including a portion ( 124 ) at which light from the light emitting area is reflected and a portion ( 128 ) at which light from the emitter can exit the cap, in order to provide a usable light cone L which includes light recycled from the more divergent emitted light, and is thereby more intense.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the generation of increased useable light from a light emitting diode (LED) for illumination purposes in microscopy, particularly, but not exclusively fluorescence microscopy. 
       BACKGROUND OF THE INVENTION 
       [0002]    Light microscopy, and in particular fluorescence light microscopy, relies on an intense source of illumination light that can be focused down to a very small cross section within the sample plane field of view. The intensity of the focused illumination spot is a key factor in determining the brightness of an image and the speed at which that image can be collected. LED&#39;s are a bright and easily controllable light source for microscopy, however, their substantial emitter size (1-3 millimetres across) and divergent emitted light make it impossible to focus more than about 5-10% of the total available emitted light down to the sample field of view. Due to the constraints of etendue and the limited numerical aperture (NA) of microscopy objective lenses, only the central cone of an LED&#39;s Lambertian emission profile can be collected by the objective lens. 
         [0003]    A prior attempt at increasing the light output from LEDs is described in U.S. Pat. No. 7,898,665, which describes arranging multiple LEDs and bringing their light into a single beam. This technique employs a complicated spatial arrangement of LEDs and optics which would be costly to implement. 
         [0004]    A further attempt is shown in U.S. Pat. No. 6,960,872, wherein a rectilinear box contains one or more LEDs and a reflective inner surface of the box reflects some light eventually out of an aperture. From there, the light exiting the aperture is focused by a lens or prism. This technique will cause much of the LED output light to be reflected many times, which will reduce the efficiency of the arrangement significantly. 
         [0005]    Yet another attempt is described in U.S. Pat. No. 6,144,536, wherein a cylindrical shell is formed around a light source, again having a reflective inner surface. Light exits the shell and is internally reflected along light path to form a diffuse illuminated area. 
         [0006]    Whilst these attempts might collect more light than could otherwise be collected from the bare semiconductor junction, they lack simplicity and their application to microscopy is of limited utility because the light from an LED source needs to be efficiently captured and focused in a limited volume, confined by other essential microscope components, and without causing dark spots or other light aberrations from the complicated arrangements described in the prior art above. 
         [0007]    The inventor has realised that there is a need to utilise a higher percentage of the LED&#39;s emission profile in a simple way, and thereby enable brighter microscopy images and shorter exposure times at low cost. 
       SUMMARY OF INVENTION 
       [0008]    In embodiments the above problems are addressed herein by the provision of an improved LED light arrangement, for example, including a light emitting area, the light emitting area being in light communication with a substantially part-spherical solid cap (i.e. a portion of a ball) at a flat surface of the cap, the cap having a generally mirrored or otherwise light reflective curved surface including a region of the curved surface which is not mirrored allowing light from the emitting area to escape in use. 
         [0009]    The practice, embodiments of the invention utilise a reflective layer surrounding the LED that redirects all but the central cone of emitted light back onto the LED itself. In this way, emitted light that lies within the central cone of the emission profile will proceed towards the microscope lens system, but the rays that lie outside of this cone will become redirected (through one or more reflections) until they emerge along a trajectory that lies within this central, usable cone. In this way virtually all the LED&#39;s light can be used, by virtue of a low cost and small size arrangement. 
         [0010]    The invention provides an LED arrangement according to claim  1  having preferred features defined by claims dependent on claim  1 , as well as a microscopy device employing the arrangement. 
         [0011]    The invention extends to any combination of features disclosed herein, whether or not such a combination is mentioned explicitly herein. Further, where two or more features are mentioned in combination, it is intended that such features may be claimed separately without extending the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention can be put into effect in numerous ways, illustrative embodiments of which are described below with reference to the drawings, wherein: 
           [0013]      FIG. 1  shows an LED arrangement; 
           [0014]      FIG. 2  is a ray diagram; and 
           [0015]      FIG. 3  shows microscopy device employing the LED arrangement shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The invention, together with its objects and the advantages thereof, may be understood better by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements in the Figures. 
         [0017]      FIG. 1  shows a conventional LED  110  package which includes a semiconductor light emitter  112  which is typically  1  to  9  mm squared in surface area, and therefore around 1 to 3 mm in width w. The emitter has an electrically insulating backing  114 , and, in this case, has packing  116  on either side of the emitter  112  allowing the LED to be mounted to a flat surface, for example by the use of transparent or translucent adhesive. 
         [0018]    Also shown in  FIG. 1  is a generally hemispherical cap  120  which is around 5 to 10 mm in radius. The cap is formed conveniently from a modified glass half ball lens of radius r. The LED package is adhered to or otherwise held to a flat face  122  of the cap  120 , such that the middle of the emitter  112  is close to the geometric centre C of the cap. 
         [0019]    The cap  120  has a reflective coating, for example a mirrored coating  124  formed around most of its curved surface  126 , formed by vacuum vapour deposition or other known techniques. A portion  128  of the curved surface is free from the reflective coating, to form a light exit. 
         [0020]    In use, the emitter  112  can emit light in all directions of an arc E of approximately 180 degrees, because the semiconductor die of the emitter  112  is essentially flat. Light emitted directly toward the exit  128  can escape through that unreflective area in a usable light cone L to be used in a microscope device, for example instrument  200  described in  FIG. 2 . However, as described above, limitations in the conventional optics available have led to the necessity for the cap  120  to be used to direct more light emitted across the arc E into the useable arc L. In this embodiment, light, for example light ray R is emitted from the emitter  112  and does not escape via the exit  128 , so it is internally reflected within the glass cap  120  off the reflector  124  to return toward the emitter  112 , whereat it is reflected off the emitter  112  and travels along path R′ into the usable cone of light L. A similar occurrence takes place for the majority of light which does not at first escape along the cone L, in some instances, by multiple reflections at the reflector  124  and back onto the emitter  112 . 
         [0021]      FIG. 2  shows a software generated ray trace diagram which defines the light paths of the LED arrangement shown schematically shown in  FIG. 1 . In particular, the reflective paths of light as a whole originating from the LED emitter  112  are more clearly visible. As described above, the rays emitted by the emitter  112  either pass through the exit or are reflected back to the LED via the reflector  124 . Those that are reflected back will be redirected by the LED emitter, and a substantial portion of those redirected rays will subsequently lie within the central cone L and thus will pass through the exit  128 . 
         [0022]      FIG. 3  shows schematically the LED arrangement  100  used in a microscope  200 . In the embodiment, the cone of light L described above is focused by a convex lens  210  into a point, which point focused light enters an optical fibre  220 . Light exiting the optical fibre  220  within a housing  300  passes through collimating lenses  230  which produce parallel rays of light which are in turn reflected by a polychroic mirror  240  toward and objective lens  250  and then refocused onto a sample  260 . This imaging light is more intense than light which is collected from a conventional LED, and so more light (and a thereby a better image) travels back through the objective lens  250 , straight through the mirror  240  and on to a tube lens  270  and finally into a camera  280  or other image capture means. 
         [0023]    The concept proposed here involving the use of a reflective arcuate ‘shield’ surrounding the LED emitter to redirect light back on to the LED itself provides a significant improvement in the amount of usable light. The cone angle of the usable light cone L is preferably about 30 degrees, but an angle of about 20 to 60 degrees will provide suitable results. Preferably the cone angle is between 40 and 20 degrees. Although LED semiconductor emitters are formed from materials having various colours, it is proposed that white or near white semiconductor materials will provide the best diffuse reflective properties for the arrangement illustrated. 
         [0024]    The emitter semiconductor  112  has a width, typically of 1 to 3 mm across. It has been found that the radius r of the half ball lens used should be about 10 to 20 times the width w of the emitter  112 . The use of a solid material, for example glass, adjacent or in direct contact with the lens  120  is preferred because this arrangement allows heat to be conducted away from the LED more efficiently than if the LED were in air. 
         [0025]    Although two embodiments have been described and illustrated, it will be apparent to the skilled addressee that additions, omissions and modifications are possible to those embodiments without departing from the scope of the invention claimed. For example, although a cap  120  in the form of a glass half ball lens has been described and illustrated, the concept will work with any approximately hemispherical shape, and any transparent or translucent material, which may include a band pass filter to reduce the bandwidth of the light in cone L. A reflective coating  124  has been described, but a separate shell, or applied film, for example, would suffice. The exit  128  could be formed by a mask used when applying the coating/film, or may be formed by removing a portion of the coating once applied. The light exit  128  is, most conveniently, circular to provide a regular conical usable light source. However, the exit  128  could be other shapes, for example the exit could be a slit, to provide a line of exit light, suitable for other optical techniques. For increased efficiency, the flat face  122  of the cap  120  could be made reflective also, at regions other than the emitter area  112 . The light emitter  112  is preferably adhered to the flat face  122 , but it could be held in place by friction, for example applied by a mechanical clamp. It is intended that the cap  120  is solid i.e. is formed from a homogeneous material such as glass. However other homogeneous materials could be employed, such as clear moulded plastics or composite materials which include liquid filled cavities.