Patent Publication Number: US-9904045-B2

Title: Integrating cone for an illumination device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of, and claims priority to, U.S. application Ser. No. 14/691,987 entitled  Integrating Cone for an Illumination Device,  which was filed on Apr. 21, 2015, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to illumination, and more particularly, is related to solid state illumination devices. 
     BACKGROUND OF THE INVENTION 
     Solid state technology has progressed very far in recent decades. However there is still significant room for improvement in the green/yellow/amber range of the electromagnetic spectrum, approximately 520-600 nm. Direct light emitting diodes (LEDs) or laser diodes in this range are traditionally very low powered and/or inefficient from an electro-optical perspective. This issue with solid state lighting producing light at wavelengths in the green/yellow/amber range is fundamentally limited by the physics of the semiconductor materials used in the construction of these devices, as the band-gap of the materials does not favor emission of light in this spectral range. This problem is commonly referred to in the industry as the ‘green gap’. 
     Solid state lighting solutions are sought after in all areas of general lighting to improve energy efficiency and increase luminaire lifetime. However, existing solid state apparatuses and methods for generating green light that meets optical power requirements for fluorescence imaging applications are generally costly. The conventional approach has been either devices having low output in the green-yellow band or low coupling efficiency for large area LEDs. LED array or laser pumped crystal or phosphor solutions are relatively expensive compared to arc lamps for some applications with low cost requirements. 
     A conventional approach to producing broadband light, such as white, is to use ultra violet light, royal blue, or near-ultra violet light from LEDs which have a phosphor powder deposited onto the LED surface. The most popular of these methods is creation of a ‘white’ LED which includes a phosphor of Ce:YAG (cerium doped yttrium aluminum garnet, Y 3 Al 5 O 12 :Ce 3+ ) suspended in an encapsulating material such as silicone, embedded in a transmissive ceramic, or deposited directly onto a blue LED die or die array with a peak wavelength between about 445 nm and 475 nm. The light absorbed by the phosphor is converted to a broadband green/yellow/amber light, which combines with un-absorbed scattered blue light to produce a spectrum that appears white. The brightness of white light is limited by the blue light intensity from the LED, phosphor quantum efficiency, and thermal quenching, especially in the yellow (approximately 560 nm) and amber (approximately 590 nm) spectral bands. Higher power LEDs are available, but the increase in power scales with an increase in the LED emitting area. The coupling efficiency from the illumination source (LED surface) to the objective plane of a microscope objective is inversely proportional to the source size at the same light intensity. Thus, the power delivered to the microscope objective plane cannot typically be increased by simply increasing the LED surface area (it is an Etendue limited optical system). 
     Another way to achieve bright yellow and amber light is using single crystal Ce: YAG LED pumped by an LED array. The efficiency of such a device is limited by the total internal reflection of such a luminescent material due to its high index of refraction, and more importantly, coupling from LED to crystal. This results in a need of a large number of LEDs to achieve the brightness needed, which increases cost, size, and thermal/electrical requirements on systems employing this method. For example, see U.S. Pat. No. 12/187,356. 
     A third way of generating high powers of light in the green gap consists of using Ce:YAG in crystal form and pumping this structure with a blue (approximately 440-490 nm) laser from the front or back of the crystal. For example, U.S. patent application Ser. No. 13/900,089 describes an optical system using this concept and the predicted improvement in electro-optical coupling efficiency to the focal plane of a microscope has been validated. This approach is very effective in producing a scalable amount of power in the green/yellow band but can be costly. The particular shape and size of the crystal, multiple laser diodes, and cooling methods lead to increased assembly and manufacturing costs. 
     Other combinations of lasers and phosphors have also been suggested for many high brightness applications including fluorescence illumination, such as U.S. patent application Ser. No. 13/897,237, and other applications such as automotive headlights, for example, U.S. patent application Ser. No. 13/697,782, and digital projection systems, such as U.S. patent application Ser. No. 13/942,603, but these methods are still unable to meet the same cost targets as mercury or xenon arc lamps, which are currently the industry standard. Therefore, there is a need in the industry to address one or more of the above mentioned shortcomings. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide an integrating cone for an illumination device. Briefly described, the present invention is directed to a device for increasing the optical power and coupling efficiency of a solid state light source in the green and/or yellow bands. The device has an integral body having an ingress surface configured to receive light from an emitting portion of the solid state light source, an egress surface substantially opposite the ingress surface, and a recess formed within the body. The recess has an input opening in the ingress surface, an output opening in the egress surface, and a recess surface within the body extending between the input opening and the output opening. The recess surface is configured to reflect visible light with Lambertian scatter characteristics. 
     Other systems, methods and features of the present invention will be or become apparent to one having ordinary skill in the art upon examining the following drawings and detailed description. It is intended that all such additional systems, methods, and features be included in this description, be within the scope of the present invention and protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principals of the invention. 
         FIG. 1  is a schematic diagram of a first exemplary embodiment of an illumination system with increased optical power in the green band. 
         FIG. 2  is a schematic diagram showing a side view of the detail structure of a cone cavity of the cone device of the system of  FIG. 1 , shown beside a front view scale. 
         FIG. 3A  is a schematic diagram showing the light throughput of the first embodiment of an illumination system. 
         FIG. 3B  is a schematic diagram of the light throughput of a prior art illumination system. 
         FIG. 4  is a plot comparing the relative power output of the optical systems of  FIG. 3A  and  FIG. 3B . 
         FIG. 5  is a schematic diagram of a second embodiment of an illumination system with increased optical power in the green band. 
         FIG. 6  is a schematic diagram of a third embodiment of an illumination system with increased optical power in the green band. 
         FIG. 7  is a flowchart of an exemplary method for increasing the optical power of a solid state light source with a phosphor layer in the green and/or yellow bands for improved fluorescence microscopy 
     
    
    
     DETAILED DESCRIPTION 
     The following definitions are useful for interpreting terms applied to features of the embodiments disclosed herein, and are meant only to define elements within the disclosure. No limitations on terms used within the claims are intended, or should be derived, thereby. Terms used within the appended claims should only be limited by their customary meaning within the applicable arts. 
     As used within this disclosure, “cone” refers to an inset surface having generally conically shaped profile. The terms cone and conical may refer to sections of a cone, for example, a cone omitting the apex. Some variations to a purely conical shape are also considered to be described with these terms, as described further below. The term “cone cavity” refers to a device having a cone shaped cavity formed therein. 
     As used within this disclosure, “substantially” means “very nearly,” or within expected manufacturing tolerances. For example, substantially parallel surfaces refer to surfaces that are parallel within any manufacturing tolerance expected by a person having ordinary skill in the art. 
     As used within this disclosure, “Lambertian scatter properties/characteristics” refers to Lambertian reflectance, a property that defines an ideal “matte” or diffusely reflecting surface. The apparent brightness of such a surface to an observer is the same regardless of the observer&#39;s angle of view. In particular, the surface&#39;s luminance is isotropic, and the luminous intensity substantially obeys Lambert&#39;s cosine law. 
     Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     Exemplary embodiments of the present invention disclose a system, device and method for increasing the optical power in the green band and coupling efficiency to a microscope using low cost solid state LED components combined with an integrating cone. The embodiments increase the total power in the green band but also improve the coupling efficiency to a microscope objective plane. 
     Generally, as described previously, the output intensity of an LED is limited by the maximum LED driving current density. Increasing the LED area can increase total optical power from the LED. But the optical output at an objective plane is not proportional with the LED area increasing. Most optical power increasing by increasing the LED area is stopped by the aperture inside the microscope. The present embodiments use a cone component to increase coupling efficiency from a large area LED to an objective plane of the microscopy system. 
     There is competition to provide a basic solid state lighting (SSL) illumination system, and many systems on the market today still do not offer the performance of the traditional arc lamp systems. The first embodiment, described below, approaches an arc lamp system, such as the X-Cite 120 for example, in terms of performance and cost. 
       FIG. 1  is a schematic diagram of a first embodiment of an illumination system  100  with increased optical power in the green band. The overall optical layout is generally typical for a microscopy illumination system, however, as described below, the first embodiment includes features not present in previous systems. 
     A conventional large area white LED  101 , for example, having an emitting area greater than 9 mm 2 , is low cost and may utilize a phosphor layer deposited on a blue or UV LED to create a broadband white light spectrum, for example, Luminius CBT-140. Although the power output of such a large LED  101  is relatively high compared to a more standard 1 mm 2  LED, the coupling efficiency in prior art systems to a microscope  107  is relatively poor. Under the first embodiment, the output of the LED  101  is integrated and reshaped by a cone cavity  102 . The cone cavity  102  has high reflectivity and Lambertian scattering properties. The dimensions and shape of the cone cavity  102  transfer an input beam, namely the light emitted from the large area LED  101 , to an output beam emitting from the cone cavity  102 , as described further below. The output beam has a smaller diameter than the input beam, improving the coupling efficiency to a microscope  107 . The output beam of the cone cavity  102  is collected by a lens  103 . For example, the lens  103  may be made from BK7 material, and be a ½ ball lens with a diameter of 10 mm (radius of 5 mm). A lens output beam  104  emitting from the lens  103  may be collimated by a collimating lens  105 , and collimated light  106  output by the collimating lens  105  may be coupled to the microscope  107 . 
       FIG. 2  shows a side view of the detail structure of the cone cavity  102  beside a bulls-eye scale shown from a front view. The cone cavity  102  includes an integral body  220  formed of a rigid material, with an ingress side  221  and an egress side  222  opposite the ingress side  221 . The cone cavity  102  may be formed as an aperture, or cavity  207 , in the body  220 , extending from the ingress side  221  to the egress side  222 . The input opening  202  of the cone cavity  102  is formed in the ingress side  221  of the body  220 , and has similar size to the emitting area of the large LED  101 , for example, greater than 9 mm 2 . The input opening  202  of the cone cavity  102  may be round  204  as shown in  FIG. 2 , such as a Luminus Devices CBT-140, or square (not shown), such as a Luminus Devices CBT-90. 
     An output opening  203  of the cavity  207  is formed in the egress side  222  of the body  220 . The output opening  203  may be round  205  or square, corresponding to the shape of input opening  202 . Other shapes of the input opening  202  and output opening  203  are possible, for example, such that the shape of the input opening  202  corresponds to the shape of the emitting area of the large LED  101 . For example a cone may correspond to a round LED, while a pyramidal shape may correspond for square, or rectangular LEDs. By extension, for any shaped LED, the cone would preferably mimic the geometric shape the LED is based on. The input opening  202  and the output opening  203  may have different shapes. Typically, the output shape is circular since this corresponds to the typical aperture stop geometry in a microscope optical train. In general the cone geometry will preferably match the LED source shape on the input side  202 , and be matched the aperture stop geometry in a microscope  107  ( FIG. 1 ) on the output side  203 . 
     The diameter of the output opening  203  is a smaller than the input opening  202 , for example the output to input area ratio may generally be in range of 1.5 to 10 times depending on applications, but not limited to this range. While several exemplary dimensions are provided herein, a person having ordinary skill in the art will recognize the dimensions and ratio of the output opening to the input opening will depend on several factors, for example, the LED  101  size and coupling requirements to the microscope  107  ( FIG. 1 ). 
     The material used for the cone cavity  102  may generally be any suitable base material and a scattering coating applied to it. Alternatively, the cone cavity  102  may be molded as an integral part from the scattering material. For example, scattering materials with the desirable optical properties includes Spectraflect and/or Spectralon from Labsphere. Such materials may be coated or molded, and have a typical reflectance of 0.99 within 400-700 nm. The internal surface  206  of the cavity  207  is highly reflective, for example, where the reflectivity is preferably higher than 99% and with Lambertian scatter characteristics. The internal surface  206  in the first embodiment may be reflective to light in the visible spectrum. However, alternative embodiments using a similar process/methodology may have a UV LED with a phosphor coating, effectively extending the reflectivity requirement more generally to include UV to near IR (eg, 350-800 nm). 
     Under the first embodiment, the cavity  207  is generally conically shaped, where the input opening  202  and output opening  203  are substantially concentric, with centers of openings  202 ,  203  sharing a common axis of the cavity  207 . The profile of a cutaway of the internal surface  206  may generally be formed from straight lines, but other embodiments may employ a curved surface with respect to the center axis of the cavity  207 , for example, a convex curved surface, or a concave profile curved surface. Other curves are also possible, for example, a cavity an elliptical or parabolic profile. 
     The cone cavity  102  with Lambertian scatter characteristics and high reflectivity of the internal surface  206  combined with the highly reflective LED surface (due to the high index refractivity of the LED substrate) act as integrating sphere Multiple reflections of the light emitted from the LED  101  occur in the cavity  207 . An exemplary material used for the body  220  forming the cavity  207  is Spectralon from LabSphere. The internal reflectivity of the cone cavity  102  is greater than 99% and with Lambertian scattering characteristics. For example, using a Luminous CBT-140 the input diameter of the cone  202  may be 4.5 mm, and the exit diameter of the cone  203  may be 1.8 mm. 
     The green or yellow light beam  212  emitted from a phosphor layer  201  on the LED directly exits  209  the cone cavity  102  or indirectly exits though multiple reflections  211  in the integrating cavity. The output beam  203  from cone cavity  102  is smaller in size compared with the yellow/green beam  212 . Similarly, a portion of the blue light directly exits the cone  203 , but most of the blue light from the large area LED  101  is reflected from cavity surface  207  back to the phosphor layer  201 , thus generating more light in the yellow and green band than is produced by light directly emitted by the LED  101 , which only interacts with the phosphor layer once. The blue light is therefore recycled, and at the output opening  203  of the cone cavity  102  the ratio of green to blue light is increased. The combination of smaller source size at the output opening  203  and increased light in the green band significantly improves the green light coupled to the objective plane of the microscope  107  ( FIG. 1 ) and allows for improved fluorescence imaging. It is desirable to have equal brightness in all wavelength bands and the white LED  101  alone has insufficient light in the green and yellow bands. 
       FIG. 3A  shows a diagram of a fluorescence microscope illumination system with the cone cavity  102  to boost light output to the microscope  107 . The LED  101  may be, for example, a large area low cost white LED or a lime LED. The output of LED  101  is integrated and reshaped by the cone cavity  102  which has high reflectivity and Lambertian scattering characteristics as described above. The output from the cone cavity  102  is collected by the lens  103 . The lens output beam  104  is collimated by collimating lens  105 . The collimated light  106  is coupled to a microscope relay lens  307 . A partial beam  308  from lens  307  is directed toward an aperture  309 , where most of the beam  308  passes through the aperture  309  and becomes the input to an objective lens  310 . The output beam  311  is directed to an objective plane  312 . 
       FIG. 3B  is a schematic diagram of the light throughput of a prior art illumination device, which is similar to  FIG. 3A , but omits the cone cavity  102 . As shown by  FIG. 3A , the small size of the source beam emitted from the cone cavity  102  results in a small beam size  308 . In contrast the beam size of the  308  of  FIG. 3B  is much larger than the beam size  308  of  FIG. 3A , resulting in the aperture  309  of  FIG. 3B  blocking much more light than the aperture  309  of  FIG. 3A . In  FIG. 3A , more light from the illuminator with the cone cavity  102  reaches to objective plane  312  than the prior art illuminator of  FIG. 3B  without the cone cavity  102 . 
     Experimental results are consistent with the above description. An experiment was conducted using a luminous round CBT-140 LED with a 4.25 mm diameter, a cone cavity  102  with reflectivity greater than 99% and Lambertian scattering characteristics. The diameter of the cone input opening  202  ( FIG. 2 ) was 4.5 mm, and the diameter of output opening  203  ( FIG. 2 ) of the cone cavity  102  ( FIG. 2 ) was 1.8 mm. The microscope was a Zeiss axio-observer Al with objectives: 10×/NA0.25, 20×/NA0.8, and 40×/0.75.  FIG. 4  shows a spectral comparison 40× objective plane between the same illuminator with and without the cone cavity  102 . The spectra in the 500-700 nm range are clearly improved with the cone cavity  102 . 
     Table 1 shows the optical output power comparison between illumination paths with and without a cone cavity for same LED. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Peak wavelength/ 
                 10x/NA: 
                 20x/NA: 
                 40x/NA: 
               
               
                   
                 band width 
                 0.25 
                 0.8 
                 0.75 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 With Cone 
                 545 nm/25 nm 
                 43 
                 81 
                 33 
               
               
                   
                 562 nm/40 nm 
                 77 
                 146 
                 59 
               
               
                   
                 470 nm/40 nm 
                 40 
                 79 
                 29 
               
               
                   
                 628 nm/40 nm 
                 55 
                 109 
                 44 
               
               
                 Without Cone 
                 545 nm/25 nm 
                 28 
                 68 
                 19 
               
               
                   
                 562 nm/40 nm 
                 50 
                 121 
                 34 
               
               
                   
                 470 nm/40 nm 
                 40 
                 95 
                 26 
               
               
                   
                 628 nm/40 nm 
                 34 
                 85 
                 24 
               
               
                 Improvement 
                 545 nm/25 nm 
                 50% 
                 19% 
                 74% 
               
               
                   
                 562 nm/40 nm 
                 53% 
                 21% 
                 75% 
               
               
                   
                 470 nm/40 nm 
                 −1% 
                 −17%  
                 11% 
               
               
                   
                 628 nm/40 nm 
                 62% 
                 28% 
                 85% 
               
               
                   
               
            
           
         
       
     
     As seen by the data in Table 1, the optical power at objective plane of a microscope was improved from 19% to 70% in the green and yellow bands depending on the microscope objective selected. Because the different objectives have different numerical aperture and input diameter, the coupling improvement is variable. For the blue band, the power at the objective plane is increased for 40× but it slight decrease for 10× and 20× because some of the blue light is reflected back onto the phosphor which increases the emission in the green and yellow bands. 
       FIG. 5  is a schematic diagram of a second embodiment of an illumination system  500  with increased optical power in the green band. As shown in  FIG. 5 , the illumination system  500  may include multiple LED light sources  501 , where integrating cones  502   a ,  502   b  may be coupled with one or more of the multiple LEDs  501   a ,  501   b . The LEDs  501   a ,  501   b  may have different properties, for example, producing light having of different wavelengths, different beam sizes, and/or different power characteristics. The light of the LEDs  501   a ,  501   b  and integrating cones  502   a ,  502   b  may be combined by a blending means  530 , for example, a prism or dichroic plates. After light of multiple LED sources  501   a ,  501   b  is combined by the blending means  530 , the combined light may be collimated by a collimating lens  505 . The collimated beam may be coupled to a microscopy system  507 . While  FIG. 5  shows two LEDs  501   a ,  501   b  and two integrating cones  502   a ,  502   b , alternative embodiments may have two, three, or more LEDs, and two, three or more integrating cones. Furthermore, there is not necessarily a one-to-one relationship between the number of LEDs and the number of integrating cones. For example, a first LED may be coupled with an integrating cone, and a second LED may not be coupled with an integrating cone. 
     As shown in  FIG. 6 , under a third embodiment of an optical system  600 , one or more integrating cones  602   a ,  602   b  can also be used in light guide coupled applications to increase the green optical power coupled to a light guide  606 . The light of the LEDs  601   a ,  601   b  and integrating cones  602   a ,  602   b  may be combined by blending means  630 . Collimated light from a collimating lens  604  is coupled to the light guide  606  by using a focus lens  605 . The output of the light guide  606  may be collimated and coupled to a microscopy system  607  by an optical lens system  610 . The blending means  630  may include one or more dichroic filters/plates. The blending means  630  may also blend in light from light sources in addition to or instead of LED  601   a , which may or may not incorporate one or more additional integrating cones. 
       FIG. 7  is a flowchart of an exemplary method  700  for increasing the optical power of a solid state light source with a phosphor layer in the green and/or yellow bands for improved fluorescence microscopy. It should be noted that any process descriptions or blocks in flowcharts should be understood as representing modules, segments, portions of code, or steps that include one or more instructions for implementing specific logical functions in the process, and alternative implementations are included within the scope of the present invention in which functions may be executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those reasonably skilled in the art of the present invention. The method is described with reference to  FIG. 2 . 
     An integrating cone cavity  102  having an input opening  202 , an output opening  203 , and a reflective interior surface  206  formed between the input opening  202  the output opening  203  is provided, as shown in block  710 . Substantially blue light from a large area solid state lighting device  101  is received into the input opening  202 , as shown by block  720 . Substantially green and/or yellow light from a phosphor layer  201  on the solid state lighting device  101  is received into the input opening  202 , as shown by block  730 . A first portion of received blue light is directly emitted through the output opening  203 , as shown by block  740 . The reflector interior surface  206  reflects a second portion of the received blue light back to the phosphor layer  201 , as shown by block  750 . The phosphor layer  201  converts at least some of the second portion of the reflected blue light to green and/or yellow light as shown by block  760 . The received and the converted green and/or yellow light is emitted through the output opening  203  as shown by block  770 . 
     The embodiments described above offer a balance between cost and performance and for cost limited applications, providing a solution with increased optical power in critical wavelength bands compared to previous solutions. The embodiments can be used for directly coupled or light guide coupled illumination products where a larger LED chip is utilized in order to meet the optical power requirements of an application. The embodiments may be particularly useful in fluorescence imaging application where it is desirable to have equal power across all visible wavelength bands. 
     While the above embodiments have generally described an integrating cone in the context of a microscopy system, the use of the integrating cone is not limited to such applications. Persons having ordinary skill in the art will recognize the integrating cone may be used in many applications that would benefit from low cost and/or passive increase in the green/yellow/amber band. 
     While the above embodiments describe generate more green/yellow light, the generalized approach described above may be used to generate more light in other wavelength ranges, for example, using different colored LEDs as the primary light source, coupled with different secondary light sources providing complementary wavebands. 
     In summary, it will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.