Abstract:
A ring illuminator ( 20 ) has a light channeling section ( 22 ) with an input face ( 24 ) optically coupling the light channeling section ( 22 ) to a flash unit ( 16 ) of a camera ( 12 ) for obtaining flash illumination therefrom. A ring section ( 26 ) is optically coupled to the light channeling section and extends around an aperture for fitting the ring illuminator around a lens ( 14 ). The ring section has a rear surface ( 30 ) for positioning toward the body of the camera. The rear surface has a plurality of light redirecting features ( 28 ) for redirecting at least a portion of the flash illumination from within the ring section outward from the camera, around the lens of the camera.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an illumination apparatus for flash photography and more particularly to a ring light attachable to a camera for redistributing illumination from the camera flash unit.  
       BACKGROUND OF THE INVENTION  
       [0002]     In conventional camera design, the flash illumination source of the camera is off-axis with respect to the objective lens of the camera and illuminates from one angular orientation only. While this may be acceptable for the bulk of imaging applications, this arrangement is particularly troublesome for close-up imaging, such as in dental imaging and imaging of small objects in near distance. In close-up camera work, the conventional flash illumination arrangement often results in undesirable shadow and overexposure effects.  
         [0003]     In response to the need for improved light distribution for minimizing shadows and other effects in close-up imaging, ring illumination has been developed. In ring illumination, a ring of light, ideally centered about the objective lens optical axis, is projected outward toward the imaging object. There have been a number of ring illumination solutions, including active ring light solutions that provide an alternate light source from the flash unit of the camera itself. Exemplary active ring light solutions include commercial products such as the Digi-Slave L-Ring Ultra LED Ring Light, from Digi-Slave, from SR Inc., Irving, Tex. This device threads onto the camera lens assembly and provides flash illumination from a circular array of bright LEDs. Another commercial solution is the Samigon Halo-Light FRL-1 Fluorescent Ring Light from Argraph Corp., Carlstadt, N.J. This device provides a continuous, bright ring of light when installed onto the camera lens. Other active ring light solutions are disclosed in U.S. Pat. No. 4,921,344 (Duplantis) and U.S. Patent Application Publication 2001/0010760 (Saito).  
         [0004]     While such active ring light devices provide improved illumination for many types of close-up photography applications, there are some drawbacks. For example, many conventional ring lights require a separate source of electrical power, cabling for flash synchronization, and light sources of some type. Because of this, a conventional ring light system can be complex and costly. Another key disadvantage of such active ring light solutions relates to the suitability of the illumination provided to any particular camera. In most types of cameras, particularly cameras using a built-in flash mechanism, the spectral and intensity characteristics of the flash illumination are at least somewhat matched to the camera itself. Devices that substitute other types of light sources for flash illumination may indeed minimize shadows and other effects but, at the same time, may compromise image quality, particularly with respect to color balance. Other drawbacks with many conventional devices include excessive size and weight.  
         [0005]     Using an alternative approach to the active ring light, various ring light solutions have been proposed for redirecting light from a conventional flash illumination mechanism into a ring light arrangement. For example: 
        U.S. Pat. No. 6,430,371 (Cho) discloses a ring light that uses a cluster of optical fibers to re-route light from the camera flash unit to an output ring.     U.S. Pat. No. 4,085,436 (Weiss) discloses an attachment with a translucent ring, having silvered reflective surfaces for distributing light around the ring. A plate member of this ring is clamped onto a separate electronic flash unit for redistributing light as ring illumination. The assembled and clamped ring light is then installed onto the camera lens.     U.S. Pat. No. 3,474,242 (Forrant) discloses a photographic ring light accessory that can be fitted around the camera lens, consisting of a somewhat horseshoe-shaped translucent light guide encased within a housing structure, the light guide ring having a gap at the base for accommodating its own flash source.        
 
         [0009]     While these solutions may provide light with spectral characteristics that are better suited to the individual camera type, drawbacks of complexity, cost, size, and usability remain. Ideally, for example, a ring light should be easy for a relatively unskilled photographer to carry, install, and use. Thus, it can be seen that there is a need for a ring light that is compact and inexpensive, fits readily onto a camera, allows straightforward use by an amateur photographer, and provides effective redistribution of light from the camera flash unit itself, without the complexity of a separate power source, light source, electrical connection, or synchronization components.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention provides a compact ring light suitable for many types of close-up photography. To do this, the present invention provides a ring illuminator comprising: 
        a) a light channeling section comprising an input face for optically coupling the light channeling section to a flash unit of a camera for obtaining flash illumination therefrom;     b) a ring section optically coupled to the light channeling section and extending around an aperture for fitting the ring illuminator around a lens of the camera;     the ring section comprising a rear surface for positioning toward the body of the camera; and     the rear surface comprising a plurality of light redirecting features for redirecting at least a portion of the flash illumination from within the ring section outward from the camera, around the lens of the camera.        
 
         [0015]     The ring illuminator of the present invention thus creates uniform illumination in angular and spatial domains by reconfiguring light from a camera flash unit and redirecting the light to a light ring surrounding a camera lens. The ring light of the present invention is advantaged in that its geometry can be fairly complex to minimize light leakage and to allow a high degree of optical efficiency and uniform illumination, yet the unit cost of the ring light can be relatively low.  
         [0016]     The ring illuminator of the present invention is advantaged over conventional ring light solutions by its relative simplicity, allowing ease of use, low-cost fabrication, and disposability for applications such as with single-use cameras. The ring illuminator according to the present invention can be a single piece unit, without any supporting casing, housing, or surface coatings, without any electrical connections, and without fasteners or parts needing replacement.  
         [0017]     It is another advantage of the present invention that it allows the design of a ring illuminator that utilizes the built-in light output and light sensing components of a camera, minimizing or eliminating the need to make manual adjustments to camera operation to compensate for ring lighting use.  
         [0018]     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein:  
         [0020]      FIG. 1  is a perspective view of a system for close-up imaging according to the present invention;  
         [0021]      FIG. 2  is a front view of a system for close-up imaging according to the present invention;  
         [0022]      FIG. 3  is a side view of a system for close-up imaging according to the present invention;  
         [0023]      FIG. 4  is a plan view showing a ring light guide in one embodiment;  
         [0024]      FIG. 5  is a rear perspective view of the ring light guide according to one embodiment;  
         [0025]      FIGS. 6A and 6B  are side views of a ring light guide in slightly different embodiments;  
         [0026]      FIG. 7  is a front perspective view of a ring light guide in a tapered embodiment;  
         [0027]      FIG. 8  is a rear perspective view of the tapered ring light guide of  FIG. 7 ;  
         [0028]      FIG. 9  is a rear view of the tapered ring light guide of  FIG. 7 ;  
         [0029]      FIG. 10  is a side view of the tapered ring light guide of  FIG. 7 ;  
         [0030]      FIGS. 11A, 11B ,  11 C, and  11 D are perspective front, front, perspective rear, and side views, respectively, of a single-piece ring light guide in an alternate embodiment;  
         [0031]      FIG. 12  is a diagram showing key dimensional relationships for maintaining total internal reflection within a curved structure; and  
         [0032]      FIGS. 13A, 13B ,  13 C, and  13 D show alternative features for light redirection from the rear surface of the right light guide of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     The present description will be directed in particular to elements forming part of, or in cooperation more directly with the apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.  
         [0034]     Referring now to  FIG. 1  an image capture system for close up imaging is referred to in general by numeral  10 . Image capture system  10  comprises a camera  12  having an attached ring light guide  20 . Camera  12  has a close-up lens  14  that may be integral to camera  12  or may be a separable accessory. A flash unit  16  is preferably integral to camera  12  and may be built into the main body of camera  12  as a fixed-position device or may be a pop-up flash unit or other type of protruding flash unit. Alternately, flash unit  16  may be an accessory unit attached to camera  12  when needed. A light sensor  18  provides feedback on scene illumination and can be used to control flash duration and various settings within camera  12 , as is well known in the photographic imaging arts. In the preferred embodiment, camera  12  is a digital camera; however, ring light guide  20  could be similarly used with a conventional film-based camera.  FIGS. 2 and 3  show front and side views, respectively, of image capture system  10 .  
         [0035]     In order to provide a simple and inexpensive solution for ring lighting, ring light guide  20  redirects light from flash unit  16  around lens  14  and outward from camera  12 , toward the subject. Ring light guide  20  advantageously requires no moving parts or separate light sources, power supplies, synchronization signals or components, or complex fasteners. Instead, ring light guide  20  can be fabricated from low-cost materials, preferably as a single-piece unit, designed to snap into place to fit snugly around close-up lens  14 , as shown in  FIGS. 1, 2 , and  3 . In one embodiment, for example, ring light guide  20  is a molded acrylic device.  
         [0036]     As shown in  FIGS. 1, 2  and  3 , and called out more particularly in  FIG. 4 , ring light guide  20  has one portion that is placed proximate to or directly against flash unit  16  and a curved portion that fits around close-up lens  14 . As is best shown in  FIG. 2 , the structure of ring light guide  20  leaves at least some portion of sensor  18  unobstructed, allowing the built-in light-sensing circuitry of camera  12  to operate normally when right light guide  20  is installed. Referring to  FIGS. 5, 6A , and  6 B, there are shown rear perspective and alternate side views respectively that more clearly show the structure of ring light guide  20 . Ring light guide  20  has two primary functional sections: a light channeling section  22  for obtaining light from flash unit  16  at an input face  24  and a curved ring section  26  for redistributing the light. A rear surface  30  of ring section  26  has an arrangement of discrete features  28  for redirecting light that has been routed through ring section  26  outward through a front surface  32 , toward the subject being imaged.  
         [0037]     Unlike prior art solutions that use reflective coatings applied to the light ring surface for light redistribution, ring light guide  20  of the present invention redirects light from flash unit  16  using total internal reflection (TIR). This redirected light is then redistributed around ring section  26  and then, by frustrating TIR at rear surface  30 , is directed outward toward the subject being imaged by camera  12 . The mechanism of TIR, a key principle used for light transmission in optical fibers, is widely known in the illumination art for implementing other types of light pipes as well. Briefly, for TIR, inherent behavior of a light pipe structure made of translucent material such as glass or plastic is used. Light is reflected at the interface of the light pipe structure with a surrounding material that has a lower refractive index, such as air, provided that certain conditions of incident angle and refractive index are met. Stated mathematically, TIR occurs when the following relationships are satisfied:  
         θ   i     ≥         sin     -   1       ⁡     (       n   2     /     n   1       )       ⁢           ⁢   and   ⁢           ⁢     n   2       ≤     n   1         
 
 where θ i  is an incident angle of a light ray within the translucent light pipe structure on the interface, n 1  a refractive index of the structure, and n 2  a refractive index of the surrounding material at the interface. (Typically the surrounding material is air, whose refractive index n 2  is 1.) 
 
         [0038]     It is instructive to note that the entire structure of ring light guide  20  operates as a type of light pipe, channeling light throughout its length using TIR. The specific portion of ring light guide  20  identified as light channeling section  22  in  FIGS. 5, 6A ,  6 B, and elsewhere has this primary function, providing a channel for light obtained from flash unit  16  using TIR. The curved portion of ring light guide  20 , identified as ring section  26  is more complex. Here, light is directed around a curve using TIR. Features  28  on rear surface  30  then frustrate TIR at that surface to cause the redirection of light in a ring.  
         [0039]     TIR can be maintained over a relatively straight section of translucent material as well as around a curve, provided that the overall curvature is suitably constrained. Referring to  FIG. 12 , there are shown key geometrical parameters influencing TIR behavior over a curved section. Here, a portion of ring section  26  has an inner radius r 1  and outer radius r 2 . In order to maintain TIR for any light directed about this curve, the following relationship must hold: 
 
 r   2   /r   i   ≦n 
 
 where n is the index of refraction of the translucent material used to form ring section  26 . 
 
         [0040]     It is important to emphasize that the design of ring light guide  20  uses TIR, but that the efficiency of this component may vary. For many types of close-up imaging applications, only a percentage of the light from flash unit  16  would be needed. Thus, strict adherence to the relationship of radii r 1  and r 2  given above would not be necessary for redistribution of sufficient light for ring illumination. For similar reasons, input face  24  need not fully cover the surface of flash unit  16 ; input face  24  need only direct sufficient light from this source for distribution around ring section  26 .  
         [0000]     Design Options  
         [0041]     Given the basic overall design of ring light guide  20  as shown in  FIGS. 1 through 6 B, a number of variations in shape, thickness, light distribution, and in mechanisms for TIR frustration by different types of features  28  are possible.  FIGS. 7, 8 ,  9 ,  10 , and  11 A- 11 D show some of these variations from different views.  
         [0042]     Various types of features  28  can be formed on rear surface  30  for frustration of TIR. Features  28  are non-continuous and non-planar, that is, are not flat, smooth reflective surfaces or coatings. Instead, features  28  are discrete elements such as indentations, printed or otherwise applied patterns, or protrusions that provide some type of light-scattering surface discontinuities for frustrating TIR at rear surface  30 . In one embodiment, features  28  can be a plurality of indented grooves or a sawtooth arrangement that effectively provides prisms for light redirection.  FIGS. 7, 8 ,  9 , and  10  show an alternate embodiment in which features  28  present a generally sawtooth profile, as shown in the perspective view of  FIG. 13D . Alternately, features  28  could have the shape of circularly cylindrical grooves as shown in  FIG. 13A , as well as using some other type of groove shape. Yet another configuration uses an array of discrete indentations or protrusions.  FIG. 13B , for example, shows features  28  as an array of ellipsoidal indentations formed into rear surface  30 .  FIG. 13C  shows yet another alternative embodiment in which features  28  are pyramidal indentations into rear surface  30 . As  FIGS. 13B and 13C  particularly show, a regular distribution of features  28  having equal pitch is not a requirement; there may be optimal arrangements of features  28  using irregular feature-to-feature spacing for more uniform distribution of illumination, for example. Also, combinations using different types of features  28  could also be employed. Certainly, some portion or all of features  28  could be coated or provided with a backing material that provides reflectivity to supplement light redirection; however, the primary redirection mechanism disrupts TIR on rear surface  30 . Scattering dots, printed or applied in some other manner, could also be used as features  28  or to supplement grooves or other structures that act as features  28 . While scattering dots could be used, however, they are not highly directional and can tend to diffuse light.  
         [0043]     Another variable that can be optimized is the overall thickness of ring section  26 .  FIGS. 8 and 10  show embodiments in which ring section  26  is tapered, having its thinnest point near the base, in the area that is most distant from light channeling section  22 . A tapered arrangement, in combination with an optimized arrangement of features  28 , may be most favorable for providing a uniform distribution of light around the lens.  
         [0044]      FIGS. 11A-11D  show an alternate embodiment of ring light guide  20  that could be described as a single light pipe, curved appropriately for the geometric configuration of the camera. Unlike the embodiments of  FIGS. 1-10 , this embodiment does not split the light from light channeling section  22  down each side of lens  14 . Instead, the embodiment shown in  FIGS. 11A-11D  directs light around lens  14  in a counterclockwise orientation, as viewed from the perspective of the subject. The operation of ring light guide  20  in this embodiment is similar to that described with reference to  FIGS. 1-10 . The use of a single curved piece of material, as shown in the embodiment of  FIGS. 11A-11D , can provide advantages for positioning ring light guide  20  on lens  14  ( FIGS. 1-3 ), for maintaining this accessory in place, and for leaving at least a portion of sensor  18  unobstructed.  
         [0045]     There are a number of composition and fabrication options for the manufacture of ring light guide  20 . Typical translucent materials that could be used include glass or various plastics having suitable refractive indices, typically at least about 1.4. Acrylic and other plastic materials are particularly advantageous for fabrication, optical qualities, overall flexibility, cost, adaptability, and durability. The generally rectangular cross-sectional shape of ring light guide  20 , as is shown in the various embodiments of this disclosure, is advantaged for providing TIR in this ring light application; however, other cross-sectional shapes could be used..  
         [0046]     As noted in the above description, light channeling section  22  and ring section  26  are shaped differently and have different functions, however, these two portions of ring light guide  20  could be continuous, such that ring light guide  20  is fabricated as a single piece. The embodiment of  FIGS. 11A-11D , for example, would be well suited for single-piece fabrication. Alternately, both portions of ring light guide  20  could be separately fabricated. In general, the use of the same material for both sections with single-piece construction would be optimal, providing the most robust solution.  
         [0047]     A key design consideration for shaping ring light guide  20  is to allow normal operation of camera  12 , so that lens  14  could be easily adjusted when ring light guide  20  is in place, for example. Another shape consideration, as noted above, is to maintain any needed clearance to prevent obstruction of sensor  18 , to take advantage of the built-in light sensing functions of camera  12 .  
         [0048]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. For example, features  28  of various types may be used to frustrate TIR and cause light redirection. The spacing from one discrete feature  28  to the next may be varied depending on distance from the light source, or the types of features  28  used could be varied, for example. Ring light guide  20  could be provided as a detachable unit or could be implemented as an accessory that is ordinarily mounted on the camera or zoom lens and is rotated into place when needed.  
         [0049]     Unlike prior art solutions, ring light guide  20  of the present invention can be designed as a single piece unit. Because it uses TIR to route light from the flash source to its ring illumination output, ring light guide  20  does not need a complex arrangement of surfaces, nor does it need planar reflective coatings on any of its surfaces. Ring light guide  20  can be clipped onto lens  14  as desired, with or without fasteners and without the need for any type of electrical connection to flash synchronization circuitry. A protective casing is not needed during operation, although a soft cloth carrying bag may be desirable to eliminate scratches, particularly along front surface  32 . Because ring light guide  20  uses light from built-in camera flash unit  16 , the spectral characteristics of the flash illumination are already matched to the camera. In addition, no electrical power connection is needed and no replacement bulb is required. Light sensor  18  of camera  12  and associated flash lighting control circuitry of camera  12  can operate normally, since the simple design of ring light guide  20  allows sensor  18  to be substantially unobstructed with ring light guide  20  installed.  
         [0050]     Thus, what is provided is a ring light guide attachable to a camera for redistributing illumination from the camera flash unit.