Abstract:
A device and a method for uniformly illuminating transparent or opaque objects while confining the illumination to those objects. The device consists of a radiation source and a transparent block. The device exploits total internal reflection to ensure that radiation introduced to the block by the radiation source propagates only within the block except where the block is in contact with the object to be illuminated. Where there is contact with the object, some of the radiation enters transparent objects, illuminating them from within or is diffusely reflected from opaque objects.

Description:
FIELD AND BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a device and a method for uniform illumination and, more particularly, to a device and method for illuminating target objects with radiation in a way that restricts the illumination to the target objects.  
           [0002]    There exist many applications where it is desirable to illuminate only a target object. In some such applications this is desirable in order to detect subtle features of the target object with greater contrast and without interference from radiation scattered by other illuminated targets. One such application is the detection of flaws in cut gemstones. These flaws can be detected by the radiation that the flaws scatter which differs in character from the radiation reflected and refracted by the facets of the gemstones. This detection of flaws can be made more reliable by restricting illumination to the gemstone so that radiation scattered by the background is not confused with radiation scattered by flaws.  
           [0003]    Another such application is in the automated recording and identification of fingerprints by imaging of fingertips. Confining the illumination to the fingertips allows homogenous illumination of the object and reduces noise which arises from background radiation.  
           [0004]    In all such applications, it is desirable that the target object be illuminated uniformly so that the intensity of radiation reflected or scattered from the target depends only on the properties of the target and not on the properties of the radiation source. Scanning the target with a stable radiation source such as a laser can simulate uniform illumination. In principle, the sum of images obtained by sequentially illuminating (scanning) contiguous portions of the target is equivalent to the image that would obtained by uniform illumination. However, scanning increases the complexity of the imaging device and decreases the confidence in the results obtained. In addition to the radiation source, the recording medium and the image processor, the imaging device must also have a scanning mechanism and means for synchronizing the scanning and the recording. Furthermore, illumination of an irregularly shaped target requires that either the scanning mechanism or the image processor have means such as an edge detection system for excluding images recorded while the radiation source illuminates past the edges of the target from the sum. To avoid the problems inherent in scanning, a radiation source for simultaneous uniform illumination must be two-dimensional.  
           [0005]    Two-dimensional uniform radiationing in as of itself is not difficult. One simple way to achieve it is to cover a closely spaced array of point sources of radiation with a diffusing screen. These point sources could be as simple as incandescent radiation bulbs. The diffusing screen smears out the lateral variation in the intensity of radiation that impinges on it from the point sources and the radiation emerging from the other side of the screen is substantially uniform. The problem with such unsophisticated two-dimensional sources in the applications envisaged here is that it is difficult to confine the illumination to the target object. If all targets had the same shape a system of baffles could be used to limit the illumination. This is difficult when the targets are objects like gemstones (transparent to radiation) or fingertips (opaque to radiation).  
           [0006]    There is thus a widely recognized need for a source of radiation that uniformly illuminates only a target object.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention exploits the phenomenon of total internal reflection to provide simultaneous uniform illumination with radiation waves of only a target object. As used herein, the term “radiation waves” refers to energy that propagates as wave, such as radiation or sound energy. Total internal reflection is a mode of propagation of radiation waves at the interface between two media. The first of the two media is termed the medium wherethrough the waves are propagating at an angle relative to the interface. The second of the two media is termed the surroundings. If the angle is equal to or greater than the critical angle of the interface, Θ critical , the radiation does not exit the medium into the surroundings, but rather is reflected from the interface back into the medium. Θ critical  is determined by the index of refraction of the medium, n medium , and of the surroundings, n surroundings , according to equation 1:  
         sin        (     θ   critical     )       =       n   surroundings       n   medium                             
 
           [0008]    From equation 1 it is clear that for a critical angle to exist, n medium  must be greater than n surroundings . Typical indices of refraction for electromagnetic radiation are n vacuum =1.0000, n air =1.0003, n water =1.333, n plexiglas =1.51, n crown glass =1.52, n flint glass =1.66, n diamond =2.417 and n gallium phosphide =3.50.  
           [0009]    According to the present invention there is provided a device made up of a) a block that is substantially transparent to the type and range of frequencies of radiation used to illuminate, the block having at least one entry surface and at least one surface of total reflection so that the radiation introduced into the block via one of the entry surfaces at a suitable angle is totally reflected by a surface of total reflection and such that a portion of the at least one surface of total reflection is substantially uniformly irradiated by the radiation; and (b) a radiation source for introducing the radiation into the at least one entry surface at the suitable range of angles.  
           [0010]    According to the present invention there is provided a suitably shaped block of material that is transparent to the appropriate wavelength, and a source of radiation generating the appropriate wavelength, hereinafter called the “radiation source”, that introduces radiation into the block through one or ore surfaces of the block, hereinafter called entry surfaces, in such a way that the radiation is incident on other surfaces of the block hereinafter called “surfaces of total reflection”, at angles greater than or equal to the critical angle of the material, and in such a way that the intensity of the radiation incident on the surfaces of total reflection is laterally uniform. In most applications envisaged, the radiation is visible radiation, but it could also be electromagnetic radiation with frequencies in the infrared or ultraviolet range or other type of radiation, such as ultrasonic waves.  
           [0011]    When a transparent object, having an index of refraction n object  that is greater than that of the surroundings, is placed in contact with one of the surfaces of total reflection, the critical angle at the area of contact changes. If the index of refraction of the object is greater than the index of refraction of the block, then the conditions for total internal reflection are not satisfied and some of the radiation incident at the area of contact escapes the block into the object. If the index of refraction of the transparent object is less than the index of refraction of the block, then the critical angle at the area of contact is greater than the critical angle elsewhere along the surface of total reflection and some of the incident radiation on the area of contact at angles between the two critical angles may be transmitted into the transparent object.  
           [0012]    The mechanism of total internal reflection assumes that the radiation incident on the surfaces of total reflection is reflected specularly. When an opaque object is placed in contact with one of the surfaces of total reflection, some of the radiation incident on the are of contact is reflected diffusely, rather than specularly. This radiation reenters the block and, according to the present invention, exits the block via other surfaces, hereinafter called “exit surfaces”.  
           [0013]    The entry surfaces, the exit surfaces and the surfaces of total reflection may have any suitable shape and curvature. In most of the preferred embodiments of the invention described hereinbelow, the exit surfaces and surfaces of total reflection are substantially flat or cylindrical.  
           [0014]    The phenomenon of total internal refection has been used in devices known in the art.  
           [0015]    U.S. Pat. No. 4,668,861 describes a sandwich of a transparent sheet, a resilient sheet and a separator that can be used as a tactile sensor: radiation introduced into the transparent sheet undergoes total internal reflection except where the resilient sheet touches the transparent sheet.  
           [0016]    U.S. Pat. No. 5,355,213 describes a device that uses total internal reflection to find surface flaws of a transparent block.  
           [0017]    The present invention addresses the shortcomings of presently known means for uniform illumination of a transparent or an opaque object while confining the illumination to the object. The object is illuminated by placing the object in contact with at least one surface of total reflection when the radiation source is activated. Suitable means are then used to detect and process the radiation emerging from the object, in the case of a transparent object or from the corresponding exit surface in the case of an opaque object. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:  
         [0019]    [0019]FIG. 1 is a conceptual sketch of the invention, illustrating the phenomenon of internal reflection;  
         [0020]    [0020]FIG. 2 is a conceptual sketch illustrating the use of the present invention to illuminate a transparent object with an index of refraction that is greater than that of the transparent block;  
         [0021]    [0021]FIG. 3 is a conceptual sketch illustrating the use of the present invention to illuminate a transparent object with an index of refraction less than that of the transparent block;  
         [0022]    [0022]FIG. 4 is a conceptual sketch illustrating the use of the present invention to heat a pan of water;  
         [0023]    [0023]FIG. 5 is a conceptual sketch illustrating the use of the present invention to illuminate an opaque object;  
         [0024]    [0024]FIG. 6 is a conceptual sketch illustrating the use of the present invention to illuminate an object using sonic waves;  
         [0025]    [0025]FIGS. 7A and 7B are conceptual sketches of the invention, illustrating how uniform illumination is achieved using a point source of radiation;  
         [0026]    [0026]FIG. 8 is a conceptual sketch of the invention, illustrating how uniform illumination of the surfaces of total reflection is achieved using two collimated beams of radiation;  
         [0027]    [0027]FIG. 9 is a perspective view of a preferred embodiment of the invention wherein the transparent block has the shape of a parallelopiped;  
         [0028]    [0028]FIG. 10 is a perspective view of a preferred embodiment of the invention wherein the transparent block has the shape of a cylinder;  
         [0029]    [0029]FIG. 11 is a perspective view of a preferred embodiment of the invention wherein the transparent block has the shape of a cylindrical tube; and  
         [0030]    [0030]FIG. 12 is a side view of a preferred embodiment of the invention wherein the transparent block is saucer shaped. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    The present invention is of an illumination device. Specifically, the present invention can be used to uniformly illuminate transparent or opaque objects while restricting the illumination only to those objects.  
         [0032]    The principle and operation of a uniform illuminator according to the present invention may be better understood with reference to the drawings and the accompanying description.  
         [0033]    Referring now to the drawings, FIG. 1 illustrates the phenomenon of total internal reflection of visible electromagnetic radiation. Transparent flint glass block  10  has an index of refraction of n block =1.66 is surrounded by air, n air =100. As a result, Θ critical  at the glass/air interface is 37°. Radiation source  12  shines radiation  14  through entry surface  16  at an angle of 40° from normal. Radiation  14  repeatedly reflects off the glass/air interface of upper surface  18  and lower surface  20 . Under the conditions of FIG. 1, both upper surface  18  and lower surface  20  are surfaces of total reflection for radiation  14 .  
         [0034]    In FIG. 2, a diamond  22  with an index of refraction n diamond =2.42 is placed onto upper surface  18  of glass block  10 . Since n diamond &gt;n glass ,, not all radiation  14  from radiation source  12  which impinges on the area of contact between diamond  22  and glass block  10  is reflected from the glass/diamond interface. Some of the radiation  14   b  is refracted upwards into diamond  22 . As a result, diamond  22  is selectively illuminated from within. In a darkened chamber, diamond  22  will appear to glow from within while glass block  10  will be dark. A flaw  24 , present in diamond  22 , scatters some radiation,  14   b.  Scattered radiation  14   b  can be easily detected by means known to one skilled in the art, such as direct observation or a camera  26 . It is important to note that it is preferable that upper surface  18  be substantially rigid, that is that it does not deform when in contact with an object that is placed thereupon.  
         [0035]    In FIG. 3, a cut glass swan  28  with an index of reflection n swan =1.52 is placed on upper surface  18  of glass block  10 . From equation 1, it is found that the critical angle at the glass/swan interface is 66°. Since radiation  14  from radiation source  12  impinges on the area of contact between swan  28  and glass block  10  at an angle of 40°, some radiation,  14   c  penetrates upper surface  18  and is refracted into swan  28 . Swan  28  is selectively illuminated from within.  
         [0036]    In FIG. 4, the present invention is used to selectively heat water  30  confined in glass vessel  32 . Glass block  10  is transparent to infrared radiation and radiation source  12  is configured to produce a substantial percentage of radiation  14  with infrared frequencies. Since n water =1.33, Θ critical  at a glass/water interface is 62°. When vessel  32  is placed on glass block  10 , some radiation,  14   d,  penetrates through the surface of vessel  32  into water  30  and is absorbed by water  30 , thus heating water  30 .  
         [0037]    It is clear to one skilled in the art that embodiments of the present invention, analogous to the embodiment described in FIG. 4, can be applied to chemical substances that react under the influence of irradiation. Such reactions include fluorescence for use in quantitative analysis, radiation-induced polymerization, ultrasonic cleaning or other radiation enhanced processes.  
         [0038]    In FIG. 5, the use of the present invention in illuminating semi-opaque object  34  is depicted. Just as in FIG. 1, radiation source  12  shines radiation  14  through entry surface  16  of glass block  10  at an angle of 40° from normal. Radiation  14  repeatedly reflects off the glass/air interface of upper surface  18  and lower surface  20 . Where opaque object  34  in contact with upper surface  18 , some of radiation  14  is reflected diffusely  14   e.  Some of diffusely reflected radiation  14   e  penetrates through lower surface  20  to be detected by detector  36 . In the case where opaque object  34  is a finger, detector  36  detects a clear image of a fingerprint  38 . The device depicted in FIG. 5 is further equipped with a detector baffle  37  to shield detector  36  from any radiation excepting diffusely reflected radiation  14   e.    
         [0039]    In FIG. 6 the use of the present invention to illuminating an object  35  using sonic radiation is depicted. An ultrasonic transducer  13  acts as a radiation source to direct sound waves  15  through entry surface  16  of plastic block  11 . Sound waves  15  repeatedly reflect off the plastic/air interface of upper surface  18  and lower surface  20  due to the difference between the acoustic impedance (the sonic equivalent of index of refraction for electromagnetic radiation) of plastic and air. Where sonically-transparent object  35  in contact with upper surface  18 , some of sound waves  15  penetrates object  35 . Features  39  within object  35  that are opaque to sound waves  15  reflect sound waves  15   a  to detector  36 . Images of features  39  produced from reflected sound waves  15   a  are displayed on monitor  38 .  
         [0040]    It is important to note that despite that two modes of operation of the present invention have been described separately hereinabove, both modes can be applied simultaneously. Thus an object that is not completely transparent will reflect radiation that can be detected as in the device depicted in FIG. 5. Simultaneously, some of radiation will penetrate the object that is not completely transparent and illuminate the object from within, as depicted in FIG. 3.  
         [0041]    For objects, whether transparent or opaque to be uniformly illuminated by devices of the present invention such as those depicted in FIGS. 1 through 6, it is necessary that radiation  14  impinging on upper surface  18  (more generally, the surface of total reflection with which the object to be illuminated makes contact) be uniformly distributed.  
         [0042]    In FIG. 7, one way for this to be achieved is illustrated. In FIG. 7A, radiation source  12  is a point radiation source. Different radiation rays  14   f,    14   g  and  14   h  enter block  10  at a wide range of angles. Radiation ray  14   f  enters at an angle that is less than Θ critical , whereas radiation rays  14   g  and  14   h  enter at an angle that is greater than Θ critical . Radiation rays  14   g  and  14   h  reflect off upper surface  18  and lower surface  20 . Due to the different angles of entry of  14   g  and  14   h,  the frequencies with which  14   g  and  14   h  reflect off the surfaces of total reflection are different. As is clear to one skilled in the art, radiation source  12  produces a plurality of radiation rays  14  which enter block  10  with a continuum of angles, ensuring that the radiation rays which undergo total reflection are homogeneously distributed along the surfaces of total reflection of block  10 .  
         [0043]    When radiation rays such as  14   f,  which do not fulfil the conditions for total internal reflection, impinge on upper surface  18  or lower surface  20 , the radiation ray is partially reflected back into block  10  and partially escapes out through either upper surface  18  (e.g.  14   f   1 ) or lower surface  20  (e.g.  14   f   2 ). At a sufficient distance from entry surface  16 , radiation rays such as  14   f,  which do not meet the conditions for total internal reflection, are sufficiently dim to be substantially non-interfering for the purpose of illuminating an object.  
         [0044]    In FIG. 7B, entry surface  16  is flanked by entry baffle  40 . Entry baffle  40  ensures that only radiation rays  14  that meet the conditions for total reflection (such as  14   g  and  14   h ) enter through entry surface  16 .  
         [0045]    As is clear to one skilled in the art, ordinary diffuse sources of radiation, such as fluorescent lamps behave substantially as a dense array of point sources of radiation. Thus one suitable radiation source  12  for a device of the present invention, analogous the device depicted in FIG. 7, is a standard tubular fluorescent lamp.  
         [0046]    [0046]FIG. 8 shows an additional method to achieve uniform illumination of the surface of total reflection with which the object to be illuminated makes contact be uniformly distributed is through the use of two substantially collimated beams,  42  and  44 , as the radiation source. Collimated beams  42  and  44  are symmetric, that is they are of equal intensity and are symmetrically disposed about block  10 . Further, collimated beams  42  and  44  enter block  10  via entry surface  16  at an angle so that the conditions for total internal reflection are met. Lastly beams  42  and  44  have a width so that each one of beams  42  and  44  complementarily illuminate half of the surfaces of block  10 .  
         [0047]    In FIG. 8, beam  42 , bound by substantially parallel rays  401  and  402  penetrate entry surface  16  and reflect from surfaces of total reflection  20  and  18  of block  10  at points  411 ,  421 ,  431 ,  441 ,  451  and  412 ,  422 ,  432 ,  442 ,  452  respectively. Beam  42  uniformly illuminates surface of total reflection  14  between points  411  and  412 , between points  421  and  422 , between points  431  and  432 , and so on (indicted by shading). Beam  44 , is bound by substantially parallel rays  405  and  406 . Although the path of beam  44  through block  10  is not explicitly traced, study of FIG. 8 makes it clear to one skilled in the art that beam  44  uniformly illuminates the remainder of surfaces  18  and  20 .  
         [0048]    As is clear to one skilled in the art, a radiation source such as depicted in FIG. 8 can be made, for example using a laser, a beam splitter and a suitably disposed arrangement of lenses and mirrors.  
         [0049]    The radiation source depicted in FIG. 8 has one primary advantage over the radiation source depicted in FIGS.  7 A and  7 B: all radiation rays are incident on the surfaces of total reflection at an identical angle. This can be an advantage when illuminating a transparent object whose index of refraction is less than the index of refraction of the block by guaranteeing that the angle of incidence of the radiation is always large enough to avoid total internal reflection at the block/object interface.  
         [0050]    As clear to one skilled in the art, in some cases it is advantageous to use a radiation source that uses a number of radiation beams that is greater than two to uniformly illuminate a block of the device the present invention. As is clear to one skilled in the art, such a radiation source is fashioned in a manner analogous to that of the two-beam radiation source depicted in FIG. 8.  
         [0051]    The transparent block of the present invention can have a variety of shapes, four non-limiting examples appearing in FIGS. 9, 10,  11  and  12 .  
         [0052]    In FIG. 9, transparent block  10  is a parallelopiped. Entry surface  16  is one of the faces of block  10 . Two parallel faces act as surfaces of total internal reflection: face  18  and the face parallel to it (not seen in FIG. 9). In FIG. 9, radiation source  12  is a fluorescent lamp accompanied by baffle  40 , configured to allow radiation produced by radiation source  12  to enter block  10  through entry surface  16  only under conditions of total internal reflection.  
         [0053]    In FIG. 10, transparent block  10  is cylindrical with entry surface  16  being one of the ends of block  10 . Curved outer surface  46  of block  10  is a unique surface of total internal reflection. As is clear to one skilled in the art, the raypaths in block  10  of FIG. 10 resemble the raypaths in an optical fiber. Radiation source  12  is a floodlight with a diffusive coating on lens  48 .  
         [0054]    In FIG. 11, transparent block  10  has the shape of a cylindrical tube, with entry surface  16  being one of the ends of block  10 . Radiation source  12  is a circular fluorescent bulb. Entry baffle  40  is shaped as a plug inside the end of transparent block  10 , preventing the entry of radiation produced by radiation source  12  into transparent block  10  from any surface excepting entry surface  16  and only under conditions of total internal reflection. Curved outer surface  46  and the parallel inner surface (not seen in FIG. 11) of block  10  are the surfaces of total internal reflection.  
         [0055]    In FIG. 12, transparent block  10  has a saucer shape with a top face  18  a bottom face  20 , and a side face  50 . Entry surface  16  is a circular region of bottom face  20  in proximity of the edge of bottom face  20 . Top face  18 , bottom face  20  and side face  50  are surfaces of total reflection. Radiation source  12  is a circular fluorescent tube or a plurality of appropriately arranged point sources of radiation. Ring shaped entry baffle  40  prevents radiation from radiation source  12  entering transparent block  10  excepting under conditions of total internal reflection. As described in FIG. 5, when an object  34  is placed in contact with top face  18 , radiation rays reflect from object  34  to be detected by a detector  36 .  
         [0056]    In FIGS. 9, 10,  11  and  12  specific shapes of a transparent block of the present invention have been described. As is clear to one skilled in the art it is possible, by using an appropriate arrangement of radiation sources, to homogeneously illuminate a surface of total reflection of a transparent block of the present invention where the transparent block has virtually any shape. For example, although saucer shaped transparent block  10  has, by implication, a round shape illuminated by circular fluorescent tube  12 , an analogous device of the present invention can be designed wherein transparent block  10  is not round.  
         [0057]    While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations and modifications of the invention may be made.