Abstract:
An apparatus includes an optical filter having first and second passbands that are different, the optical filter including selectively operable first passband adjusting structure that varies a characteristic of the first passband without influencing the second passband. According to a different aspect, a method includes filtering radiation with an optical filter having first and second passbands that are different, and selectively varying a characteristic of the first passband without influencing the second passband.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates in general to filtering techniques and, more particularly, to optical filtering techniques. 
       BACKGROUND 
       [0002]    There are applications for which it is desirable to have an optical bandpass filter with two distinct passbands. One approach would be to fabricate a multi-layer coating that has two separate passbands. But as a practical matter, it can be difficult to actually manufacture such a coating with two separate passbands that each have the desired frequency range. Further, the spacing between the passbands becomes fixed at the time of manufacture. In addition, although the filter can be tilted from normal incidence in order to achieve a degree of tuning, this causes both passbands to move simultaneously toward shorter wavelengths. It is not possible to significantly vary the center wavelength of one passband without affecting the other passband, nor is it possible to vary the bandwidth of either passband. Thus, although existing dual-passband optical filters have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawings, in which: 
           [0004]      FIG. 1  is a diagrammatic view of an apparatus that is part of a fluorescence microscope, and that includes a dual-passband optical filter embodying aspects of the invention. 
           [0005]      FIG. 2  is a graph showing a curve that represents the transmissivity of a bandpass filter element that is a component of the embodiment of  FIG. 1 . 
           [0006]      FIG. 3  is a graph similar to  FIG. 2 , but also showing in broken lines two additional curves that represent shifted positions of a passband caused by tilting movement of the bandpass filter element. 
           [0007]      FIG. 4  is a graph depicting a broken line curve representing the transmission characteristic of one edge filter element and a solid line curve representing the transmission characteristic of another edge filter element, where the edge filter elements are both components of the embodiment of  FIG. 1 . 
           [0008]      FIG. 5  is a graph that is similar to  FIG. 4  but that shows the same two curves  131  and  132  from the perspective of the reflection characteristic rather than the transmission characteristic of each. 
           [0009]      FIG. 6  is a graph showing a curve that represents the transmissivity of a bandpass filter defined by the two edge filter elements. 
           [0010]      FIG. 7  is a graph similar to  FIG. 5  that shows the same curve as  FIG. 6 , and that also shows the effect on a passband of tilting movement of the two edge filter elements. 
           [0011]      FIG. 8  is a graph showing both passbands of the dual-passband optical filter of  FIG. 1 . 
           [0012]      FIG. 9  is a diagrammatic view of an apparatus that is a portion of a fluorescence microscope, and that includes a dual-passband optical filter that is an alternative embodiment of the dual-passband optical filter of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  is a diagrammatic view of an apparatus  10  that is part of a fluorescence microscope, and that includes a dual-passband optical filter  12  embodying aspects of the invention. A source  14  of a known type emits a broadband beam within a selected waveband, and this radiation is filtered by the filter  12  in a manner discussed in more detail later. The filtered radiation from the source  14  is ultimately directed by the filter  12  to a specimen or target  16  that has been treated with a conventional fluorescent dye. This radiation causes the dye in the target  16  to fluoresce, and to emit radiation at a wavelength different from the wavelength of the radiation received through the filter  12  from the source  14 . Radiation emitted due to the fluorescence is filtered by the filter  12  in a manner discussed in more detail later, and then directed through imaging optics  18  of a known type to a detector array  19  of a known type. 
         [0014]    The optical filter  12  includes two cascaded optical filter sections  26  and  28  that operate independently. The optical filter section  26  includes a gear  33  that is supported for limited rotational movement about a pivot axis  34 . A drive mechanism  36  is provided to selectively pivot the gear  33 . In the disclosed embodiment, the drive mechanism  36  includes a not-illustrated stepper motor having a rotatable shaft with a not-illustrated pinion gear thereon, the pinion gear engaging the teeth of the gear  33 . The drive mechanism  36  includes two not-illustrated switches that can be manually actuated to cause the stepper motor to rotate its shaft in either of two opposite directions, and thus pivot the gear  33  in either one direction or the other. The drive mechanism  36  could, however, have some other configuration. In  FIG. 1 , the gear  33  is shown in a center position. The drive mechanism  36  can selectively pivot the gear  33  a few degrees away from the illustrated center position about the axis  34 , in either of two opposite rotational directions. The drive mechanism  36  can also releasably maintain the gear  33  in any angular position. 
         [0015]    A V-shaped support member  41  is fixedly secured on the gear  33  for pivotal movement therewith. Two substrates  43  and  44  each have one end fixedly but detachably secured to a respective leg of the V-shaped support member  41 . The facing surfaces of the substrates  43  and  44  extend at an angle of approximately 45° with respect to each other, and intersect at a line that is coincident with the axis  34 . 
         [0016]    A bandpass filter element  46  is provided on the surface of substrate  43  that faces the substrate  44 . The bandpass filter element  46  is a multi-layer filter coating of a known type, and in particular a multi-cavity Fabry-Perot structure, but it could alternatively have some other suitable structure. The filter element  46  is transmissive to radiation inside a passband having a center wavelength, and is reflective to radiation above and below this passband.  FIG. 2  is a graph of a curve  51  representing the transmissivity of the filter element  46  of  FIG. 1  with respect to radiation arriving from the source  14 , when the gear  33  is in the center position shown in  FIG. 1 . For clarity, the curve  51  in  FIG. 2  has a somewhat idealized shape. When the gear  33  and the filter element  46  are in the center position, the filter element  46  has a passband that is between two wavelengths A and C, and has a bandwidth  52  and a center wavelength B. The filter element  46  is transmissive to radiation within the passband (between wavelengths A and C), and reflective to wavelengths above or below the passband. 
         [0017]    When the gear  33  and filter element  46  are in the center position of  FIG. 1 , the beam of radiation from the source  14  propagates along a path of travel  71  and then impinges on the filter element  46  at an angle of approximately 22.5° with respect to a not-illustrated reference line perpendicular to the filter element  46 . If the gear  33  and filter element  46  are pivoted away from the center position of  FIG. 1  in either direction, there will be a change in the angle at which radiation from the source  14  impinges on the filter element  46 . As a result, the entire passband will move leftwardly or rightwardly in  FIG. 2 , without any significant change in the bandwidth  52 . This means that the center wavelength will either increase or decrease by the same amount.  FIG. 3  is a graph that is similar to  FIG. 2 , but that also shows in broken lines two curves  56  and  58  that represent shifted positions of the passband  51  caused by limited pivotal movement of the gear  33  away from the center position in respective directions that are opposite. The curves  56  and  58  have respective bandwidths  57  and  59  that are each approximately equal to the bandwidth  52 . When the passband shifts from  51  to  56  or  58 , the center wavelength also increases or decreases from wavelength B, as indicated diagrammatically by arrows  61  and  62  in  FIG. 3 . The curves  56  and  58  in  FIG. 3  each represent a small and exemplary amount of shift of the passband  51  in each direction, but do not represent upper and lower limits on the shifting of passband  51 . The passband  51  can in fact be shifted significantly farther in either direction. 
         [0018]    A reflective element  66  is provided on the surface of the substrate  44  that faces the substrate  43 . In the disclosed embodiment, the reflective element  66  is a mirror coating of a known type that has a multi-layer design utilizing multiple dielectric materials. Alternatively, however, the coating  66  could be made from any other suitable material or combination of materials, and could for example be made from a metallic material. The reflective element  66  is reflective to all wavelengths within the operating range of the optical filter section  26 . Wavelengths that are emitted by the source  14  and that are outside the current passband of the filter element  46  are reflected by the filter element  46 , then travel to and are reflected by the reflective element  66 , and then propagate along a path of travel  73  to an optical beam dump  74  of a known type. The geometry of the optical filter section  26  is such that, as the filter element  46  and reflective element  66  rotate with the gear  33 , the path of travel  73  does not move. 
         [0019]    The substrate  43  is made of a material that is transmissive to all wavelengths within all possible wavelength ranges of the passband of the filter element  46 . Wavelengths emitted by the source  14  that are within the current wavelength range of the passband of filter element  46  pass through the filter element  46  and through the substrate  43 , and then propagate along a path of travel  72 . The pivotable support  41 , substrates  43 - 44 , filter element  46  and reflective element  66  are equivalent to an arrangement disclosed in U.S. Ser. No. 12/363,527 filed Jan. 30, 2009, the entire disclosure of which is hereby incorporated herein by reference. 
         [0020]    The optical filter section  26  also includes a further gear  81  supported for limited rotational movement about an axis  82  that is spaced from and parallel to the axis  34 . The gear  81  has a diameter substantially equal to that of the gear  33 , and has teeth that engage the teeth on gear  33 . Thus, when the gear  33  is pivoted, it pivots the gear  81  through an equal but opposite angular movement. A compensating element  86  is fixedly supported on the gear  81  for pivotal movement therewith, at a location so that the path of travel  72  extends through the compensating element. The compensating element  86  is made of the same material and has the same thickness as the substrate  43 . The compensating element  86  is oriented so that the facing surfaces of the substrate  43  and compensating element  86  form equal but opposite angles with respect to the path of travel  72 . The compensating element  86  realigns radiation propagating along the path  72 , in order to compensate for any deviation that may be caused by refraction as the beam travels through the substrate  43 . After passing through the compensating element  86 , the radiation continues along a path of travel  88  to the optical filter section  28 . 
         [0021]    The optical filter section  28  includes a gear  101  that is supported for limited rotational movement about a pivot axis  102  that is spaced from and parallel to the axes  34  and  82 . A drive mechanism  104  is similar to but independent from the drive mechanism  36 , and can effect limited pivotal movement of the gear  101  about the axis  102 . The gear  101  is shown in a center position in  FIG. 1 . The drive mechanism  104  can selectively pivot the gear  101  a few degrees away from the illustrated center position about the axis  102 , in either of two opposite rotational directions. The drive mechanism  104  can also releasably maintain the gear  101  in any angular position. 
         [0022]    A V-shaped support member  108  is fixedly supported on the gear  101  for pivotal movement therewith. Two substrates  111  and  112  each have one end fixedly but detachably coupled to a respective leg of the V-shaped support member  108 . The facing surfaces of the substrates  111  and  112  extend at an angle of 45° with respect to each other, and intersect at a line that is coincident with the axis  102 . The facing surfaces of the substrates  111  and  112  each have thereon a respective edge filter element  113  or  114 . The edge filter elements  113  and  14  are each a multi-layer coating of a known type. The edge filter elements  113  and  114  are discussed in more detail later. 
         [0023]    The optical filter section  28  includes a further gear  118  supported for limited rotational movement about an axis  119  that is spaced from and parallel to the axes  34 ,  82  and  102 . The gear  118  has a diameter substantially equal to the diameter of the gear  101 , and has teeth that engage the teeth on gear  101 . Thus, whenever the gear  101  is pivoted, it pivots the gear  118  through an equal but opposite angular movement. A compensating element  122  is fixedly supported on the gear  118  for pivotal movement therewith. The compensating element  122  is made of the same material as the substrate  111 , and has the same thickness as the substrate  111 . The substrate  111 , the substrate  112  and the compensating element  122  are each transmissive to all radiation within the operating range of the filter  12 . 
         [0024]    The compensating element  122  is positioned so that radiation propagating along the path of travel  88  will pass through the compensating element  122 , and then continue along a path of travel  126 . The facing surfaces of the compensating element  122  and the substrate  111  are oriented so that they form equal but opposite angles with respect to the path of travel  126 . The compensating element  122  realigns beams of radiation that pass through it, in order to compensate for any deviation or offset that may possibly be induced into the radiation by refraction as the beam travels through the substrate  111 . The compensating elements  86  and  122  and the associated gears  81  and  118  are optional, but are included in the apparatus  10  of  FIG. 1  for enhanced beam alignment. 
         [0025]      FIG. 4  is a graph depicting a broken line curve  131  that represents the transmission characteristic of the edge filter element  113 , and also depicting a solid line curve  132  that represents the transmission characteristic of the edge filter element  114 , when the gear  101  is in the center position shown in  FIG. 1 .  FIG. 5  is a graph that is similar to  FIG. 4  except that it shows the same two curves  131  and  132  from the perspective of reflection rather than transmission. In other words, in  FIG. 5  the broken line curve  131  represents the reflection characteristic of the edge filter element  113 , and the solid line curve  132  represents the transmission characteristic of the edge filter element  114 , when the gear  101  is in the center position shown in  FIG. 1 . For clarity, the curves  131  and  132  are each shown with a somewhat idealized shape in  FIGS. 4 and 5 . For the purpose of this discussion, the transmission characteristic in  FIG. 4  of each edge filter element  113  and  114  (considered alone) identifies the wavelengths that are transmitted through that edge filter element, and the reflection characteristic in  FIG. 5  of each edge filter element identifies the wavelengths that are reflected by that edge filter element. 
         [0026]    As evident from the curve  131  in  FIGS. 4 and 5 , the edge filter element  113  is transmissive and reflective to wavelengths respectively below and above an edge wavelength D. Similarly, as evident from the curve  132  in  FIGS. 4 and 5 , the edge filter element  114  is reflective and transmissive to wavelengths respectively below and above an edge wavelength F. In the embodiment of  FIG. 1 , the wavelengths D and F are both significantly longer than the wavelengths A, B and C in  FIGS. 2 and 3 . The pivotable support  108 , substrates  111 - 112 , and filter elements  113 - 114  are equivalent to an arrangement disclosed in U.S. Ser. No. 12/169,808, filed Jul. 9, 2008, the entire disclosure of which is hereby incorporated herein by reference. 
         [0027]    With reference to  FIG. 1 , radiation from the optical filter section  26  that is propagating along the path  88  will arrive at the optical filter section  28 , and will pass successively through the compensating element  122 , substrate  111  and edge filter element  113 , and then travel along a path of travel  136  to the target  16 . This radiation will cause the fluorescent dye in target  16  to fluoresce and emit radiation with a longer wavelength that is approximately halfway between wavelengths D and F. This radiation propagates away from the target  16  along a path of travel  138 . In the disclosed embodiment, the path of travel  138  is actually coincident with the path of travel  136  but, for clarity, these paths are shown in  FIG. 1  with a slight lateral offset. 
         [0028]    When the gear  101  is in the center position shown in  FIG. 1 , the radiation traveling along path  138  impinges on the edge filter element  113  at an angle of approximately 22.5° with respect to a not-illustrated reference line perpendicular to the edge filter element  113 . The portion of this radiation below the wavelength D passes through the edge filter element  113  and the substrate  111 , and travels through the compensating elements  122  and  86  and through the substrate  43  to the bandpass filter element  46 , where it is reflected because it is outside the passband of the bandpass filter element  46 . This reflected radiation then travels to and is absorbed by an optional beam dump  141 . 
         [0029]    In contrast, radiation that arrives along the path  138  and that is above wavelength D is reflected by the edge filter element  113 , and then travels to the edge filter element  114 , where it impinges on the edge filter element  114  at an angle of incidence of 22.5° with respect to a not-illustrated reference line perpendicular to the filter element  114 . The portion of this radiation that is above wavelength F ( FIGS. 4 and 5 ) passes through the filter element  114  and the substrate  112 , and then travels to and is absorbed by an optional beam dump  144 . In contrast, radiation that impinges on the edge filter element  114  and that is below wavelength F is reflected by the edge filter element  114  and then propagates along a path of travel  147  through the imaging optics  18  to the detector array  19 . 
         [0030]    The edge filter elements  113  and  114 , operating together, serve as a bandpass filter as to radiation that arrives on path  138  and departs on path  147 , where the passband is between wavelengths D and F when the gear  101  and filter elements  113  and  114  are in the center position shown in  FIG. 1 .  FIG. 6  is a graph showing a curve  161  that represents the transmissivity of the bandpass filter section  28 , or in other words the combined reflectivity of the filter elements  113  and  114  with respect to radiation arriving on path  138  and departing on path  147 . For clarity, the curve  161  in  FIG. 6  has a somewhat idealized shape. When the gear  101  and filter elements  113 - 114  are in the center position of  FIG. 1 , the filter elements  113  and  114  together define a passband that is between two wavelengths D and F, and that has a bandwidth  162  and a center wavelength E. Radiation between wavelengths D and F is transmitted through the bandpass filter and exits at  147 , whereas radiation outside this passband does not exit at  147  but instead is routed to either the beam dump  141  or the beam dump  144 . 
         [0031]    If the gear  101  and edge filter elements  113  and  114  are pivoted in either direction away from the center position of  FIG. 1  by the drive mechanism  104 , the path  147  will not move, but the angles of incidence of radiation impinging on each of the edge filter elements  113  and  114  will change.  FIG. 7  is a graph that is similar to  FIG. 6  and shows the same curve  161  depicted in  FIG. 6 , but that also shows the effect on the passband of pivotal movement in either direction away from the center position. 
         [0032]    More specifically, if the gear  101  and edge filter elements  113  and  114  are pivoted in one direction away from the center position, the edge wavelength of the filter element  113  will increase from D while the edge wavelength of the filter element  114  will decrease from F, as indicated by a broken-line curve  163  in  FIG. 7 . In contrast, if the gear  101  and filter elements  113  and  114  are pivoted away from the center position in the opposite direction, the edge wavelength of the filter element  113  will decrease from D and the edge wavelength of the filter element  114  will increase from F, as indicated by a broken-line curve  164  in  FIG. 7 . Consequently, as shown in  FIG. 7 , if the gear  101  and filter elements  113  and  114  are pivoted away from the center position in one direction, the bandwidth of the passband will decrease from the bandwidth  162  to a bandwidth  166 , whereas if they are pivoted away from the center position in the opposite direction, the bandwidth of the passband will increase from the bandwidth  162  to a bandwidth  167 . During pivotal movement of the gear  101  and filter elements  113  and  114  in either direction, the center wavelength E does not change significantly. 
         [0033]      FIG. 8  is a graph showing both passbands of the dual-passband filter  12  of  FIG. 1 , including the curve  51  of  FIGS. 2-3  involving a passband with a fixed bandwidth and a tunable frequency, and the curve  161  of  FIGS. 6-7  involving a passband with a tunable bandwidth and a fixed center wavelength. 
         [0034]      FIG. 9  is a diagrammatic view of an apparatus  210  that is a portion of a fluorescence microscope, and that is an alternative embodiment of the apparatus  10  of  FIG. 1 . Components in  FIG. 9  that are similar or identical to components in  FIG. 1  are identified in  FIG. 8  with the same reference numerals used in  FIG. 1 . The following discussion focuses primarily on differences between the embodiments of  FIGS. 1 and 8 . 
         [0035]    The imaging optics  18  and detector array  19  of  FIG. 1  have been replaced with a source  213  in the embodiment of  FIG. 9 . The source  213  is a known device that emits a broadband beam within a selected waveband. In the disclosed embodiment, the source  213  is essentially identical to the source  14 . 
         [0036]    In  FIG. 9 , the dual-passband filter  12  of  FIG. 1  has been replaced by a dual-passband filter  212 . The filter  212  of  FIG. 9  is effectively identical to the filter  12  of  FIG. 1 , except for differences discussed below. The filter  212  includes an optical filter section  226  that differs from the optical filter section  26  of  FIG. 1  only in that the filter section  226  does not include the beam dump  141 . The filter  212  also includes an optical filter section  228  that differs from the optical filter section  28  of  FIG. 1  only in that the beam dump  144  has been slightly repositioned, and an optional beam dump  215  has been added for radiation that passes through the edge filter element  113  and the substrate  111 . 
         [0037]    The apparatus  210  of  FIG. 9  operates as follows. Radiation emitted by the source  14  enters the filter  212 , and is filtered in basically the same manner discussed above in association with the filter  12  of  FIG. 1 . The resulting filtered radiation exits the filter  212  along the path  136  and impinges on the target  16 . 
         [0038]    Radiation from the source  213  propagates along a path of travel  238  to the edge filter element  114 , where radiation above the wavelength F ( FIGS. 4 &amp; 5 ) passes through the edge filter element  114  and substrate  112 , and then travels to and is absorbed by the beam dump  144 . In contrast, radiation arriving along the path  238  that is below the wavelength F is reflected and travels to the filter element  113 . The portion of this radiation below the wavelength D passes through the filter element  113  and the substrate  111 , and then travels to and is absorbed by the beam dump  215 . In contrast, radiation that arrives at filter element  113  and is above wavelength D is reflected, and then travels along a path of travel  247  to the target  16 . The edge filter elements  113  and  114  together function as a bandpass filter as to radiation arriving at  238  and departing at  247 . 
         [0039]    The target  16  is thus illuminated by radiation within two different passbands that are each separately and independently controlled by a respective one of the two optical filter sections  226  and  228 . This radiation arriving at the target  16  may illuminate and/or cause fluorescence from the target, and radiation emitted by the target  16  as a result of illumination and/or fluorescence can then be collected and imaged in a known manner by known structure that is not shown in  FIG. 9 . 
         [0040]    The radiation beam traveling along path  73  in  FIGS. 1 and 9  is a notch that is the inverse of the radiation beam traveling along the path  72 . In each of  FIGS. 1 and 9 , it would be possible to replace the beam dump  74  with the not-illustrated detector of a known spectrometer. The spectrometer could then be used as an optical wavelength monitor that indicates the range of wavelengths currently within the passband of the bandpass filter section  26  or  126 . This information could then in turn be used to operate the drive mechanism  36  in order to adjust the rotational position of the gears  33  and  81 , and thereby tune the bandpass filter section  26  or  126  so as to change the range of wavelengths falling within the passband. 
         [0041]    As evident from the foregoing discussion, the embodiments of  FIGS. 1 and 9  are each configured so that the bandpass filter section  26  or  226  with tunable wavelength and fixed bandwidth passes wavelengths that are shorter than the wavelengths passed by the bandpass filter section  28  or  228  with fixed wavelength and variable bandwidth. Alternatively, however, either embodiment could be configured so that the bandpass filter section  28  or  228  with fixed wavelength and variable bandwidth passes wavelengths that are shorter than the wavelengths passed by the bandpass filter section  26  or  226  with tunable wavelength and fixed bandwidth. 
         [0042]    In the embodiments of  FIGS. 1 and 9 , the substrate  44  supports the reflective element  66  that reflects radiation to the beam dump  74 . Alternatively, however, the reflective element  66  and the beam dump  74  could be omitted, and the substrate  44  could be made from a material that is absorptive to the wavelengths of radiation reflected by the bandpass filter element  46 , so that the substrate  44  serves as a beam dump. This would avoid the expense of the reflective element  66  and the beam dump  74 . Similarly, in the embodiments of  FIGS. 1 and 9 , radiation passing through the edge filter element  114  also passes through the substrate  112  and then travels to the beam dump  144 . Alternatively, however, the beam dump  144  could be omitted, and the substrate  112  could be made from a material that is absorptive to the wavelengths of radiation that pass through the edge filter element  114 , so that the substrate  112  serves as a beam dump. This would avoid the expense of the beam dump  144 . 
         [0043]    Although selected embodiments have been illustrated and described in detail, it should be understood that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the claims that follow.