Abstract:
A detector configuration determines the direction of illumination incident on a photosensitive device. Multiple mask layers include holes which form an interlayer optical path through which radiation reaches a photodetector. The interlayer optical path provides a selected nominal maximum signal angle and the detector senses when radiation is received at or near that angle. In one embodiment, three holes in three metallization layers provide an arbitrarily narrow interlayer optical path with improved angular detection relative to that provided by two holes. An illumination direction-sensing array may use multiple instances of the detector configuration. The detector configuration may provide enhanced utility and economy by being adapted to use only those fabrication steps used for fabricating other primary circuits on an IC.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to a detector configuration for determining an angle of incidence of light on a photodetector, and in one example embodiment, the detector configuration may be utilized to indicate the degree of photodetector and/or light source misalignment in position encoders or the like. 
       BACKGROUND OF THE INVENTION 
       [0002]    Various angle of incidence detectors are known, such as those disclosed in U.S. Pat. No. 4,857,721 to Dunavan, et al., European Patent Specification EP 0747719 to Cantin, et al., and U.S. Patent Application Publication No. 2002/0053635 to Schroter, et al., each of which is incorporated herein by reference. However, each of the preceding references teaches that at least one “shadow mask” critical to the operation of the disclosed devices is formed separately from the photodetector elements of the disclosed devices. For at least this reason, at least one aspect of a combination of features including fabrication cost, size, and/or the measurement resolution of the disclosed detector configurations is undesirable for many applications. 
         [0003]    U.S. Pat. No. 6,875,974 to Muesch and Budde (the &#39;974 patent), which is hereby incorporated herein by reference, discloses an apparatus wherein each respective radiation-transmitting area of a shading mask is arranged with a respective degree of overlap or displacement relative to a respective pair of photodiodes. The various respective degrees of overlap cause the null signal from each respective pair of photodiodes to correspond to a respective angle of incidence. The photodiodes of each pair are connected to one another. Each photodiode comprises p- and n-regions arranged symmetrically relative to an insulating trench between the pair of photodiodes. The shading mask may be fabricated as a metallization layer of a photodiode IC, spaced apart from the photodiodes by an insulation layer of the photodiode IC. Alternatively, an analogous device using an array of quadrant detectors (instead of detector pairs) is disclosed for detecting the angle of incidence “in two dimensions”, that is, in two different planes. However, the device configurations disclosed in the &#39;974 patent impose photodiode design and fabrication constraints that may be undesirable in various applications and/or IC fabrication processes. Improved angle of incidence detector configurations that overcome any or all of the foregoing disadvantages would be desirable. 
       SUMMARY OF THE INVENTION 
       [0004]    The following summary is intended to present only a brief overview of certain aspects of the invention. The features of the invention that are described in this summary are exemplary only, and not limiting. 
         [0005]    The present invention is directed to providing a detector configuration which is usable in a photosensitive device such as an IC or “chip” to determine the direction of illumination incident on the device. In one specific embodiment, a detector (or a plurality of detectors) formed according to the detector configuration may be utilized to adjust or compensate for the alignment of components of an optical sensing arrangement that is used in a metrology instrument or other optical system. In metrology systems that are utilized for determining precise physical measurements (e.g., with micron or sub-micron resolution), proper component alignment is often necessary to provide the best signal-to-noise (S/N) ratio and accuracy. For example, in one type of an optical encoder system, a light source is part of the measurement readhead electronics and the signal is provided by light reflected from a scale surface, in which case a proper orientation of the readhead light source relative to the scale surface is important in order for accurate measurements to be obtained, and to maintain the energy efficiency of the system. 
         [0006]    In various embodiments, a detector configuration is provided for determining the direction of incident light relative to a surface. In one embodiment, a number of mask layers include a first layer and a second layer that include respective first-layer and second-layer holes that define an interlayer optical path through which radiation reaches a photodetector. The interlayer optical path may have a selected nominal maximum signal direction or angle. Thus, in operation, a detector according to the invention may sense when an illumination direction is near, or coincides with the nominal maximum signal direction. In a further embodiment, a third mask layer is included between the first and second mask layers and includes a third-layer hole arranged in an offset manner relative to first- and second-layer holes such that a cross-section through the resulting interlayer optical path along a plane approximately parallel to a plane of the first mask layer and a plane of the second mask layer has a smaller area than the input area of the second-layer hole and the output area of the first-layer hole. Such an embodiment may provide improved angular selectivity and/or sensitivity relative to a two-hole interlayer optical path. In some such embodiments, the cross-section through the resulting interlayer optical path has a smaller area than the area of the minimum hole size specified for the fabrication processes used to fabricate the detector. 
         [0007]    In various embodiments, a detector according to this invention may be included in a photosensitive device, wherein the photosensitive device includes circuit elements that are not used for sensing the illumination direction, and the first mask layer and the second mask layer may be economically formed from material layers that are used to fabricate some of the circuit elements that are not used for sensing the illumination direction. In a further embodiment, the circuit elements that are not used for sensing the illumination direction may include a primary photodetector array that does not include the photodetectors that are used in the illumination direction-sensing detectors, and the photodetectors that are used in the illumination direction-sensing detectors may be economically formed during the same process steps that are used to form the photodetectors of the primary photodetector array. 
         [0008]    In various embodiments, at least one material layer is formed between the first mask layer and the second mask layer, and the at least one material layer comprises at least one optically transmissive insulating layer. In a further embodiment, each material layer formed between the first and second mask layers is formed by a process step that is used to fabricate one of a circuit element, an insulator for a circuit element, and a protective layer for a circuit element, for a circuit element that is not used for sensing the illumination direction. In a further embodiment, the interlayer optical path is filled with optically transmissive insulating material, to mitigate potential unwanted diffraction effects. 
         [0009]    In various embodiments, the detector is configured for sensing over a two-dimensional range, comprising an angle of incidence range and an azimuthal angle range, that includes the nominal peak signal direction. 
         [0010]    In various embodiments, a plurality of similar detectors are provided for sensing the illumination direction. Each respective detector may have a respective nominal peak signal direction comprising an angle of incidence component and an azimuthal angle component. The photodetector of each detector may output a respective output signal depending on an amount of radiation it receives, and a combination of the respective outputs may be indicative of an angle of incidence component and an azimuthal angle component of the direction of illumination. In a further embodiment, a circuit may be included with the plurality of detectors to process a combination of respective outputs and provide an indication when the direction of illumination approximately coincides with a predetermined direction of illumination. 
         [0011]    In various embodiments, a wavelength selective filter may be provided to filter the illumination reaching the photodetector(s) of the illumination direction-sensing detector(s). 
         [0012]    In one embodiment, each of the first and second interlayer optical paths may transmit radiation to the same photodetector, wherein the first interlayer optical path has a first nominal peak signal direction and the second interlayer optical path has a second nominal peak signal direction, and for a first range of illumination directions that includes the first nominal peak signal direction the amount of radiation received by the photodetector is a maximum when the illumination direction approximately coincides with the first nominal peak signal direction, and the amount of radiation received by the first photodetector decreases as a function of increasing deviation between the illumination direction and the first nominal peak signal direction when the illumination direction is within the first range of illumination directions. Furthermore, for a second range of illumination directions that includes the second nominal peak signal direction the amount of radiation received by the first photodetector is a maximum when the illumination direction approximately coincides with the second nominal peak signal direction, and the amount of radiation received by the first photodetector decreases as a function of increasing deviation between the illumination direction and the second nominal peak signal direction when the illumination direction is within the second range of illumination directions. 
         [0013]    In one embodiment, two similar detectors are provided for sensing an illumination direction, wherein the two detectors have different respective interlayer optical paths, different respective nominal peak signal directions and different respective photodetectors. 
         [0014]    The various features outlined above may be used separately or in combination to provide a desired combination of benefits in various embodiments. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0016]      FIG. 1  is a side-view cross-section diagram of a first embodiment of a detector configuration according to this invention, wherein multiple metal layers provide holes forming an angled interlayer optical path that transmits incident light to a photodetector as a function of its angle of incidence; 
           [0017]      FIG. 2  is a side-view cross-section diagram of a second embodiment of a detector configuration according to this invention, wherein multiple metal layers provide holes that are configured to form an angled interlayer optical path that has a reduced width for transmitting incident light to a photodetector; 
           [0018]      FIG. 3  is a plan-view diagram of an array of differently angled detectors having configurations of the type illustrated in  FIG. 1 ; 
           [0019]      FIG. 4  is a diagram of a first dual angle detector configuration wherein multiple metal layers with holes form two angled interlayer optical paths that transmit incident light to a single photodetector for two different angles of incidence; and 
           [0020]      FIG. 5  is a diagram of a second dual angle detector configuration wherein multiple metal layers with holes form two angled interlayer optical paths that transmit incident light to two respective photodetectors for two different angles of incidence. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    In various exemplary embodiments of a detector configuration according to this invention, a plurality of metallization layers with holes form an angular filter which determines that only light input approximately along a respective nominal peak signal direction will reach a photodetector (e.g., a photodiode) located under the angular filtering arrangement. Thus, it should be appreciated that a nominal peak signal angle α referred to in the following description is dictated by a particular detector configuration, in practice. It should be also appreciated that in the general case, when light from a light source is not well aligned at the nominal peak signal angle α, the light is blocked by the angular filtering arrangement of the detector configuration, and the photodetector will therefore only provide a maximum signal when the light source is properly aligned at the angle α. 
         [0022]      FIG. 1  is a diagram of a first embodiment of a detector configuration  100  with an angled interlayer optical path  180  directed toward a photodetector  105  for sensing radiation at an angle of incidence corresponding to an angle α. The detector configuration  100  may be one of a plurality of similar configurations included a photodetector IC. The function of the detector configuration  100  may be an auxiliary function of the photodetector IC (e.g., an imaging array IC) as described in greater detail below. Each reference line n shown in  FIG. 1  represents a vector normal to a nominal surface plane of the photodetector IC. While a photodiode type photodetector is illustrated, it will be appreciated that any light sensitive element could also be utilized, such as a photogate, photo-transister, etc. The term light, as used herein, is not restricted to visible radiation. As shown in  FIG. 1 , the photodetector  105  includes a surface  120 , an N+ diffusion layer  122 , a P well  124 , a P substrate  126  and a field oxide layer  128 . It will be appreciated that while a P well and a P substrate implementation is illustrated, any of the many other suitable photodetector fabrication processes may be utilized, such as are known in the art. The photodetector  105  is connected by an electrical connection post  125  to a first connection metal layer  130 . Additional metal layers  140   a ,  140   b , and  140   c  are configured to block incident light in the vicinity of the photodetector  105 , except for the holes  160   a ,  160   b  and  160   c , respectively, which are configured to form the angled interlayer optical path  180  at a nominal angle α, through which light may reach the photodetector  105 . 
         [0023]    Each of the holes  160   a ,  160   b  and  160   c  has a horizontal opening dimension w a , w b  and w c , respectively. Each of the metal layers  130 ,  140   a ,  140   b  and  140   c  has a thickness  132 ,  142   a ,  142   b  and  142   c , respectively. Below each metal layer  130 ,  140   a ,  140   b  and  140   c  may be respective supporting and insulating layers  129 ,  139   a ,  139   b ,  139   c , that have nominal thicknesses  127 ,  137   a ,  137   b ,  137   c , respectively, and are formed of a material having good or sufficient optical transmission characteristics (e.g., SiO 2 ). The insulating layer material may also fill each of the holes in the metal layers such that the layers merge, depending on the particular IC fabrication processes used. Filling the holes with the insulating material may be desirable because the holes may be small enough compared to the wavelength of the light that diffraction effects may be significant. Filling the holes with a material having an index of refraction higher than air helps to reduce unwanted diffraction effects. 
         [0024]    The various interlayer optical path configurations and techniques disclosed herein may be used in an IC dedicated to detecting the angle of incident illumination. Alternatively, the interlayer optical path configurations and techniques disclosed herein may be implemented using layers and/or process steps that are different than those used to fabricate other structures on a photodetector IC (e.g., a camera array). However, in various embodiments, the thicknesses and materials of each metal layer and each insulating layer are advantageously the standard thicknesses and materials used to fabricate other structures on the photodetector IC. For example, each metal layer may be fabricated by the same process step(s) used to fabricate a connection layer used by other circuits on the photodetector IC. Likewise, each insulating layer may be fabricated by the same process step(s) used to fabricate an insulating layer that insulates or protects other circuit components on the photodetector IC. In addition, in various embodiments, the photodetector  105  may advantageously be fabricated by the same process step(s) used to fabricate other photodetector components on the photodetector IC. While metallization layers are described as defining the interlayer optical path  180 , it will be appreciated that other relatively opaque material layers may also be used, such as second-poly, etc., depending on the process rules for forming the integrated circuit, the desired separation (thickness) between masking layers, etc. 
         [0025]    In the embodiment shown in  FIG. 1 , the metallization layers  140   a ,  140   b , and  140   c  are configured such that for the horizontal opening dimensions w a =w b =w c  and each opening  160   a - 160   c  is horizontally offset from the adjacent opening by an amount that defines the desired angle α. In this case, the interlayer optical path  180  provides a maximum signal at the angle α. As shown in  FIG. 1 , the effective unobstructed width of the interlayer optical path  180  along a direction perpendicular to the surface normal n is designated w eff , which, for approximately collimated illumination incident at the angle α, corresponds to a signal area  170  on the photodetector  105 . More generally herein, the signal area  170  refers to or indicates that actual illumination area or signal area on the photodetector  105  for any given angle of illumination. Also shown are the limiting rays r 1  and r 2 , which are rays at the most extreme angles φ 1  and φ 2 , respectively, which may pass through the interlayer optical path  180 . It can be seen that an effective height associated with the layer features which define the interlayer optical path  180  is designated as height h eff . The ratio (w eff /h eff ) of a detector configuration according to this invention may be adjusted such that the detector configuration provides an analog signal over a wider angular range or narrower angular range, according to the needs of a particular application. In particular, the ratio may be adjusted by selecting a desired opening dimension or dimensions to determine w eff , and/or by the “offset” hole technique described below with reference to  FIG. 2  and the detector configuration  200 . 
         [0026]    In the embodiment shown in  FIG. 1 , it may be seen that the opening  160   b  actually plays no role in defining the interlayer optical path  180  or the limiting rays r 1  and r 2 . Therefore, in various embodiments, the “interior” metal layer  140   b  may be optional (that is, omitted, or etched away) in the vicinity of the detector configuration  100 , if desired. In other embodiments, if it does not interfere with its primary function as a circuit connection to photodetector  105 , the metal layer  130  may be configured to mask the photodetector  105  and provide a hole that may partially define the interlayer optical path  180 . In such a case, the metal layer  140   a  may become an “interior” layer that may be optional in the vicinity of the detector configuration  100 , if desired. In various embodiments, it may be advantageous if the photodetector  105  has dimensions such that the limiting rays r 1  and r 2  fall within its active area. For example, this may allow the photodetector  105  to have a simpler structure or isolation and still provide desirable signal characteristics, or may result in more predictable or usable signals from one or more incident angle detectors when the illumination is not well-collimated, or has an unknown angular intensity distribution, or the like. However, in other applications, the photodetector  105  may have smaller dimensions and various benefits of the invention will be retained. 
         [0027]    In operation, assuming approximated collimated illumination, for angles of incidence other than the angle α, the signal area  170  on the photodetector  105  is diminished, resulting in a relatively diminished output signal. For example, for the angles of incidence φ 1  and φ 2  corresponding to the limiting rays R 1  and R 2 , the signal area is reduced to approximately zero. Thus, in one example embodiment, if it is desired to orient the detector configuration  100  at the angle α with reference to an illumination direction, the detector configuration  100  (that is, the host photodetector IC), or the illumination source, may be rotated or otherwise moved until the signal output from the photodetector  105  is at a maximum, thus indicating a correct alignment. 
         [0028]    In one embodiment, the distances  139   a - 139   c  between the metallization layers  140   a - 140   c , respectively, as well as their thicknesses  142   a - 142   c , may be on the order of approximately 1 micron, the opening dimensions w a , w b  and w c  may be approximately 1.5 microns, and the photodetector  105  may have an active area dimension of approximately 6 microns. 
         [0029]    In one embodiment, an incident angle detector according to the detector configuration  100 , or alternative configurations described herein, may be fabricated by itself on a single chip. In another embodiment, the incident angle detector may be fabricated on a chip with other devices and circuits, as would be compatible with a linear array, quadrature detector arrays such as those described in U.S. Pat. No. 4,857,721 to Kojima, and U.S. Patent Application Publication No. 2005/0017158 to Kojima, which are hereby incorporated herein by reference, or a full imaging array (a camera). The chip may included circuits to process the incident angle detector data and report angles, angular deviations, wavelengths, surface tilt, tilt correction signals, or to be used in tilt calculations in order to compensate in the image processing or measurement results obtained from other devices on the chip. The fabrication of the detector may also be combined with MEMS devices. Wavelength selective filtering layers may also be added over any or all of the incident angle detectors on a chip, if desired. 
         [0030]      FIG. 2  is a diagram of a second embodiment of a detector configuration  200  with an angled interlayer optical path  180 ′ directed toward a photodetector  105  for sensing radiation at an angle of incidence corresponding to an angle α. The illustrated components of the detector configuration  200  are similar to those of  FIG. 1 , except as otherwise described below. Each of the metal layers  130 ,  140   a ,  140   b ′ and  140   c  and insulating layers  129 ,  139   a ,  139   b ,  139   c , may be similarly fabricated and have similar thicknesses. 
         [0031]    In contrast to the detector configuration  100 , in the embodiment shown in  FIG. 2 , the “interior” metal layer  140   b ′ provides an important function in defining the angled interlayer optical path  180 ′, and is not optional in the vicinity of the photodetector  105 . It should be appreciated that when the thicknesses and materials of each metal layer and each insulating layer are advantageously the standard thicknesses and materials used to fabricate other structures on the photodetector IC, that the number of layers and their thicknesses may generally be constrained. Thus, the height h eff  may be effectively predetermined or “fixed”. In addition, there is typically a minimum allowed or practical dimension for an opening through a metal layer, for example due to physical or economic fabrication constraints (e.g., process capability or process time limitations). In contrast to the detector configuration  100 , by intentionally “offsetting” the interior layer hole  260  to occlude part of the interlayer optical path that would otherwise be established by the exterior layer holes  160   a  and  160   c , the detector configuration  200  provides a dimension w eff  for the interlayer optical path  180 ′ that is less than any of the opening dimensions w a , w b  or w c , even if the opening dimensions are the minimum allowed IC process dimensions. Described another way, the interior layer hole  260  is offset relative to the exterior layer holes  160   a  and  160   c  such the effective cross-sectional area of interlayer optical path  180 ′ is nominally less than the area of the input hole  160   c  and the area of the output hole  160   a  of the interlayer optical path  180 ′. 
         [0032]    As may be seen by comparison of  FIGS. 1 and 2 , the resulting limiting angles φ 1 ′ and φ 2 ′ for the “offset interior hole” detector configuration  200 , which correspond to the limiting rays that may pass through the interlayer optical path  180 ′, define a narrower angular range than the limiting angles φ 1  and φ 2  provided by the detector configuration  100 . In such a case, the dimensions of the photodetector  105  could be reduced to correspond to the resulting limiting rays r 1 ′ and r 2 ′, if desired. It should be appreciated that offset holes in additional metal (or opaque) layers, if available, would allow further reduction of the angular range without a further reduction in the nominal width of the interlayer optical path  180 ′ or the resulting maximum signal, if desired. It should be appreciated that although  FIG. 2  shows a “one dimensional” view, in various embodiments one or more offset holes may be arranged to reduce the angular range of an interlayer optical path along two directions (e.g., orthogonal directions), simultaneously. It should be appreciated that, in general, during alignment or setup operations relatively long signal integration times may be used if necessary in order to accumulate a sufficient signal from the photodetector  105 . Therefore, the interlayer optical path effective width, and the associated limiting angular range, may be made very narrow, such that the angle of incidence may be detected with very high angular resolution using a detector according to the detector configuration  200 . It should be appreciated that if an interlayer optical path with the desired angular resolution outputs too little light to a detector, then a plurality of similar and parallel interlayer optical paths may be fabricated adjacent to each other, with all of the parallel optical paths outputting light to the same detector. 
         [0033]      FIG. 3  is a schematic plan view of one exemplary detector array  310  in which multiple detectors  300  are utilized for detecting radiation incident at multiple directions on the array  310 . As shown in  FIG. 3 , the array  310  comprises detectors  300  in five rows  331 - 335 , and five columns  321 - 325 . Each of the respective detectors  300  may have a configuration similar to the detector configurations  100  or  200 , as schematically represented by the three metal layers  340   a ,  340   b  and  340   c , and the respective sets of holes for forming respective interlayer optical paths  380 . The photodetectors associated with each detector are not shown, but will be understood based on  FIGS. 1 and 2 . The detectors  300  may be spaced apart at any convenient spacing that is consistent with their dimensions. It will be understood that the metal layers  340   a ,  340   b  and  340   c  may each be continuous between the detectors  300 , if desired. 
         [0034]    The general azimuthal angle β shown in  FIG. 3  may be defined about an axis that is approximately normal to the nominal plane of the detector array  310  and the associated photodetector plane, that is, normal to the plane of  FIG. 3 . As shown in  FIG. 3 , each respective detector  300  may be formed with their respective interlayer optical path  380  aligned along a respective nominal azimuthal angle β, as well as at a respective nominal angle of incidence α (as shown in  FIGS. 1 and 2 ). Thus, an individual detector  300  may accurately determine an angle of incidence and an azimuthal angle of illumination with high resolution, while an array as a whole may cover a full 360 degrees of azimuthal angle and a very wide range of angles of incidence. It will be appreciated that the “offset” hole design principles described with reference to  FIG. 2 , may be used to decrease the angular sensing range and/or increase the angular sensing sensitivity with respect to the azimuthal angle, in a manner analogous to that used to increase the angle of incidence sensing sensitivity, by narrowing the interlayer optical path along the direction of the azimuthal angle variation. 
         [0035]    More generally than shown in  FIG. 3 , the number and location of detectors in an array, as well as their nominal angles α and β, and their individual angular range limits, may be designed according to previously described principles in order to best meet the illumination analysis needs of a particular application. It will be appreciated that each of the detectors shown in  FIG. 3  could be arranged along a single row or column to form a linear array, or in any other convenient layout on a chip, if desired. It will be appreciated that the signals from various combinations of detectors may undergo signal processing to enhance the detection of the illumination direction, for example, to normalize all the signals for common-mode illumination intensity variation, for mitigating the effects of a common mode angular distribution of illumination intensity (e.g., a Lambertian distribution), or the like. It will be appreciated that the various interlayer optical paths may be fabricated to have the same effective cross-sectional area, such that the detector having the maximum signal indicates the nominal direction of the illumination. Alternatively, the relative signal strengths of the various detectors may be calibrated and compensated to provide a similar result. It will be appreciated that while the holes shown in  FIG. 3  are generally shown as being square, other potential shapes may also be utilized for the holes, such as circular, rectangular, grill, polygon, angled slot, etc. It will be appreciated that signals from various combinations of detectors may undergo signal processing to provide a feedback signal to a user (or an automatic control system), that is performing setup or alignment, to indicate which direction a tilt adjustment should be made in order to achieve a desired alignment. In various applications, this may facilitate the alignment of a photodetector chip, or an assembly including the chip, or a source that is providing the illumination, or a reflective surface that is providing reflected illumination. For example, such applications are facilitated when the detectors disclosed herein are fabricated and/or assembled in conjunction with devices disclosed in U.S. patent application Ser. No. 11/396,368 to Sesko, U.S. patent application Ser. No. 10/804,636 to Shimomura et al., U.S. Pat. No. 5,995,229 to Omi, and U.S. Pat. No. 6,642,506 to Nahum and Masreliez, each of which is hereby incorporated herein by reference. 
         [0036]      FIG. 4  is a schematic diagram of a detector configuration  400  which includes two interlayer optical paths  480 - 1  and  480 - 2  which are directed toward a shared photodetector  105 . The components and configuration of each of the interlayer optical paths  480 - 1  and  480 - 2  may be similar to those previously described for the detector configurations  100  or  200 . The nominal angles α- 1  and α- 2  may be designed to be of equal magnitude, or different magnitudes, if desired. 
         [0037]    In operation, the detector configuration  400  is able to detect the direction of radiation incident on the detector in a limited angular range around either or both of the nominal angles α- 1  and α- 2 . The detector configuration  400  may be used whenever additional information (such as a signal from an additional detector, or a host system set up or timing constraint, etc.) is available to determine which of the interlayer optical paths  480 - 1  or  480 - 2  is producing the signal on the photodetector  105 . In one application, the detector configuration  400  may be mounted to rotate about an axis very roughly aligned with the normal vector n, and receive illumination within its angular sensing range. Assuming the interlayer optical paths  480 - 1  or  480 - 2  have substantially identical cross-sections, when each path produces an identical signal during rotation, then the normal vector n is aligned precisely parallel to the rotation axis. 
         [0038]      FIG. 5  is a schematic diagram of a detector configuration  500  which includes two interlayer optical paths  580 - 3  and  580 - 4  which are directed toward two photodetectors  105 - 3  and  105 - 4 , respectively. The components and configuration of each of the interlayer optical paths  580 - 3  and  580 - 4 , and the photodetectors  105 - 3  and  105 - 4 , may be similar to those previously described for the detector configurations  100  or  200 . In operation, the detector configuration  500  is able to use a single input hole to illuminate two photodetectors along two different interlayer optical paths. 
         [0039]    The signals from the photodetectors  105 - 3  and  105 - 4  may be processed as differential signals. When the nominal angles α- 3  and α- 4  have the same magnitude and the interlayer optical paths  580 - 3  and  580 - 4  have substantially identical cross-sections, and the signals from the photodetectors  105 - 3  and  105 - 4  are balanced, then the illumination direction is parallel to the normal vector, assuming the illumination direction is within the angular sensing range of each of the interlayer optical paths  580 - 3  and  580 - 4 . Conversely, the amount of imbalance between the signals from the photodetectors  105 - 3  and  105 - 4  may be indicative of the angle of incidence over the combined angular sensing ranges of the interlayer optical paths  580 - 3  and  580 - 4 . If the nominal angles α- 3  and α- 4  are designed to be different magnitudes, then the signals will balance at a corresponding angle of incidence that is not along the normal vector. 
         [0040]    While the preferred embodiment of the invention has been illustrated and described, numerous variations in the illustrated and described arrangements of features and sequences of operations will be apparent to one skilled in the art based on this disclosure. Thus, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.