Abstract:
A focus sensing system adaptable for use in microscopes or other optical systems incorporates a selective beam block. An outgoing reference beam, incident upon a target to be inspected, is reflected to become an incoming reference beam. A beam block inserted within the optical system selectively rejects unwanted reflections from surfaces other than the target allowing the desired incoming reference beam to be photodetected without interference from reflections off of surfaces that are not of interest. The photodetector generates an electronic signal corresponding to the displacement of the target from the ideal focal point. The electrical signal may be used to drive a servomechanism to displace either the target or the microscope objective lens to bring the target into focus.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   U.S. patent application Ser. No. 11/312,034 filed Dec. 20, 2005, now U.S. Pat. No. 7,348,528, entitled “Distance Measuring System,” is incorporated here by reference. 
   U.S. patent application Ser. No. 11/614,234 filed Dec. 21, 2006, published as U.S. Pre-Grant Publication No. 2007/0177470, entitled “Distance Measuring System,” is incorporated here by reference. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   FIELD OF THE INVENTION 
   The present invention relates generally to optical systems for sensing the distance to a surface. In particular, this invention is of an optical apparatus and methods for detecting and maintaining focus on an object. 
   BACKGROUND OF THE INVENTION 
   A focus sensing system is an optical device that senses the state of focus of an image-forming device by emitting one or more beams of light that are made to reflect from the surface under examination. The reflected light is then collected and processed to determine the distance between the objective lens and the target object being examined. Such a focus sensing system may simply measure the distance between the objective lens and the target, or it may be used to enable control of the state of focus. In the latter instance, it may be used as a component of a focus controlling system, that is, an autofocus system. 
   Many devices in the prior art project a spot of light through a circular objective lens onto a target surface. Other focus sensing devices have used one or more toric lenses to form a line focus which is projected onto a target surface. In general, a toric lens is one which has different curvatures in two principle sections. A cylinder lens is a limiting case of a toric lens where the radius of curvature in one section is infinite, that is, the section is flat, while the radius of curvature in the other section is finite, but non-zero. Toric lenses introduce astigmatism into the outgoing beam of light. Instead of having a single point focus, an astigmatic system has two line foci. Where astigmatic lenses have been used previously by others, only one of the two astigmatic line foci has been used. In those applications the line focus has been used to perform an averaging function in order to reduce the effect of local asperities in the surface under observation. 
   While such an arrangement can be optimized for measuring the distance to a single reflective surface, it has fundamental difficulties when it is used to measure distance to an object or surface that is buried in a transparent medium. These problems result from the fact that the discontinuity in the refractive index at boundaries, even between transparent interfaces, still produces a reflection. Such a reflection creates its own signal, confusing the returned position signal from the intended target. The result is degraded image quality because of imperfect focus. 
   Various techniques have been used in attempts to overcome these error mechanisms. For example, an electrical, mechanical or optical offset may be introduced to offset the signal from an unwanted surface. Unfortunately this offset can only correct the net focus error signal if the reflectance of the unwanted surface does not change, or the distance between the surfaces remains constant. The general result is that previously used methods remain susceptible to many factors, resulting in errors in the distance measurement and a consequential degradation in the quality of the image. 
   BRIEF SUMMARY OF THE INVENTION 
   The presently described systems and methods provide a means of autofocus that remains highly reliable even when confronted by effects that would be detrimental to a proper focus. Some such effects are thickness of the specimen, reflectance from various surfaces, and tilt of the target. When not properly addressed, these effects will degrade the ability to retain correct focus, diminishing the quality of the image. 
   The described autofocus system reliably measures focus inside of a transparent medium by optically blocking the light reflected from a surface that is not of interest while allowing light from the target to pass through to the focus sensor. In one embodiment a single point focus with suitable relay optics is used to form an intermediate aerial image of one surface. Another embodiment uses a toric element to create a pair of line foci in which one focus is projected onto one surface, and the second focus is projected onto a second surface. In either embodiment most of the light reflected from the surface of interest is allowed to pass unblocked through the system to be used to measure the focus while a beam block removes the light reflected from another surface. 
   The described embodiments may be adapted for use in a microscope or other industrial inspection or measurement systems with a minimum of impact on the optical characteristics of the original system. The distance information may be used to drive a servomechanism to adjust focus, or alternatively, to generate data regarding the topography of the target surface. 
   Embodiments of the present invention include a light source emitting a collimated reference beam of light. A portion of the reference beam is blocked by a knife-edge element. The remaining outgoing reference beam travels to the target surface, where it is reflected to become an incoming reference beam on the opposite side of an optical axis. The incoming reference beam is diverted by the reflecting surface of the knife-edge element to a photodetector that generates an electronic output signal according to the position of the incoming reference beam on the surface of the photodetector. The signal generally corresponds to the distance from the objective lens of the microscope to the target surface and may be used to generate an output signal. The knife-edge element enables the incoming reference beam to generally follow the path that would have been taken by the blocked portion of the outgoing reference beam. 
   A scanning mirror may be included in any embodiment of the presently described system to controllably deflect the outgoing reference beam and the incoming reference beam. Because the two reference beams generally follow the same optical path, the deflection of the incoming reference beam compensates for the deflection of the outgoing reference beam, so that the position of the incoming reference beam on the surface of the photodetector is generally independent of the deflection of the outgoing reference beam made by the scanning mirror. In this way, the measured distance is unaffected by scanning the outgoing reference beam along the target. With some care being given to placement of a beam blocking element, the described systems can be made to scan in two axes for characterization of a surface. 
   Due to the complementary nature of the optical paths for the outgoing and incoming reference beams, the resulting system is relatively unaffected by thickness of the specimen and reflectance from surfaces other than the actual target. The described system also generally compensates for tilt of the target. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The particular features and advantages of the invention briefly described above as well as other objects will become apparent from the following description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  shows an embodiment of the present invention in its most basic configuration prior to insertion of a beam blocking element; 
       FIG. 2  is a side view of the reference beam impinging upon the target surface where another surface produces an undesirable secondary reflection; 
       FIG. 3  shows a side view of an embodiment using a beam block within an afocal relay to remove an image produced by an undesirable reflection from the far surface of a transparent target; 
       FIG. 4  is a plan view showing the layout of the components of an implementation of the system of  FIG. 3  for use in a microscope; 
       FIG. 5  shows a side view of an embodiment using toric lenses with a beam block to remove any image produced by an undesirable reflection from the near surface of a transparent target; and 
       FIG. 6  is a simplified block diagram of a servo-system showing a focus sensing system as described here when used as a component of an autofocus system. 
   

   The following Reference Numbers may be used in conjunction with one or more of the accompanying  FIGS. 1-6  of the drawings:
       100  Focus System     110  Light Source     120  Photodetector     130  Objective Lens     140  Beamsplitter     145  Beam Block     150  Afocal Relay     152  Relay Lens, first     155  optical center of relay     158  Relay Lens, second     160  Cylinder Lens, first     165  Beam Block     170  Knife-edge element     172  Mirror, folding     175  Mirror, detector     180  Collimating Lens     185  Focusing Lens, detector     190  Cylinder Lens, second     200  Target     210  Surface of interest     215  Focus of interest     220  Surface to be rejected     225  Apparent location of reflected spot     230  Focus to be rejected     300  Light Beam, outgoing     305  Optical axis     310  Reference Beam, outgoing     320  Reference Beam, incoming     330  Incident Beam     340  Reflected Beam     350  Real Image     400  Servomechanism   

   DETAILED DESCRIPTION OF THE INVENTION 
   The focus sensing systems and methods described here may be used within an autofocus system as part of a microscope or other industrial inspection or measurement system. The described systems maintain focus with high reliability even when confronted by many challenging situations that would be detrimental to a proper focus. One such challenge is the detection of a surface that is buried in a transparent medium which causes reflections from surfaces along the optical path that are near enough to the target to cause a competing focus signal. One class of problems that is overcome by the present system relates to reflections off of transparent interfaces resulting from discontinuities in the refractive index. Another class of problems relates to opaque reflective surfaces below the target of interest. Such reflections degrade focus signals and diminish image quality, however, the presently described system maintains a high quality image in spite of variations in the thickness, reflectance or tilt of the target. 
   These error mechanisms have prompted others to use a technique in which a reflection from the outer surface is used as the focus sensing reference, and then either an electrical or optical offset is introduced to maintain focus on a buried target. Such an approach is used in U.S. Pat. No. 7,071,451 issued to Ishikawa et al. However, an error will occur in the distance measurement if any of the following factors occurs: 
   (1) The thickness between the surface and the target changes; 
   (2) the reflectance of either the surface or the target varies, so that there is a different combination of the signal from the front and rear surface; or 
   (3) if the local tilt of the target changes. 
   This will degrade the ability to retain correct focus, and with it the quality of the image will be degraded. 
   The present system addresses the class of applications where it is necessary to focus a beam inside of a transparent surface under conditions where one or more of the above three parameters degrade focus sensing. This system makes it possible to reliably measure focus inside of a transparent medium in spite of the thickness of the medium, its reflectance, and variations in tilt. This goal is accomplished by optically blocking the light reflected from the unwanted surface while allowing light from the target to pass through to the focus sensor. 
   An embodiment of this system uses a point focus, either scanned or stationary, and suitable relay optics so that an intermediate aerial image of one surface is formed. It is then possible to introduce a beam block that removes all of the light reflected from one surface while letting most of the light from the other surface propagate onward where it can be used to measure the state of focus of the beam. 
   Each focus retains certain properties of a focused spot separately from the other. As a result, the light from one focus can be used to generate a focus error signal and the light from the other can be blocked with a narrow beam block or spatial filter. It is important to note that the light from either surface may be removed. Therefore, it becomes possible to study either surface without errors caused by light reflected from the other. 
   A basic focus sensing system is shown in  FIG. 1 . Light source  110  emits an outgoing beam  300  of light, a portion of which is blocked by a knife-edge element  170 . The unblocked portion of outgoing light beam  300  passes through a collimating lens  180  before entering an optical relay  150 . One advantage of inserting a relay into any optical system is to provide spatial separation between optical elements without substantially impairing the focus sensing capability of the system. In the present system this spatial separation will be used for an additional purpose as will be described later. 
   The unblocked portion of outgoing beam  300  becomes an outgoing reference beam  310  which is reflected from beamsplitter  140  to then pass through objective lens  130  before impinging upon target surface  210 . Reflection from the target  200  causes the outgoing reference beam  310  to cross the optical axis  305  to become incoming reference beam  320 . 
   The inclusion of the beamsplitter  140  is optional, its purpose being to provide visual access through an alternate viewport as a portion of the light reflected from the target  200  passes straight through to a human user, either directly by means of viewing eyepieces, or indirectly through a camera system. Where alternate access to the optical path is not required, the beamsplitter  140  may be eliminated, resulting in an unfolded, straight optical path. When included, the beamsplitter  140  reflects the incoming reference beam  320  back through the optical relay  150  and collimating lens  180 . The incoming reference beam  320  is then reflected from the backside of knife-edge element  170  to photodetector  120  which produces a focus error signal. By precisely subtending the incoming reference beam  320  over the portion of the optical path not taken by outgoing reference beam  310 , knife edge element  170  performs multiple functions, acting as a knife-edge in the outgoing path, as a knife-edge in the incoming path, as a lossless beamsplitter, and as a fold mirror. Although  FIG. 1  shows a single lens  180  being used both in the forward path and the return path, it is also possible to use two separate lenses: one for collimating the outgoing light and one for focusing the return light. In this alternate configuration the lenses would be placed on the other side of mirror  170 , that is, the collimating lens would be placed between the light source  110  and the knife-edge element  170  while the focusing lens would be placed between the mirror  170  and the photodetector  120 . 
   Since the paths traced by the outgoing and incoming reference beams ( 310  and  320 , respectively) are parallel and complementary, aberrations and distortions tend to be automatically compensated resulting in a high quality image. In U.S. Pat. No. 7,348,528 Marshall describes inclusion of a scanning mirror located between optical relay  150  and collimating lens  180  whereby the target  200  may be scanned by outgoing reference beam  310  without affecting the position of the returned incoming reference beam  320  with respect to the photodetector  120 . Marshall also points out that with appropriate design and placement of the collimating lens ( 180 ) and the mirrored surfaced of the knife-edge element ( 170 ), the use of an afocal relay ( 150 ) further acts to prevent artifacts in the focus error signal that might be introduced by a longer optical path. 
   Building upon the above-described system, it is now desired to remove unwanted reflections from surfaces or refractive interfaces other than the one of interest. Refer to  FIG. 2  for a discussion of the problem and the presently introduced solution. After incident beam  330  passes through the objective lens  130 , it reflects off of the target  200  at the surface of interest  210 . However, depending upon the refractive index of the target  200  some of the incoming beam penetrates the first surface of the target  200  to be reflected off of its backside at surface  220  to become reflected beam  340 . Tracing the reflected beam  340  backwards shows that it appears to have been reflected from a virtual focal point  225  which is beyond (below) surface  220  by a distance that may be substantially different from the thickness of the target  200 , due to the fact that the longitudinal magnification of lens pair  152  and  130  may be other than one. Also, the unwanted light is reflected from the surface to be blocked  220  and the spot  225  appears to be twice as far away from the first surface  210  as the distance between the surfaces ( 210  and  220 ) of the target  200 , a 2× multiplier of distance. 
   As the unwanted image beam  340  passes through the objective lens  130  and is reflected by the beamsplitter  140  the change in position of the apparent focus  225  results in a real image  350  that is offset from the optical center  155  of the relay  150 . This offset is predictable as it is a function of the distances between the desired and unwanted surfaces ( 210  and  220 , respectively) of the target  200  multiplied by the square of the lateral magnification of the first relay lens  152 , and objective lens  130  in a first-order approximation, assuming that the first relay lens  152  has a focal length that is different from that of the objective lens  130 . 
   Since the location of the real image  350  is predictable for a target  200  of a given thickness, a beam block  145  may be placed at that point in the optical path to remove the light reflected from the unwanted surface  220  from the incoming reference beam  320  before it reaches the photodetector  120 . Since the light that is to be removed has been focused into the real image  350 , in theory the beam block  145 , if precisely placed, would only need to block a point. 
   In practice, however, the target  200  will generally exhibit some variation in thickness. While it is desirable to keep the beam block  145  small in order to avoid unnecessary loss of light reflected from the desired surface, in order to maintain a high signal to noise ratio, this design offers a fair amount of latitude. As the beam block  145  is displaced from the actual location of the real image  350  light that ought to be rejected will leak around the beam block. Increasing the size of the beam block  145  increases the capture range at the penalty of less signal light reaching the photodetector  120 . In one example, a beam block is used to block a second surface that is 1.25 mm from the surface of interest and measurements show that the capture range is 1 mm in one direction and 0.5 mm in the other direction which is more than adequate for many applications. 
   It is to be noted here that proper placement of the beam block  145  allows for the rejection of either of the near or far surfaces. To reject light from the upper surface  210  while focusing on the lower surface  220  the beam block  145  must be shifted nearer the second relay lens  158  in which case the objective lens  130  will also need to be refocused. Furthermore, if the target consists, for example, of multiple layers of refractive material wherein the approximate thickness and variation of each layer is known, it is possible to place multiple beam blocks in the optical path to reject multiple images reflected from more than one unwanted surface. It will be recognized by a person skilled in the art that such a design must take into account reflection coefficients and distances. 
     FIG. 3  applies the teachings of  FIG. 2  to the optical configuration shown in  FIG. 1 . To better accommodate real world applications, the optical system of  FIG. 3  has been enhanced to include a folding mirror  172  placed between the optical relay  150  and the collimating lens  180 . The first effect of this rearrangement is to reduce the spread of the overall system  100 . This configuration allows the knife-edge element  170  to be relocated into a region of collimated light. Adding a second mirror  175  and a decollimating lens  185  to focus the incoming reference beam onto the photodetector  120  enables the light source  110  and the photodetector  120  to be constructed on a common electronic circuit card or substrate. 
   By driving mirror  172  with a scanner mechanism the system may be made to scan a surface of the target while still rejecting unwanted reflections. Introduction of a scanner divides the optical system into two optical spaces, a scanned space and a non-scanned space. The scanned space includes the optical path between the scanner ( 172 ) and the target  200 , whereas the non-scanned space extends from the scanner to the detector  120 . 
   The location of images within the scanned space will follow the motion of the scanner. This means that for a single-axis scanner an image that focuses to a point when unscanned will be spread into a line focus. Accordingly, the small disk that would have served as beam block  145  in an unscanned system will need to be replaced with a bar or wire having sufficient length and width to block the spread image. A two-axis scanner complicates matters but a block of appropriate size and shape may still be inserted to replace beam block  145  for a functional system. 
   In the non-scanned space images remain stationary; they are not affected by scanner motion. This allows the use of a scanner while retaining the small disk that served as beam block  145  of the unscanned system. However, this requires relocation of the beam block into the non-scanned space, generally near the detector  120  or its companion focusing lens  185 . 
   Afocal relay  150  illustrates only one type of afocal relay, where lenses  152  and  158  have nominally equal focal lengths and unity magnification. However, relays of other magnifications may be used. 
   In a focus sensing system that uses a relay there is a location within the relay tube where there is a real image of the focused spot just as it is reflected by the unwanted surface. As has been described, this is a major advantage of using a relay. An alternate embodiment of the presently described system may be built without an afocal relay, in which case it becomes necessary to use a toric, or more specifically a cylinder lens. 
   As illustrated in  FIG. 5  the optical path of the toric-based system is very similar to that described above. The afocal relay  150  has been replaced by a toric, or cylindrical, lens  160 . The astigmatism introduced as the outgoing beam  310  passes through the cylindrical lens  160  produces a pair of line foci rather than a single point focus. While others have designed systems around astigmatic lenses, they have used only one of the two astigmatic line foci. In those applications the line focus has been used to perform an averaging function in order to reduce the effect of local asperities in the surface under observation. The presently described system uses both line foci created by one or more toric elements in such a way that one astigmatic focus is projected on, or near to, one surface, while the second astigmatic focus is projected on, or near to the other surface. In principle either surface can be the target surface, but in practice it is usually the buried surface  210  that is of interest while it is desired to reject reflections from the nearer surface  220 . 
   A cylinder lens  160  placed in the outgoing reference beam  310  generates a pair of line foci which are perpendicular to one another. One of these line foci is projected onto an upper surface  220  of the target  200  while its perpendicular counterpart is adjusted to focus on the lower surface  210  of a transparent substrate. After reflection from the surfaces of the target  200 , the incoming reference beam  320  passes back through cylinder lens  160 . The complementary nature of this return trip through the astigmatic cylinder lens  160  effectively removes the astigmatism introduced on the forward pass while the spatial offset of the two foci is maintained. 
   After reflection from the knife-edge element  170  the image beam passes through a second cylinder lens  190  which again produces a pair of line foci. Photodetector  120  is located to intercept the line focus returned from the surface of interest  210 . A beam block  165  is placed in the path of the incoming reference beam  320  to block the image returned from the unwanted surface  220 . Since the two images are perpendicular to one another, the beam block  165  will be rotated 90-degrees from the image of interest. Since the beam block  165  has a finite width, it will produce a gap in the middle of the image on the photodetector  120 . Having thus blocked the unwanted image the light reflected from the surface of interest  210  can be used to generate a focus error signal that depends only on the location of the surface of interest. Because the beam block has non-zero thickness, the system has also been desensitized at some level to tilt caused by light reflected from both surfaces, from target thickness variation and from reflectance variation. The remaining line focus at the photodetector  120  averages the focus information to reduce the effect of local asperities, but only at the surface of interest. 
   One disadvantage of this toric-based alternate embodiment is that it is not effective where there are more than two surfaces, such as with a multi-layered target. However, this approach is preferable to previous toric-based methods where the light collected from both surfaces was averaged and an offset was applied. The focus sense signal of that method was corrupted by any tilt of the surface of interest, and variations in the optical properties of the glass substrate. 
   Both the afocal relay and toric-based embodiments require a minimum separation between the two surfaces in question. While there is no hard limit to the minimum separation, it becomes progressively more difficult to construct a workable system as the distance between the two surfaces decreases. This is due to the close spacing of the two images and the very small size of the necessary beam block. In the case of the afocal relay system it is advantageous to increase the magnification of the pair of lenses consisting of the objective and the relay lens nearest to it. 
   The focus sensing system  100  will typically be used as part of a more complete autofocus system. As seen in the minimal autofocus system of  FIG. 6 , a focus sensing system  100  may supply focus error data directly to servomechanism  400  to control the position of objective lens  130  with respect to the target  200 . Alternately, the servomechanism  400  may be made to reposition the target  200  with respect to the objective lens  130 . 
   While the present invention has been described with respect to various preferred and alternate embodiments, there is no implication to restrict the present invention to preclude other implementations that will be apparent to those skilled in the related arts. It is easily recognized that the described invention may be implemented with a variety of alternative subsystems created in part by rearrangement of certain optical elements. Therefore, it is not intended that the invention be limited to the disclosed embodiments or to the specifically described details insofar as variations can be made within the spirit and scope of the appended claims.