Device for optical investigation of an object

A two-dimensional illuminating grid (11b) illuminates an object (14). Its spacial form is measured by imaging through a prism grid (66) on a receiver array (17), with the prism grid performing a pupil division for the image of each of the illuminated object points. For this purpose, for each measurement point, the difference signal from each of four adjacent pixels (8a, 8b, 9b, 9a) of a receiver array (17) is evaluated. For example, 65,000 points in space can be measured in 20 ms.

BACKGROUND
 The present invention relates to a device for three-dimensional
 investigation of an object according to the preamble of claim 1.
 A device of this kind is known from DE 40 35 799. In that patent, an
 illuminating grid with a pixel size is imaged on a receiver array, which
 matches the pixel size of the photosensitive areas of the receiver array
 or is a whole-number multiple of said array. The diaphragm action of the
 receiver array is utilized. A system of this kind has the disadvantage
 that during the evaluation of images taken from various focal planes to
 determine the depth values, the absolute brightness values are always
 evaluated and their maximum must be determined.
 A device is known from JP 1-55513(Feb. 3, 1989) which is suitable for
 rapidly focusing a microscope using incident light from the aid of a laser
 beam. For this purpose, a photodiode is utilized with four receiver
 segments and the object point illuminated by the laser is imaged through
 an optical system with cylindrical lenses on the four-quadrant receiver.
 This principle differs however basically in its effect from pupil
 division, since no focused image of the object point is produced. The
 device according to JP 1-55513 is unsuitable for rapid 3-D measurement.
 DD 265 224 likewise describes a device with point-wise object illumination,
 but it only makes it possible to measure an object location
 simultaneously. It is likewise unsuited for rapid measurement of large
 sample areas.
 DE 26 34 655 teaches a device for focusing a single-lens reflex camera. It
 is unsuited for measuring 3-D objects.
 In the book "Technische Optik" [Technical Optics] by Gottfried Schroeder,
 Vogel-Verlag, 1977, page 145, a device from the field of photography is
 described that uses a double prism and serves to focus a camera. Since
 only one object position is detected, this system cannot be used for
 automatic measurement of larger object areas.
 A device that makes it possible to determine the focus of an individual
 measurement point with a zero signal detection is known from the company
 publication "Microfokus, Beruehrungslos messen" [Microfocus, Measurement
 with Zero Contact] from UBM Messtechnik, D 76275 Ettlingen. In that
 publication, the light from a laser diode is imaged on the object to be
 illuminated and the light reflected from this object is split by a pair of
 prisms so that the two half pupils are imaged on two receiver pairs. The
 light from a laser diode is imaged on the object to be measured and the
 light reflected from the object is split by the pair of prisms in such
 fashion that two half pupils are imaged on two receiver pairs*. A system
 of this kind, apart from the focus, supplies a directional signal that
 indicates the direction in which the position of the measured object
 differs from the focus position. However, only a single point on the
 object is detected. Measurements with a system of this kind therefore
 require a great deal of time, especially when large areas of the object
 are to be measured.

FNT *Evident repetition. Translator's note.
 SUMMARY OF INVENTION
 The present invention has the goal of providing a device that makes it
 possible to determine focus with zero signal detection and in this way to
 detect many measurement points at the same time. This goal is achieved
 according to the invention by the combination of features in claim 1.
 Advantageous embodiments of the invention are shown in claims 2 to 6.
 The system according to the invention has the advantage that the focus
 position of the individual measurement points is determined by zero signal
 detection and that very many measurement points are detected
 simultaneously. This is performed economically because receiver arrays,
 preferably CCD arrays, and prism arrays in the prism plane are used as the
 radiation receivers. The arrays can be produced economically by
 compression molding for example.
 Zero signal detection is implemented by the arrangement of the receiver
 array relative to the prism array being so designed that in the focus case
 the image of the light reflected from the illuminated point on the sample
 and deflected by a prism falls symmetrically on the boundary line between
 two radiation receivers (pixels) of the receiver array.
 The system according to the invention differs from the system described in
 Patent DE 40 35 799 in that the diaphragm function of the radiation
 receivers arranged in an array is utilized in the latter, with the
 photosensitive areas of said receivers being separated from one another by
 gaps. Focus detection is performed in this patent by evaluating the
 intensity maximum detected on each pixel of the radiation receiver array.
 For this purpose, a measurement series is performed that stores the values
 for several different z-positions of the object in the computer. For each
 pixel, this determines the z-position at which its intensity has its
 maximum. It supplies the depth value to be determined. In the present
 invention, on the other hand, the difference between the signals of two
 adjacent pixels and the receiver array is evaluated. When it is equal to
 zero, the focus position exists that supplies the depth of value to be
 determined. Receiver arrays are used for this purpose that have no zone
 that is insensitive to light, or only a small zone of this kind, between
 adjacent pixels. Instead, for detecting the difference signal in the
 present invention, it is necessary to use receiver arrays with
 photosensitive areas that are directly adjacent to one another. For this
 reason, the system according to the present invention differs basically
 from that described in the abovementioned DE.
 The system according to the invention is especially suitable for
 three-dimensional measurement of mechanical parts in incident light and
 for recording 3-D images of fluorescent objects. For mechanical parts, a
 semi-transparent mirror is placed in known fashion in the beam path to
 separate the illuminating light from the light reflected from the object.
 For fluorescence applications, a dichroic mirror is used in a manner known
 of itself, and possibly light filters as well, to separate the
 illuminating light and the light emitted by fluorescence from the object.

In FIG. 1, (11) refers to a light source, for example a halogen lamp,
 which, with the aid of condenser (11k) and possibly with a filter (11f)
 (to separate out a sufficiently narrow range of the spectrum), holes (12l)
 in a layer (12s). A layer of this kind can be produced in known fashion
 for example from chromium on a glass plate (12g). The holes (12l) are
 arranged in a layer (12s) in the same grid shape as the photosensitive
 areas of receiver array (17). For example, if a receiver array with
 512.times.512 receivers is used, said receivers being arranged in the form
 of a grid at a distance of 11 .mu.m, the layer then has 256.times.256
 holes spaced 22 .mu.m apart and with a hole size of 4 .mu.m.times.4 .mu.m
 for example. Hence, the holes are much smaller than their spacing. The
 spacing of the holes or areas from center to center is termed the grid
 size.
 The illuminating grid that is generated by illuminated holes (12l) in layer
 (12s) lies in illumination plane (11b). This plane is imaged by lenses
 (13o, 13u) in focal plane (13f), so that in the latter object (14) is
 illuminated with points of light arranged in the form of a grid. In the
 case of objects that are not transparent, only surface (14o) can be
 illuminated, while in transparent objects, layers (14s) inside can also be
 illuminated with the light points. The light beams reflected from the
 object in focal plane (13f) are focused by lenses (13u, 13o) through a
 beam splitter (16) in diaphragm plane (17b). Diaphragms are produced in
 prism plane (66) by the edges of the prism pairs that are separated from
 one another by gaps. Between lenses (13o, 13u), a so-called telecentric
 diaphragm (13t) is usually provided, said diaphragm ensuring that center
 beam (13m) strikes object (14) parallel to optical axis (10) so that the
 positions of the points of light on the object do not change if object
 (14) is moved in the direction of optical axis (10).
 The above-mentioned beam splitter (16) is made in the form of a
 semi-transparent mirror for incident light applications. For fluorescence
 applications, a dichroic mirror is used in known fashion.
 Object (14) can be moved by an adjusting device (15) in all three
 directions in space, so that various layers (14s) of object (14) can be
 scanned. The movement in the x- and y- directions can be made smaller than
 the grid size of light points (12). Of course, the movement of object (14)
 in the z-direction can also be produced by shifting lenses (13o, 13u) in
 the direction of optical axis (10) and similarly, instead of moving the
 object in the x- and y- directions, the layer (12s) with the holes (12l)
 and the receiver array (17) can be moved accordingly as well.
 The signals from receiver array (17) are transmitted through a connecting
 line (17v) to a computer (18) which performs the evaluation in known
 fashion and displays the results of the evaluation on a screen (18b), for
 example in the form of graphic images. Computer (18) can also control the
 shifting of focal plane (13f) in the object and scanning in the x- and y-
 directions through connecting line (18v). This control can be provided in
 the computer as a fixed program or can be performed as a function of the
 results of the evaluation.
 FIG. 2 shows a glass plate (12g) in a top view, with an illumination point
 (12l) being shown enlarged. The provision of the illumination points in
 the shape of an array is merely indicated; in reality, as already
 mentioned, with a grid-type arrangement, for example, there are 256 lines
 with 256 illuminated points each.
 In FIG. 3, a single prism pair is shown in a side view. It consists of two
 wedged-shaped partial prisms (60a) and (60b) which are arranged opposite
 one another so that the light falling on different partial prisms is
 deflected in opposite directions.
 FIG. 4 shows a prism array (66) in a top view. A single prism pair (60) is
 shown enlarged and consists of prisms (60a and 60b) located opposite one
 another. It is advantageous to provide as many pairs of prisms as there
 are illumination points.
 FIG. 5 shows, on the same scale, the corresponding receiver array (17). A
 receiver quadruple (6) is associated with each prism pair, said quadruple
 consisting of receiver pair (8a, 8b) and receiver pair (9a, 9b) and hence
 of a total of four individual receivers (8a, 8b, 9a, 9b).
 FIG. 6 shows the images (2l, 2r) of the two half-pupils generated by a
 prism pair in the focused state. It is evident that the focus images fall
 symmetrically on the receiver pairs, in other words receivers 8a and 8b
 receive equal amounts of light. The same is true of receivers 9a and 9b.
 Outside the focus however, more light falls on one of the receivers of a
 pair than on the other. This is shown in FIG. 7. Receiver 8b receives more
 light than receiver 8a and receiver 9a receives more light than receiver
 9b. The lack of symmetry is reversed if the position of sample (14)
 deviates in the other direction from the focus position. Then receiver 8a
 receives more light than receiver 8b and receiver 9b receives more light
 than receiver 9a. Thus the direction signal is obtained. For example, it
 can be used to measure objects (14) that are larger than the visual field
 of the system by scanning in a manner known of itself. In this way it is
 possible, as object (14) is moving, to calculate a signal for the average
 deviation from the focus and to adjust the z-position of the object
 relative to the measuring system in a coordinate measuring device in such
 a way that it follows the surface contours of the object. If this takes
 place at a certain speed, so that the computer is always aware, on which
 receiver the partial areas of the object that are in the visual field are
 imaged at different points in time, it is possible, in a form of a
 "trail", to detect an entire strip on the object quickly and to evaluate
 the peaks from the recorded data.
 FIGS. 8 and 9 serve to explain in detail the beam path as it is split by
 the prisms. First of all, it is important to locate the prism array at a
 suitable distance from the receiver array. This is clear in FIG. 8, which
 shows the light cones of several simultaneously illuminated object points
 in front of the detector array and their midlines (81). In area (87), the
 light cones overlap while in area (86) they are separate from one another.
 The prism array is advantageously located approximately in the center
 (86m) of area (86). Then the distance between the light cones is large
 enough and the distance from the receiver array is likewise sufficient. It
 is necessary in order for the two partial images of the pupils to reach
 the receiving grid sufficiently far apart.
 FIG. 9 shows one of the light cones with its midline (81) and a prism pair
 consisting of prisms (60a and 60b). The two images (82 and 83), offset
 laterally from midline (81) of the light that strikes through the cone,
 are in focus in the example shown, in other words they are sharply imaged
 on receiver plane (17b). One can also see the lateral offset (2s) of the
 two focal points.
 FIG. 10 shows the full pupil of the telecentric diaphragm. It is based on
 the explanations above. Since the middle beams contribute little to the
 z-resolution, however, it can be advantageous to screen them out, using an
 annular diaphragm. An example of this is shown in FIG. 11. Only ring (4t)
 is permeable to light, so that the light beams close to the center are not
 imaged. The dynamic range of the radiation receiver array can be better
 utilized as a result, so that a greater deviation signal is obtained
 outside the focal position.
 FIG. 12 shows this. It shows the images of the half-pupils in a position in
 which the corresponding object point is located outside the focus, whereby
 have a half-moon shape [sic]. This can have considerable practical
 advantages because with equal control, the deviation signal, in other
 words the difference between the light striking the two receivers of a
 pair is greater than when a full pupil is used.
 FIG. 13 shows another embodiment of the prism array. The prism pair (64) on
 prism array (68) are here delimited in the shape of a circle and each of
 the two prisms (64a and 64b) fills a semi-circle. The gaps (88) are not
 transparent to light. Thus a portion of the light that comes from object
 points located far outside the focus is screened from the receiver array.
 As a result, noise signals are suppressed. This noise signal suppression,
 as indicated by the above remarks, is produced by a diaphragm function
 that is exercised by the marginal boundaries of the circular areas. In
 contrast to the previously known arrangement according to DE 40 35 799,
 the diaphragm function follows from the prism array according to the
 invention and not from the receiver array. In addition, the diaphragm
 function in the present invention is not absolutely necessary. It has only
 a function-improving effect.
 FIG. 14 shows that two prism pairs (65) are arranged so that they are
 rotated through 90.degree. relative to the others. Thus it is possible,
 with objects with a structured surface, to record the structure properties
 as well. Direction-dependent reflection properties of the object can be
 detected in this manner.
 In FIG. 15, a lens array (22a) is located between condenser (11k) and
 filter (11f) and layer (12s) with holes (12l), said array containing the
 same number of small lenses (22l) as layer (12s) has holes (12l). Lenses
 (22l) have the purpose of imaging the images of the luminous filaments of
 light source (11) in the holes, thus giving the points of light a greater
 intensity.
 Lens array (22a) and layer (12s) with holes (12l), as indicated, can be
 combined into one common part (22g). The manufacture of suitable lens
 arrays is known for example from a publication by K. Koizumi (SPIE, Volume
 1128, 74 (1989).
 An especially advantageous version of the illuminating grid is shown in
 FIG. 16. In that figure, (31) represents a light source array, with can
 consist for example of light-emitting diodes (LEDs) (31). In this case
 also, it can be advantageous to locate in illumination plane (11b) a layer
 (32s) with holes (32l) so that the points of light have dimensions that
 are sufficiently small. Except for lens (31o) for imaging, a field lens
 (31f) is advantageous for additional imaging in the beam path.
 It is advantageous to use integrated LED arrays for the illuminating grid,
 like those described for example in a paper by J. P. Donnelly (SPIE 1043,
 92 (1989)). Such LED arrays have the advantage that certain partial
 quantities of LEDs can be switched on and off. In both cases, the
 switching on and off is controlled by computer (18) through switching
 device (19).
 The beam path shown in FIGS. 1, 15 and 16 between illuminating plane (11b),
 focal plane (13f) and diaphragm plane (17b) is only a special embodiment
 of several known beam paths in which the invention can be used in a manner
 that is immediately apparent to the individual skilled in the art. In
 addition, in the beam path shown, an image of illuminating plane (11b) is
 not necessary in focal plane (13f) on a scale of 1:1. Instead, not only
 reduction, as is known from microscopes, but also enlargements are
 possible, for which reason the term "microscope" was not used in the
 above.
 In FIG. 17, the illuminating grid is formed by a lens array (53), which,
 thanks to its sufficiently good imaging properties, produces sufficiently
 small points of light (54) in illuminating plane (11b) from a nearly
 point-shaped light source (51). Condenser lens (52) causes lens array (53)
 to be traversed by a parallel bundle, so that each individual lens (53l)
 is utilized optimally.
 FIG. 18 shows a system in which a diaphragm (61) is multiply imaged in
 illuminating plane (11b) by a lens array (53). This diaphragm is
 illuminated by light source (11) through condenser (62) and diffuser (63).
 A wide variety of embodiments is possible for the diaphragm. As an
 example, FIG. 19 shows a diaphragm (61) with a square boundary for
 light-permeable area (71) and a light-impermeable center (72) for an
 illuminating grid.
 FIG. 20 shows schematically the curve (103) of the difference signal from a
 sensor pair as a function of the focal position. It is evident that the
 signal is equal to zero at the focus (100) and runs approximately linearly
 in a range (101) to (103). By a calibration process, the slope can be
 determined relative to the focus shift and the location of the focus can
 be determined in the computer even without scanning the focal plane
 itself. This is a very important advantage over known confocal systems.
 FIGS. 21 and 22 show a mechanical part (105) in two views that serves as an
 example for explaining advantageous measurement strategies.
 In FIG. 23, section A--A through part (105) is shown enlarged and the
 sequence of different image planes (110) is shown, said planes being
 superimposed on one another in the focal direction (z-direction).
 FIG. 24, like FIG. 24 [sic], shows another sequence (110) of different
 image planes superimposed on one another for measuring another area of
 mechanical part (105).
 In FIG. 25, a sequence 1 to 22 of image planes is shown of which some (5 to
 11) are located on top of one another and others (11 to 22 for example)
 are arranged so that they overlap at an angle. Recording images that
 overlap at an angle is advantageous if the measuring device in which the
 sensor according to the invention is used does not have a rapid drive for
 x-y movement of the sensor relative to the object or of the object
 relative to the sensor. Thus, the shifts can take place discontinuously or
 quasi-continuously without a rapid drive being required that would permit
 rapid acceleration and rapid stops. Without adversely influencing the
 measurement speed, more economical systems can be built.