DE9301901U1 - Device for measuring topography using projected patterns - Google Patents

Description

Description: 93009 G

Device for measuring topography using projected patterns

The use of so-called strip projection measuring devices is known for large-scale quantitative optical measurement of the topography of diffusely reflecting surfaces. Such measuring devices have a projector that projects a pattern, normally a strip pattern with a sinusoidal intensity curve perpendicular to the direction of the strip, onto the measuring surface. The strip pattern, which is deformed according to the topography to be measured, is recorded with a video camera. By subsequently evaluating the recorded camera images using the evaluation algorithms known from interferometry and using the assumed known geometric arrangement of the projector and camera to each other, the topography of the measuring surface, i.e. the surface coordinates relative to a coordinate origin given by the arrangement of the camera and projector to each other, can be calculated. Such measuring devices are described, for example, in EP-Bl 0 182 469, DE-Al 4 0 07 500 and in the applicant's older application P 41 30 237. It is essential for the measuring principle that for each object point the projection direction and the observation direction have an angle other than zero, since the parallax is responsible for the measurement resolution in the direction of the optical axis of the camera. With a fixed period length of the projected pattern, the measurement resolution in the direction of the optical axis of the camera is therefore finer the larger the angle between the projection and observation directions.

For measuring large surfaces, e.g. body parts in the automotive industry, it is also

It is useful to use central perspective projection optics for both the projector and the camera, since with central projection and simultaneous central perspective observation the size of the available measurement volume is independent of the aperture widths of the optics. However, if the central perspective projector and central perspective camera are arranged at large angles to each other, a pronounced drop in the illumination intensity, i.e. the basic brightness of the patterns in the camera images, can be observed, which means that part of the camera dynamics cannot be used for the actual phase evaluation.

A special camera for observing molten uranium during isotope separation is known from US-Al 4 815 828. A filter arranged in the camera beam path, the transmission of which decreases with increasing light intensity, compresses the intensity range of the radiation hitting the camera sensor so much that the camera sensor is not overloaded. However, this document does not contain any information on the use of such cameras in topography measurement using pattern projection, in which the recorded patterns are subjected to a subsequent phase evaluation.

An autofocus camera with an active autofocus is also known from US-Al 4 771 308. The camera contains a strip grid and a projection lens with which the strip grid is projected into a plane containing the optical axis of the camera lens. To adjust the focus on a subject arranged in the image area of the camera lens, the camera lens is moved in a known manner along the optical axis so that a sharp pattern image is created on a sensor. This ensures that there is enough measuring light for the autofocus system even for subjects that are far away.

is detected, the grating carrier has, in addition to the strip grating, a transmission gradient such that the intensity of the projected pattern increases with distance.

Topography measurement is not intended with this camera, and even if the camera were used for topography measurements, the object coordinates could at best be determined in a very small area around the optical axis of the camera lens.

The present invention is intended to provide a measuring device for topography measurement by means of projected patterns, in which a large part of the camera dynamics is available for pattern evaluation even at large angles between the optical axis of the projector and the optical axis of the camera.

To solve this problem, according to the present invention, a measuring device with the features of claim 1 is proposed.

The measuring device according to the invention therefore contains at least one projector for projecting a pattern onto an object surface, the projector having a first optical axis, and at least one camera for recording the pattern deformed on the object surface, the camera optics having a second optical axis inclined to the optical axis of the projector. Furthermore, an evaluation computer is provided for evaluating the camera images, with the aid of which the coordinates of the object surface are calculated from the strip phases of the deformed patterns relative to a device-fixed coordinate system. In addition, a device is provided which influences the light energy density integrated by the camera differently at different locations within the image plane. This device

It can be achieved that the average intensity of the recorded pattern images is approximately constant within the entire image plane.

In a first embodiment, the filter is a transmission filter that has locally different transmission levels. The transmission level of the filter should only have different values in the direction that lies in the plane spanned by the optical axes of the camera and projector. In the direction perpendicular to this plane, however, the transmission level of the filter should have constant values.

In principle, the transmission filter can be arranged either in the projector or in the camera. However, arranging the filter in the camera has the advantage that no or only very few heat problems occur. If the filter is arranged in the camera, the filter can also consist of a phototropic material whose transmission factor automatically decreases as the light intensity increases. Such a filter must then be arranged outside the image or intermediate image planes of the test object. This ensures that the locally different transmission factor of the filter has no harmful influence on the average intensity of the pattern within one or a few pattern periods, but only takes effect over areas that are significantly larger than the pattern period.

In a second embodiment, a device is provided which causes a location-dependent exposure time or integration time of the camera. A CCD sensor with optoelectronic detectors arranged in rows and columns can be provided as the camera sensor. Within each detector column running perpendicular to the plane of the optical axes of the camera and projector, the integration time is then constant, while the individual

Detector columns have different integration times. Instead of the filter, an electronic circuit is provided in this embodiment, with the help of which exposure-controlling erase pulses are applied to the individual detector columns.

The present invention is particularly advantageously applicable to strip projection measuring devices, in which the angle between the optical axis of the camera and the projector is greater than 45°. This is because with such large parallax angles, the illumination, i.e. the average intensity across the image field, drops particularly sharply.

In a particularly preferred embodiment, a stripe grid, for example a Ronchi grid, is provided in the projector, which is projected onto the surface of the measuring object by a projection lens. The stripe grid is arranged within the projector in such a way that the Seheimpflug condition for planes perpendicular to the optical axis of the camera is fulfilled. This ensures optimal stripe contrast within this plane. The stripe grid should also have a non-equidistant division that is adapted to the angle between the optical axis of the camera and that of the projector in such a way that patterns with equidistant stripes are created in planes perpendicular to the optical axis of the camera.

In the following, details of the invention are explained in more detail with reference to the drawings.

In detail:

Fig. 1 is a schematic diagram of the measuring device according to the invention with a transmission filter arranged in the camera;

Fig. 2a-2b Diagrams of the transmittance of the filter of Figure 1 as a function of location;

Fig. 3a a top view of a CCD sensor for a second embodiment with different integration times for each column and

Fig. 3b is a diagram of the integration time as a function of location.

The strip projection measuring device in Figure 1 has a measuring head (1) with a projector (Ib) and a camera (la). The projector (Ib) contains a projection lens (2) which projects a strip grid (3) illuminated by a light source (4) and an illumination optic (5) in central projection onto the object surface (6) to be measured. The strip pattern deformed on the surface of the measuring object (6) is imaged with the aid of the camera lens (7) in central projection onto the camera sensor (8), which is designed as a CCD sensor.

An evaluation computer (10) is connected to the measuring head (1), into which the strip images recorded with the camera sensor (8) are read and subsequently evaluated. The topography of the object surface is quantitatively recorded by evaluating the strip phases in the recorded patterns. The evaluation itself is carried out using the phase evaluation algorithms known from interferometry. Details of these evaluation algorithms are described in the publications mentioned above, in particular in DE-Al 40 07 500, EP-Bl 0 182 469 and additionally in DE-Al 40 14 019, to which reference is hereby expressly made. The result of the phase evaluation is then displayed graphically or numerically on a monitor (11).

In contrast to the previously known stripe projection measuring devices, here the optical axis (2a) of the projection lens (2) and the optical axis (z) of the camera lens (7) have a very large parallax angle ((f), which is 55° here. At the same time, the stripe grid (3) is arranged at an angle to the optical axis (2a) of the projection lens (2) such that the Scheimpflug condition is fulfilled for a plane (12) lying perpendicular to the optical axis (z) of the camera lens (7) (the main plane of the projection lens (2) and the plane of the grid (3) intersect the plane (12) along a straight line (12a) lying perpendicular to the plane of the drawing). The grid (3) is thus sharply imaged in the plane (12). In addition, the grid (3) has a non-equidistant division, which is dimensioned such that a grid image with equidistant stripes is created in the plane (12). The stripe direction itself is perpendicular to the drawing plane.

Due to the different imaging ratios of the grid (3) in the image field of the camera (la) lying between (6a) and (6b), the projected pattern in the plane (12) or on the object surface (6) has very different basic brightnesses. In particular, the brightness at (6b) is significantly lower than at (6a). Correspondingly different brightnesses would then also be present at the image points (8a) or (8b) assigned to the points (6a) and (6b) on the camera sensor (8). To compensate for these different brightness ratios within the image field, a transmission filter (9) with a transmittance that increases linearly in one direction is arranged in front of the camera sensor outside an intermediate image plane. The transmittance of the filter (9) decreases, as shown in Figure 2a, continuously from (9b) to (9a) in the direction x, which lies in the plane of the drawing. The light scattered by the object in area (6b) that the filter (9) transmits in area (9b) is therefore filtered out by the filter

(9) is less attenuated than the light scattered at (6a) and transmitted by the filter (9) at (9a).

In the y-direction perpendicular to the plane of the drawing in Figure 1, the transmission of the filter (9) has a constant value that depends only on the x-position. This is shown in Figure 2b for the x-positions belonging to the areas (9a) and (9b). The drop in transmission in Figure 2a between (9b) and (9a) is chosen so that the drop in the basic brightness over the image field caused by the different image scale of the grating (3) on the object surface (6) is compensated. The strip brightness of the grating image on the sensor (8) is therefore constant for all positions in the image field.

In principle, it would be possible to use a suitably selected transmission filter in the projector instead of the filter (9) in the camera, for example in a plane parallel to the grid (3). However, due to the light absorption in the filter and the relatively high light output within the projector, heat problems can then very easily arise.

When arranging the transmission filter in the camera, it is also possible to use a filter made of phototropic glass instead of the filter described above with a defined, preset, locally different degree of transmission. The degree of transmission of the glass then automatically decreases with increasing light intensity. With such a filter arranged outside an image plane, a locally different degree of transmission is then automatically established, which corresponds to the curves shown in Figures 2a and 2b. However, the arrangement of the filter (9) outside of the image or intermediate image planes of the object (6) is then important, since otherwise the filter would erase or weaken the stripe patterns to be evaluated.

In an alternative embodiment, instead of a physical transmission filter, a camera sensor (80) (Figure 3a) with a different integration time for each column is provided. The camera sensor (80) has a large number (for example 512 x 512 or 1024 x 1024) of photoelectric detector elements arranged in rows and columns. For the sake of simplicity, only eight sensor columns (80s) and eight sensor rows (80z) are shown in Figure 3a. Different integration times for the released charge carriers can be set for the different detector columns (80s) using a control device (not shown here).

The manufacture of such camera sensors (80) does not pose any problems, because in principle, devices are already available in conventional CCD camera chips to control the integration time of each individual receiver element. However, the control lines are initially connected column by column and then the control lines of each column are connected to the control line of the next but one neighboring column. In the sensor chip (80) shown in Figure 3a, however, only the control lines within each column (80s) are connected to one another, so that a different integration time can be set for each column (80s).

The overall arrangement of the measuring device according to the second embodiment essentially corresponds to the arrangement according to Figure 1, in which instead of the sensor (8), the sensor (80) from Figure 3a is arranged so that the detector columns (80s) are arranged perpendicular to the plane of the optical axes (2a, z) of the camera lens (7) and projector lens (2), i.e. perpendicular to the plane of the drawing. This orientation of the camera sensor (80) is indicated in Figure 3a by the two adjacent coordinate axes. As already

As stated above, in this embodiment the physical transmission filter (9) is replaced by an electronic circuit that controls the integration time of the detector columns (80s).

In the diagram in Figure 3b, the different integration times τ for the individual detector columns are plotted as a function of location. The integration time τ increases from the column (80a) on which the object point (6a) is imaged to the next column and is maximum for the column (80b) on which the object point (6b) is imaged. The individual integration times of each camera column are set so that the uneven basic brightness across the image field is balanced out and the camera is not overloaded anywhere in the image field.

Compared to the embodiment with a transmission filter, the control of the integration time provides the advantage that the integration time of each column can be selected depending on the light intensity of a previously recorded image. This means that the entire camera dynamics can always be used for the phase evaluation. The disadvantage, however, is that the different exposure times for each column must be taken into account in the subsequent evaluation.

As an alternative to the straight grid (3), a non-straight grid can also be provided in the projector, which is arranged in such a way that an equidistant stripe pattern is projected onto a non-flat test surface. When testing body parts, for example, a prismatic body with a flank profile that corresponds to the flank profile of the area to be tested is then selected as the reference surface.

Claims (10)

Protection claims: 1. Device for topography measurement with at least one projector (Ib) for projecting a pattern onto the object surface (6), the projector (Ib) having a first optical axis (2a), at least one camera (la) for recording the pattern on the object surface (6) deformed pattern, wherein the camera (la) has a second optical axis (z), an evaluation computer (10) for evaluating the camera images, wherein the coordinates of the object surface (6) relative to a device-fixed coordinate system are calculated from the strip phases of the deformed pattern, and with a device (9) which locally influences the light energy density integrated by the camera within the image plane in different ways. 2. Device according to claim 1, wherein the device influencing the integrated light energy density is a transmission filter (9). 3. Device according to claim 2, wherein the transmission (T) of the filter (9) has different values along a direction (x) which lies in the plane spanned by the optical axes (2a, z) of the camera (la) and projector (Ib) and has constant values in the direction perpendicular to this plane (y). 4. Device according to one of claims 2 or 3, wherein the filter (9) in the camera (la) is arranged outside an image plane and consists of a phototropic material. 5. Device according to claim 1, wherein the device influencing the integrated light energy density comprises a location-dependent exposure or integration time &tgr; of the camera (80). 6. Device according to claim 5, wherein the camera in lines (80z) and columns (80s) arranged optoelectronic detectors and an electronic circuit for the location-dependent control of the exposure or integration time &tgr; is provided, wherein the exposure or integration time &tgr; is constant within each detector column (80s) arranged perpendicular to the plane of the optical axes (z, 2a) of the camera (la) and projector (Ib), but the integration times of the individual detector columns (80s) are different. 7. Device according to one of claims 1-6, wherein the angle (ψ) between the optical axes (z, 2a) of camera (la) and projector (Ib) is greater than 45°. 8. Device according to one of claims 1-7, wherein the projector has a strip grating (3) with alternating transmitting and absorbing strips and wherein the grating (3) and the lens (2) of the projector (Ib) are arranged inclined to one another in such a way that the Scheimpflug condition for planes (12) perpendicular to the optical axis (z) of the camera (la) is fulfilled. 9. Device according to claim 8, wherein the grating (3) has a non-equidistant pitch which is adapted to the angle (Cf) between the optical axes (z, 2a) of the camera (la) and the projector (Ib) in such a way that in planes (12) perpendicular to the optical axis (z) of the camera (la) patterns with equidistant stripes are formed. 10. Device according to one of claims 1-9, characterized in that the projector (16) is designed for central perspective projection and the camera (1a) is designed for central perspective observation.