阅读说明：本技术 通过有意引入的光学像差使视觉伪影最小化来改进结构光投影 (Improved structured light projection by minimizing visual artifacts through intentionally introduced optical aberrations ) 是由 A·施文德纳 托马斯·延森 J·斯蒂格瓦尔 于 2021-05-07 设计创作，主要内容包括：本发明涉及通过有意引入的光学像差使视觉伪影最小化来改进结构光投影。一种三角测量装置用于通过将结构光图案投影到测量对象上来测量测量对象,该三角测量装置包括：投影仪,其被配置成将结构光图案投影到测量对象上,结构光图案能被分解成不同的空间频率,投影仪包括：像素元素矩阵,其中,伪影图案由相邻像素元素之间的伪影区域限定,以及透镜系统,其中,透镜系统确定相对于参考波前具有波前像差的波前；以及摄像头,该摄像头包括透镜系统和成像传感器,其中,摄像头被配置成接收由投影仪投影到测量对象上的结构光图案；以及处理单元,其被配置成通过评估由摄像头提供的成像信息来提供距离信息。(The present invention relates to improving structured light projection by minimizing visual artifacts through intentionally introduced optical aberrations. A triangulation device for measuring a measurement object by projecting a structured light pattern onto the measurement object, the triangulation device comprising: a projector configured to project a structured light pattern onto a measurement object, the structured light pattern being resolvable into different spatial frequencies, the projector comprising: a matrix of pixel elements, wherein the artefact pattern is defined by artefact areas between adjacent pixel elements, and a lens system, wherein the lens system determines a wavefront having a wavefront aberration relative to a reference wavefront; and a camera comprising a lens system and an imaging sensor, wherein the camera is configured to receive a structured light pattern projected onto the measurement object by the projector; and a processing unit configured to provide distance information by evaluating imaging information provided by the camera.)

1. A triangulation device (8, 9, 10) for measuring a measurement object (12) by projecting a structured light pattern (11) onto the measurement object (12), the triangulation device comprising:

● projector (8) configured to project the structured light pattern (11) onto the measurement object (12) with projection light comprising at least one wavelength λ, the structured light pattern (11) being resolvable into different spatial frequencies, the projector comprising:

□ pixel element matrix (4, 5, 6, 7), wherein the artifact pattern is defined by artifact areas (2, 3) between adjacent pixel elements, and

□ a lens system, wherein the lens system is arranged such that the projected light passing through the lens system defines a wavefront having a wavefront aberration relative to a spherical reference wavefront; and

● camera (9, 10) comprising a lens system and an imaging sensor (10), wherein the camera is configured to receive the structured light pattern (11) projected onto the measurement object (12) by the projector (8); and

● processing unit configured to provide distance information by evaluating imaging information provided by the cameras (9, 10),

it is characterized in that the preparation method is characterized in that,

the wavefront aberration is decomposed by Zernike to obtain a primary spherical aberration coefficient Z₉Defining, wherein Zernike polynomials defining the Zernike decomposition are ordered according to a Fringe Zernike coefficient ordering, wherein the primary spherical aberration coefficients Z₉Greater than one quarter of the wavelength lambda.

2. The triangulation device according to claim 1,

it is characterized in that the preparation method is characterized in that,

the primary spherical aberration coefficient Z₉Greater than one third of the wavelength λ or greater than one half of the wavelength λ.

3. The triangulation device according to claim 1 or 2,

it is characterized in that the preparation method is characterized in that,

the primary spherical aberration provides a low pass filter.

4. The triangulation device according to any of the previous claims,

it is characterized in that the preparation method is characterized in that,

the structured light pattern (11) is implemented as a sinusoidal pattern.

5. The triangulation device according to any of the previous claims,

it is characterized in that

The modulation transfer function MTF (16) of the projector's lens system is closer to the diffraction MTF value (14) than to zero at spatial frequencies of the structured light pattern (11) and closer to zero than to the diffraction MTF value (14) at spatial frequencies of adjacent pixel element artifact patterns.

6. The triangulation device according to any of the previous claims,

it is characterized in that the preparation method is characterized in that,

a maximum ratio between MTF values of a lens system of the projector at spatial frequencies of the adjacent pixel element artifact pattern and MTF values of a lens system of the projector at spatial frequencies of the structured light pattern (11) is less than 0.5, in particular less than 0.25.

7. The triangulation device according to any of the previous claims,

it is characterized in that the preparation method is characterized in that,

the MTF values of the projector's lens system at the spatial frequencies of the adjacent pixel element artifact pattern are less than half the MTF values of the projector's lens system at the spatial frequencies of the structured light pattern (11).

8. The triangulation device according to any of the claims 5 to 7,

it is characterized in that the preparation method is characterized in that,

the MTF value relates to a point along an optical axis of a lens system of the projector, wherein the point is within a predetermined measurement range.

9. The triangulation device according to any of the previous claims,

it is characterized in that the preparation method is characterized in that,

the MTF (16) remains substantially constant over the entire (1) projected structured light pattern (11), wherein two arbitrary points on the measurement object (12) onto which the structured light pattern (11) is projected have substantially equal MTF curves (16), said arbitrary points being located on the portion of the surface of the measurement object (12) illuminated by the structured light pattern (11).

10. The triangulation device according to any of the previous claims,

it is characterized in that the preparation method is characterized in that,

the lens system of the projector (8) is focused with respect to a predetermined measurement range.

11. The triangulation device according to claim 10,

it is characterized in that the preparation method is characterized in that,

the surface of the measurement object (12) projected by the structured light pattern (11) is located within the predetermined measurement range.

12. The triangulation device according to any of the previous claims,

it is characterized in that the preparation method is characterized in that,

the projector (8) is implemented as a digital light processing projector using a digital micromirror device DMD, or as an LCD projector, or as an OLED projector, the artefact area (2, 3) between two adjacent pixel elements having an extent of at most 15%, in particular at most 10%, of the width of the individual pixel elements.

13. The triangulation device according to any of the previous claims,

it is characterized in that the preparation method is characterized in that,

the triangulation device comprises a further camera comprising a lens system and an imaging sensor, and the camera and the further camera of the triangulation device are symmetrically arranged around the projector, wherein the processing unit is configured to provide distance information by evaluating imaging information provided by the camera and the further camera.

14. The triangulation device according to any of the previous claims,

it is characterized in that the preparation method is characterized in that,

the structured light pattern (11) is designed to contain spatial frequencies, the average of which or in particular the maximum of which is smaller than the minimum spatial frequency (18) of the adjacent pixel element artifact pattern.

15. The triangulation device according to claim 14,

it is characterized in that the preparation method is characterized in that,

the minimum spatial frequency (18) is a lower limit, in particular a maximum lower limit, of a set of spatial frequencies of the neighboring pixel element artifact pattern, and/or the spatial frequency of the structured light pattern (11) is separated from the spatial frequency of the neighboring pixel element artifact pattern.

Technical Field

The present invention relates to triangulation devices using structured light projection.

Background

Projecting a structured light pattern is a common technique for fast acquisition of 3D information about an object to be measured. In the simplest case, a triangulation system comprises a projector which projects a laser line onto an object and a camera which takes a corresponding scene. If the relative geometry between the projector and the camera is known, 3D information about the illuminated object can be inferred because the laser line incident on the object is located at the intersection of the plane defined by the projected laser line and the projector and a ray passing through the camera center of the camera and the location of the laser line on the image sensor of the camera. By sweeping a laser line across the object to be measured, 3D information about the object can be accurately captured. To speed up the entire measurement process, the single projected laser line is typically replaced by a more complex pattern that varies both temporally and spatially. Encoding the projected structured light pattern; therefore, the correspondence between the image points and the points of the projection pattern can be obtained, whereby the surface of the irradiated object can be triangulated. An additional benefit of using projected structured light is that it simplifies the corresponding problems typically encountered in more classical triangulation systems. Here, the same physical point at which the 3D position is to be obtained must be identified in at least two images. Finding a sufficient number of such corresponding points in a plurality of images is often a difficult computational problem.

In many different technical fields, it is desirable to accurately measure objects or to obtain further information about the internal composition of objects. This is particularly applicable to manufacturing industries that require precise measurement and inspection processes in order to control their product quality. Triangulation scanners are commonly used to inspect the surface of manufactured products. The principle of triangulation enables a fast and accurate scanning of a surface. Measuring systems using the principle of triangulation are disclosed, for example, in DE102004026090a1 and WO2011/000435a 1.

Projectors using Liquid Crystal Displays (LCDs) with a limited number of pixels require circuitry for accessing and controlling the individual pixels. Such circuits are arranged on the semiconductor chip close to the individual pixels. The area occupied by such circuitry on a semiconductor chip is generally not available for practical imaging purposes, resulting in visual artifacts in the projected image, since the lens system of the projector is generally sufficient to resolve the circuit area. In the case of a digital light processing projector, the micromirrors need to be located at a sufficient distance from each other so that each micromirror can be tilted. For metrology purposes, circuit or micromirror artifacts are problematic because such visual artifacts may lead to erroneous results in the case of 3D information to be calculated. This is especially true for neighboring pixel elements with strong gradients (where one pixel is bright and the other pixel is dark). Circuit or micro-mirror artifacts between two pixel elements can shift the local intensity centroid. In the case of a projector transmitting a structured light pattern, erroneous inferences may be made due to the presence of electrical circuitry or micro-mirror artifacts in the projected image.

Disclosure of Invention

It is therefore an object of the present invention to provide a triangulation device that reduces circuit or micromirror vision artifacts in the projected image.

This object is achieved by realizing at least part of the features of the independent claims. Features which in an alternative or advantageous manner further develop the invention are described in the dependent patent claims.

The present invention relates to a triangulation device for measuring a measurement object by projecting a structured light pattern onto the measurement object, the triangulation device comprising: a projector configured to project the structured light pattern onto the measurement object with projection light, the projection light comprising at least one wavelength λ, the structured light pattern being resolvable into different spatial frequencies, the projector comprising: a matrix of pixel elements, wherein an artifact pattern is defined by artifact regions between adjacent pixel elements, toAnd a lens system, wherein the lens system is arranged such that the projected light passing through the lens system defines a wavefront having a wavefront aberration relative to a spherical reference wavefront, and a camera comprising a lens system and an imaging sensor, wherein the camera is configured to receive the structured light pattern projected onto the measurement object by the projector; and a processing unit configured to provide distance information by evaluating imaging information provided by the camera, wherein the wavefront aberration is decomposed by a primary spherical aberration coefficient Z of Zernike₉Defining (in particular controlling), wherein Zernike polynomials defining the Zernike decomposition are ordered according to a Fringe Zernike coefficient ordering, wherein the primary spherical aberration coefficients Z₉Greater than one quarter of the wavelength lambda.

It will be appreciated that the wavefront aberration is generally not limited solely by the aberration coefficient Z₉And other aberration coefficients may also affect the shape of the wavefront aberration.

The spherical reference wavefront and the wavefront defined by the lens system of the projector may be defined at the exit pupil (exit pupil) of the lens system. The spherical reference wavefront at the exit pupil of the lens system can characterize an ideal lens because the spherical reference wavefront can be sharply focused at a point on the image plane. Characterizing wavefront aberrations by Zernike decomposition can compactly capture the relevant behavior of the lens system. The so-called "Fringe Zernike" notation is known in both lens design programs and interferometry. The coefficients of the Zernike polynomials are usually specified in units of "waves", which refer to the wavelength of the light used. Zernike polynomials represent typical wavefront aberrations. Item number 9 describes the "primary spherical aberration".

Lens systems are typically designed to achieve a maximum Modulation Transfer Function (MTF) under given constraints, i.e., the lens system is optimized for image quality. To achieve an ideal MTF, the aberrations can be combined together to compensate for each other to some extent, which is referred to as aberration balancing. For example, defocus can be used to balance spherical aberration to improve MTF. Another classical balancing task is the combination of astigmatism, field curvature and defocus to find the best compromise of MTF over the entire field of view. For larger field angles, astigmatism and curvature of field are worse, so defocusing will reduce the MTF in the center of the field, but for larger field angles, the curvature of field and astigmatism are at least partially compensated. In addition, manufacturing tolerances degrade the MTF by increasing aberrations due to detuning balance of aberrations and asymmetries caused by lens element tilt and decentration. In fact, many practical lenses have significant residual aberrations even if they are properly optimized and provide sufficient MTF for the application.

For small F-number lenses, spherical aberration is more difficult to correct. Lenses with F-numbers such as 2.8 or less and optimized for image quality tend to have more residual spherical aberration, e.g., the value of the fringe Zernike term number 9 (primary spherical aberration) is typically 0.1 wave. A lens with an F-number of 5.6 optimized for image quality may have a fringe Zernike term number 9 of 0.01 waves or less due to its smaller aperture. These values are exemplary, as they also strongly depend on the effort taken by the correction lens, on the requirements and on cost targets. Lenses with smaller F numbers (such as 2.8 or less) may also have 0.01 wave or less residuals for Fringe Zernike term 9, but such lenses require more effort to achieve smaller residuals, i.e., they require more lens elements, tighter manufacturing tolerances, and are therefore more expensive.

For modern cameras or consumer projectors optimized for image quality of 200 ten thousand or more pixels, the Fringe Zernike term number 9 is typically 0.1 wave or less, and is typically significantly smaller. For a Full-HD DLP projector with a pixel pitch of 5.4 μm, the Fringe Zernike term number 9 of the projection lens may be required to be 0.1 wave or less for projecting Full-HD images with a high definition impression.

The triangulation device according to the invention comprises a projection lens with wavefront aberrations, characterized in that the Fringe Zernike term number 9 is a quarter of the wavelength λ or higher allowing both a sufficient MTF contrast at the spatial frequencies of the projected sinusoidal pattern and a low MTF contrast at the spatial frequencies of the artefact pattern.

In one implementationIn the mode, the primary spherical aberration coefficient Z₉Greater than one third of the wavelength λ or greater than one half of the wavelength λ.

In another embodiment, the triangulation device may additionally comprise a scattering disk (dispersion disk).

In another embodiment, the primary spherical aberration provides a low pass filter.

In another embodiment, the structured light pattern is implemented as a sinusoidal pattern.

The sinusoidal pattern may be designed in such a way that it contains mainly lower spatial frequencies.

In another embodiment, a Modulation Transfer Function (MTF) of a lens system of the projector is closer to a diffraction limit MTF value than to zero at spatial frequencies of the structured light pattern and closer to zero than to a diffraction MTF value at spatial frequencies of adjacent pixel element artifact patterns.

The diffractive MTF curve corresponds to the best resolution that can in principle be achieved by the projector, which can be achieved by using a perfect lens system. According to the present invention, a structured light pattern can be projected with high contrast while an artifact pattern can be suppressed. By specifying that the MTF value of the lens system of the projector is closer to the diffractive MTF value than to zero at spatial frequencies of the structured light pattern, the structured light pattern can be transmitted through the lens system of the projector sufficiently strongly. Equivalently, the specified conditions may ensure that the artifact pattern is suppressed in a stronger manner than the structured-light pattern.

In another embodiment, a maximum ratio between MTF values of a lens system of the projector at spatial frequencies of the adjacent pixel element artifact pattern and MTF values of a lens system of the projector at spatial frequencies of the structured light pattern is less than 0.5, in particular less than 0.25.

Both the adjacent pixel element artifact pattern and the structured light pattern may include a plurality of spatial frequencies. The ratio can be taken between the value of the MTF curve evaluated at any spatial frequency of the adjacent pixel element artifact pattern and the value of the MTF curve evaluated at any spatial frequency of the structured light pattern. From all such possible ratios, the maximum ratio may be less than 0.5, in particular less than 0.25.

In another embodiment, said MTF values of said projector's lens system at spatial frequencies of said adjacent pixel element artifact pattern are less than half of said MTF values of said projector's lens system at spatial frequencies of said structured light pattern.

In another embodiment, the MTF value is related to a point along an optical axis of a lens system of the projector, wherein the point is within a predetermined measurement range.

In another embodiment, the MTF remains substantially constant (preserve) throughout the projected structured light pattern, wherein two arbitrary points on the measurement object onto which the structured light pattern is projected have substantially equal MTF curves, said arbitrary points being located on the portion of the surface of the measurement object illuminated by the structured light pattern.

The expected MTF curve characterizes the optical behavior of the projector throughout the projector field of view.

In another embodiment, the lens system of the projector is focused with respect to a predetermined measurement range.

The triangulation device is configured to operate within a predetermined measurement range. The structured light pattern projected by the projector onto the surface of the measurement object is sharp (sharp) or substantially sharp, provided that the lens system of the projector is in its first state or second state and is within a predetermined measurement range.

In another embodiment, the surface of the measurement object onto which the structured light pattern is projected is located within the predetermined measurement range.

In a further embodiment, the projector is implemented as a digital light processing projector, the artifact region between two adjacent pixel elements being of the order of at most 15%, in particular at most 10%, of the width of an individual pixel element.

In general, the invention relates to any projector having a triangulation arrangement of light emitting pixel elements.

In another embodiment, the triangulation device comprises a further camera comprising a lens system and an imaging sensor, and the camera and the further camera of the triangulation device are symmetrically arranged around the projector, wherein the processing unit is configured to provide the distance information by evaluating imaging information provided by the camera and the further camera.

Triangulation devices can evaluate the images provided by the two cameras, potentially improving the estimation of distance information. The two cameras may also be arranged asymmetrically around the projector.

In another embodiment, the structured light pattern is designed to contain spatial frequencies whose maxima are less than the minimum spatial frequencies of the adjacent pixel element artifact patterns.

If the average of the spatial frequencies present in the structured light pattern is smaller than a certain predetermined number (minimum spatial frequency) and if all spatial frequencies of the artifact pattern are equal to or larger than said predetermined number, it is guaranteed that some spatial frequencies present in the structured light pattern are not present in the artifact pattern. Thus, the additional filtering step, which is realized by the additional optical aberrations, can in principle partially separate the structured light pattern from the artefact pattern by reducing the contrast of the artefact pattern more strongly than the contrast of the structured light pattern.

In another embodiment, the minimum spatial frequency is a lower limit, in particular a maximum lower limit, for a set of spatial frequencies of neighboring pixel element artifact patterns.

Any number known to be less than all spatial frequencies of the artifact pattern may be selected as the minimum spatial frequency. However, if it is to be ensured that spatial frequencies larger than the minimum spatial frequency are sufficiently suppressed, the lower the selected minimum spatial frequency, the faster the desired MTF curve must also drop. Therefore, a conservative estimation of the minimum spatial frequency places a greater burden on the design of the lens system. The maximum lower bound is the best possible estimate when there is a spatial frequency of the artifact pattern equal to the maximum lower bound and there is no spatial frequency less than the maximum lower bound.

In another embodiment, the spatial frequency of the structured light pattern is separated from the spatial frequency of the adjacent pixel element artifact pattern.

In the case where the spatial frequencies of the structured light pattern and the artefact pattern are separated, perfect filtering will be able to separate the two patterns.

Drawings

The system of the invention is described in more detail below by way of example only, with the aid of specific exemplary embodiments which are schematically illustrated in the drawings, and further advantages of the invention are examined. In the drawings, like elements are labeled with like reference numerals. In detail:

fig. 1 shows an illustration of the effect of a lens system using a projector with wavefront aberration according to the invention on a projected structured light pattern, wherein the structured light pattern is projected onto a measurement object;

FIG. 2 shows an example of a triangulation device used; and

fig. 3 shows an illustration of the Modulation Transfer Function (MTF) and the MTF of a lens system designed for image quality according to the present invention.

Detailed Description

Fig. 1 shows an illustration of the effect of a lens system using a projector with wavefront aberration according to the invention on a projected structured light pattern, which is projected onto a measurement object.

Fig. 1 (a) shows a projected pattern, which is projected onto a flat surface. The plane onto which the pattern is projected is in focus, i.e. the pattern is clearly projected. In addition, it is assumed in (a) in fig. 1 that the lens system of the projector is optimized for image quality; the lens system is optimized in such a way that optical aberration errors are minimized. Since the projector comprises a limited number of pixel elements and the magnification of the lens system is limited, the projection matrix of pixel elements occupies a compact area on the flat surface 1.

Between the pixel elements 4, 5 there are typically regions 2, 3 that are not illuminated. This area is reserved, for example, for circuit paths for accessing the individual pixel elements of the semiconductor chip. This area may also be needed to enable the micromirror to tilt. Since the areas 2, 3 between the pixel elements are not illuminated, they look like a fine grid on a flat surface onto which the projector is projected. In fig. 1, a grid for a rectangular arrangement of pixel elements is shown. However, similar considerations apply to more general pixel element arrangements, such as hexagonal meshes.

In case the projected pattern is imaged by a high resolution camera, the fine grid corresponding to the areas 2, 3 may lead to visible artefacts. The automatic processing of the recorded image is affected by such visual artifacts, since in case of bright/dark transitions between adjacent pixels, the local intensity maximum of the projected pattern may shift between said adjacent pixels.

In fig. 1 (b), the same pattern as in fig. 1 (a) is projected onto a flat surface using a lens system of a projector designed according to the present invention. The lens system according to the invention low-pass filters the entire projected image. Sharp transitions are strongly suppressed while gradual transitions in the image are effectively preserved by the increased optical aberrations.

In case the projected structured light pattern is characterized by low spatial frequencies and the artefact grid (adjacent pixel element artefact pattern) is characterized by high spatial frequencies, the lens system of the projector according to the invention preserves the structured light pattern while substantially suppressing the artefact grid. The artifact grid 3 in (b) in fig. 1 is suppressed compared to the artifact grid 2 in (a) in fig. 1. However, the structured light pattern is also partially blurred, so that information from the pixel element 5 is moved to the other pixel elements 6, 7. The intensity of this blurring effect at any point on the surface of the projection image of the measurement object depends on the intensity of all pixel elements in its vicinity. Thus, the projected structured light pattern is better visible in the projected image than in the artefact grid. Thus, since the desired structured-light pattern can be extracted with less error, further processing benefits from partial suppression of the artifact grid.

Fig. 2 shows an illustration of a triangulation device comprising a projector 8 and a camera configured to project a structured light pattern.

In fig. 2, the projector emits a single light ray. The incident light ray 11 is generally curved when incident on the surface of the measurement object 12. The camera, which is mathematically characterized by a camera center 9 and an image plane 10, receives an image of the projected structured light pattern, which is implemented as a line in fig. 2. However, instead of lines, more general structured light patterns may be used.

The information received by the camera and the geometric a priori information about the relative position of the camera with respect to the projector 8 can then be used immediately (at once) to infer the distance from the triangulation device to the entire projection line. The 3D position of the projection line is located at the intersection of the triangle 13 associated with the projector 8 and the projection ray and a ray passing through the camera center 9 and the point in the image plane 10 to which the projection ray is imaged. The distance information may be extracted by using a processing unit operating on the image received by the camera. The processing unit may first identify the location of the line in the image received by the camera and may then use this information to infer the distance of the entire projected line to the triangulation device at once.

The lens system of the projector 8 is deliberately designed in this way according to the invention to propagate mainly the structured light pattern and strongly suppress the artefact grid.

Although the structured light pattern shown in fig. 2 is a simple light ray, requiring a light ray to be swept across the entire measurement object in the case where complete 3D information is desired, more complex structured light patterns can also be designed, allowing for the capture of the entire surface of the measurement object at once. Such complex structured-light patterns may be designed according to the invention in such a way that their spatial frequencies are well separated from the spatial frequencies of the artifact grid, as this allows the filtering step to remove one and leave the other. The corresponding filtering step is performed by designing the lens system of the projector according to the invention.

One example of a complex structured light pattern is a multi-frequency sinusoid consisting of sinusoidal patterns with different frequencies within a narrow frequency band, where each frequency is projected several times with different phase shifts. Another example of a complex structured light pattern is obtained by combining binary gray codes with sinusoidal phase shifts, where a single sinusoidal pattern (with a small amount of phase shift) for fine measurements is combined with a set of coarse binary patterns that extend the non-ambiguous range by providing a code that enumerates sinusoidal periods.

Fig. 3 shows an illustration of the Modulation Transfer Function (MTF) and the MTF of a lens system designed for image quality according to the present invention. All curves in fig. 3 relate to the same point in the projector field of view.

Assuming a perfect lens system, the best possible MTF curve is given by the diffractive MTF curve 14. The lens system of a projector optimized for image quality according to the prior art typically has an MTF curve 15, which MTF curve 15 approaches the diffractive MTF curve 14 at low spatial frequencies and deviates from the diffractive MTF curve 14 at higher spatial frequencies.

Designing the lens system of the projector according to the invention changes the MTF curve, wherein the new MTF curve 16 shows a stronger suppression at higher spatial frequencies than the MTF curve 15 of the lens system designed for image quality. In case the spatial frequency 17 of the structured light pattern is sufficiently low, the lens system according to the invention and the lens system optimized for image quality have similar MTF values at the spatial frequency 17 of the structured light pattern, whereas the minimum spatial frequency 18 of the artefact grid as well as higher spatial frequencies are more strongly suppressed by the lens system according to the invention than by a lens system designed for image quality. Thus, the lens system according to the invention preserves contrast mainly at low spatial frequencies, while effectively reducing high frequency artifacts.

It goes without saying that these illustrated figures are only schematic representations of possible exemplary embodiments.

Although the invention has been illustrated above, in part, with reference to certain preferred embodiments, it must be understood that various modifications and combinations of the different features of the embodiments can be made. All such modifications are intended to be within the scope of the appended claims.