Abstract:
The invention is based on an autofocus method in which light from a light source is focused at a measurement light focus in a sample and is reflected from there and the reflected light is guided through an optical system in two light paths onto at least two detector elements. In order to achieve fast and accurate automatic focusing on the sample, it is proposed that the measurement light focus is moved in layers of the sample which reflect light to different extents, and the detector elements are arranged in such a way that, in this case, profiles of a radiation property registered by the detector elements are different and a focus position is set in a manner dependent on the profiles.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The application is a divisional of co-pending U.S. patent application Ser. No. 13/255,827, filed Sep. 9, 2011, which claims benefit from U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/IB2010/000518, filed Mar. 11, 2010, entitled AUTOFOCUS METHOD AND AUTOFOCUS DEVICE and incorporated herein by reference. 
     
    
       [0002]    The application relates to an autofocus method, in which light from a light source is focused at a measurement light focus in a sample and is reflected from there and the reflected light is guided through an optical system and at least one aperture in two light paths onto at least two detector elements. 
       PRIOR ART 
       [0003]    Two methods are known for automatically focusing microscopes at a sample:
       The position of a sample or the distance of the sample from a reference point is measured, in that light reflected from the sample is examined interferometrically for patterns, intensity or the like.   The images of the sample are examined for contrast, resolution, autocorrelation or phase contrast.       
 
         [0006]    In microscopy, a sample is normally made up of a sample material to be analyzed, which is applied to a transparent specimen slide and is covered with a thin transparent cover slip. A position measurement of the sample material frequently leads to measuring the position of one of the reflection planes on the layer interface of the sample. Since a reflection at the air/cover slip interface layer is much stronger than a reflection at an interface layer on the sample material, the air/cover slip reflection typically outshines the reflection at an interface layer on the sample material, which is more suitable for an autofocus. 
         [0007]    Measuring the position of a strong reflecting layer above or below the sample and drawing a conclusion about the position of the sample material from the thickness of the sample, which is arranged at a known distance from the reflecting layer is known from U.S. Pat. No. 6,130,745. Typically, however, when using high-resolution systems in the case of the described sample, the tolerances in the layer thickness (e.g., of the cover slip or of specimen slide) are greater than the depth of field of the imaging system and a focusing cannot always be guaranteed with such a method. 
         [0008]    The object of the invention is disclosing an autofocus method with which an optical system, e.g., a microscope, can be adjusted in terms of focus quickly and precisely on a reflecting layer of a sample. 
       Attainment of the Object 
       [0009]    This object is attained by a method of the type cited at the outset, in which, according to the invention, the measurement light focus is moved in layers of the sample which reflect light to different extents and the detector elements are arranged in such a way that, in this case, profiles of a radiation property registered by the detector elements are different from each other and a focus position is set in a manner dependent on the profiles. Due to the different profiles of the registered radiation profiles, the position of an especially distinguished layer in the sample, e.g., a reflecting interface, may be found and focused thereon or on a target focus plane arranged at a known distance therefrom. 
         [0010]    Surfaces, e.g., interfaces, are also to be considered layers in the following. One of the layers is advantageously an interface. The light paths are expediently separated from one another at least partially, in particular they are separated from one another in the optical system. The separation is accomplished expediently by a shaded area between the light paths. The shading can be produced by a diaphragm. 
         [0011]    The profile may be detected by punctiform measurements at several positions of the measurement light focus, separated expediently by light paths. The radiation property of the reflected light may be the radiation intensity. The adjusted focus position is a desired focus position in which the optical system is arranged expediently in relation to the sample in such a way that an image acquisition of the sample produces the desired images. 
         [0012]    In addition, because of the invention, the optical path length of the light paths can be specified. The optical path length in this case may be measured from the sample to a diaphragm in front of a detector. The optical path lengths of light paths are expediently selected to be different. A deviation signal to a selected reflecting/scattering sample structure can be generated hereby through the separate analysis of the optical path lengths of the light paths and focused thereon or on a target focus plane arranged at a known distance therefrom. A layer reflecting light may be a reflecting and/or scattering sample structure and may be in particular an interface layer, especially an interface layer or interface adjacent to the sample material. 
         [0013]    The autofocus method is a method for automatically focusing the optical system on a desired focus position or target focus plane, e.g., within the sample. If focusing is on the target focus plane, the optical system can form a sharp image of an object arranged there, in an image plane in which a detector or a camera is expediently arranged. After the autofocusing, the sample may be depicted with the aid of a camera. 
         [0014]    The sample may be sample material prepared for examination, a slide to which it is applied, and a cover slip, which covers it. A layer structure that is transparent for the autofocus light, on whose layer interfaces a reflection or scattering of the autofocus light is incident, is likewise suitable. The sample does not have to have any transparency for the autofocus light after the layer intended for focusing. The reflection/scattering at an interface layer, which is described here, may also be caused by the reflecting/scattering particle layer or defective layer in the material. The layer interfaces may be pretreated (e.g., mirrored) in order to increase the signals for the autofocus system. 
         [0015]    The target focus plane is that plane within the sample on which the optical system is supposed to focus or from which the position of the desired focus is supposed to be removed at a predetermined distance. The target focus plane is expediently a reflection plane on which incident light is reflected. It may be an interface layer within the sample, e.g., the plane of a glass/sample material interface. Similarly, the scattering on the sample itself could be utilized. 
         [0016]    The light guided to the light paths expediently originates from a common light source, wherein not just the originally radiating material but also a reflective layer, an aperture or the like is designated as the light source. A one-dimensional or two-dimensional light source is typically used. The two light paths are expediently formed symmetrically to each other and are arranged in particular symmetrically to the optical axis of the optical system. 
         [0017]    The light source, which in particular is punctiform or linear or strip-shaped or is made up of several points of light, is focused at the measurement light focus in the sample by the optics. It may be depicted in this way in the sample. The measurement light focus is normally punctiform, but depending on the shape of the light source may alternatively be one-dimensional or two-dimensional and e.g., include several points. The measurement light focus is situated advantageously in the focus or near to the focus of the optical system being focused. The focus of the optical system may be a focus plane. The optical system forms a sharp image in an image plane of an object lying in the focus of the optical system. It is also possible for the measurement light focus to be located at a preset distance from the focus of the optical system. Through this, the measurement light focus may be adjusted on a reflection plane, e.g., an interface layer of the cover slip/sample material, wherein the focus of the optical system e.g., is located 20 μm away from the interface layer in the sample material. 
         [0018]    A portion of the light incident on the sample is reflected. In the following, a reflection and/or a scattering may be understood as “reflected.” The layer reflecting the light may be a reflecting and/or scattering layer. When we speak of reflection in the following, a scattering is meant to be included therein. 
         [0019]    The two light paths are expediently guided symmetrically around the optical axis of the optical system. They advantageously strike the sample in different directions so that their reflections are radiated in different directions and thus may be easily analyzed separate from one another. It facilitates the detection of the individual layers in a layer structure, if the angle of the incident light paths is selected in such a way that reflections of adjacent layers do not overlap one another. If a scattering layer is used to determine the focus position, then the separation of the light paths should first occur in the detection path. 
         [0020]    The light of the autofocus system advantageously has a different frequency than light, which may be used to examine or form an image of the sample. The light property is expediently the light intensity. 
         [0021]    The optical system may be that of a microscope. It has an optical axis, which is normally aligned perpendicularly to a sample plane in which the sample extends. 
         [0022]    The light paths between the light source and the reflection layer or between the reflection layer and the detector may be designed as illumination paths or detection paths. An autofocus light path therefore is made up of an illumination path and a detection path. The difference in the optical path length may now be generated in both the illumination path and the detection path as well as in both (sic) paths. A realization of the detection path is described in the following. 
         [0023]    The measurement of the optical path length of the paths takes place by at least one, in particular respectively a diaphragm in front of the detectors. Because of a wavelength-dependent position of the light paths at the diaphragm, it is possible to draw conclusions about the optical path length of the system. A possible realization is described in the following: 
         [0024]    The detector elements are arranged in such a way that e.g., relative to an element of the optical system, e.g., relative to a diaphragm, profiles of a radiation property registered by the detector elements are different from one another. The element of the optical system may be a diaphragm, e.g., directly in front of the detector elements, a beam splitter, a mirror or another suitable element. 
         [0025]    If light from a light path is reflected on two stacked layers of the sample, then the light path or the optical path length of the light from the one layer, e.g., to the detector or a diaphragm in front of the detector, is longer than the light path or the optical path length from the other layer. Because of this, the two light paths from the two layers, e.g., to the detector elements, may be different. The light paths expediently run so that they are blocked in different ways at a diaphragm in front of the detector elements, e.g., if the one light path is blocked completely or partially and the other is blocked partially or not at all. In this way, it is possible for the light paths to be detected individually and without another light path from another layer. 
         [0026]    The main reflection from a glass/air interface layer above the sample is expediently masked by a diaphragm in front of the detector elements while the measurement light focus is moved through layers underneath which reflect light to different extents. Through this, light from these layers may pass through the diaphragm. In this way, layers that reflect considerably weaker than the glass/air interface layer may be detected. 
         [0027]    The diaphragm or its aperture is advantageously arranged in an image plane of the optical system, i.e., in a plane in which an object focused by the optical system is projected. The aperture may be an image of the light source. 
         [0028]    The light which is reflected from the measurement light focus is expediently projected in the plane of the diaphragm in accordance with the shape of the measurement light focus. The diaphragm is advantageously arranged such that it allows light from both light paths reflected in the measurement light focus to pass through, in particular to an equal extent. Through this the diaphragm expediently blocks light, which was reflected above and below the measurement light focus completely or asymmetrically with respect to the two light paths. 
         [0029]    The aperture is expediently arranged not as usual around the optical axis of the optical system rather asymmetrically to the optical axis, in particular asymmetrically to the optical axis of the two light paths at the location of the aperture. In particular, it is arranged completely outside of the optical axis. Through this it is possible to achieve in a simple manner a selection of the one or other light path for a separate analysis at different positions of the measurement light focus. 
         [0030]    An especially precise focusing may be achieved if the profiles are detected continuously. 
         [0031]    In an advantageous embodiment of the invention, a focus of the optical system is adjusted such that the signals of the detector elements are in a fixed ratio to one another. In the case of an incidence of light on the detector elements in a fixed ratio, a position of symmetry between the light paths and thus the target focus plane may be detected in a simple way. This may be accomplished even more simply if the signals are equally strong. The difference in the path length of the paths is selected in such a way that, when the signals are superimposed, the signals overlap on a flank and therefore have a point of intersection. The signals are equally strong at this point of intersection. With the aid of a zero crossing of the differential signal, the same strength of the signals may be detected in a simple manner. 
         [0032]    Another embodiment of the invention provides for a target position of a focus of the optical system to be detected the aid of the signals of the detector elements and for the focus to be adjusted with the aid the detected target position by an actuator. The target position may be a position output by the actuator, e.g., the position at which the signals of the detector elements are equal. It is also possible to use this adjustment only as a pre-adjustment. Alternatively or e.g., as a precision adjustment, it is conceivable to reach the target position by a regulating process, wherein the detector signals are used as regulating input signals and a signal for controlling the actuator is used as a regulating output signal. 
         [0033]    A simple and reliable automatic focusing may be achieved if the detector elements are calibrated so that the strength of their signal, which is caused by light reflected by an interface layer, is the same. Through this the focus position is expediently adjusted in the reflecting layer or the layer reflecting light. Alternatively, the detector elements may be adjusted in such a way that their signal strength is different in a targeted manner, e.g., in order to achieve a targeted focus offset. 
         [0034]    A good orientation when searching for the target position of the focus or the target focus plane may be achieved if the measurement light focus is moved through the target focus plane toward a sample/air interface and the reflection of the sample/air interface is used for a rough orientation. 
         [0035]    To examine a sample it may be necessary that it be examined at different locations, e.g., if it is larger than the field of view of a microscope. To do so, after a first examination, it is moved perpendicularly to the optical axis of the optical system after and then reexamined. A quick automatic focusing after such a movement can be achieved, [in that] the signals of the detector elements after movement of the sample perpendicular to the optical axis of the optical system are checked for plausibility with respect to the rough adjustment still in effect on the target focus plane. If there is plausibility, it is possible to dispense with a time-consuming complete re-focusing. The plausibility may be a limit value in the difference of the signals, which may not be exceeded. In addition, plausibility testing may also be used for making a rough adjustment so that, if there is plausibility, only a fine adjustment still needs to be made. 
         [0036]    Another advantageous embodiment of the invention provides for the light source to have a light pattern, which is projected in the sample. The light pattern may be one-dimensional, two-dimensional or three-dimensional and is expediently projected in a plane perpendicular to the optical axis of the optical system in the sample. In this case, the reflected light is detected from several pattern points of the light pattern respectively separated by light paths. As a result, a tilting of the target focus plane, e.g., to the optical axis, can be detected from several target positions of the several pattern points. The signals generated in this way may be utilized for regulating the autofocusing. 
         [0037]    Furthermore, the invention is directed at an autofocus device with an optical system for focusing light at a measurement light focus in a sample and for guiding light reflected from there through an aperture onto at least two detector elements. 
         [0038]    It is proposed that the autofocus device include an actuator and control means for moving an element of the optical system or the sample via the actuator in such a way that the measurement light focus is moved in layers of the sample which reflect light to different extents, wherein the detector elements are arranged in such a way that, in this case, profiles of a radiation property registered by the detector elements are different and the control means is provided for evaluating the profiles at several positions of the measurement light focus. 
         [0039]    In the case of the movement of the element of the optical system relative to the sample, the actuator may move the element or the sample relative to a fixed reference point, e.g., the ground. 
         [0040]    The control means is advantageously designed to control one, several or all of the above-mentioned process steps. 
         [0041]    The autofocus device advantageously includes a measuring system which is provided to detect the distance of the element of the optical system from the sample or a distance dependent thereon, in particular in a non-optical way. As soon as the focus position is found optically, the distance may be measured with the further measuring system and maintained during illumination of the sample. 
         [0042]    Using a color camera with a color-sensitive detector is known for making color images. A color-sensitive detector is normally limited to three colors. When using a Bayer pattern, a pixel is respectively made up of one blue, one red and two green-sensitive detector cells from whose signals all intermediate colors are composed. Because of the required four detector cells per pixel, the resolution of this type of a color detector is low. 
         [0043]    A line spectrograph is known for achieving a high image resolution in conjunction with a high color resolution. An object is scanned line-by-line, wherein the image of a line is spread out spectrally, for example by a prism, so that a three-dimensional image of the line develops. In this way, a three-dimensional image is acquired and saved line-by-line and the individual images are assembled to form a three-dimensional color image. 
         [0044]    Also known is moving several color filters in succession in front of the detector and thus successively making several images of an object in different frequency ranges. The shots may be combined into a hyperspectral image. 
         [0045]    An object of the invention is disclosing a method for taking an image of an object with which high-resolution color images can be made. 
         [0046]    This object is attained by a method for taking an image of an object in which the object is projected by an optical system onto several detector elements of a detector and the image is separated with a light filter with several filter areas that filter differently into several image areas that filter differently. According to the invention, it is proposed that the differently filtered image areas of the image are projected simultaneously onto the detector, in particular side by side. It is possible to dispense with changing filters in front of the detector and it is also possible to take shots of the object in rapid succession. 
         [0047]    The light filter may be a spectral filter or a polarizing filter. It may be arranged immediately in front of the detector or directly on the detector. The light filter is expediently arranged in an image plane of the optical system, wherein 1/10 of the focal distance of the optical system is tolerable on the light filter as the distance from the mathematical image plane and can be viewed as still in the image plane. The light filter may be a cut-off filter, interference filter or absorber filter. The filter areas may be different in terms of spectral filtering so that the image areas are filtered differently spectrally. They may adopt concrete forms, for example strips, a checkerboard pattern and/or a micropattern, in which the filter areas have a length and/or width less than 100 μm. In terms of their expansion, the filter areas are expediently greater than two, in particular greater than ten, detector elements or pixels. A spatially continuously varying filter characteristic is likewise possible. 
         [0048]    The detector may be designed as a chip and is expediently sensitive in all spectral ranges of the filter areas. It may be a CCD (charge-coupled device) or a CMOS sensor (complementary metal oxide semiconductor). The detector elements are advantageously arranged as a two-dimensional lattice in the form of a matrix. The detector is designed expediently as a monochrome detector and is sensitive in the spectral range of the structured filter. 
         [0049]    The dimensions of the filter areas are advantageously adapted to the dimensions of the detector elements, for example in that a width and/or length of a filter area are respectively an integer multiple of a quantity of one of the detector elements of detector, for example m×n detector elements. The light filter may be fastened directly on the detector, for example directly on a detector chip, or be deposited directly on the sensitive areas of the chip. 
         [0050]    The filter areas correspond expediently to a structure and/or organization of the sample image or the sample or the sample receptacles. They may be as large as regular structural sections of the sample image and/or have their shape. 
         [0051]    In an advantageous embodiment of the invention, the filter areas are moved from one to the next shot of the object via an image pattern of the image so that every point of the image pattern is recorded in several light characteristics, in particular spectra. These values are expediently allocated to the point and can be depicted and/or stored. The image pattern in this case may be the entire image or a section of the image. The movement is a relative movement, wherein the filter areas may rest, e.g., relative to a detector housing, and the image pattern is moved, or vice versa. 
         [0052]    In the case of multiple shots of each point in several light characteristics, for example colors, a color image may be composed from the several shots. Due to the separation of the light filter into the filter areas, when moving the light filter, only one small movement in the size of a dimension of a single filter area is sufficient so that it is not necessary for the entire light filter to be moved away from the detector and a new light filter to be moved to the detector. Because of the short path of movement, the movement may be executed very quickly. 
         [0053]    To achieve a high light yield, it is advantageous if signal contributions in the individual detector elements within a filter area, which may be allocated with a movement of the sample image relative to the detector of a sample area, are accumulated in a value. Such accumulated values of a sample area from different filter areas may be combined into overall information about the light characteristics of the sample area. 
         [0054]    The image pattern may remain at rest at least during the movement of the filter areas to the detector, so that the filter areas are moved relative to the detector. It is also possible to move the image over the detector so that the filter areas are at rest relative to the detector. Moving the image over the detector may be accomplished with a movement of the optical system or a portion of the optical system relative to the detector. A further possibility is moving the filter and the detector relative to the optical system, which, for example, is at rest relative to a camera housing. Generally speaking, one or two of the three elements of detector, light filter and optical system may be kept at rest relative to a camera housing for example, whereas the remaining two elements or the remaining one element are/is moveable relative to the other elements. 
         [0055]    The optical system is advantageously part of a microscope, whose housing is solidly connected to a specimen stage on which the object can be moved in the form of a sample, e.g., on a moveable tray, in particular with the aid of motorized drive and a position control via a control means. 
         [0056]    The object is advantageously moved relative to the optical system and the light filter and the object is acquired in several shots respectively image-section-by-image-section, wherein the position of the filter areas in the image sections is respectively unchanged. The optical system in this case may be at rest in a housing, for example of the microscope, and the object and with it the light pattern of the object are guided past the optical system, wherein the image sections and with them the filter areas move over the entire image. 
         [0057]    If, for example, the object is elongated, e.g., in the form of a row of samples then it is possible to record the entire object through a plurality of successively recorded image areas, wherein each image point of the object was recorded in many colors or in each color or through each filter area. In this way, it is possible to make a color image of the entire object very rapidly. It is possible to dispense with moving the color filter relative to the optical system or to the detector. Because a device for taking images of samples is frequently guided via an actuator for controlled movement of samples along the recording device, for example the microscope, the recording device may be remain held hereby in an especially simple manner. 
         [0058]    The filter areas are expediently designed as strips, which extend from one side of the image to the opposite side of the image and are aligned in terms of their longitudinal direction perpendicular to the movement direction. Also an extension from one side of the image section to the opposite side of the image section is sufficient. Every image point of the object may hereby be guided over all filter areas of the light filter in an especially simple manner. 
         [0059]    The movement is advantageously such that an image point is moved by the width of a filter area from one shot to the next shot. The width is expediently respectively several pixels. A small overlapping area, e.g., corresponding to double the precision of the moved actuator, is meaningful in this case. 
         [0060]    In order to achieve an especially high resolution, in particular in the case of especially interesting image sections, it is advantageous if the movement from one image acquisition to the next image acquisition is less than one image pixel. Because of the movement in the subpixel range, a subpixel resolution can be computed. 
         [0061]    The default for the movement is advantageously specified by a control means, which in particular independently detects especially interesting image areas and triggers a subpixel movement. The movement may be carried out in different modes, e.g., a subpixel mode, a filter area-wide mode in which the movement from one shot to the next shot amounts to the width of a filter area, or a multi-pixel mode in which the movement amounts to several pixels of the detector. A control of only two of the three described modes is also possible. 
         [0062]    In an advantageous embodiment of the invention, the light filter is a cut-off filter, whose cut-off frequency varies in the spatial profile of the light filter perpendicular to the cut-off and in particular perpendicular to the movement. In this way, by controlling the movement increment from one shot to the next shot, the color resolution of an entire image can be controlled from the shots. The light filter is connected expediently to an actuator and a control means, which is used to control a movement of the light filter. 
         [0063]    In addition, it is proposed that the light filter include two cut-off filters arranged in succession in the optical path of the image, whose cut-off frequency in the spatial profile of the cut-off filters respectively perpendicular to the cut-off (and in particular perpendicular to the movement) varies with a frequency response that is opposite from one another. With a corresponding arrangement of the cut-off filters with respect to one another, a spatial transmission window can be generated hereby, which may be both spatially as well as spectrally enlarged and reduced by moving the cut-off filters to one another. As a result, this allows a high variability to be achieved in the frequency and spatial area of the recorded images. 
         [0064]    An especially good spectral adaptation of the detector and light filter may be achieved, if the detector has several detector regions that are different in terms of color sensitivity and respectively sensitive in one color area and at least one respective filter area is arranged in front of every detector region. This is advantageously adapted in terms of its color area to the color area of the detector so that the color areas of the light filter are different. The adaptation is accomplished advantageously in that the transmission of the filter area is in the sensitivity range of the corresponding detector region and is not in a color area of one of the other detector regions. 
         [0065]    The different detector regions may be arranged spatially directly side by side, for example in a cohesive matrix of detector elements, or set up spatially separated from one another so that the optical system includes one or more elements for guiding the image of the object to several detector regions, for example a dichroic mirror or the like. The detector regions are advantageously operated synchronously so that an image of the object at the detector regions is taken simultaneously in several color channels. 
         [0066]    Another advantageous embodiment of the invention provides that the filter areas have different transmission values and the transmission values are respectively adapted to a recording characteristic of the detector, in particular to achieve a uniform image exposure with an inconstant spectral sensitivity of the detector elements. In this way, it is possible to achieve an especially good image result. The adaptation may be effected by a different size of the filter areas. Another possibility is adapting a different frequency transmission width of the filter areas to the detector. Thus, a frequency transmission width may be greater in a frequency range in which the detector is less sensitive, and less in a frequency range in which the detector is more sensitive. 
         [0067]    In addition, it is also possible to adapt the transmission strength, i.e., a damping of the filter area, to the detector so that a higher damping is selected in a frequency range in which the detector is more sensitive than in other frequency ranges. 
         [0068]    A high image quality may likewise be achieved if the filter areas have different transmission values and a triggering of the detector elements is adapted to a transmission value of the filter area shading it. Thus, an amplification can be boosted, an integration time can be lengthened or pixels can be consolidated, if a filter area has a high damping as compared with another filter area. This makes it possible to achieve a uniform exposure of the image in all frequency ranges. If the transmission value of a filter area is especially high, it is also possible to read out only every second pixel. 
         [0069]    The different triggering of the detector elements advantageously follows a movement of the filter areas over the detector. If, for example, the light filter is moved over the detector, this movement may be detected so that each filter area may be allocated the detector elements that are covered by the area. Control of the detector elements can be adapted hereby pixel-by-pixel to the respectively allocated filter area. 
         [0070]    Furthermore, the invention is directed at a device for taking an image of an object with a detector, which has several detector elements, an optical system for projecting the object onto the detector and a light filter with several filter areas that filter differently. 
         [0071]    Especially high-resolution color images can be made, if the light filter is arranged so that several image areas of the image of the object are projected simultaneously onto the detector; these image areas are filtered through the filter areas differently. 
         [0072]    The device includes a control means, which is advantageously provided to control one, several or all of the above-mentioned process steps. 
         [0073]    The invention will be explained in more detail on the basis of exemplary embodiments, which are depicted in the drawings. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0074]      FIG. 1  is a schematic representation of a microscope with an autofocus device. 
           [0075]      FIG. 2  is a schematic representation of an optical path or illumination paths of the autofocus device on a sample. 
           [0076]      FIG. 3 - FIG. 6  are reflection optical paths or detection paths of the sample on two detector elements. 
           [0077]      FIG. 7  is a diagram of signals of the detector elements and a modified operating distance plotted over time. 
           [0078]      FIG. 8  is a schematic diagram of the signal with a differential signal. 
           [0079]      FIG. 9  is a projection of a point of light on a moved oblique sample. 
           [0080]      FIG. 10  is a projection of a light source pattern on a resting oblique sample. 
           [0081]      FIG. 11  is a separation of an optical path by a semi-transparent mirror. 
           [0082]      FIG. 12  is a separation of an optical path by a dichroic mirror. 
           [0083]      FIG. 13  is a schematic representation of a microscope directed at a sample with a camera having a light filter on a detector. 
           [0084]      FIG. 14  is an optical path of a sample onto three detectors. 
           [0085]      FIG. 15 - FIG. 18  are a detector with a light filter in four different positions with respect to a sample. 
           [0086]      FIG. 19  is a diagram of an cut-off filter with a continuously migrating cut-off in the filter surface. 
           [0087]      FIG. 20  is a transmission diagram resulting from two cut-off filters arranged in succession. 
           [0088]      FIG. 21  is a sensitivity diagram of a detector. 
       
    
    
     DETAILED DESCRIPTION 
       [0089]      FIG. 1  depicts an autofocus device  2  which is integrated into an optical imaging system  4 . The optical imaging system in this special embodiment is a microscope for fluorescence analysis of biological material in a sample  6 . For this, the optical imaging system  4  includes an image detector  8  or a camera, which is connected to a control means  10  for acquisition control and saving of images taken, or an eyepiece for observing the sample directly. The control means  10  is part of both the optical imaging systems  4  as well as the autofocus device  2  and is used to control the autofocus methods described in the following. 
         [0090]    The autofocus device  2  includes a light source  12 , which makes light available for the autofocus method. It may also provide the light for the fluorescence analysis, wherein, as a rule, it is more expedient for the optical imaging system  4  to have another light source (not shown) for this. The light source  12  has a light generator  14 , e.g., a LED (light emitting diode), and optics  16  for shaping the radiated light, which may include a light diffuser. A diaphragm  18  with an opening pattern generates a one-dimensional or two-dimensional light source pattern, which is expediently symmetrical to an optical axis  20  of an optical system  22 , which may include additional optical elements  24  and an objective  26  of the optical imaging system  4  besides the optics  16 . A spatially defined light source may also replace the elements  16  and  18 . A means  28 , which amounts to an aperture, separates the illumination of the sample  6  from the light source  12  into several light paths, which run separated from one another from the means  28  to the sample  6  and are brought to a common measurement light focus (illumination paths) in the sample  6 . The means  28  may alternatively be attached in the detection path (see below) between elements  30  and  46 , in particular when focusing on scattering objects. 
         [0091]    Light from the light source  12  is directed to the objective  26  of the optical imaging system  4  via two beam splitters  30 ,  32  in the form of dichroic or semi-transparent mirrors; the optical imaging system is mounted in a microscope housing  34  and focuses the light on the sample  6 . To do so, the objective  26  has an optical element  36 , e.g., a lens, which is movable in a controlled manner along the optical axis  20  of the objective  26  by means of an actuator  38 . Controlling the position of the optical element  36  and therefore of the focus in the sample  6  is accomplished by the control means  10 . The actuator itself may include an independent distance meter. 
         [0092]    Light reflected from the sample  6  passes through the objective  26  in the opposite direction, as indicated by a dashed arrow, and is guided via the beam splitter  32 , on the one hand, to optics  40  and to the image detector  8  and, on the other hand, via the beam splitter  30  and additional optics  42  to a detector  44 , which includes several detector elements (detection path). The detector elements may be individual sensors, e.g., photodiodes, or a lattice of sensors. Arranged in front of the detector  44  is a diaphragm of the optical system  22  with an aperture  46 , which is shaped in accordance with the aperture of diaphragm  18  and is arranged in the image plane of the optical system  22  in which an image of the sample  6  is generated and therefore of the light source pattern projected on the sample  6 . The diaphragm opening  46  may include one or several openings and is designated in the following only as aperture  46 . The detector  44  supplies its signals to the control means  10 , which evaluates them and uses them as a control or regulation input for controlling the actuator  38 . In addition, the control means may process the independent distance signal of the actuator  38  and optionally use it for regulation. 
         [0093]      FIG. 2  shows a schematic representation of an optical path (illumination path) of the autofocus device  2  in two light paths  48 ,  50  on the sample  6 . In this exemplary embodiment, the light pattern of the light source  12  is reduced to a point of light, which radiates through two openings of the means  28  for separation into the light paths  48 ,  50 . In the case of a diaphragm  18 , which generates for example two points of light, as depicted in  FIG. 1 , light from each point of light is divided into two light paths  48 ,  50 , as indicated by the means  28  in  FIG. 1 . 
         [0094]    Light from both light paths  48 ,  50  is focused at a punctiform measurement light focus  52  in the sample  6 , which may have the shape of the light source and e.g., is punctiform, is elongated corresponding to a slit-shaped light source or has another optional shape. Since both the light for the measuring light from the light source  12  and the light for examining the sample are guided through the objective  26 , the measurement light focus  52  may be in the focus of the camera or the optical imaging system  4 , which may be a focus plane. However, it is also possible for the measurement light focus  52  to be removed from a focus  56  of the camera by a pre-known distance  54 . 
         [0095]    The typical sample  6  includes a specimen slide  58  on which biological sample material  60  is applied, which is covered with a thin transparent cover slip  62 . This sample  6  reflects incident light on three interfaces  64 ,  66 ,  68 , namely the strongly reflecting air/glass interface  64 , the considerably less strong reflecting glass/sample material interface  66  and the sample material/glass interface  68  (which is not considered further in the following), wherein, in the case of very thin sample materials, the signals develop a combination from the interfaces  66  and  68 . In this case, the glass/sample material interface  66  forms a target focus plane  70  described in this first exemplary embodiment, in which the measurement light focus  52  is supposed to be guided by the autofocus method. 
         [0096]    The autofocus method carried out for this is described on the basis of  FIGS. 3-8 .  FIGS. 3-6  show the optical system  22  and the objective  26  in a very simplified way located above the sample  6 , which is indicated only on the basis of interfaces  64 ,  66 . The detector  44  is represented on the basis of two detector elements  72 ,  74 , which are arranged on both sides of the optical axis  20 . In the case of an arrangement as in  FIG. 1  with two point light sources, there would be four detector elements. The aperture  46  in front of the detector  44  expediently has the same form as the light source, i.e., punctiform or circular in this exemplary embodiment. It is arranged in such a way that it is located asymmetrically offset from the optical axis  20 , wherein the axis  20  lies outside of the aperture  46 , i.e., does not pass through it. 
         [0097]    The portions of the two light paths  48 ,  50  that are incident on the sample  6  are depicted by thin dots and are directed at the measurement light focus  52 , which is in the specimen slide  58 , i.e., beneath the target focus plane  70 , which is identical to interface  66 . The different light paths from the different interfaces  64 ,  66  to the aperture  46  or to the detector elements  72 ,  74  are depicted in different ways. The light path of the main reflection that is reflected from the strongly reflecting interface  64  is represented by solid lines and the light path of the light that is reflected by the less strongly reflecting interface  66  is represented by dashed lines. It is evident that, for one, no light or negligibly little light is reflected in the measurement light focus  52  and, secondly, the light reflected by the interfaces  64 ,  66  misses the aperture  46  so that no light from it reaches the detector elements  72 ,  74 . 
         [0098]    In  FIG. 4 , the sample  6  is moved downwards as compared to the depiction in  FIG. 3 , as indicated by arrows, so that the measurement light focus  52  was moved upwards relative to the sample  6 . Moving the sample  6  is equivalent to moving the objective  26  using the actuator  38 . In the position depicted in  FIG. 4  of the sample  6  relative to the objective  26 , the measurement light focus  52  is located just beneath the interface  66 . Because of the asymmetry of the aperture  46  to the optical axis  20 , in this position reflected light from the light path  48  passes through the aperture  46  and hits the detector element  72 , whereas light from light path  50  misses the aperture  46  so that the detector element  74  remains shaded. 
         [0099]    With a further movement of the sample  6  downward or of the measurement light focus  52  in the sample  6  upwards, the measurement light focus  52  reaches the interface layer  66  and the target focus plane  70 , as depicted in  FIG. 5 . The reflections of both light paths  48 ,  50  cross at the image plane, in which the diaphragm and the aperture  46  are arranged. Because of the asymmetrical aperture  46  outside of the optical axis  20 , both light paths  48 ,  50  are largely shaded, but not completely due to the flat aperture of the light paths  48 ,  50 . Both detector elements  72 ,  74  respectively receive a little and the same amount of light and transmit an identical signal to the control means  10 . 
         [0100]      FIG. 6  shows the light paths  48 ,  50  with an even further movement of the sample  6  downward or of the measurement light focus  52  in the sample  6  upward. The measurement light focus  52  exits the interface layer  66  and approaches the interface layer  64  so that the reflection of the interface layer  66 , which reaches only the detector element  74 , continues to be shaded and the reflection of the interface layer  64  falls ever more strongly through the aperture  46  on the detector element  72 . 
         [0101]    The aperture  46  is arranged in the image plane of the objective  26 . Light reflected from the measurement light focus  52  passes through the aperture  46  and namely expediently to an equal extent from both light paths  48 ,  50 . The aperture  46  in this case is arranged so that light which is reflected from above or below the measurement light focus  52  passes through the aperture  46  from the two light paths  48 ,  50  to an unequal extent. An equally strong illumination of the detector elements  72 ,  74  therefore means that one of the interface layers  64 ,  66  lies in the measurement light focus. The aperture in this case is advantageously only so large that light from an interface layer  64 ,  66  which is further than 100 μm away from the measurement light focus  52  cannot pass through the aperture  46  from any of the light paths  48 ,  50 . 
         [0102]    The aperture  46  makes it possible to select the light from different light paths according to the optical path length. Similarly, a selection of the light from different light paths is made possible according to their different direction[s] toward the detector elements  72 ,  74 . 
         [0103]      FIG. 7  plots the amplitudes A of the signal  76  of the detector element  72  and of the signal  78  of the detector element  74  over the time t in the case of movement of the measurement light focus  52  in the sample  6  as described in  FIGS. 3-6 . In addition, the movement of the position  80  of the measurement light focus  52  in z-direction, which is parallel to the optical axis  20  of the objective  26 , is plotted over the time t correlated to the signals  76 ,  78 . Four points in time III, IV, V, VI are marked, which correspond to the positions  80  of the measurement light focus  52  in  FIGS. 3, 4, 5 and 6 . 
         [0104]    To automatically focus the sample  6 , first of all the light generator  14  of the autofocus light source  12  is switched on and the objective  26  or its optical element  36 , which is moveable via the actuator  38  into its initial position (in the figures completely downward in the direction of the sample  6 ), moves so that the measurement light focus  52  is located within the sample  6  and there it is expediently located within the specimen slide  58 . 
         [0105]    Now the actuator  38  is moved in such a way that the measurement light focus  52  is moved completely through the sample material  60  and through the target focus plane  70 . At the same time, the signals  76 ,  78  of the detector elements  72 ,  74  are continuously recorded and a position signal of the actuator  38  is expediently recorded as well. To begin with, the signal  76  of the detector element  72  increases and then quickly drops again. Then the signal  78  of the detector element  74  increases and drops again, both according to the incidence of light through the aperture  46  as described in  FIGS. 4-6 . 
         [0106]    In particular, the position of the intersection of the flanks of the signals  76 ,  78 , called the target position in the following, is recorded, in which the measurement light focus  52  is located in the target focus plane  70 . This target position is detected by the control means  10 , which is connected to the actuator  38 , which transmits its position or that of the optical element  36  to the control means  10  continuously or at the request of the control means  10 . 
         [0107]    Again the sharp increase first of the signal  76  and then of the signal  78  over a limit value g is taken as a sign and orientation that the measurement light focus  52  is approaching the strongly reflecting interface  64  and therefore is located above the target focus plane  70 . The movement of the measurement light focus  52  upwards is stopped. 
         [0108]    Now the actuator  38  may be adjusted in a simple process step in accordance with the detected target position and the sample  6  is focused very swiftly. The measurement light focus  52  is adjusted to the target focus plane  70  and thus also the focus of the microscope  4 , when the measurement light focus  52  is located in this focus. Otherwise, the focus is adjusted to a desired plane, which is removed by a known distance from the target focus plane  70 . 
         [0109]    A more precise focusing is achieved if the movement of the measurement light focus  52  is reversed and this time the measurement light focus  52  is guided into the sample material  60  more slowly, as shown in  FIG. 7 . The maximum of the signal  76  forms again, and an adjustment of the signals  76 ,  78  to signal equality guides the measurement light focus  52  into the target focus plane  70 . 
         [0110]    An alternative method may be begun so that the measurement light focus  52  is located above the sample  6  and run into the sample  6  from there. The first incident main reflection from the glass/air interface layer  64  is clearly identified. Because the thickness of the cover slip  62  is known, e.g., 170 μm, the measurement light focus  52  may be moved downwards swiftly by this or a somewhat shorter distance. Then the movement speed can be reduced and the measurement light focus  52  moved further downwards until the signals  76 ,  78  are equally strong. 
         [0111]    A regulation to the target position based on the signals  76 ,  78  is explained in the following on the basis of  FIG. 8 . A differential signal  82  is formed from the difference between the signals  76 ,  78 , e.g., by subtracting the signals  76 ,  78 , and used as the control variable, with the zero crossing  84  as the control target value. In the zero crossing  84 , the measurement light focus  52  is located in the target position  86 . The detector  44  is advantageously calibrated for this so that the signals  75 ,  78  are equal when the measurement light focus  52  is located in the target focus plane  70 . If the measurement light focus  52  is supposed to be located somewhat outside the reflecting interface layer  66 , then an offset to a signal  76 ,  78  may be given or a signal  76 ,  78  may be more or less amplified. The zero crossing  84  hereby displaces in the z-direction. If the relation of the offset or amplification to the displacement is known, then the target focus plane  70  may be correspondingly adjusted around the interface  66  without the autofocus method described in regard to  FIGS. 7 and 8  having to be changed. The corresponding adjustment of the detector  44  may be carried out as a calibration prior to an autofocus method or during the autofocus method upon appropriate instruction by the control means  10 . 
         [0112]    After the adjustment or setting of the focus position, the light generator  14  is switched off and the focus position is regulated or maintained by means of the position signal of the actuator  38 . The advantage of this is that the autofocus light pattern is not projected with the camera during the exposure. Optionally, the light generator  14  may remain switched on continuously and regulation is carried out according to the differential signal  82 . 
         [0113]    Now, images of the sample  6  or of the sample material  60  may be recorded, if need be at several z-positions. Said positions may be approached by a corresponding control of the actuator  38 . It is also possible to reach these via a signal shift of one or both signals  76 ,  78 . 
         [0114]    To record several images of a large sample  6 , said sample is moved in the x-y direction  88 , i.e., perpendicular to the s-axis or the optical axis  20 , as indicated in  FIG. 9 . The focusing may be retained in the process. However, if the sample  6  is oblique, the measurement light focus  52  slips by a distance  90  in z-direction within the sample  6 . In order to identify this, the signals  76 ,  78  are checked for plausibility at the new x-y position. If the signals  76 ,  78  do not meet expectations, i.e., they are outside limit values, rough locating of the target focus plane  70  is initiated as described with regard to  FIG. 7 . If the signals  76 ,  78  are acceptable, it is possible to start directly with regulation, e.g., to the zero crossing  84 . 
         [0115]      FIG. 10  shows a projection of a light source pattern on a resting oblique sample  6 . On the basis of a single autofocus point of light, it is not possible to detect whether the sample  6  is oblique in relation to the optical axis  20 . However, if the measurement light focus  52  includes several focus points  92 , e.g., in that a light pattern is projected onto several focus points  92  in the sample, reflections from every focus point  92  may be analyzed separately via respectively at least two light paths, as described above. In this way, it is possible to identify that the respectively located target focus planes are not identical to the individual focus points  92 . An error signal may be output so that the sample  6  is again inserted in a straight manner into its mount. 
         [0116]      FIG. 11  and  FIG. 12  show alternative detection schemes, which use two optical paths that are not separated in the optical system  22 . In  FIG. 11  a beam is first separated in the detection path after the optical system  22  and before the detectors  72 ,  74  by means of a semi-transparent mirror  94 . Using two apertures  46 , which are arranged asymmetrically to the mirror  94  in front of the detectors  72 ,  74 , the distance signal of the slightly different paths is detected. The asymmetry is illustrated by the different distances  96 ,  98  of the apertures  46  perpendicular to the mirror  94 . 
         [0117]    In  FIG. 12  the light generator  14  emits rays with two different frequencies (λ 1 , λ 2 ), which are separated in front of the detectors  72 ,  74  by means of a dichroic mirror  200 . The distance signal is generated in turn with the aid of the apertures  46 . In this case, the apertures  46  may be arranged symmetrically to the dichroic mirror  100  if the index of refraction of the optical system  22  adequately spatially separates the light paths of the different frequencies, as depicted in  FIG. 12  by the distances of the two light paths in front of the mirror  100 . 
         [0118]    Also in the exemplary embodiments in  FIG. 11  and  FIG. 12 , a selection of the light from different light paths according to the optical path length is rendered possible by an optical means, in these cases the semi-transparent mirror  94  or dichroic mirror  100 . Similarly, a selection of the light from different light paths is made possible according to different directions towards the detector elements  72 ,  74 . 
         [0119]      FIG. 13  shows a schematic representation of e.g., an optical imaging system  102  designed as a microscope, which is directed at a sample  106  located on a specimen stage  104 . The optical imaging system  102  includes a light source  108 , whose beams of light are directed at the sample  106  in an optical path (indicated by a solid arrow  110 ) with the aid of an optical system  112  and a dichroic mirror  114 . The optical system  112  includes an objective  116 , which is movable with the aid of an actuator  18  relative to a microscope housing  120  along the optical axis  122  of the optical path for focusing the sample  106 . 
         [0120]    A ray reflected or scattered by the sample  106  is directed in an optical path (indicated with a dashed arrow) through the dichroic mirror  114  and optical elements  124  (indicated only generally) of the optical system  112  into a camera  126 , which features a detector  128  with a light filter  130 . The detector  128  includes a plurality of detector elements  132  arranged in a two-dimensional matrix, which are designed as CCD elements and attached on a chip. The light filter  130  is a spectral filter with several filter areas  134  that are different in terms of spectral filtering, which are also arranged on the chip and in the optical path directly in front of the detector elements  132 . 
         [0121]    The specimen stage  104  and along with it the sample  106  are movable with the aid of an actuator  136  perpendicularly to the optical axis  122  of the objective  116 , as indicated by arrows  138 , so that several shots of the sample  106  may be taken in different positions of the sample  106  in relation to the microscope  102 . The actuator  136  may be triggered by a control means  140  of the microscope  12  in such a way that a travel distance of the sample  106  from shot to shot may be adjusted to a predetermined value or a value calculated by the control means. The control means  140  may also be the control means  140  of the camera  126  or an additional control means of the microscope  102  outside of the camera  126 . 
         [0122]    Because of the control means  140 , an actuator  142  of the light filter  130  and/or an actuator  144  of the detector  128  may be triggered as an alternative or in addition to the actuator  136  so that the filter areas  134  and/or the detector elements  132  may be triggered and are movable relative to the optical system  112  perpendicular to the optical axis  122  of the optical path incident in the camera  126 . An image of an object of the sample  16  may migrate hereby in one or more ways via the light filter  130  and/or detector  128 . 
         [0123]    An alternative embodiment of a detector  146  with several detector regions  148 ,  150 ,  12  is shown in  FIG. 14 . The following description is limited essentially to the differences from the exemplary embodiment in  FIG. 13 , to which reference is made with respect to features and functions that remain the same. Components that are essentially unchanged are identified in principle by the same reference numbers, and features that are not mentioned are adopted without describing them again. 
         [0124]    Two dichroic beam splitters  154 ,  156  direct a ray reflected by the sample  6  divided by three spectral ranges to detector regions  148 ,  150 ,  152 . The detector regions  148 ,  150 ,  152  are respectively sensitive in only one of the spectral ranges or are more sensitive than in the other spectral ranges. Arranged respectively in front of every detector region  148 ,  150 ,  152  is a filter area  158 ,  160 ,  162 , wherein the filter areas  158 ,  160 ,  162  are only transparent in one of the spectral ranges or are more transparent than in the other spectral ranges. Their transparency is adjusted spectrally to the respective detector region  148 ,  150 ,  152  allocated to them. One or all of the filter areas  158 ,  160 ,  162  may be divided in turn into sub-areas that are different in terms of spectral filtering, as shown in  FIG. 14 . Because of the division into detector regions  148 ,  150 ,  152  that are different in terms of spectral sensitivity and that have respective filter areas  158 ,  160 ,  162  that are different in terms of spectral filtering, an especially high light yield may be achieved over a wide spectral range. 
         [0125]      FIGS. 15-18  respectively depict the image of a sample  106 , the 3×10 sample receptacles  164 , which also may represent sample surfaces; even considerably greater numbers are conceivable containing respectively the same or different sample substances. The sample receptacles  164  are arranged in a rectangular matrix and fastened on the specimen stage  104 . The to-be-examined objects  166  are located in the sample substance. The sample  106  shows an image pattern in its entirety and in its objects. 
         [0126]    The detector  128  and its 11×15 rectangular detector elements  132  are depicted with dashed lines for the sake of clarity, whereas the light filter  130  with its 5 strip-shaped filter areas  134  is depicted with solid lines. The strips of the filter areas  134  are arranged perpendicularly to the movement direction of the specimen stage, which is depicted by an arrow  138 . To better differentiate the lines, the image of the sample is depicted by dashed-and-dotted lines. 
         [0127]      FIG. 15  shows the detector  128  and the image of the sample  106  in a position relative to each other, in which an image is being made of the sample  106 , but not the first one as explained in the following. The image represents an image section having five image areas, in which twelve sample receptacles  164  including content are depicted completely and three sample receptacles  164  are depicted only partially. Three sample receptacles  164  are respectively depicted by a filter area  134  and therefore are in its spectral range. A filter area  134  depicts respectively one of five image areas of the image section. Each filter area  134  and each image area in this case overlaps precisely three detector elements  132  perpendicularly to the direction of movement of the sample  106 , expressed more generally: precisely an equal number of detector elements  132 . 
         [0128]    For a next image, the image of the sample  106  is moved further by the distance of the width of the filter areas  134 , wherein the width is viewed in the direction of the movement of the sample  106 . Now another image section of the sample  106  is taken, wherein this image section covers another sample section and other objects  166 . The position of the filter areas  134  in the image sections remains the same, but not relative to the sample sections and objects  166 . With the second image, the sample receptacles  164  that are depicted again are depicted in another spectrum, i.e., in another color. 
         [0129]      FIG. 17  shows the sample  106  offset in turn by the width of the filter areas  134  so that the sample receptacles that have now been depicted three times are depicted in three different spectra. In this manner, all areas of the sample  106  and all sample receptacles  164  are depicted at least as often as there are filter areas  134 , at least five times in the exemplary embodiment shown, so that each sample area is recorded in five spectra. A five-color image can be assembled for each sample area from these five images. In order to depict every sample area five times, in the first shot the sample  106  is only recorded by one filter area  134 , in the second shot by two filter areas  134 , etc. Therefore,  FIG. 15  shows the fifth shot of the sample  106 . 
         [0130]    The sample  106  is depicted in its entirety, in that the sample  106  is depicted image-section-by-image-section on the detector  128  and several partially overlapping images of the sample  106  and the objects  166  are made. In this case, at least as many images as there are different filter sections  134  are taken. A multi-color image of the sample  106  or of an object  166  is respectively generated, e.g., by the control means  140 , from as many overlapping images as there are different filter sections  134 . 
         [0131]    The shots are evaluated in this case by an evaluation means, which may be the control means  140 , which is connected to the detector  128  by signals. This process identifies when an object  168  is of special importance and should be depicted in high resolution. If this requirement is detected, then the sample  106  is moved from one shot to the next by only less than one pixel length, i.e., the length of a detector element  132 , as  FIG. 18  shows in comparison to the  FIG. 17 . In this case the object is moved from shot to shot subpixel-by-subpixel over a boundary between two filter areas  134 . A resolution may be achieved from the shots in the area, which was run over by the boundary, which lies in the subpixel range so that the object  168  may be depicted in especially high resolution. 
         [0132]    As an alternative to a movement of the sample  106  to the microscope  102 , the light filter  130  and/or the detector  128  may be moved relative to the sample  106  and for example relative to the microscope housing  120 . 
         [0133]    In a further embodiment, the charge of the individual detector elements within a filter area may be displaced with the image of the sample detector element-by-element and read out only after one or more displacements. Or the charges that are allocated to a sample position during the displacement of the sample image within a filter area may be assigned a pixel spectral value. In this way, the charge generated by the light may be accumulated by the sample over a longer time. 
         [0134]      FIG. 19  shows a double diagram in which the filter surface of a light filter  170  is depicted in the x-direction and y-direction. 
         [0135]    The z-direction is the direction of the optical axis  122  at the entrance to the camera  126 . In addition, the absorption A of the light filter  170  is depicted. The higher the transmission of the light filter  170 , the smaller the absorption A. In the hatched area, the absorption is ideally close to 100% and the light filter  170  is not transparent. The light filter  170  is a cut-off filter with an edge  172  with a specific wavelength λ. The wavelength λ, is a function of the position of the edge  172  in the x-direction of the filters  170 . The wavelength λ of the edge  172  is higher further to the right in the filter than further to the left. In the depicted example, the change of the wavelength of the edge per distance of the light filter is constant in the x-direction. Other relations with linear or non-linear changes are also conceivable. In the case of the light filter  170 , a great many or infinite numbers of filter areas that are different in terms of spectral filtering are located very close or infinitely close side by side. 
         [0136]    When using the light filter  170  instead of the light filter  130  in  FIGS. 15-18 , each sample area may be depicted as often as required in different spectra so that a spectral resolution of the overall image of the sample  106  is dependent on the path of movement of the sample  106  from shot to shot. In this way, the spectral resolution of the overall image may be freely selected. 
         [0137]    If two cut-off filters  174 ,  176  with an opposing edge profile are arranged in succession, as shown in  FIG. 20 , a transmission window  178  may be adjusted both in its spatial expansion Δx as well as in its spectral expansion Δλ by a movement of the cut-off filters  174 ,  176  against each other. Spectral ranges may be excluded and a spectral resolution may be adjusted. 
         [0138]    Adapting the light filter  130  to the detector  128  is shown in  FIG. 21 . In  FIG. 21  a graph of the sensitivity E of the detector  128  is plotted over the wavelength λ of the registered light. The sensitivity E is a function of the wavelength λ of the light and is less by a wavelength λ 1  than by a wavelength λ 2 . In order to achieve the most uniform possible exposure of the shots of the sample  106  over the entire relevant spectral range, the transmissive filter area  134  of the light filter  130  on wavelength λ 1  is transmissive in a greater wavelength range Δλ 1  than the transmissive filter area  134  of the light filter  130  on wavelength λ 2 , which is transmissive only in a smaller wavelength range Δλ 2 . 
         [0139]    Another possibility for achieving the most uniform possible exposure of the shots of the sample  106  over the entire relevant spectral range is undertaking an electronic adaptation of the detector elements  132  on the filter area  134  located in front of it. In the case of a less transmissive filter area  134 , a detector element  134  allocated to this filter area  134  may be triggered in a different manner than a detector element  132 , which is allocated to a higher transmissive filter area  134 . The different triggering may be achieved by a different adjustment of the gain and/or the integration time of the detector elements  312 . A pixel binning, i.e., combining two or more pixels or detector elements  132 , is conceivable, just like a subsampling, i.e., a reading out of only every n th  detector element  132 , with n=1, 2, 3, etc. The corresponding control may be undertaken by the control means  140 . 
         [0140]    In an especially advantageous exemplary embodiment, in the case of the electronic adjustment of the detector elements  132 , a displacement of the filter areas  134  in front of the detector elements  132  is taken into consideration. To this end, the position of the light filter  130  in relation to the detector  128  must be known, e.g., through position signals from one of the actuators  142 ,  144 . 
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                 2 
                 Autofocus device 
               
               
                 4 
                 Microscope 
               
               
                 6 
                 Sample 
               
               
                 8 
                 Image detector 
               
               
                 10 
                 Control means 
               
               
                 12 
                 Light source 
               
               
                 14 
                 Light generator 
               
               
                 16 
                 Optics 
               
               
                 18 
                 Diaphragm 
               
               
                 20 
                 Optical axis 
               
               
                 22 
                 Optical system 
               
               
                 24 
                 Optical element 
               
               
                 26 
                 Objective 
               
               
                 28 
                 Means 
               
               
                 30 
                 Beam splitter 
               
               
                 32 
                 Beam splitter 
               
               
                 34 
                 Microscope housing 
               
               
                 36 
                 Optical element 
               
               
                 38 
                 Actuator 
               
               
                 40 
                 Optics 
               
               
                 42 
                 Optics 
               
               
                 44 
                 Detector 
               
               
                 46 
                 Aperture 
               
               
                 48 
                 Light path 
               
               
                 50 
                 Light path 
               
               
                 52 
                 Measurement light focus 
               
               
                 54 
                 Distance 
               
               
                 56 
                 Focus 
               
               
                 58 
                 Specimen slide 
               
               
                 60 
                 Sample material 
               
               
                 62 
                 Cover slip 
               
               
                 64 
                 Interface 
               
               
                 66 
                 Interface 
               
               
                 68 
                 Interface 
               
               
                 70 
                 Target focus plane 
               
               
                 72 
                 Detector element 
               
               
                 74 
                 Detector element 
               
               
                 76 
                 Signal 
               
               
                 78 
                 Signal 
               
               
                 80 
                 Position 
               
               
                 82 
                 Differential signal 
               
               
                 84 
                 Zero crossing 
               
               
                 86 
                 Target position 
               
               
                 88 
                 Direction 
               
               
                 90 
                 Distance 
               
               
                 92 
                 Focus point 
               
               
                 94 
                 Mirror 
               
               
                 96 
                 Distance 
               
               
                 98 
                 Distance 
               
               
                 100 
                 Mirror 
               
               
                 102 
                 Microscope 
               
               
                 104 
                 Specimen stage 
               
               
                 106 
                 Sample 
               
               
                 108 
                 Light source 
               
               
                 110 
                 Arrow 
               
               
                 112 
                 Optical system 
               
               
                 114 
                 Mirror 
               
               
                 116 
                 Objective 
               
               
                 118 
                 Actuator 
               
               
                 120 
                 Microscope housing 
               
               
                 122 
                 Optical axis 
               
               
                 124 
                 Optical elements 
               
               
                 126 
                 Camera 
               
               
                 128 
                 Detector 
               
               
                 130 
                 Light filter 
               
               
                 132 
                 Detector element 
               
               
                 134 
                 Filter area 
               
               
                 136 
                 Actuator 
               
               
                 138 
                 Arrow 
               
               
                 140 
                 Control means 
               
               
                 142 
                 Actuator 
               
               
                 144 
                 Actuator 
               
               
                 146 
                 Detector 
               
               
                 148 
                 Detector region 
               
               
                 150 
                 Detector region 
               
               
                 152 
                 Detector region 
               
               
                 154 
                 Mirror 
               
               
                 156 
                 Mirror 
               
               
                 158 
                 Filter area 
               
               
                 160 
                 Filter area 
               
               
                 162 
                 Filter area 
               
               
                 164 
                 Sample receptacle 
               
               
                 166 
                 Object 
               
               
                 168 
                 Object 
               
               
                 170 
                 Light filter 
               
               
                 172 
                 Edge 
               
               
                 174 
                 Cut-off filter 
               
               
                 176 
                 Cut-off filter 
               
               
                 178 
                 Transmission window 
               
               
                 A 
                 Absorption 
               
               
                 E 
                 Sensitivity 
               
               
                 λ 
                 Wavelength 
               
               
                 Δλ 
                 Wavelength range 
               
               
                 A 
                 Amplitude 
               
               
                 g 
                 Limit value 
               
               
                 t 
                 Time 
               
               
                 z 
                 Direction of the optical axis