MEASURING DEVICE AND METHOD FOR DETERMINING A DEPTH OF FIELD OF AN OPTICAL ASSEMBLY

Description

[0001] The present invention relates to a device for determining the depth of field of an optical structure, in particular of optical microscopes.

[0002] Determining the depth of field of an optical setup such as an optical microscope is extremely important in many optical investigations, e.g. in biology, metallurgy, process engineering or in experimental setups. This can provide information about the depth range at which an acceptably focused image can be expected in the investigations. This is important, among other things, when the object to be measured cannot be placed easily and deterministically in the optical setup or in front of an objective of the optical setup, when the objects to be observed extend into the depth or can move (rapidly). Examples of this include investigations of droplet sprays or the impact of droplets on surfaces, as well as the investigation and tracking of particles and organisms in microchannel flows.In these applications, autofocus and multifocus methods using a sensor, a lens or liquid lenses are usually not feasible due to the speed of the focusing method.

[0003] In some cases, especially with very expensive microscopes, a simple calibration object, such as a line pattern, can be shifted in depth using a very precise shifting mechanism. The upper and lower shift points, at which the sharpness of the calibration pattern is just acceptable, define the depth of field.

[0004] In addition, the depth of field (Depth of Field, DOF ) can also be determined mathematically, see for example: Merklinger, H. M: The INs and OUTS of FOCUS (v1.03e).

[0005] Published by the author. P. 13 ff. (2002) ISBN 0-9695025-0-8, where the depth is the difference between the two distances D 1 and D 2 results in: DOF = D 2 − D 1 with: D 1 = f 2 D + gfD − gf 2 f 2 − gf + gD D 2 = f 2 D − gfD + gf 2 f 2 + gf − gD g = Na

[0006] This corresponds to D 1 the distance of the lens to the near point and D 2 to the far point on the object side, with previously defined, accepted diameters a of the circle of confusion. Other constants included in the equations are the focal length f , the aperture number N and the distance of the focus point D in front of the lens. Since optical systems in scientific applications sometimes do not allow for the unambiguous definition of parameters, an experimental determination of the depth of field is often almost essential.

[0007] Furthermore, the document US 2007 / 279621 A1 describes a setup for the direct measurement of a vertical intensity profile through a focal plane along an illumination beam, a determination of the depth of the focal plane, and a maximum intensity of the intensity profile. The setup comprises a plurality of focusing indicators mounted relative to a substrate, wherein the focusing indicators are distributed at various locations along the illumination beam. The focusing indicators are configured to be illuminated with an intensity corresponding to the position relative to the focal plane along the axis of the illumination beam. The position of the respective focusing marks can be predetermined, e.g., along a specific scale with a specific inclination of the scale and the path of the illumination beam, or it can be initially unknown and subsequently determined.

[0008] The object of the invention is therefore to simplify the experimental determination of the depth of field of an optical setup and to improve its accuracy. This object is achieved by a device having the features specified in the independent claim. Preferred embodiments are the subject of the dependent claims.

[0009] Thus, in one aspect, the invention relates in particular to a measuring device for determining a depth of field of an optical structure, in particular a light microscope. For this purpose, the measuring device comprises a device body with a measuring axis, wherein the device body is designed such that it can be placed stable in a measuring position on an object-side support plane (in particular a specimen slide plane) of the optical structure such that the measuring axis of the device body coincides with an (object-side) optical axis of the optical structure. In particular, the device body preferably has a base surface for this purpose, which serves to place the measuring device in the optical structure in the measuring position. Due to the stable placement, the measuring device can be reproducibly brought into the measuring position for an accurate measurement of the depth of field.

[0010] In order to be able to measure the depth of field, the device body has a measuring scale applied along a scale line in such a way that the scale line forms a scale angle with the direction of the measuring axis φgreater than 0° (preferably at least about 10°, more preferably at least about 20°, even more preferably at least about 30°) and less than 90° (preferably not more than about 80°, more preferably not more than about 70°, even more preferably not more than about 60°) and the measuring scale is optically detectable in the measuring position of the device body by the optical structure for determining the depth of field. In other words, in the measuring position of the measuring device or the measuring body, the measuring scale is visible and readable through the optical structure. For this purpose, the device body is preferably optically transparent at least partially along the measuring axis. This can be achieved in that the device body is open in the optically transparent region or has an opening or in that the volume material of the device body is optically transparent in this region, e.g. in the form of glass.In particular, the device body may comprise glass or consist essentially of glass.

[0011] The measuring position thus defines the position of the measuring device relative to the optical structure or within the optical structure in which the depth of field can be determined. In particular, the measuring position is determined by an object-side focal point of the optical structure, i.e. the area in which an object to be examined is sharply imaged by the optical structure. The measuring device is designed such that the object-side focal point of the optical structure lies in the area of the scale line when the measuring device is in the measuring position. The scale line can represent a line that is not separately marked optically and that (only) describes the course of the measuring scale. In a preferred embodiment, however, the scale line can also be designed as an optically visible (continuous or partially interrupted) line (for example as part of the measuring scale).

[0012] By arranging the measuring scale at an angle (oblique) relative to the optical axis of the optical setup, the measuring scale extends simultaneously in an axial directional component of the optical setup and in a lateral direction. Thus, the measuring scale (due to an axial progression along the scale line) covers the entire focal range of the optical setup, i.e., in particular between a near and far limit of the focal range. At the same time, the measuring scale (due to a lateral progression along the scale line) can be read and evaluated through the optical setup. This allows the depth of field to be read very precisely and with high reproducibility from a single, instantaneous image of the measuring scale through the optical setup (e.g., from a single glance or a single snapshot).In particular, it is therefore not necessary to shift the focal plane of the optical setup for the purpose of measuring the depth of field.

[0013] Furthermore, the device body comprises an optically transparent block or it can preferably be formed from such an optically transparent block, in the interior of which the measuring scale is formed. The device body or the optically transparent block is regarded as optically transparent in particular when light in at least one measuring wavelength range (which is used in particular as the wavelength range of an optical measurement by means of the optical structure) that runs out of the measuring scale leaves the measuring device in the direction of the measuring axis (towards the optical structure) to a proportion of at least 50%, preferably at least 75%, even more preferably at least 90%, most preferably at least 95% of the light output. The measuring wavelength is particularly preferably in the visible range, in particular in a range from approximately 380 nm to 780 nm. In a preferred embodiment, the optically transparent block comprises glass orIt is preferably formed essentially of glass. Soda-lime glass, for example, is considered as a preferred glass material. In another aspect, Schott-Glas N-BK7 ®< is particularly suitable.

[0014] The device body is thus formed in particular with an optically transparent block with the base surface, a light exit surface and the measuring axis perpendicular to the base surface and the light exit surface such that the device body, in the measuring position, can be placed firmly with the base surface on the support plane of the optical structure such that the measuring axis of the device body coincides with the optical axis of the optical structure, wherein the device body has the measuring scale applied along the scale line in the interior of the optically transparent block such that the scale line encloses the said scale angle with the direction of the measuring axis and the measuring scale can be optically detected by the optical structure in the measuring position of the device body for determining the depth of field in that the light detected by the optical structure passes from the measuring scale through the light exit surface.

[0015] The optically transparent block is preferably formed as a straight, in particular four-sided prism, particularly preferably as a cube. This is particularly preferred and very useful when the support plane of the optical structure is perpendicular to its (object-side) optical axis, and when the measuring axis of the measuring device is perpendicular to a base surface of the prism. This ensures that the light emanating from the measuring scale leaves the device body, in particular the optically transparent block, essentially symmetrically in the direction of the optical structure, thereby achieving the most distortion-free and thus error-free reading and evaluation of the sharpness representation of the measuring scale.

[0016] Additionally or alternatively, the optically transparent block is designed as a microscope slide, in particular as a microscope slide. A microscope slide is considered to be a support on which small objects (objects) can be observed microscopically, in particular under a light microscope. Such microscope slides are conventionally available in this size and material, although as microscope slides for light microscopy they often consist of a glass plate (also called a support glass) measuring 76 mm × 26 mm (DIN ISO 8037-1), and this size is also used for the transparent block in a preferred embodiment of the invention. Alternatively, the sizes 76 mm × 52 mm, 76 mm × 51 mm, 76 mm × 38 mm, 76 mm × 25 mm, 48 mm × 28 mm and / or 46 mm × 27 mm are also preferred. In this case, values in the range of about 0.5 mm to about 3 mm, particularly preferably up to about 2 mm, most preferably in the range of about 1 mm to about 1.5 mm are preferably used as the slide thickness.In the embodiment as an object micrometer, a (lateral or flat) micrometer scale is additionally provided in or on the object carrier, which is provided in particular in addition to the measuring scale which serves to measure the depth of field and in particular runs perpendicular to the measuring axis.

[0017] Furthermore, the scale line runs along a straight line which, with a normal direction to a base surface of the transparent block (in particular the prism), forms the scale angle φ which preferably lies in a range of about 30° to about 60°, particularly preferably in a range of about 40° to about 50°, most preferably about 45°. The measuring scale marks periodic distances d along the scale line according to an optical refractive index n of the optically transparent block. d = n 10 − m L / cos φ with an integer m as a decimal multiple of a standardized unit of length L, where the unit of length is 1 meter according to the International System of Units (SI). With this scaling, the periodic distances correspond essentially to equivalent axial distances d' a = 10 -< m<L in a vacuum (or approximately in air). They thus provide a direct measure of the depth of field in vacuum or air in the standardized unit of length L or in a decimal multiple thereof. This allows the effective depth of field of the optical setup in air or vacuum to be read directly and easily. For example, L = 1 m and m = 6, the depth of field can be read in units of µm. It is of course possible that the measuring scale, in addition to these periodic intervals, dperiodic subdivisions of these distances into, for example, 2 or 5 equal subsections. Particularly preferably, the measuring scale includes numerical labeling, which significantly simplifies reading or counting the marked periodic distances.

[0018] This scaling is fundamentally applicable even if the device body is open, at least in the area between the measuring scale and the objective of the optical assembly, or at least if the measuring scale itself is not covered by an optical medium such as glass. In this case, the refractive index n is to be set to the refractive index of the surrounding medium (e.g., air, or approximately 1).

[0019] In a particularly preferred embodiment, the measuring scale is formed as a laser engraving. The technique of laser engraving is particularly well-established for glass and can be applied very efficiently within the scope of the present invention. This provides the measuring scale with special protection in the device body and prevents it from being damaged during use.

[0020] As an alternative to the aspect described above, a measurement method for determining the depth of field of an optical assembly is described below, which can solve the technical problem and contribute to the understanding of the claimed invention. The method initially comprises providing an optically scattering medium in a measurement area around the object-side focal point of the optical assembly, i.e., where the depth of field of the optical assembly is to be measured. Preferably, providing an optically scattering medium includes introducing fog and / or smoke into the measurement area around the object-side focal point of the optical assembly.

[0021] In addition, the method comprises projecting a measuring scale along a scale line within the measuring range such that the scale line forms a scale angle with the direction of the object-side optical axis of the optical structure φgreater than 0° (preferably at least about 10°, more preferably at least about 20°, even more preferably at least about 30°) and less than 90° (preferably not more than about 80°, more preferably not more than about 70°, even more preferably not more than about 60°) and the measuring scale is optically detectable by the optical structure for determining the depth of field. In particular, the measuring scale is detectable by the light scattering on the optically scattering medium for the optical structure. In this case, the measuring scale is projected into the measuring area in particular as a 3-dimensional light or laser projection in the manner of a hologram, wherein the light is scattered on the optically scattering medium and detected by the optical structure.The optically scattering medium is preferably only dense enough to result in sufficient scattering of the projected light in the area of the projected measuring scale, but so that the scattered light representing the measuring scale can still be detected for the most part outside the measuring range for the optical setup. The majority of the light from the projected measuring scale should therefore preferably be detectable by the optical setup without further scattering. Finally, the method comprises detecting the boundaries of the area of the projected measuring scale that is sharply imaged by the optical setup within the framework of a tolerance criterion. These boundaries represent, in particular, the near limit and the far limit of the focus range and are identified by the fact that the measuring scale itself transitions from a blurred to a sharp area (or vice versa). The structure of the measuring scale (in particular the direction, shape, size, divisions, etc.) depends on the optical setup.) the method preferably applies the features analogous to those of the measuring device described here.

[0022] Particularly preferably, the method is used, in particular in one of the preferred embodiments described here, in a setup for flow measurement in at least one flow channel, wherein the measuring range lies at least partially within the at least one flow channel. In such an application, the optically scattering medium used to measure the depth of field can simultaneously be a medium whose flow behavior is to be determined or which is used to determine the flow pattern of a carrier medium. A particularly preferred application is a PIV measurement (particle image velocimetry).

[0023] The invention is described below by way of example using preferred embodiments with reference to the accompanying figures. Herein: Fig. 1 a schematic comparison of a conventional measurement of a depth of field ( Fig. 1A ) and an inventive measurement of a depth of field ( Fig. 1B ) of an optical structure; Fig. 2 shows a measuring device according to a preferred embodiment of the present invention; Fig. 3 shows an exemplary measuring scale which can be used in a measuring device or a method according to a preferred embodiment of the invention; Fig. 4 shows a representation of a recording of a measuring scale in the image area of an optical structure; Figs. 5 - 6 show further measuring devices according to preferred embodiments of the present invention; and Fig. 7 shows geometric representations to illustrate measurement errors.

[0024] In particular, insofar as the same reference numerals are used in the partially separately described embodiments, corresponding statements regarding the respective components, structures and functions are preferably also applicable in the respective other embodiments.

[0025] Fig. 1Aillustrates a conventional procedure for determining the depth of field of an optical assembly 100. In the example shown, the optical assembly relates to a light microscope with several interchangeable objectives 104 mounted in a lens revolver. The respectively active objective 104 defines an (object-side) optical axis 102 of the optical assembly 100. For microscopy of an object, the object is positioned, in particular placed, on a specimen carrier device 106 of the optical assembly. In order to determine the depth of field of the optical assembly, a conventional calibration pattern 108 is conventionally placed on the specimen carrier device 106 such that the calibration pattern 108 lies essentially parallel to a focal plane of the optical assembly 100.By moving the object carrier device 106 parallel to the optical axis 102, the calibration pattern 108 can be brought into the focal plane of the optical structure 100, where it is sharply imaged by the optical structure (within the scope of its optical resolution). An image sharpness that is still sufficient within a predetermined or desired tolerance is also achieved in the vicinity of the focal plane. The axial extent of this vicinity along the optical axis 102 is regarded as the depth of field. In order to determine this depth of field, the calibration pattern 108 is conventionally moved by means of the object carrier device 106 between the two sharpness limits (a near limit and a far limit, on either side of the focal plane, respectively), and the axial displacement required for this is determined. This is usually carried out by a user who determines the respective impression of sharpness at the near or far end.The distance limit is assessed and the respective position at the near and far limits is determined sequentially. Since the two positions are approached one after the other, the two sharpness impressions cannot be compared directly side by side, which may result in slightly different sharpness settings. Furthermore, this procedure requires a correspondingly precise calibration of the axial displacement of the slide mechanism.

[0026] Fig. 1BIn comparison, this illustrates a preferred embodiment of a procedure according to the invention for the same optical structure 100. For this purpose, instead of a conventional calibration pattern 108, a measuring device 10 according to a preferred embodiment of the present invention is placed on the object carrier device 106. In particular, this measuring device 10 is placed stably by means of a base on the object carrier device 106 in such a way that a measuring axis of the measuring device essentially coincides with the optical axis 102 of the optical structure 100. In the illustrated embodiment, the measuring device comprises a cuboid-shaped device body (block), within which a measuring scale 18 is formed obliquely or diagonally such that the measuring scale runs along a scale line (preferably a straight line), wherein the scale line forms a scale angle with the direction of the measuring axis (i.e., with the direction of the optical axis 102). φ greater than 0° and less than 90°. In addition, the device body (block) is in particular transparent in such a way that the measuring scale can be viewed via the activated lens 104. Now, the object carrier device 106 is preferably positioned in such a way that the scale line of the measuring device intersects the focal plane of the optical structure 100 in the region of the optical axis 102. In other words, the measuring scale 18 preferably passes through the focal plane of the optical structure 102 in the region of the measuring axis (i.e., in the region of the optical axis 102) - in such a way that it also passes through both the near limit and the far limit of the focal range of the optical structure within a field of view of the optical structure. This can be achieved in particular by the scale angle φ small enough, preferably not larger than about 65°. Conversely, it is advantageous if the scale angle φis not chosen too small, since otherwise the points of passage of the measuring scale or the scale line through the near and far limits of the focus range within the field of view will be so close together that reading is difficult, which can then also impair the reliability of the evaluation and thus the achievable accuracy. Preferably, the scale angle φ not less than about 25°.

[0027] With a procedure according to the invention, an evaluation of the depth of field can be carried out after a single positioning of the measuring device with a single glance or on the basis of a single image recording, without having to move a calibration pattern during or in between.

[0028] Fig. 2shows a schematic perspective view of a measuring device 10 according to a preferred embodiment of the invention. In this embodiment, the measuring device 10 essentially consists of a preferably glass, cube-shaped device body 12, in the interior of which a corresponding measuring scale 18 is embedded along a diagonal scale line 16. In this case, the scale line 16 preferably runs as a straight line between the centers of two diagonally opposite cube edges. The measuring scale 18 preferably lies within a plane spanned by the two diagonally opposite cube edges. The measuring scale 18 marks positions and distances in at least one direction parallel to the scale line. An exemplary embodiment of such a measuring scale is presented below.

[0029] For the use of this measuring device 10, the cube can be placed in a stable position with one side surface, the base surface 20. This is particularly suitable when the object-side optical axis of the optical structure to be measured runs vertically. For example, the perpendicular bisector of the base surface forms a measuring axis 14 of the measuring device 10, which can be aligned with the aforementioned optical axis of the optical structure. As a result, the measuring scale 18 can be optically detected along the measuring axis by the measuring structure in such a way that the light detected by the optical structure passes from the measuring scale through a light exit surface 22 of the device body 12 essentially symmetrically about the measuring axis.Since the measuring axis 14 in a device body with a prismatic shape (in particular in the form of a straight prism), in particular in a cube, is also perpendicular to the light exit surface 22 precisely when it is perpendicular to the base surface 20, distortions of the measurements due to the refraction of light at the light exit surface are minimized.

[0030] The material used for the device body 12 is preferably a dimensionally stable material that is transparent in the visible light range. Glass, for example, is suitable for this purpose. The measuring scale can be created, for example, by laser-induced internal glass engraving. This allows the glass cube itself to be manufactured with very high quality in terms of material homogeneity, surface planarity, and angles. Furthermore, glass is mechanically, thermally, optically, and chemically stable and quite durable over time. Laser internal engravings can also be produced with very high precision. Finally, such internal glass engravings are protected against external influences and are durable.

[0031] Fig. 3A to Fig. 3C show the design of an exemplary measuring scale 18, as it can be used, for example, in a cube-shaped measuring device. Fig. 3Aa parallel projection of the measuring scale in a direction along the measuring axis, i.e., the view of the measuring scale that is fundamentally observable from the direction of the optical structure. In this preferred embodiment, the measuring scale even comprises two (essentially parallel) subscales 18a and 18b. It is conceivable in one embodiment that the two subscales can be simultaneously detected by the optical structure in the measuring position of the measuring device. Alternatively or additionally, it is also possible for the measuring device to be displaced perpendicular to the optical axis of the optical structure in such a way that one of the two measuring scales is selectively detected to determine the depth of field.

[0032] Fig. 3B shows an enlarged section of the measuring scale 18 from Fig. 3Ain the area marked there A. The two subscales 18a, 18b run parallel to each other and parallel to the scale line. The entire measuring scale 18 lies essentially in a plane that runs as a spatial diagonal, as shown in Fig. 3C as a section through the device body (as a cube) along the line BB from Fig. 3A A scale line in this case can be understood as any line that runs along the gradient of this plane. Along such scale lines, the subscales mark periodic intervals, which in the projection of Fig. 3Bare labeled "0.1". In any case, with a straight scale line, as in this case, the periodicity of the markings along the scale line also represents a periodicity of these markings with respect to their respective positions parallel to the measuring axis – and thus, in the measuring position, parallel to the optical axis of the optical setup. Furthermore, the markings can also be recognized and distinguished in the image field of the optical setup, at least when they are within the focal range of the optical setup. Thus, the boundaries of the focal range can be determined in the image field of the optical setup as the outermost markings that are just (sufficiently) sharply imaged. Due to the periodicity of the spacing of the markings, the entire size of the focal range, i.e., a measure of the depth of field, can be very easily counted or read directly.Direct reading is preferably supported and simplified by labeling the markings (with numbers). In the representation of . Fig. 3B For example, the two subscales are shifted from each other by half of their periodic distances in order to improve the resolution of the entire measuring scale 18.

[0033] The measuring scale is thus positioned along the scale line or measuring section in such a way that it runs diagonally, allowing optical measurement of the depth of field for both incident and transmitted light setups. The measuring scale can be adjusted and labeled so that the depth of field can be read directly. This is comparable to measuring a length. Conversion using angles due to possible projections is thus eliminated.

[0034] Using the previously described equations (1) - (4), the depth of field in air can be calculated analytically. Using Snell's law of refraction in equation (5), the influence of introducing a medium with a refractive index can also be taken into account. n 2 into the surrounding medium ( n 1 ) on the position of the focal plane and the front and rear focus point (equations (2) and (3)). Here, α the angle of incidence and reflection of the light rays into the medium. n 1 sin α 1 = n 2 sin α 2

[0035] The ambient medium is preferably air, but can also be an immersion liquid or another gas depending on the specific application. The relative shift of the focal plane, caused by the introduction of a medium with a refractive index n 2 different from the surrounding medium ( n 1 ) can be analytically derived to the following relationship: Δ d d = tan α 1 1 − n 1 n 2 sin α 1 2 n 1 n 2 sin α 1 − 1

[0036] Here, d corresponds to the distance of the focal plane to the effective lens or objective plane and α 1 corresponds to the angle of incidence of the rays at the optical transition from the ambient medium to the scale-bearing medium. Using the numerical aperture for the ambient medium (index 1), defined in equation (8), equation (7) can be transformed so that equation (9) can be used to calculate the relative displacement of the focal plane as a function of the refractive indices and the numerical aperture. A N , 1 = n 1 sin α 1 Δ d d = n 2 n 1 1 − A N , 1 n 2 2 1 − A N , 1 n 1 2 − 1

[0037] Due to the angular relationships, the angle of incidence α 1 is equal to half the aperture angle used in equation (8). It can be shown with the help of equation (9) that the focal plane for the case n 2 > n1 shifts backward or away from the lens. This behavior applies analogously to the front and rear focal points or the front and rear focal limits, which are defined in particular by the enveloping rays. Using the corresponding angles in equation (9), the exact position of the depth of field can thus be calculated.

[0038] It can be assumed that the depth of field depends on the refractive index to the same extent as the displacement of the focal point in equation (8). The scale for determining the depth of field can thus be defined according to two criteria. It can be defined as a universally valid scale from which the depth of field values can be read. By inserting the read values into equations (7) or (9), the exact depth of field can be calculated.

[0039] Alternatively, and particularly preferably, the measuring device and scale can be precisely designed for a specific measuring task using equations (7) and (9), respectively. The scale then already contains absolute values of the depth of field, so that the appropriate length measurements can be determined directly upon reading. An example of this is shown in particular in equation (5). For example, when using a medium with the refractive index n 2 = 1.5 markings of the measuring scale, which represent a measure for a depth of field of 1 mm in the case of a scale angle of 45°, preferably arranged at a geometric distance of approximately 2.1 mm from each other.

[0040] The possible optical image of an exemplary measuring scale, as it could be used in a measuring device according to the invention, is shown in Fig. 4The measuring scale in this example comprises two subscales, each of which represents periodic intervals—specifically, shorter and longer lines. For example, the longer, numbered lines can be understood as representing positions with mutual axial intervals of 1 mm. The shorter lines in between could represent positions with mutual axial intervals of 0.1 mm in one subscale and 0.05 mm in the other subscale. The numbered markings further simplify counting and measuring.

[0041] In particular, Fig. 4It is evident that a central area of the measuring scale, namely the area located in the object space near the focal plane, is sharply displayed in the image plane. As the distance from the sharpest point increases, the measuring scale moves further and further away from the focal plane and passes through the limits of the sharpness range. These limits and their distance from each other can be read at a glance or in a single image of the measuring scale thanks to the optical design, without mechanically shifting the measuring device after a single positioning.

[0042] As already mentioned, Fig. 3Ca sectional view through the device body. This shows exemplary side lengths of 25 mm for a corresponding cube. Particularly preferably, the side length of the device body, especially in the case of a cube shape, is in the range of approximately 5 mm to approximately 40 mm. At this size, the device body is sufficiently large to be easily handled manually and small enough for many applications.

[0043] Especially for applications in the field of microscopy with high magnifications and very short focal lengths and focus distances, it is also desirable to be able to use smaller or thinner measuring devices. Examples of such measuring devices for high-magnification microscopes are described in Fig. 5 and Fig. 6shown. Thus, in these preferred embodiments, the device body is designed in particular in the form of a slide plate. It is even particularly preferred to design the device body according to standardized slide plates in terms of size and material. It is even possible to use such existing slide plates directly as the device body and to provide them, for example by laser engraving, with a corresponding measuring scale. When the device body is placed on a slide device of a microscope, this scale then runs at an angle of more than 0° and less than 90°, particularly preferably in the range between approximately 30° and approximately 60° relative to the optical axis of the microscope objective, and can be read directly through the microscope.Such measuring devices in the dimensions of microscope slides are, on the one hand, very easy to handle, are generally highly compatible with the holders for microscope slides available on many microscopes, and are particularly well suited for optical setups with very short focal lengths.

[0044] The special design of Fig. 6 In a central region of the light exit surface 22, it even has a recess for receiving an immersion liquid. The measuring scale is preferably arranged between this recess and the opposite base surface 20. This embodiment is therefore also directly applicable to immersion microscopy by filling the recess with an immersion liquid.

[0045] Using the inventive method, depth of field can be determined with particularly high levels of accuracy and reproducibility, and with minimal effort. For example, glass bodies can be manufactured with high precision as device bodies and thus as carriers of the measuring scale. Laser-assisted (holographic) application of the measuring scale is also possible with very high precision. Overall, manufacturing tolerances are thus possible that lie below the resolution limits of the measuring devices to be calibrated. At the same time, the measuring devices according to the invention can be manufactured very cost-effectively. Due to the laser technology used, virtually unlimited size scaling is possible, so that calibration and measuring devices can be manufactured on a scale from a few hundred micrometers up to several centimeters. High-quality glass bodies (e.g., cuboids or cubes) also exhibit very high edge and surface parallelism.In addition, the measuring scale (calibration pattern) has a known and very precisely manufactured angle, so that angle and distance errors are practically eliminated or negligible if the measuring device is well positioned in the measuring section.

[0046] To better understand the possibility of embedding a calibration pattern or a measuring scale in a fixed pattern carrier, the following describes, as an alternative to this, the 3-dimensional or holographic introduction of a characteristic calibration and measuring pattern, for example a (length) scale and possibly other patterns commonly used in image processing (Siemens star, line pairs, etc.), into a measuring space or object space, which is captured by an optical structure.

[0047] This can be achieved, for example, by incorporating a light projection (or hologram) into smoke or liquid mist. Especially when using such a method, a fixed sample carrier is preferably not required. This is particularly useful when the object space (measurement space) is very limited in space or, due to the nature of the measurement to be performed there, is very difficult to access for arranging or even attaching a sampled device body.

[0048] For holograms or 3D light projections in liquid mist, smoke, or similar, the arrangement can otherwise be considered analogous. Such introduction of the pattern also results in similar advantages to those obtained with the use of a measuring device according to the invention. Using a simple image, the depth of field can be measured immediately, without moving the sample carrier or camera. Due to the light or laser technology used, almost unlimited size scaling is possible even with projections in smoke, liquid mist, or similar. Thus, an optical magnification arrangement (e.g., for microchannel flows) can be calibrated and measured in the same way as an optical reduction arrangement (e.g., for droplet experiments).

[0049] Preferred areas of application for a measurement method for determining the depth of field of an optical setup by projecting a measuring scale into an optically scattering medium are, for example, pressure chambers in which a spray is to be measured. In certain measurements, the deviation of the particles from the focal plane is used to determine the position of particles in depth. Therefore, precise knowledge of the depth of field is crucial for such measurements. With this method, the depth of field can be remeasured very easily and quickly (especially without opening the pressure chamber), for example when changing the optics or changing the optical setup. In particular, the spray already present in the pressure chamber can be used as an optically scattering medium.Through an optical window in the pressure chamber, the measuring scale can be projected into the pressure chamber by light projection in such a way that the optical structure can detect this measuring scale due to the light scattering by the optically scattering medium.

[0050] Other particularly preferred areas of application for this method include PIV measurements (particle image velocimetry) or measurements of flows in channels. Especially in many measurement configurations for flow measurement, it is very difficult or even impossible to measure the depth of field using a movable calibration pattern, either due to the internal geometry of the flow arrangements or due to the external geometric boundary conditions. It is already difficult or almost impossible to even position the calibration pattern correctly in the focal plane. Moving it in a controlled manner along the optical axis to measure the depth of field is even more difficult or only possible with considerable inaccuracy.

[0051] In technical systems and also in scientific investigations, pipe flows of various types are often measured. Measurement techniques such as PIV, shadow images or schlieren are often used here. A particularly challenging example is the measurement of a round, conical vortex tube made of Plexiglas. Due to the conically opening inner diameter, the wall thickness varies locally. For the measurement, a laser cut through the tube illuminates the plane to be measured. The plane of the laser cut can be shifted as desired. One or two cameras with appropriate optics record the necessary images. The problem for measuring the depth of field of the camera(s) in this application lies in the accessibility of the vortex tube. The tube is typically a complete, non-openable (Plexiglas) tube with dimensions of, for example, approximately 5-15 cm for the diameter and a length of 1.5 m.

[0052] Calibration plates for such pipes are usually only available commercially as custom-made products. The complexity and thus the cost of custom-made calibration plates are extremely high due to the accuracy requirements. Furthermore, mounting the calibration plates in the center of the pipe is very challenging, and precise adjustment of their position is virtually impossible.

[0053] It is precisely under such conditions that the method can fully exploit its potential by projecting a measuring scale into the measuring chamber. However, even the use of a measuring device according to the invention can offer significant advantages in this case, since no controlled mechanical displacement of a calibration pattern is required during depth-of-field measurement. The measuring device only needs to be positioned once. For this purpose, the device body can also be adapted to the internal shape of the pipe.

[0054] Error limits and their potential reduction are discussed below. In particular, the error of the measuring device consists of the geometric errors caused by deficiencies in the device body (e.g., glass body) as a sample carrier and the samples introduced as a measuring scale along the scale line. This includes, in particular, the geometric error due to the imperfect shape of the device body, using a glass cube as a sample carrier as an example. The following influencing factors play a role: Surface flatness: typical values: ±0.08µm Surface parallelism: very high accuracies can also be achieved here.

[0055] As a useful comparison, the beam deviation data for beam splitter cubes can be used. This is a much more difficult manufacturing process due to the bonded interface between two prisms arranged in a cube. The typical value for the deviation of the orthogonal laser beam is assumed to be <±5 arcmin.

[0056] If the pattern is not supported by a glass body, but is introduced solely by projection, e.g., into smoke, the accuracy of the projecting pattern generator must be taken into account. No general estimate can be given in this case. However, due to high-quality mechanical construction, even these tolerances are extremely small.

[0057] In Fig. 7illustrates the error calculation due to angular errors when viewing the samples. This takes into account any refraction of the non-orthogonal light beam at the interface of the sample carrier due to the expected very small angles of well below one degree ( Δ θ « 1°) is neglected. The result is the following: d + Δ d d = sin β + Δ θ sin β with β = 90° - φ as sample angle, preferably β = 45°, and the angular error Δ θ . The errors for an angular deviation between -1° and +1° are summarized in the following table (for β = 45°). D θ -1° -0,5° -5' 0° +5' 0,5° 1° d + Δ d d -1,76% -0,88% -0,15% 0% +0,15% +0,87% 1,73%

[0058] The following applies to possible pattern errors due to imperfect pattern placement: The pattern is preferably applied to the measuring section by engraving the glass from the inside using a laser. However, the pattern can also be applied as a projection in fog or spray. In both cases, the dot diameter and resolution of the dot grid are crucial in relation to the dimensions of the measuring section.

[0059] For the insertion into a vitreous body, a comparison of representative data on available systems from various manufacturers (CERION laser GmbH and Wisely Laser Machinery Limited) shows: Cerion C-professional WLASER Crystal / Glass Engraving Machine 3D Laser (4KB) Minimum point size: 20µm 20µm Repeatability: 30µm 30µm Resolution: Not specified 800-1200 dpi

[0060] A dot size of 20 µm and a resolution of 800-1200 dpi can therefore be assumed. This corresponds to approximately 30-45 dots per millimeter. At the lower resolution of 800 dpi, a maximum error of ± 32 µm can be expected. At a resolution of 1200 dpi, this corresponds to a mere ± 21 µm. Such an error in determining the depth of field is perfectly sufficient for most metrological tasks. For example, with a depth of field of approximately 500 µm, as in the measurement setup of an exemplary drop test rig with very high optical magnification, the error is 4.2%. Other, more commonly used test rigs have greater depths of field, e.g., in the range of 25 mm, so the error is only 0.084%. Depths of field of several centimeters are also common in experimental test rigs for heat transfer tests.In addition to the commercially available systems, cavities in the glass with a distance of only 6 µm have already been created in the academic sector, which can therefore significantly reduce the error.

[0061] In the case of holographic insertion using a projector or similar device, the considerations are analog and therefore dependent on the pixel noxel size and the DPI resolution of the pattern-generating generator. The total error is dominated in particular by the insertion of the pattern into the measuring section or object space. In this case, the errors can be in the range of up to approximately ± 30 µm, which is perfectly sufficient for measuring the depth of field. Other methods for determining the depth of field exhibit significantly higher errors for comparable effort, for example, when using a meter scale (approximately one division of the scale, e.g., ± 500 µm). In this case, a measurement of the depth of field is not possible because the error is of the same order of magnitude as the depth of field itself. List of reference symbols

[0062] 10Measuring device 12Device body, block 14Measuring axis 16Scale line 18Measuring scale 20Base area 22Light exit surface 100Optical structure 102Optical axis 104Objectives 106Slide device 108Conventional calibration pattern

Claims

1. Measuring device (10) for determining a depth of focus of an optical structure (100), comprising a device body (12) with a measurement axis (14), wherein the device body (12) is designed such that it can be placed stably in a measurement position on a support plane of the optical structure in such a way such that the measurement axis (14) of the device body (12) coincides with an optical axis of the optical structure, and wherein the device body (12) has a measurement scale (18) applied along a scale line (16) such that the scale line (16) forms a scale angle φ greater than 0° and less than 90° with the direction of the measurement axis (14) and the measurement scale (18) is optically detectable through the optical structure (100) in the measurement position of the device body (12) for determining the depth of focus, wherein the device body (12) comprises an optically transparent block, inside which the measurement scale (18) is formed, the optically transparent block is formed as a prism and / or is designed as a specimen slide, the scale line (16) runs along a straight line which, with a normal direction to a base surface (20) of the transparent block, encloses the scale angle φ, characterized in that the measurement scale (18), at an optical refractive index n of the optically transparent block, marks periodic intervals d along the scale line (16) according to d = n 10 − m L / cos φ with an integer m as decimal multiples of a standardized unit of length L, wherein the unit of length is 1 meter according to the international system of units SI.

2. Measuring device (10) according to claim 1, wherein the optically transparent block is formed substantially of glass.

3. Measuring device (10) according to claim 1 or 2, wherein the optically transparent block is formed as a straight, in particular four-sided prism, particularly preferable as a cube.

4. Measuring device (10) according to any one of the preceding claims, wherein the optically transparent block is formed as a specimen micrometer.

5. Measuring device (10) according to any one of the preceding claims, wherein the scale angle φ is in a range of about 30° to about 60°, particularly preferable in a range of about 40° to about 50°, in particular at about 45°.

6. Measuring device (10) according to any one of the preceding claims, wherein the measurement scale (18) is formed as an inner laser engraving.