Measuring Device and Method of Determining a Depth of Field of an Optical Setup

Abstract

The present invention relates to a measuring device (10) and a method for determining a depth of field of an optical structure (100). In this case, the measuring device comprises a device body (12) with a measuring axis (14), the device body (12) being formed such that, in a measuring position, it can be placed in a stationary manner on a deposit plane of the optical structure such that the measuring axis (14) of the device body (12) coincides with an optical axis of the optical structure, wherein the device body (12) has a measurement scale (18) arranged along a scale line (16) such that the scale line (16) encloses with the direction of the measuring axis (14) a scale angle φ greater than 0° and less than 90° and the measurement scale (18) can be optically detected in the measuring position of the device body (12) by the optical structure (100) for determining the depth of field.

Claims

1. A measuring device for determining a depth of field of an optical structure, comprising: a device body with a measuring axis, wherein the device body is formed such that, in a measuring position, it can be placed in a stationary manner on a support plane of the optical structure such that the measuring axis of the device body coincides with an optical axis of the optical structure, and wherein the device body comprises a measurement scale arranged along a scale line such that the scale line encloses with a direction of the measurement axis a scale angle φ greater than 0° and less than 90°, and the measurement scale can be optically detected in the measuring position of the device body by the optical structure for determining the depth of field.

2. The measuring device of claim 1, wherein the device body comprises an optically transparent block within which the measurement scale is formed.

3. The measuring device of claim 2, wherein the optically transparent block is formed substantially of glass.

4. The measuring device according to claim 2, wherein the optically transparent block is a straight, four-sided prism.

5. The measuring device according to claim 2, wherein the optically transparent block is designed as a microscope slide comprising a specimen micrometer.

6. The measuring device according to claim 4, wherein the scale line runs along a straight line which, with a normal direction to a base area of the transparent block, encloses the scale angle φ, which lies in a range from about 30° to about 60°.

7. The measuring device according to claim 6, wherein the measurement scale at an optical refractive index of the optically transparent block has periodic distances d along the scale line according to
d=n10−mL/cos φ marked with an integer m as decadic multiples of a standardized unit of length L according to the International System of Units SI.

8. The measuring device according to claim 1, wherein the measurement scale is formed as an internal laser engraving.

9. A method for determining a depth of field of an optical structure, the method comprising: positioning an optical dispersing medium in a measurement area around a specimen-side focal point of the optical structure; projecting a measurement scale along a scale line within a measurement range in such a way that the scale line includes a scale angle φ greater than 0° and less than 90° with a direction of a specimen-side optical axis of the optical structure, and the measurement scale is optically detectable by the optical structure for determining the depth of field; and detecting limits of the measurement area of the projected measurement scale which is shown as sharp by the optical structure within the a scope of a tolerance criterion.

10. The method according to claim 9, wherein positioning the optical dispersing medium comprises introducing fog and/or smoke into the measurement area around the specimen-side focal point of the optical structure.

11. The method according to claim 9, wherein projecting the scale 3—is a dimensional light comprising laser projection.

12. The method according to claim 9, wherein the method is performed in a setup for flow measurement that includes at least one flow channel, wherein the measurement area is at least partially within the at least one flow channel.

Description

THE FIGURES SHOW THE FOLLOWING

[0029] FIGS. 1A—a schematic comparison of a conventional measurement of a depth of field (FIG. 1B 1A) and a measurement of a depth of field according to the invention (FIG. 1B) of an optical structure;

[0030] FIG. 2 a measuring device according to a preferred embodiment of the present invention;

[0031] FIGS. 3A—an exemplary measurement scale which can be used in a measuring device or 3B method according to a preferred embodiment of the invention;

[0032] FIG. 4 the representation of an image of a measurement scale in the image area of an optical structure;

[0033] FIGS. 5-6 further measuring devices according to preferred embodiments of the present invention; and

[0034] FIG. 7 geometric representations to illustrate measurement errors.

[0035] In particular, insofar as the same reference numbers are used in the partially separately described embodiments, corresponding explanations regarding the respective components, structures and functions are preferably also applicable in the respective other embodiments.

[0036] FIG. 1A illustrates a conventional approach to determining the depth of field of an optical structure 100. In the illustrated example, the optical structure relates to an optical microscope having a plurality of interchangeable lenses 104 mounted in an lens nosepiece. The respective active lens 104 defines an (specimen-side) optical axis 102 of the optical structure 100. In order to microscope a specimen, the specimen is positioned, in particular deposited, on a microscope slide device 106 of the optical structure. Now, to determine the depth of field of the optical structure, conventionally, for example, a conventional calibration pattern 108 is placed on the microscope slide device 106 such that the calibration pattern 108 is substantially parallel to a focal plane of the optical structure 100. By moving the microscope slide 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 its optical resolution). An image sharpness that is still sufficient within a specified or desired tolerance is also achieved in a vicinity of the focal plane. The axial extent of this environment along the optical axis 102 is considered the depth of field. To determine this depth of field, the calibration pattern 108 is conventionally displaced between the two limits of focus (a near limit and a far limit, respectively on this side and on the other side of the focal plane) by means of the microscope slide device 106, and the axial displacement required for this is determined. This is usually performed by a user who assesses the respective impression of sharpness at the near or far limit and thus determines the respective position at the near or far limit in succession. Since the two positions are approached one after the other, the two sharpness impressions cannot be compared directly next to each other, so that a slightly different sharpness setting can occur here. In addition, this procedure requires a correspondingly accurate calibration of the axial displacement path of the microscope slide device.

[0037] In comparison, FIG. 1B illustrates a preferred embodiment of an approach according to the invention for the same optical structure 100. For this purpose, instead of a conventional calibration pattern 108, a measuring device 10 is placed on the microscope slide device 106 in accordance with a preferred embodiment of the present invention. In particular, this measuring device 10 is placed in a stationary manner on the microscope slide device 106 by means of a base area such that a measuring axis of the measuring device substantially coincides with the optical axis 102 of the optical structure 100. In the embodiment shown, the measuring device comprises a cuboid device body (block) within which a measurement scale 18 is formed obliquely or diagonally such that the measurement scale runs along a scale line (preferably a straight line), the scale line enclosing with the direction of the measuring axis (i.e. with the direction of the optical axis 102) a scale angle φ greater than 0° and less than 90°. In addition, the device body (block) is transparent, in particular, such that the measurement scale can be viewed through the activated lens 104. Now the microscope slide 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 measurement 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)—and such that it also passes through both the near limit and the far limit of the focus range of the optical structure while still within a field of view of the optical structure. This can be achieved in particular by selecting the scale angle φ small enough, i.e. preferably not greater than about 65°. Conversely, it is advantageous if the scale angle φ is not selected too small, since otherwise the points of passage of the measurement scale or scale line through the near and far limits of the focus range within the field of view are 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 is not smaller than about 25°.

[0038] With a procedure according to the invention, an evaluation of the depth of field can thus be carried out after positioning the measuring device once with a single glance or on the basis of a single image recording, without having to carry out a displacement of a calibration pattern during or in between.

[0039] FIG. 2 schematically shows a perspective view of a measuring device 10 according to a preferred embodiment of the invention. In this embodiment, the measuring device 10 consists essentially of a preferably glass, cube-shaped device body 12, inside of which a corresponding measurement scale 18 is embedded along a diagonal scale line 16. Preferably, the scale line 16 in this case runs as a straight line between the centers of two diagonally opposite cube edges. Preferably, the measurement scale 18 lies within a plane which is defined by the two diagonally opposite edges of the cube. The measurement scale 18 marks positions and distances in at least one direction parallel to the scale line. An exemplary embodiment of such a measurement scale is shown below.

[0040] For the application of this measuring device 10, the cube can be placed with one side surface, the base area 20, stationary. This is particularly the case when the specimen-side optical axis of the optical structure to be measured is vertical. Here, for example, the central perpendicular to the base area forms a measuring axis 14 of the measuring device 10, which can be brought into line with the said optical axis of the optical structure. As a result, the measurement scale 18 can be optically detected along the measurement axis by the optical structure such that light detected by the optical structure passes from the measurement scale substantially symmetrically about the measuring axis through a light exit surface 22 of the device body 12. Since the measurement axis 14 in a device body with a prismatic shape (in particular in the form of a straight prism), especially in a cube, is also perpendicular to the light-emitting surface 22 exactly when it is perpendicular to the base area 20, distortions of the measurements due to the refraction of light at the light-emitting surface are minimized

[0041] The material of the device body 12 is preferably a dimensionally stable material that is transparent in the range of visible light. For example, glass is suitable for this purpose. The measuring scale can be produced, for example, by laser-induced internal glass engraving. Both the glass cube itself can be produced with very high quality in terms of material homogeneity, surface planarity and angles. In addition, 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 glass interior engravings are protected and resistant to external influences.

[0042] FIG. 3A to FIG. 3C show the embodiment of an exemplary measuring scale 18, such as can be used in a cube-shaped measuring device. FIG. 3A shows a parallel projection of the measurement scale in one direction along the measurement axis, i.e. the view of the measurement scale that can be observed from the direction of the optical structure. In this preferred embodiment, the measurement scale even comprises two (substantially parallel) sub-scales 18a and 18b. In one embodiment, it is conceivable that the two subscales in the measuring position of the measuring device can be detected simultaneously by the optical structure. Alternatively or additionally, it is also possible that the measuring device is shifted perpendicular to the optical axis of the optical structure in such a way that selectively one of the two measuring scales is detected for determining the depth of field.

[0043] FIG. 3B shows an enlarged section of the measurement scale 18 from FIG. 3A in the area marked there A. The two partial scales 18a, 18b run parallel to each other and parallel to the scale line. The entire measurement scale 18 lies substantially in a plane extending as a spatial diagonal, as shown in FIG. 3C as a section through the device body (as a cube) along line B-B of FIG. 3A. A scale line in this case can be understood as any line that runs along the slope of this plane. Along such scale lines, the subscales mark periodic distances, which are labeled “0.1” in the projection of FIG. 3B. At least in the case of 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 position in the direction parallel to the measurement axis—and thus in the measurement position parallel to the optical axis of the optical structure.

[0044] In addition, the markings can also be detected and distinguished in the image field of the optical structure at least when they are in the focus range of the optical structure. In this way, the limits of the sharpness range can be determined in the image field of the optical structure as the outermost markers that are still (sufficiently) sharp. Due to the periodicity of the distances between the markings, it is thus very easy to count or directly read off the total size of the area of focus, i.e. a measure of the depth of field. Direct reading is preferably supported and simplified by labeling the markings (with numbers). In the illustration of FIG. 3B, for example, the two sub-scales are shifted relative to each other by half of their periodic distances in order to improve the resolution of the entire measurement scale 18.

[0045] The measurement scale is thus inserted along the scale line or measurement section in such a way that it runs diagonally and permits optical measurement of the depth of field for both incident light and transmitted light setups. The measurement scale can be adjusted and labeled so that the depth of field can be read directly. This is comparable to measuring a linear dimension. A conversion by means of angles due to possible projections is therefore not necessary.

[0046] Using the previously described equations (1)-(4), the depth of field in air can be calculated analytically. Using the Snellius law of refraction in equation (5), the influence of introducing a medium with refractive indesx n.sub.2 into the surrounding medium (n.sub.1) on the position of the focal plane and the front and rear focal points (equations (2) and (3)) can also be described. Here, a represents the angle of incidence and angle of reflection of the light rays into the medium.

n.sub.1 sin(α.sub.1)=n.sub.2 sin(α.sub.2) (6)

[0047] The surrounding medium is preferably air, but can also be an immersion liquid or other gas depending on the specific application. The relative displacement of the focal plane, due to the introduction of a medium with a refractive index n.sub.2 different from the ambient medium (n.sub.1) can be analytically derived to the following ratio:

[00002] $\begin{matrix} \frac{Δ d}{d} = \frac{\tan (α_{1}) \sqrt{1 - {(\frac{n_{1}}{n_{2}} \sin (α_{1}))}^{2}}}{\frac{n_{1}}{n_{2}} \sin (α_{1})} - 1 & (7) \end{matrix}$

[0048] Here, d corresponds to the distance of the focal plane to the effective lens or lens plane and α.sub.1 corresponds to the angle of incidence of the rays at the optical transition from 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.

[00003] $\begin{matrix} A_{N . 1} = n_{1} \sin (α_{1}) & (8) \end{matrix}$

[0049] Here, due to the angular relationships, the angle of incidence α.sub.1 is equal to half the opening angle, which is used in equation (8). It can be shown using equation (9) that the focal plane shifts backward or away from the lens for the case n.sub.2>n.sub.1. This behavior applies analogously to the front and rear focal points and the front and rear focus limits, respectively, 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.

[0050] It can be assumed that the depth of field is dependent on the refractive index to the same extent as given for the shift 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 where the values of the depth of field can be read. By inserting the read values into equation (7) or (9), the exact depth of field can be calculated.

[0051] Alternatively, and particularly preferably, the measuring device and scale can be accurately designed for a particular measurement task using equations (7) and (9), respectively. The scale then already contains absolute values of the depth of field, so that the appropriate linear dimensions can be determined directly when reading off the scale. In particular, an example of this is shown in equation (5). For example, when using a medium with the refractive index n.sub.2=1.5, markings on the measuring scale representing a measure of a depth of field of 1mm are preferably placed at a geometric distance of about 2.1 mm from each other in the case of a scale angle of 45°.

[0052] A possible optical image of an exemplary measurement scale as it could be used in a measuring device according to the invention is shown in FIG. 4. In this example, the measurement scale again comprises two sub-scales, each scale section of which is a marker of periodic distances, in particular shorter and longer lines. For example, the longer lines with digits can be understood as representing position with mutual axial distances of 1 mm The shorter lines in between could represent position with mutual axial distances of 0.1 mm in one sub-scale and 0.05 mm in the other sub-scale. The marking with numbers makes counting and measuring even easier.

[0053] In particular, it can be seen in FIG. 4 that a central region of the measurement scale, namely that located in the specimen space near the focal plane, is shown in focus in the image area.

[0054] As the distance from the sharpest point increases, the measurement scale is further and further away from the focal plane and passes through the limits of the focus range. These limits and their distance to each other can be read at a glance or in a single image of the measurement scale through the optical structure without mechanical displacement of the measuring device after one-time positioning.

[0055] As mentioned above, FIG. 3C represents a sectional view through the device body. In it, exemplary side lengths of 25 mm of a corresponding cube are indicated. Particularly preferably, the side length of the device body, especially in the case of a cube shape, is in the range of about 5 mm to about 40 mm At this size, the device body is large enough to handle well manually and small enough for many applications.

[0056] However, especially for applications in the field of microscopy with high magnifications and very short focal lengths and focal distances, it is also desirable to be able to use smaller or thinner measuring devices. Exemplary variants of such measuring devices for microscopes with high magnification are shown in FIG. 5 and FIG. 6. Thus, in these preferred embodiments, the device body is particularly in the form of a small microscope slide plate. It is even particularly preferred to design the device body also according to the size and material of standardized microscope slide plate. It is even possible to use such already existing microscope slide plates directly as device bodies and to provide them, for example by laser engraving, with a corresponding measurement scale which, when the device body is placed on a microscope slide device, then runs at an angle of more than 0° and less than 90°, particularly preferably in the range between about 30° and about 60° relative to the optical axis of the microscope lens and can be read directly through the microscope. Such measuring devices in the dimension of microscope slide plates are on the one hand very easy to handle and on the other hand highly compatible with the microscope slide plates of many microscopes. They are especially suitable for optical structures with very short focal lengths.

[0057] The particular embodiment of FIG. 6 even has a recess in a central region of the light-emitting surface 22 to accommodate an immersion liquid. The measurement scale is preferably arranged between this recess and the opposite base area 20. Thus, this embodiment is also directly applicable for immersion microscopy by filling the well with an immersion liquid.

[0058] With the method according to the invention, the depth of field can be determined with particularly high accuracy and reproducibility and with little effort. For example, glass bodies can be manufactured as device bodies and thus as carriers of the measuring scale with high accuracy. Laser-assisted (holographic) insertion of the measuring scale in particular is also possible with very high accuracy. Overall, this allows manufacturing tolerances that are below the resolution limits of the measuring equipment 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 that can be used, almost free size scaling is possible, so that calibration and measurement devices can be manufactured in a size scale of a few hundred micrometers up to several centimeters. High-quality manufactured glass bodies (e.g. cuboids or cubes) also have very high edge and surface parallelism. In addition, the measurement scale (calibration pattern) has a known and very precisely manufactured angle, so that angular and distance errors are practically excluded or negligible if the measuring device is well placed in the measuring section.

[0059] However, the invention does not only concern the possibility of embedding a calibration pattern or a measurement scale into a fixed pattern carrier, but rather in general the 3-dimensional or holographic insertion of a characteristic calibration and measurement pattern, for example a (length) scale and possibly further patterns common from image processing (siemens star, line pairs, etc.), into a measurement space or specimen space, which is detected by an optical structure.

[0060] This can also be achieved, for example, the introduction of a light projection (or hologram) in smoke, liquid mist. Especially when using such a method according to a preferred embodiment of the invention, no fixed pattern carrier is required. This is particularly useful when the specimen space (measuring room) is very limited in space or very difficult to access for arranging or even fixing a sampled fixture due to the nature of the measurement to be made there.

[0061] For holograms or 3D light projections in liquid mist, smoke or similar, the arrangement is otherwise to be considered analogously. When the pattern is introduced in this way, there are also analogous advantages to using a measuring device according to the invention. By means of a simple image, a measurement of the depth of field can be performed immediately, without moving the specimen carrier or camera. Due to the light or laser technology used, almost free size scaling is possible even with a projection in smoke, liquid mist, or similar. Thus, an optical magnification setup (e.g. for microchannel flow) can be calibrated and measured in the same way as an optical reduction setup (e.g. for droplet experiments).

[0062] Preferred areas of application for a measurement method according to the invention for determining a depth of field of an optical structure by projecting a measurement scale into an optically dispersing medium are, for example, pressure chambers in which a spray is to be measured. In certain measurements, the deviation of particles from the focal plane is used to determine the position of particles in depth. Therefore, an accurate knowledge of the depth of field is quite crucial for such measurements. With the method according to the invention, for example, the depth of field can be re-measured very easily and quickly (especially without opening the pressure chamber) when the optics are changed or the optical structure is changed. In particular, the spray already present in the pressure chamber can be used as an optically dispersing medium. Through an optical window in the pressure chamber, the measurement scale can be projected into the pressure chamber by light projection such that the optical structure can detect this measurement scale due to the light dispersing by the optically dispersing medium.

[0063] Other particularly preferred areas of application for a method according to the invention concern PIV measurements (particle image velocimetry) or measurements of flows in ducts. Especially in many measurement constellations for flow measurement, it is very difficult or even impossible to measure the depth of field with a movable calibration pattern either due to the internal geometry of the flow arrangements or due to the external geometric boundary conditions. Thus, it is already difficult or hardly possible to bring the calibration pattern into the correct position in the area of the focal plane at all. Moving this along the optical axis in a controlled manner in order to measure the sharpness limits is very difficult or only possible with great inaccuracies.

[0064] In technical plants and also in scientific investigations, pipe flows of various kinds are very often measured. Measurement techniques such as PIV, shadowgraphs 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 is locally different. For the measurement, a LASER section through the tube illuminates the plane to be measured. The plane of the LASER section can be shifted as desired. One or two cameras with appropriate optics take the necessary images. The problem for a measurement of the depth of field of the camera(s) in this application is the accessibility into the vortex tube. For example, the tube is typically a complete, non-openable (Plexiglas) tube with dimensions of, for example, approx. 5-15 cm for the diameter and a length of 1.5 m.

[0065] Commercially, calibration plates for such tubes are usually only available as custom-made products. The effort and thus the costs of an individual production of calibration plates are extremely high due to the accuracy requirements. Moreover, holding the calibration plates in zthe middle of the tube is very challenging and precise adjustment of the position is also almost impossible. It is under such conditions that the method according to the invention can show its potential by light projection of a measurement scale into the measuring chamber. But even the use of a measuring device according to the invention can in this case already bring substantial advantages, since here no controlled mechanical displacement of a calibration pattern has to be performed during the measurement of the depth of field. The measuring device only has to be positioned once. For this purpose, the device body can also be adapted to the inner shape of the tube.

[0066] In the following, error limits and their possible reduction are still discussed. In particular, the error of the measuring device is composed of the geometric errors generated by inadequacies of the device body (e.g. glass cube) as pattern carrier and the patterns introduced as measurement scale along the scale line. This includes in particular the geometric error due to imperfect shape of the fixture body using the example of a glass cube as pattern carrier. The following influencing variables play a role: [0067] Surface flatness: typical values: ±0.08 μm [0068] Surface parallelism: very high accuracies are also achievable here.

[0069] As an alternative consideration, the beam deviation data for beam splitter cubes can be used for comparison here. This is a much more difficult fabrication because there is a bonded interface between two prisms arranged to form a cube. The typical value for the deviation of the orthogonal laser beam are assumed here to be <±5 arcmin

[0070] If the pattern is not carried by a glass body but is introduced by projection alone, e.g. in smoke, the accuracies of the projecting pattern generator have to be considered. No lump-sum estimate can be given here. However, due to mechanically high-quality constructions, these tolerances are also extremely small.

[0071] FIG. 7 illustrates the error calculation due to angular errors when viewing the patterns. Here, any refraction of the non-orthogonal light beam at the boundary surface of the pattern carrier is neglected due to the very small angles of well below one degree (Δθ<<1°) to be expected. The following facts emerge:

[00004] $\frac{d + Δ d}{d} = \frac{\sin (β + Δθ)}{\sin β}$

with β=90°−φ as the sample angle, preferably β=45°, and the angular error Δθ As an example, the errors for an angular deviation between −1° and +1° are summarized in the following table (for β=45°).

TABLE-US-00001 Δθ −1° −0.5° −5’ 0° +5’ 0.5° 1° [00005] $\frac{d + Δ d}{d}$ −1.76% −0.88% −0.15% 0% +0.15% +0.87% 1.73%

[0072] The following applies to possible pattern errors due to imperfect insertion of the pattern: The pattern is preferably introduced into the measuring section by engraving glass from the inside using a LASER. However, the pattern can also be introduced as a projection in mist or spray. For both, the point diameter and the resolution of the point grid in relation to the dimension of the measurement section is decisive.

[0073] For insertion into a glass body, a comparison of representative data on available equipment from various manufacturers (CERION laser GmbH and Wisely Laser Machinery Limited) yields the following results:

TABLE-US-00002 WLASER 3D Crystal/Glass Laser Engraving Machine Cerion C-professional (4 KB) Minimum Point Size: 20 μm 20 μm Repeat accuracy: 30 μm 30 μm Resolution: No information 800-1200 dpi

[0074] A dot size of 20 μm and a resolution of 800-1200 dpi can therefore be assumed. This thus corresponds to approx. 30-45 points per millimeter. At the lower resolution of 800 dpi, a maximum error of ±32 μm can thus be assumed. At a resolution of 1200 dpi, this corresponds to only ±21 μm. Such an error in determining the depth of field is perfectly adequate for most metrological tasks. As an example: With a depth of field of about 500 μm, as the measurement setup of an exemplary drop test rig with very high optical magnification has, the error is 4.2%. Other, more commonly used test stands have greater depths of field, e.g. in the range of 25 mm, so that the error is only 0.084%. Depths of focus of several centimeters are also common in experimental test rigs for heat transfer tests. In addition to the commercially available equipment, it has already been possible in academia to create cavities in the glass that have a spacing of only 6 μm, which can therefore reduce the error even further.

[0075] In case of holographic insertion by means of a beamer or similar devices, the considerations here are analogous and therefore depend on the pixel/voxel 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 measurement section or specimen space. Here, for example, the errors can be in the range of up to about ±30 μm, which is perfectly sufficient for measuring the depth of field. Other measures for determining the depth of field have significantly higher errors with comparable effort, for example when using a meter scale (approximately one division unit of the scale, e.g. ±500 μm). In this case, it is not possible to measure the depth of field because the error is of the same scale as the depth of field itself.

LIST OF REFERENCE NUMERALS

[0076] 10 Measuring device [0077] 12 Device body, block [0078] 14 Measuring axis [0079] 16 Scale line [0080] 18 Measurement scale [0081] 20 Base area [0082] 22 Light emission surface [0083] 100 Optical structure [0084] 102 Optical axis [0085] 104 Lens [0086] 106 Microscope slide device [0087] 108 Conventional calibration pattern

Measuring Device and Method of Determining a Depth of Field of an Optical Setup

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G01M11/0207

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G02B21/24

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