DE102012214019B3 - Measuring system for determination of reflectance characteristics of materials of e.g. solar mirror in solar power plant, has hood comprising recess or area over which beam is introduced, and tape pivotably arranged at surface of hood - Google Patents

Abstract

The system (1) has a radiation coupler (39) i.e. reflectance collimator, connected with a light source (35) for generating a light beam (31). The radiation coupler is arranged at a guide device (43) for guiding the radiation coupler along a preset guide track (45) aligned in a direction of a mirror material sample (19). A hood (3) comprises a recess (29) or a transparent area over which the light beam is introduced into an inner space (5) of the hood. A shadow tape (55) is pivotably arranged at an inner surface (7) of the hood. An independent claim is also included for a method for quality determination of a mirror material sample.

Description

The present invention relates to a measuring system for determining reflection characteristics of solar mirror materials.

In modern solar power plants, concentrating mirrors are often used. In the design of solar power plants, therefore, the reflection characteristic of different mirrors of interest to compare different mirror materials can. The solar mirror materials used are highly reflective. Most are silvered glass mirrors, rolled and polished aluminum or metal mirrors, polymer coating systems, and other highly specular mirror materials.

In analyzing the reflection characteristics of solar mirror materials, the hemispherical reflectance and the directional reflectance of the material are of particular interest. Further characteristics are the wavelength-dependent scattering as well as the reflection distribution at different angles of incidence and their polarization dependence.

The most important parameter for the suitability of a mirror for concentrating solar technology is a solar-weighted, directed reflectance. For this, the hemispherical measurement of the reflection spectrum in the solar wavelength range (λ = 250-2500 nm) is necessary so that a weighting of this with the standardized solar spectrum (ASTM G 273) can be performed.

The directional reflectance is usually measured with conventional reflectometers. In this case, the mirror material sample is illuminated with a defined light beam having an angle of incidence θ _i and the light reflected in the direction of the directed reflection with the angle of reflection θ _a = θ _i is received by a detector.

In such a measurement, the range defining the directional reflection is always dependent on the measurement aperture (either the detector size or a pre-set acceptance aperture). The radius of the measuring aperture produces an acceptance angle φ, which can be chosen in fixed steps in some measuring methods. However, such a measurement not only contains the narrow distribution of the directed reflex, but also an indeterminate portion of possibly existing scattering. On the other hand, a very wide, directed reflex can also be cut off or evaluated differently depending on the centering in the measuring aperture.

Spectrophotometers or Fourier transform spectrometers are currently used to determine the reflection spectrum in the solar wavelength range. Together with an integration sphere, the measurement of the hemispherical reflectance is spectrally resolved and with high accuracy. However, such measurements are less suitable for directional reflectance measurements of samples whose reflectivity is not perfectly reflective. The determination of the solar-weighted, directed reflectance φ _s of solar mirrors at a certain acceptance angle is therefore currently done by a combination of both measurements, but only an approximate value can be determined. This approximate value is used as a quality parameter for the evaluation of solar mirror materials, but this involves greater inaccuracies and may disadvantage some mirror materials.

The directional reflection behavior of the mirror sample is considered only under restricted conditions with respect to all three relevant parameters, the acceptance angle, the wavelength and the angle of incidence. For some types of mirrors, however, large measurement uncertainties occur and their properties are only insufficiently characterized by the limited acceptance angles.

For diffuse reflective materials, it is also known to determine a bidirectional distribution function. For example, a rasterizing goniometer can be used. In this case, the sample is illuminated at a certain angle of incidence and the reflected light is received by a detector. The detector moves the space above the sample step by step and determines an intensity matrix of many measuring points with which the distribution function of the reflected radiation can be derived. The light source can also be made movable, so that different angles of incidence can be realized. In order to ensure a very fine solid angle resolution, the system must be designed to be large and it requires a very precise mechanism. With the increasing number of measuring points and thus increasing angular resolution, the measuring time is quadratic.

There is thus basically the problem that a qualitative evaluation of the reflection properties of mirror materials for solar application is only approximated. There is no known system with which the solar-weighted, directional reflectance can be measured directly at any defined acceptance angle.

In practice, the measurement of the directed reflection often leads to large acceptance angles is used so that the directional reflected beam and adjacent scattering can not be distinguished from each other. In addition, commercially available reflectometers often have only a few discrete acceptance angles. However, measurement with such a device does not do justice to the use of a mirror for use in concentrating solar technology, as scattered components may also contribute to the energy harvesting process in addition to the main directional reflex, but may be lost in other cases. This applies above all to anisotropic reflection properties, which lead to greater losses for point-focusing solar collectors than with line-focusing systems. Small acceptance apertures also significantly degrade the measurement accuracy for non-perfect mirrors.

Furthermore, in the case of the known measuring methods, the directional reflectance can only be measured in an insufficiently wavelength-dependent manner. The known method for determining the directional reflectance is based on the assumption that reflection losses due to scattering are wavelength-independent and therefore approximately constant over the spectrum. However, this does not correspond to reality.

Finally, measurements with conventional reflectometers are usually measured at angles of incidence near vertical incidence (between 5 ° -15 ° from vertical). In reality, however, significantly larger angles of incidence occur at a solar collector and some mirror materials differ significantly, especially at shallow angles of incidence.

The determination of the bidirectional distribution function over a goniometer can theoretically solve these problems at least partially. However, the mirror material samples are highly reflective, so that conventional goniometers are generally only suitable to a limited extent.

It has also been found that a very high solid angle resolution is necessary for the determination of the shape of the very narrow, directed reflection, so that the determined distribution function can be correlated with the hemispherical reflectance. Due to the size of the mechanism and the size of the detector, a large measuring distance is required to achieve high solid angle resolutions so that suitable goniometers would have very large device dimensions. Furthermore, the duration of a measurement with such a goniometer would be several minutes to hours due to the large amount of data.

From Gregory J. Ward: Measuring and Modeling Anisotropic Reflection, Computer Graphics 26, 2 July 1992, 265-272, a measurement system for determining reflection characteristics is known, having the features of the preamble of claim 1.

However, this measurement system is of limited use for determining reflection characteristics of highly polished specular materials used for solar mirrors. US 6,577,397 discloses a measuring system with a hood forming a projection screen.

It is therefore an object of the present invention to provide a measuring system for determining reflection characteristics of solar mirror materials, with which the reflection characteristics can be determined with high accuracy and very quickly. In addition, the measuring system should enable a high solid angle resolution and still have the smallest possible external dimensions.

It is a further object of the present invention to provide an improved method for determining the quality of a specular material sample.

The measuring system according to the invention is defined by the features of claim 1. The method according to the invention is defined by the features of claim 14.

The measurement system according to the invention for determining reflection characteristics of solar mirror materials has a mirror holder for receiving a mirror material sample and a radiation detector with an image sensor and a receiving optical system for receiving light reflected by the mirror material sample. Further, a hood is provided which defines an interior space and which has a mirrored inner surface, wherein the mirrored inner surface is semi-ellipsoidal-shaped and has a first and a second focal point, wherein the first and the second focal point disposed in the interior of the hood are. The mirror material sample can be arranged via the mirror holder in the first focal point and the receiving optics of the radiation detector is arranged in the second focal point. The measuring system has a light source and a radiation integrator connected to the light source for generating a light beam directed onto the specimen material. The radiation input coupler is arranged on a guide device for guiding the radiation coupler along a predetermined guideway. The semi-ellipsoidal-shaped hood further has at least one recess or at least one transparent region, via which the light beam can be introduced into the interior of the hood. The transparent area can also be semi-transparent.

The measurement system according to the invention enables a new approach for the determination of the directional reflectance. The light beam introduced into the interior of the hood is reflected by the mirror material sample and irradiated in the direction of the mirrored inner surface. This in turn reflects the light in the direction of the radiation detector. Since the radiation detector with image sensor with its receiving optics is arranged in the second focal point, almost all the light reflected by the mirror inner surface can be detected via the receiving optics. By providing an image sensor, almost the entire interior space is simultaneously recorded by means of the radiation detector. The measurement system according to the invention thus enables a continuous measurement of the reflection distribution, which avoids the loss of information present in the prior art method as well as the measurement inaccuracy in the directional reflection measurement with undefined or limited acceptance angles. By using the image sensor, anisotropic scattering characteristics of the mirror material sample can be taken into account, since the use of an image sensor enables resolution in two orthogonal angular directions.

By providing a guide device for guiding the radiation incoupler along a predetermined guideway, virtually any angle of incidence of the light beam directed onto the specimen of mirror material can be made possible.

In contrast to scanning goniometers, which usually have a very long measurement duration, a determination of reflection characteristics can be carried out very quickly by means of the measuring system according to the invention, since by means of the radiation detector with image sensor usually a single or a few images suffice. The pixel position of the reflected light on the image sensor corresponds to the reflection space angle at the sample, defined by the azimuth angle and the polar angle. The recess in the semi-ellipsoid-shaped hood allows the introduction of the light beam in the interior of the hood in a simple manner. Depending on the intended use of the measuring system according to the invention, however, it may also be advantageous to provide a transparent region for introducing the light beam into the interior of the hood. Of course, the recess may be filled by a transparent material, so that the light beam can be introduced into the interior of the hood in an advantageous manner without foreign bodies can get into the interior.

In a preferred embodiment of the invention it is provided that the at least one recess is slit-shaped. When using a measuring system with a transparent region, the transparent region may be strip-shaped. In this way, the light beams can be directed advantageously at different angles of incidence on the mirror material sample.

In a particularly preferred embodiment of the invention, it is provided that the at least one recess is arranged in the half of the hood arranged above the specimen, the recess being up to a point of the hood, which by an intersection of an axis orthogonal to the specimen Hood is formed extends. In this way, it is ensured that the light beam can be directed at the mirror specimen in a large angle of incidence range and the rays reflected by the specular specimen are reflected by the mirrored inner surface in the direction of the second focal point and thus to the radiation detector.

The invention advantageously provides that the inner surface of the hood has an angular deviation of the surface normal at each point of the inner surface of at most 0.5 mrad from the ideal surface normal and / or a surface roughness of at most 150 nm, preferably at most 30 nm. It has been found that such a high precision of the optical shape enables a particularly accurate measurement. The low roughness of the inner surface is necessary to minimize the scattering of the light in the reflection from the inner surface. Such precise surfaces can be achieved for example by a very accurate production of a green body and subsequent finishing and polishing with a high-precision diamond lathe.

According to the invention, it is provided that the radiation input coupler is designed as a collimator, preferably a reflection collimator, wherein the collimator is connected to the light source via a light guide. Of course, it is also possible that the Strahlenseinkoppler is disposed directly on the light source and the light source is movable together with the Strahlininkoppler means of the guide device along the guideway.

The provision of a collimator as a radiation integrator allows the generation of a high-precision light beam onto the specimen of mirror material. Because the collimator is connected to the light source via a light guide, the radiation incoupler can advantageously be moved along the guide path independently of the light source. In addition, different light sources can be connected to the light guide, so that the measuring system according to the invention can be used very flexibly. It can, for example, measurements in both the white light or Sun spectrum as well as in certain wavelength bands by the use of spectral filters or monochromatic light sources are made possible.

The guide track of the guide device can be arranged outside the hood and / or curved, wherein the guide track preferably runs parallel to the inner surface of the hood. As a result, the radiation incoupler can be moved in a particularly advantageous manner during a measurement, so that the setting of different radiation angles of incidence on the sample of mirror material is made possible in a particularly simple manner.

The receiving optics of the radiation detector may include a fisheye lens. The use of a fisheye lens allows light rays with a viewing angle of almost 180 ° per angular direction to be imaged into the image sensor.

In a particularly preferred embodiment of the present invention, it is provided that a shadow band is arranged pivotably on the inner surface of the hood, wherein the shadow band is parallel to the inner surface and preferably has the width of the light beam. The shadow band can thus be pivoted through the beam path, so that the main reflex is hidden. As a result, the complete dynamic range of the detector can be used for the relatively weak scattering proportion of the specular specimen, which considerably increases the signal-to-noise ratio in the lateral areas of the scattering distribution. This allows a very accurate measurement of specimen samples.

The mirror holder may be formed as a three-point holder. As a result, the sample holder is very variable, so that the mirror plane of the sample of mirror material can be positioned precisely in the first focal point of the semi-ellipsoid. The mirror material sample can thus be aligned very accurately.

It can further be provided that the mirror holder is designed so as to be tiltable about a pivot axis extending through the first focal point, which axis runs orthogonally to a light incidence plane on the mirror. At very shallow angles of incidence, the main reflex is thrown to the lower edge of the ellipsoid when the sample is aligned parallel to the focal plane. The reflection thus takes place at a very shallow angle to the radiation detector and the receiving optics. As a result, the beam path is subject to greater optical errors on the inner surface and on the receiving optics. By being able to tilt the mirror holder, the main reflex for shallow angles of incidence can be directed into a more central region of the semi-ellipsoid so that the beam path is directed at a more favorable angle towards the radiation detector. As a result, optical errors can be avoided.

The invention advantageously provides that the radiation detector has an adjusting device, via which the radiation detector is adjustable along an axis running through the second focal point. The radiation detector is thus height adjustable. The position of the entrance area of the receiving optics of the radiation detector can thereby be moved into and out of the interior space. Thus, rays reflected from the outer edge of the ellipsoid can be imaged with the same quality as rays coming from the center region. For example, a measurement with the measuring system according to the invention can consist of three to five recordings, with the radiation detector being moved further into the interior space for each photograph. The different images are combined at a later time in the determination of the bidirectional distribution function. In this way, a correct bidirectional distribution function of the entire interior can be accommodated.

By way of example, the image sensor may have a minimum size of one-quarter inch, with the image sensor preferably having a fill factor of> 80%. The image sensor has a resolution of at least 8 megapixels. By using such an image sensor, a very high solid angle resolution can be achieved. For a precise determination of the bidirectional distribution function, for example, a resolution of approximately 785 nsr per pixel is necessary. This is achieved by the chip image sensor used in accordance with the invention. In this case, the image sensor may be formed, for example, as a monochrome chip. The high fill factor is advantageous because the optically effective area of the image sensor is as little as possible interrupted by readout pixels or inactive areas.

In a particularly preferred embodiment of the invention it is provided that the first and the second focal point have a distance between 60 mm and 80 mm, preferably 74 mm. In other words, the semi-ellipsoid shape of the inner surface approximates a hemisphere. The distance between the two focal points is chosen so that a mirror material sample with a diameter of about 50 mm can be arranged next to the receiving optics of the radiation detector.

As a light source, for example, a halogen lamp can be used. Before the coupling into the light guide, spectral filters as well as Be polarization filter upstream. Alternatively, there is the possibility that the light source consists of different monochromatic LED light sources, to which the light guides are coupled successively.

The radiation detector may be, for example, a camera.

• – Bestimmung des spektralen Gesamtreflexionsgrades der Spiegelmaterialprobe sowie dessen solare Gewichtung,
• – Bestimmung einer Verteilungsfunktion der reflektierten Strahlung,
• – Korrelieren des Gesamtreflexionsgrades mit der Verteilungsfunktion zur Bestimmung des gerichteten Reflexionsgrades als Funktion des Akzeptanzwinkels,
• – Charakterisierung der Spiegelmaterialprobe anhand dieser Funktion.

The invention further relates to a method for determining the quality of a mirror material sample. The following steps are provided:

• Determination of the total spectral reflectance of the mirror material sample and its solar weighting,
• Determination of a distribution function of the reflected radiation,
• Correlating the total reflectance with the distribution function to determine the directional reflectance as a function of the acceptance angle,
• - Characterization of the sample of mirror material by means of this function.

The directional reflectance as a function of the acceptance angle is thus independent of fixed acceptance angles or their accuracy of measurement. One thus obtains a reflectance in each solid angle. Characterized a characterization of the mirror material sample is possible in a particularly advantageous manner. The method according to the invention can provide that an inventive measuring system is used in the determination of the distribution function of the reflected radiation.

Furthermore, the inventive method can provide that in the characterization of the mirror material sample, an evaluation of the directional reflectance using a virtual round aperture or a virtual slit diaphragm occurs. As a result, a specific analysis can be carried out with respect to point-focusing or line-focusing solar collectors. The use of a virtual slit diaphragm results in a more meaningful rating for mirrors used in line-focusing systems. In such systems, anisotropic reflectance distributions have less impact on effectiveness.

In one exemplary embodiment of the method according to the invention, when determining the distribution function of the reflected radiation, the mirror material sample is illuminated in different spectral ranges or in white light to determine a wavelength dependence of the scattering behavior of the reflection, wherein the directional reflectance is additionally determined as a function of the spectral range. Thus, a directional reflectance with spectral information is obtained, whereby a characterization of the mirror material sample can be additionally performed on the basis of the spectral information. The method according to the invention is limited in this respect only by the sensitivity of the radiation detector. With currently available detectors thus the dependence is not possible in the entire spectral range of sunlight, but at least in the range of the largest weighting up to a wavelength of about 1000 nm.

In the following, the invention will be explained in more detail with reference to the following figures.

Show it:

1 a schematic sectional view of a measuring system according to the invention and

2 a schematic plan view of an inventive measuring system.

In the figures, an inventive measuring system 1 for the determination of reflection characteristics of solar mirror materials shown schematically. The measuring system 1 has a hood 3 on that has an interior 5 limited. The hood 3 has a mirrored inner surface 7 on, with the inner surface 7 semi-ellipsoid-shaped. On the inner surface 7 opposite side is the interior 5 through a cover plate 9 limited. The cover plate 9 is to the interior 5 designed to absorb light.

The mirrored inner surface has a first focus 11 and a second focus 13 on that on a common focal plane 15 are arranged.

On the cover plate 9 is a mirror holder 17 arranged, which is a mirror material sample 19 holds. The mirror holder 17 is in this way on the cover plate 9 arranged that the sample 19 with its mirror surface 21 in the first focus 11 is arranged. Furthermore, the measuring system according to the invention 1 a radiation detector 23 on, who has an image sensor 25 and a receiving optics 27 has. The radiation detector 23 is in this way on the cover plate 9 arranged that the receiving optics 27 in the second focus 13 is arranged. The receiving optics 27 may be, for example, a fisheye lens.

The hood 3 has a slot-shaped recess 29 on, over which a ray of light 31 in the interior 5 the hood 3 can be initiated. The recess is arranged on the hood, that the recess up to a point of the hood 3 extends through an intersection of an orthogonal to the specimen 19 extending axis 33 is formed.

For the introduction of the light beam 31 in the interior, the measuring system has a light source 35 and one via a radiation conductor 37 with the light source 35 connected radiation Einkopoppler 39 on. The light guide 37 may be, for example, a fiber optic cable. The light guide 37 can also filter 41 , such as color filters or polarizing filters, upstream. Of course, the filters 41 be arranged at a different location in the radiation path. The radiation Einkopoppler 39 may be, for example, a reflection collimator, over which a directed light beam 31 can be generated.

The radiation Einkopoppler 39 is on a guide device 43 arranged by means of the radiation Einkopplpler 39 on a guideway 45 can be moved. This is the radiation Einkopoppler 39 on a sledge 47 attached, along the guideway 45 can be moved. In this way, different angles of incidence of the light beam can be 31 to the test 19 produce. For example, angles of incidence of about 5 ° -60 ° can be set.

The light source 35 can produce white light or be a monochromatic light source.

The guideway 45 extends along a circular path with a center in the first focal point 11 ,

The mirror holder 17 can be designed as a three-point mount, with over screws 49 a very accurate positioning of the mirror material sample 19 in the first focus 11 is possible. Furthermore, the mirror holder 17 a tilting device, not shown, via which the mirror holder 17 one by the first focal point 11 extending pivot axis which is orthogonal to the light incident plane extending on the mirror material sample, tiltable.

The radiation detector 23 has an adjustment 51 on, over which the radiation detector along one through the second focal point 13 extending axis 53 is adjustable.

In the interior 5 is a swiveling shadow band 55 arranged parallel to the inner surface 7 runs. The shadow band 55 preferably has the width of the light beam 31 , About the shadow band 55 can be the main reflex of the light beam 31 Hide what's in the 1 is shown schematically.

The mirror holder 17 For example, a magnetic holder for the mirror material sample 19 so that the mirror material sample 19 magnetically on the underside of the mirror holder 17 is fastened.

The first and the second focal point 11 . 13 may for example have a distance of 74 mm. It can through the mirrored inner surface 7 formed semi-ellipsoid having a size in which the large half-axis has a length of 345 mm and the small semi-axis has a length of 343.01 mm. The measuring system according to the invention thus has a diameter of about 70 cm and is thus convenient to use in a laboratory.

The hood 3 can be made from a solid aluminum body in lightweight construction. The mirrored inner surface 7 can be created for example by diamond polishing, with no further mirrored coating is required.

The first and the second focal point 11 . 13 For example, they may be 74mm apart. This distance is sufficient that the mirror material sample 19 may have a diameter of about 50 mm and still the radiation detector 23 and the receiving optics 27 in the second focal point 13 can be arranged.

The semi-ellipsoid-shaped inner surface 7 deflects all after reflection from the specimen of mirror material 19 outgoing radiation on the receiving optics 27 and thus on the image sensor 25 , Each pixel corresponds in each case to a reflection angle at the mirror material sample 19 ,

With the measuring system according to the invention 1 Thus, a bidirectional distribution function of the light reflected in the entire half-space can be measured, over which an alternative possibility of characterizing the sample of mirror material is created. For this purpose, the total spectral reflectance of the specimen of mirror material is also determined. The overall reflectance is correlated with the distribution function of the reflected radiation and a directional reflectance as a function of the acceptance angle is obtained. In addition, the mirror material sample can be characterized in an advantageous manner.

Claims (17)

Measuring system ( 1 ) for determining reflection characteristics of solar mirror materials with a mirror holder ( 17 ) for receiving a mirror material sample ( 19 ), with a radiation detector ( 23 ) with image sensor ( 25 ) and a receiving optics ( 27 ) for receiving the sample of mirror material ( 19 ) reflected light, with a hood ( 3 ), which has an interior ( 5 ) and a mirrored inner surface ( 7 ), wherein the mirrored Inner surface ( 7 ) is semi-ellipsoid-shaped and has a first focal point ( 11 ) and a second focal point ( 13 ), wherein the first and the second focal point ( 11 . 13 ) in the interior ( 5 ) the hood ( 3 ), the mirror material sample ( 19 ) via the mirror holder ( 17 ) in the first focal point ( 11 ) and the receiving optics ( 27 ) of the radiation detector ( 23 ) in the second focal point ( 13 ) and with a light source ( 35 ) and one with the light source ( 35 ) radiation input coupler ( 39 ) for producing a sample of the mirror material ( 19 ) directed light beam ( 31 ), wherein the radiation input coupler ( 39 ) on a guide device ( 43 ) for guiding the radiation injection coupler ( 39 ) along a predetermined guideway ( 45 ), and wherein the hood ( 3 ) at least one recess ( 29 ) or at least one transparent region over which the light beam ( 31 ) in the interior ( 5 ) the hood ( 3 ), characterized in that on the inner surface ( 7 ) the hood ( 3 ) a shadow band ( 55 ) is arranged pivotably. Measuring system ( 1 ) according to claim 1, characterized in that the at least one recess ( 29 ) slit-shaped, or that the at least one transparent region is strip-shaped. Measuring system ( 1 ) according to claim 1 or 2, characterized in that the at least one recess or the at least one transparent region in the above the mirror sample ( 19 ) arranged half of the hood ( 3 ) is arranged, wherein the recess ( 29 ) or the transparent area up to a portion of the hood ( 3 ) passing through an intersection of an orthogonal to the mirror sample ( 19 ) axis ( 33 ) with the hood ( 3 ) is formed extends. Measuring system ( 1 ) according to one of claims 1 to 3, characterized in that the inner surface ( 7 ) the hood ( 3 ) an angular deviation of the surface normal at each point of the inner surface ( 7 ) of not more than 0.5 mrad from the ideal surface normal and / or has a surface roughness of not more than 150 nm. Measuring system ( 1 ) according to one of claims 1 to 4, characterized in that the radiation input coupler ( 39 ) is designed as a collimator, wherein the collimator via a light guide ( 37 ) with the light source ( 35 ) connected is. Measuring system ( 1 ) according to one of claims 1 to 5, characterized in that the guideway ( 45 ) of the guiding device ( 43 ) outside the hood ( 3 ) and / or curved. Measuring system ( 1 ) according to one of claims 1 to 6, characterized in that the receiving optics ( 27 ) of the radiation detector ( 23 ) has a fisheye lens. Measuring system ( 1 ) according to one of claims 1 to 7, characterized in that the shadow band ( 55 ) parallel to the inner surface ( 7 ) and preferably the width of the light beam ( 31 ) having. Measuring system ( 1 ) according to one of claims 1 to 8, characterized in that the mirror holder ( 17 ) is designed as a three-point holder. Measuring system ( 1 ) according to one of claims 1 to 9, characterized in that the mirror holder ( 19 ) one by the first focal point ( 11 ) pivot axis orthogonal to a light incident plane on the specimen of mirror material ( 19 ) runs, is tiltable. Measuring system ( 1 ) according to one of claims 1 to 10, characterized in that the radiation detector ( 23 ) an adjusting device ( 51 ), over which the radiation detector ( 23 ) along one through the second focal point ( 13 ) axis ( 53 ) is adjustable. Measuring system ( 1 ) according to one of claims 1 to 11, characterized in that the image sensor has a minimum size of ¾ '', wherein the image sensor preferably has a fill factor> 80%. Measuring system ( 1 ) according to one of claims 1 to 12, characterized in that the first and the second focal point ( 11 . 13 ) have a distance between 60 mm and 80 mm, preferably 74 mm. Method for determining the quality of a mirror material sample ( 19 ) comprising the following steps: - determination of the total spectral reflectance of the specimen of mirror material ( 19 ), - determining a distribution function of the reflected radiation, - correlating the total reflectance with the distribution function of the reflected radiation to determine the directional reflectance as a function of the acceptance angle, - characterizing the sample of mirror material by means of this function. A method according to claim 14, characterized in that in determining the distribution function, a measuring system according to one of claims 1 to 13 is used. A method according to claim 14 or 15, characterized in that in the characterization of Mirror material sample an evaluation of the directional reflectance using a virtual round aperture or a virtual slit diaphragm is done. Method according to one of claims 14 to 16, characterized in that in the determination of the distribution function of the reflected radiation, an illumination of the mirror material sample ( 19 ) in different spectral regions or in the white light for determining a wavelength dependence of the scattering behavior of the reflection, wherein the directional reflectance is additionally determined as a function of the spectral range.

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