DE10356352B3 - Device for calibrating measurement devices for quantitative infrared radiation measurement has metal cavity radiator with small downward-facing aperture, collimator below aperture at distance so its focal point is exactly in aperture center - Google Patents

Abstract

The device is in the form of a standard for calibrating measurement devices for quantitative infrared radiation measurement and has a cavity radiator made of metal with a small downward-facing opening (2) acting as an aperture and a collimator (6) arranged below the aperture at a distance so that its focal point (27) lies exactly in the center of the aperture.

Description

The The invention relates to a device in the form of a standard for calibration of measuring devices for quantitative infrared radiation measurement.

Infrared sensors, such as spectrometers, radiometers, pyrometers often have a large aperture, for example, up to 30 cm in diameter and more, such as e.g. one Telescope, especially if used for remote measurements. Should with them quantitative measurements are carried out, they must, since this is scientific measurements in the non-custody area trades, be calibrated. For the field inserts typical for these devices, (mobile in the area) under harsh conditions, for example, on the plane, are repeated Calibrations are required which require field-standard calibration standards. These must usually for calibration to different temperatures be heated, very rarely also be cooled.

In DE 101 10 131 A1 The simplified automatic pyrometer calibration is to be made possible by the realization of several fixed point temperatures during a heating and cooling process without additional changes to the measuring arrangement. For this purpose, designed as a cavity radiator fixed spotlight is provided, which consists of several parts made of ceramic materials and which contains a plurality of chambers for encapsulating fixed point materials. Such a device can be used for one-cycle calibration of radiation thermometers (pyrometers).

In US 2003/0042422 A1 are more for simulating a black body Plates for absorbing and reflecting electromagnetic radiation intended. Incident electromagnetic radiation is emitted by a Plate to the next reflected until the direction of electromagnetic radiation is reversed. Once the electromagnetic radiation reflects becomes the majority the electric radiation absorbs, with the result that one negligible Amount of incident electromagnetic radiation escapes.

In US 6,390,668 B1 a thermal radiation source is provided for calibrating and testing infrared thermometers and detectors. In US 6,447,160 B1 For example, a cavity of a blackbody having two types of wall surfaces is described, the first type having a high emissivity and the second type having a low emissivity. By combining these two types of surface, the influence of ambient temperature in the black body can be reduced while at the same time the emissivity approaches unity.

As standards for the calibration of such radiation measuring devices in the infrared spectral range radiation sources are used whose construction and mode of operation should give them the property of black emitters. Ie. the emissivity of the sources becomes ε _N = 1, their temperature T is assumed to be uniform over the entire area of the normal relevant for the calibration and - during the measuring time for calibration - as being constant in time. Then, the connection between the measured variable and the measured value required for the calibration via the Planck radiation law is solely determined by the temperature T of the calibration radiator - and thus clearly and only dependent on the accuracy of the knowledge of this temperature T.

For calibration, the normal must completely cover the aperture of the radiometer and also fill its field of view angle so that clear and defined conditions exist. A high emissivity of ε _N = 1 is best achieved through a cavity with a large inner surface and small aperture ("cavity emitter"), the small aperture is the black emitter, the closer the aperture is to the inner surface, the closer At ε _H = 1, the emissivity is determined, and the emissivity of the material of the inner surface also plays a role.

However, in view of the large apertures of the devices to be calibrated, the radiant aperture must be large, namely up to 30 cm in diameter and more. If almost ε _H = 1 is to be realized, a very large cavity is required. A big, unwieldy normal would be the consequence, which would be unsuitable for field use. Furthermore, an enormous effort would be required to heat the inner surface and to stabilize at a homogeneous temperature with deviations of, for example, ΔT <0.1 K.

It As a rule, therefore, radiation standards are not used as cavity radiators. but in the form of extended surfaces, i.e. "Panel radiator", usually in the form of Metal plates, used slightly larger than the aperture of the calibrating device are. By controlled, electric heating, they should as possible homogeneous surface temperature to be brought. The bigger the radiator surface the harder it is to reach that goal. In the field complicate as well Environmental influences, such as Wind, sun, the temperature during to keep the measuring time constant.

By structuring the surfaces, for example by furrowing and coating with high-emitting materials in the form of paints, an emissivity near ε _F = 1 should be realized, which is only possible to a limited extent. Therefore, surface radiators always have a lower emissivity than comparable cavity radiators. Unintentional reflections from radiation sources, such as electronics, operating personnel, are therefore possible and falsify the calibration. Field experiments must therefore be built with particular care.

The Structure disturbs Furthermore the temperature homogeneity: the furrows are warmer in the "valley" than at the tips. Structuring and painting also distort the Temperature measurement, as the temperature sensor for the control usually in the base material the radiator is fixed, which determines the radiation Temperature but the temperature is directly at the surface.

Farther are high emissivity paints in the usable temperature range limited can be used, for example "Nextel velvet black" is specified only up to 140 ° C. Calibrations included Temperatures even up to 1000 K and more are in certain applications desirable, For example, in investigations of combustion processes. The shortcomings Calibration is often the dominant source of error quantitative measurements. The meter would be able to measure more accurately but the calibration errors prevent that.

adversely In the prior art, the emissivity and temperature homogeneity of radiation standards for the Calibration of large-aperture infrared radiation meters very often not meet the requirements of the properties of the calibrating device are predetermined, and thereby an undesirable error in the calibration caused, which in turn adversely affects the measurement result effect. It is also particularly disadvantageous that the shortcomings in terms of the emissivity and in particular the temperature homogeneity not or only with big ones Effort must be prevented or quantified. That's why they usually neglected and therefore the measurement uncertainty can not be reliably quantified become.

adversely is also that said inadequacies and problems with increasing aperture (radiating area) of the Calibration standards grow. Another disadvantage is the limited temperature range the available Floodlights. The disadvantage is with surface spotlights also the big one Need for heating power by the large, thermally uninsulated aperture is conditional, as well as the usually long time until the target temperature is achieved stable. This is especially true in environments where the available Heating power is limited, for example, in field measurements or on satellites, often a big one Disadvantage.

task The invention is a device in the form of a standard for Calibration of measuring instruments to create, so a higher reliability and accuracy of radiometric / spectral radiometric calibration allows and thereby the uncertainty in quantitative, radiometric / spectral radiometric Measurements are reduced.

According to the invention is for solution the task of a small cavity radiator made of metal with an aperture intended. To create a correspondingly large aperture of the cavity radiator is additional a collimator, i. an optical system for beam expansion, aligned under the small aperture of the cavity radiator in arranged at such a distance that its focal point exactly in the center of the aperture opening lies. Advantageous embodiments of the invention are the subject further claims.

following The invention will be explained in detail with reference to the drawings. It demonstrate:

1a a preferred embodiment of the invention a Kalibrierstrahler with holding frame and housing in the viewing direction of the radiating aperture;

1b a sectional view taken along a section line AA in 1a through calibration emitter, holding frame and housing;

2a opposite 1a about 45 degrees rotated calibration emitter with holding frame and housing again in the direction of view of the radiating aperture;

2 B a sectional view taken along a line BB in 2a through calibration emitter, holding frame and housing;

3 in section an enlarged detail view of a portion of a wall of the cavity of the Kalibrierstrahlers;

4 a sectional view through the cavity;

5 a preferred arrangement of Kalibrierstrahlers and a collimator;

6 a sectional view with an additional temperature sensor, and

7 an enlarged sectional view of a device for calibrating the temperature sensor.

In 1a to 2 B and 3 is a preferably spherical cavity 10 a hollow sphere 1 of metal, preferably copper, with an opening 2 represented. The spherical shape has advantages in terms of size, weight and heating power required, but manufacturing technology can only be realized with more effort. Other forms of cavities, such as cylinders, cones, double cones, etc. are also suitable in principle.

The hollow sphere 1 consists of two equal-sized hemispherical shells 11 . 12 of which the one half shell 11 in the apex (Pol) the preferably circular opening 2 and the other half shell 12 has no such opening. Preferably at the apex of the half shell 12 (without opening), ie at the closed pole is on the outer wall surface 4 at least one temperature sensor 70 ( 1b . 2 B ) with good heat transfer, whose supply lines, in 1b and 2 B are indicated only by short lines, outwardly through a wall of a housing 16 to a controller, not shown, as well as a memory and display device, also not shown. The temperature sensor 70 is preferably a PT100 element.

In inner wall surfaces 3 the two hemispherical shells 11 . 12 are concentric or spiral-shaped, preferably approximately V-shaped furrows 31 ( 3 ), which connect seamlessly to each other and parallel to the "equator" 13 or from the poles to the "equator" 13 run. The grooves have flank angles in the range of about 30 ° to 60 ° and a depth of preferably 1 to 2 mm. The half-shells 11 . 12 be after introducing the furrows 31 heated to about 700 ° C, and that until the inner wall surfaces 3 oxidize ie scale to copper oxide CuO.

The outer wall surfaces 4 the two hemispherical shells 11 . 12 each with a spiral from the pole to the equator 13 extending groove 41 Mistake. The groove 41 serves to accommodate an electric heating wire 5 , is preferably a straight wall recess with a straight wall 42 and semi-circular bottom 43 ,

The slope of the spiral groove 41 is such that the distances in which the heating conductor 5 to lie on the ball, are small and in the range of a few millimeters. The length of the heating conductor 5 is the length of the spiral groove 41 adapted, ie the heated part must be in the spiral groove 41 lie and only the cold ends are led to the outside.

Width and depth of the groove 41 are sized so that the inserted heating wire 5 good thermal conductivity on the walls 42 the groove 41 is present and after its insertion by caulking 44 the upper groove border 45 a permanent thermal contact given and a mechanical separation of hollow ball shell and heating wire 5 is prevented.

3 shows by way of example the cross section of the spiral groove 41 partly with inserted heating conductor 5 , In order to ensure the best possible heat transfer, the lower part of the groove ( 41 ) a semicircular bottom 43 while the walls 42 the groove 41 straight, ie, tangent to the semicircular bottom 43 run. The maximum groove depth is a few tenths of a millimeter less than the diameter of the heating element 5 ,

This allows the heating conductor 5 by caulking 44 in the grooves 41 to fix. In this case, for example, caulked at a distance of about 20mm on a width of about 5 mm. This is the heating conductor 5 with large contact surface safely in the grooves 41 held. Nevertheless, it remains in the longitudinal direction of the heating conductor 5 an expansion possibility, so that stresses due to different thermal expansion of heat conductor and ball material are avoided.

The Wall thickness The hemispheres are dimensioned so that a change in the over the Heating wires supplied heating energy no temperature inhomogeneity on the inner sphere surface caused. That an increase in temperature of the heating wire leads though to a temperature inhomogeneity on the outer wall, the wall thickness but is so big that this inhomogeneity is balanced when the temperature increase the inner wall of the ball reached.

The half-shells 11 . 12 show at the equator 13 clamping 14 and retaining flanges 15 on. For example, by screwing the clamping flanges 14 are the two half-shells 11 . 12 firm and good thermal conductivity to the hollow sphere 1 connected. Through the retaining flanges 15 is a thermally insulated holder of the hollow sphere 1 created in a preferably metallic housing 16 by means of a holding frame 17 is attached. Between retaining flange 15 and support frame 17 are thermally insulated sockets 18 intended. As insulation material for the sockets 18 For example, glass fiber reinforced Teflon is used for temperatures below 200 ° C and ceramic material for higher temperatures.

The rooms 19 between the hollow sphere 1 and walls of the housing 16 are with thermal iso lierendem material, such as filled with aluminum foil ceramic fleece or bulk filled. The housing 16 is compared to the hollow sphere 1 and dimensioned taking into account the operating temperature range of the calibration heater so that there is sufficient space for sufficient thermal insulation available.

In the case 16 is an opening 20 formed, via which the radiation of the hollow sphere 1 can escape and therefore according to the ball opening 2 is dimensioned.

The opening 2 the hollow sphere 1 is with a tube 21 in the form of a to the ball 1 attached or turned flange ( 1b ) as a transition to the housing opening 20 Mistake. The hollow sphere 1 is so in the case 16 mounted that tube 21 with the housing opening 20 flees. The insulating material is recessed in this area, so that an unimpeded flow of radiation from the hollow sphere 1 is ensured to the outside.

In operation, the radiant opening 2 directed downwards. For temperature measurement, control and calibration, Pt100 probes are used 70 ( 1b . 2 B ) intended. Preferably, a quasi-continuous control of the temperature with a digital PID controller and the ability to store the temperature (s) with a time stamp is used.

For uniform heating of the spherical surface preferably sheathed heating conductors or wires are provided, which consist of a metal tube whose diameter is a few millimeters and its wall thickness is usually 0.1 mm to 0.2 mm. In the interior of the metal tube, the actual heating conductor is a resistance wire which is electrically insulated from the metal tube by a powdery non-conductor, for example magnesium oxide. To make an electrical connection to the ends of the sheathed heat conductor 5 to allow the ends are not heated. At the ends, for example, with a length of some 10 cm, the resistance wire is replaced by a wire with good electrical conductivity.

The advantages of a jacketed heating conductor 5 are the following: The actual heating conductor / wire is insulated from the metal tube. Therefore, a direct attachment of the heating leads 5 on a metallic surface in the spiral groove 41 with straight walls 42 and semi-circular bottom 43 to no short circuit.

The sheathed heating conductor 5 is mechanically much more robust than, for example, the resistance wire alone. He can therefore much easier on the hollow sphere 1 be fixed. Due to the thin wall thickness of the metal tube and the powdered insulation is the jacketed heating conductor 5 sufficiently flexible to on the hollow ball surface, preferably in spiral grooves 41 to be relocated.

In particular, when black bodies are to be heated to high temperatures of 500 to 1000 ° C, it is important to ensure that the coefficient of thermal expansion of a jacketed heating element 5 as well as possible with that of the material of the hollow sphere (Nute 41 ) to match. However, since this is not always possible exactly, relatively hard connections such as gluing or brazing are prohibited.

The small bright opening allows the use of a relatively small cavity. Its design is as a hollow ball, based on the size of the inner spherical surface, the smallest possible Body. This fact and the choice of material, for example, from good thermally conductive copper in connection with the specific structure described above the heating and the thermal insulation make for a good one thermal homogeneity the inner surfaces and for that a homogeneous radiance at the radiating aperture. Furthermore, it is characterized a pronounced independence of environmental influences, such as Sun, wind, etc. reached.

The furrowing of the inner surfaces increases the surface area and thus the emissivity. The scaling causes an additional increase in the emissivity. A further coating of the inner surfaces is therefore not necessary, because the interaction of cavity, surface structure and surface material (CuO) leads to an emissivity at the small opening, which is sufficiently close at ε _H = 1.

Of Further, the structure allows an unadulterated temperature measurement the inner surfaces. Because the inner surfaces not coated with thermally limiting materials can The calibration lamp can also be operated at high temperatures. Overall, despite thermal insulation, a smaller, more compact Construction possible, the above In addition, a low demand for heating energy requires what, in particular In field or aircraft deployment is of great advantage.

For a calibration, a fixed positioning of the radiating opening is necessary, wherein the focal point of the collimator must be in the center of the opening. Even with mechanical vibrations, such as in an aircraft use, this optical adjustment must not change. The way of mounting the cavity in the housing causes a very robust and vibra tion-insensitive construction, which ensures a stable optical adjustment even in harsh environments.

The downwardly directed opening of the radiator ensures that the heated air remains in the interior of the cavity, resulting in improved temperature homogeneity. The measurement of the temperature T _{H of} the cavity is preferably carried out with Pt100 sensors and thus more accurate than for example with thermocouples. That's important because it's all about absolute accuracy.

A will further increase the absolute accuracy of the temperature achieved by that the actual used sensors be calibrated before installation. This excludes errors due to scatters in the manufacturing process. Failure due to aging of the temperature sensor will be avoided by post-calibration. This is a simple device provided, which simplifies this process and thus safe, fast and cheap to do power.

As in 5 is shown as a collimator 6 preferably uses an off-axis parabolic mirror, which causes a beam deflection by preferably 90 °. However, arrangements with other deflection angles are possible. Preferably, no further mirror is used; however, constructional constraints may require a deflecting mirror. The collimator 6 has a diameter that is the aperture of a gauge to be calibrated 8th completely covered. The reflecting surfaces 61 of the collimator 6 are polished and preferably coated with gold.

The reflectance of these mirrors is not ρ _S = 1, but for each surface only about ρ _S = 0.98 to 0.99. The total degree of emission ε _{ges of} the arrangement of cavity radiator and collimator is reduced by: ε _ges = ε _H ρ _S , ie ε _ges ≦ ρ _S. It is advantageous that this influence is known and - regardless of the ambient conditions - constant and therefore correctable.

The correction is done by measuring the temperatures of all mirrors. Thus, the radiation emitted by the mirrors radiation can be determined and taken into account. The prerequisite for this is that the mirror substrate has a good thermal conductivity. This is accomplished through the use of metals such as aluminum as a substrate. The measurement of the temperature T _S of the mirror / s is preferably carried out with at least one temperature sensor, Pt100 sensor 71 inside the mirror substrate, close to the reflective surface 61 , For this purpose, the substrate of the mirror has at least one bore 62 from the side or from below to near the reflective surface 61 is guided.

By additional thermal isolation of the mirror (s) is thermal inhomogeneity and especially fast changes the mirror temperature during a calibration counteracted. These measures cause a high temperature stability of the mirror.

It can be used for metals such as aluminum or copper. The mirror is on a bracket 63 so mounted and optically adjusted that its focal point 27 in the center of the opening 2 of the cavity 1 lies where it is with the inner surface 3 flees. The device to be calibrated 8th is close to the collimator 6 and is mounted and optically adjusted so that the optical axis 28 of the collimated beam with the optical axis 101 of the device 8th is congruent. The bundle cross section 29 covers the aperture of the device, which is not visible in the side view 8th Completely.

In order to prevent errors of the temperature sensors due to aging, the sensors are recalibrated at certain intervals. 6 shows a further sectional view with an additional temperature sensor 72 and a device for calibrating the temperature sensor. This is on the wall surface 4 the hollow sphere 1 , Preferably at the closed pole, a small annular flange 46 provided (turned on or soldered), preferably made of the same material as the hollow ball 1 , For example, from copper.

The flange 46 serves together with a pipe 9 made of poorly heat-conductive material, such as ceramic, as a guide for a in 6 Reference sensor, not shown, only for calibrating the built-in temperature sensor 72 is used, whose supply in 6 are shown shortened, but not their further course through the housing wall 16 outward.

This guide made of flange 46 and pipe 9 Prevents the calibration probe from being accidentally applied to the built-in probe when it is inserted into the assembly 72 is attached and damaged or destroyed. At the same time, the arrangement ensures that the calibration probe is very close (a few millimeters) next to the permanently installed Pt100 probe 72 on the hollow sphere 1 can measure. The pipe 9 flows into a box-shaped lock 91 on the housing wall 16 is mounted. In the lock 91 is removable thermal insulation material 92 in the form of a lock 91 densely filled blocks. A preferably by pivoting openable lid 93 closes the lock 91 outward.

In normal operation is the insulating material 92 in the lock 91 and the lid 93 is closed. For recalibration of the permanently installed sensor 72 becomes the lid 93 opened and the insulation 92 away. A high-precision temperature sensor is through the pipe 9 introduced and touches the hollow sphere surface 4 inside the flange 46 , To ensure a good heat transfer the sensor is fixed on the hollow sphere surface 4 to press.

By heating the hollow sphere 1 and comparing the sensor temperatures then becomes a calibration curve for the built-in sensor 72 added. Disassembling the probe 72 and calibration in an additional heat bath, such as water or silicone oil, is not required. The calibration is thus considerably simplified. Their reproducibility is ensured by the feeler 72 not manipulated. The arrangement of pipe 9 and lock 91 with insulating material 92 and lid 93 it is not necessary to remove the insulation 19 in the case 16 , Damage to the built-in sensor 72 or its connections is also excluded in this way and the entire process of recalibration can be performed quickly by untrained personnel and thus cost.

There may be other temperature sensors elsewhere on the surface 4 the hollow sphere 1 be attached. These Kings nen serve any deviations from the temperature homogeneity of the hollow ball surfaces 3 . 4 capture. They can also be used for temperature control and redundancy in case of sensor failure 70 . 72 Offer.

If deviations from the temperature homogeneity are detected, then these are taken into account in the calibration process in such a way that the inner wall surface 3 is calculated as being composed of segments of different temperature. The area of the segments results from the arrangement and position of the additional temperature sensor. The total radiance is the sum of the radiances of all segments. For this purpose, the additional sensors 70 . 72 evenly, ie at equal distances from or among each other, on the spherical surface 4 distributed mounted.

For a highly accurate calibration, the emissivity ε _{H of} the cavity radiator and the reflectance ρ _S of the mirror / s are also determined by measurement or calculation and used in the following expression: L N = ε H ρ S L H + (1 - ρ S ) L S ,

In this case, L _{N is} the radiance of the normal consisting of cavity radiator and collimator, with L _H the radiance of a black radiator of the temperature T _{H of} the cavity, with L _S the radiance of a black radiator of the temperature T _S / of the mirror / s, with ε _H denotes the emissivity of the radiating opening of the cavity radiator and with ρ _S the reflectance of the reflecting surface (s) of the mirror / s.

With the help of the collimator, the following effects and improvements have been achieved. Due to the optics, reflected ambient radiation, which otherwise would be very disturbing, since it can only be quantified very inaccurately, does not reach the measuring device to be calibrated, as is the case when using a surface radiator. By measuring the mirror temperatures their influence is monitored, recorded and thus correctable. Due to the low emissivity of the mirror (ε _S = 0.01 to 0.02), a temperature inhomogeneity on the mirror surface has a much smaller effect than with an equally large area radiator with a high emissivity ε _F.

following are given algorithms for sizing a calibration unit. The emissivity of a cavity radiator is calculated as follows.

Here, with ε _H, the emissivity of the radiating opening of the cavity radiator, with ε _O the emissivity of the inner surface of the cavity, with s the area of the aperture opening 2 of the cavity radiator 1 and S denotes the area of the inner surface of the cavity. The formula for ε _H is taken from
Wolfe, WL Zissis, GJ, editors: The Infrared Handbook, Environmental Research Institute of Michigan, Office of Naval Research, 1978) and verified by experiments:
Bauer, G .: For determining the emissivity of cavities by reflection measurements, optics 28, Issue 2 (1968/69), and:
Bauer, G., Bischoff, K., Evaluation of the emissivity of a cavity source by reflection measurements, Applied Optics, Vol. 12, 1971.

Of the Value of emissivity for different surfaces can be taken from the literature. For sizing the collimator on the one hand is the geometry of the optical system of the calibrated radiometer on the other hand, the geometry of the radiating aperture of the Cavity radiator. For the sake of simplicity, here is of circular symmetrical Geometries of both assumed that far outweigh in practice.

The aperture of the optics of the measuring system is thus circular and has an area A _M with a radius r _M. The half angle of the cone-shaped field angle of the measuring instrument is β _M , which is the solid angle covered with it: Ω M = 2π (1 - cosβ) sr ≈ πβ 2 , where the approximation is for small angles β.

The aperture of the collimator, more precisely its projection onto the aperture of the measuring system, is then preferably also circular and has the area A _K and the radius r _K. The radius must be at least so large that at a given distance L between the measuring device and the collimator, the instrument receives only radiation from the collimator. So, r K > r M + Ltanβ.

wobei mit f_K die optische Brennweite des Kollimators bezeichnet ist.The solid angle covered by the collimator is:

where f _{K is} the optical focal length of the collimator.

Then dimension: Ω _K > Ω _M. Thus, assuming correct optical adjustment, it is ensured that the aperture and the solid angle of the measuring device are covered by the aperture and the solid angle of the calibrating device.

Above Allow equations the dimensioning while observing optical laws in the Kollimatorauslegung or by using data sheets of commercially available collimators. The Production of the cavity body and their thermally insulated assembly, as above described. The decor leaves of course also for calibration of small aperture gauges.

1

hollow sphere

10

spherical cavity in 1

11 12

 Hemispherical shells

13

equator

14

clamping flange

15

retaining flange

16

metal housing

17

holding frame

18

sockets

19

rooms

2

Opening (aperture)

20

housing opening

21

tube

27

focus

28

optical axis

29

Beam cross-section

3

inner wall surface

31

V-shaped furrows

4

outer wall surface

41

spiral groove

42

straight wall of 41

43

semicircular bottom of 4

44

caulking

45

upper slot edge

46

flange

5

heating conductor

6

collimator

61

Surface of 6

62

drilling

63

bracket

70 71, 72

 temperature sensor

8th

to calibrating device

9

pipe

91

lock

92

block insulating material

93

cover

101

optical axis of 8th

Claims (20)

Apparatus in the form of a standard for the calibration of quantitative infrared radiation measuring instruments, characterized by a metal cavity radiator with a small downwardly opening aperture ( 2 ), wherein under the aperture of the cavity radiator a collimator ( 6 ) and is arranged at such a distance that its focal point ( 27 ) right in the middle of the aperture ( 2 ) lies. Apparatus according to claim 1, characterized in that the cavity radiator one of two half-spherical shells ( 11 . 12 ) formed metal hollow sphere ( 1 ) and at the apex of a ( 11 ) of the two hemispherical shells ( 11 . 12 ) the aperture serving as an aperture ( 2 ) is provided. Apparatus according to claim 2, characterized in that for connecting the two hemispherical shells ( 11 . 12 ) of these a good heat-conducting, the ball diameter corresponding clamping flange ( 14 ) protrudes outwards, Device according to one of claims 2 or 3, characterized in that in addition to the two hemispherical shells ( 11 . 12 ) extending in the radial direction retaining flanges ( 15 ) are formed, by means of which the hollow ball ( 1 ) via a holding frame ( 17 ) thermally isolated in a hollow sphere ( 1 ) enclosing metal housing ( 16 ) is held. Apparatus according to claim 4, characterized ge indicates that between the retaining flanges ( 15 ) and the holding frame ( 17 ) thermally insulated sockets ( 18 ) are formed. Device according to one of claims 4 or 5, characterized in that the spaces ( 19 ) between the hollow sphere ( 1 ) and the surrounding housing ( 16 ) are filled with thermally insulating material. Device according to claim 6, characterized in that that the thermally insulating material laminated with aluminum foil Ceramic fleece or bulk material is. Device according to one of claims 2 to 7, characterized in that in the inner wall surfaces ( 3 ) of the two hemispherical shells ( 11 . 12 ) concentric or spiral-shaped, in cross-section approximately V-shaped furrows ( 31 ) are formed. Device according to claim 8, characterized in that the V-shaped grooves ( 31 ) have a flank angle in the range of 30 ° to 60 °. Device according to one of claims 2 to 9, characterized in that in the outer wall surfaces ( 4 ) of the two hemispherical shells ( 11 . 12 ) extending from the respective vertex, spirally extending groove ( 41 ) for receiving a heating conductor ( 5 ) is trained. Device according to claim 10, characterized in that the grooves ( 41 ) in the two hemispherical shells ( 11 . 12 ) each incisions with a straight wall ( 42 ) and semi-circular bottom ( 43 ) are. Device according to one of claims 2 to 11, characterized in that on the outer surface ( 4 ) of the open-ended hemisphere shell ( 12 ) at least one temperature sensor ( 72 ) is provided. Apparatus according to claim 12, characterized in that the temperature sensor ( 72 ) at the apex of the open-ended hemisphere shell ( 12 ) is permanently installed. Device according to one of claims 10 to 13, characterized in that the heating conductors ( 5 ) comprise a resistance wire accommodated in a thin-walled metal tube which is insulated from the metal tube by powdery non-conductive insulating material such as magnesium oxide. Device according to claim 14, characterized in that the ends of the resistance wire are electrically good conducting wires. Device according to claim 1, characterized in that the collimator ( 6 ) is an off-axis parabolic mirror whose diameter is the aperture of the measuring instruments to be calibrated ( 8th ) covered. Device according to one of claims 1 or 16, characterized in that at or close to the reflective surface ( 61 ) of the collimator ( 6 ) in its substrate at least one in a bore ( 62 ) accommodated temperature sensor ( 71 ) is provided. Device according to one of claims 13 to 15, characterized in that for calibration of the permanently installed temperature sensor ( 72 ) in its immediate vicinity for introducing a reference temperature sensor on the outer surface ( 4 ) of the open-ended hemisphere shell ( 12 ) ending pipe ( 9 ) is provided. Apparatus according to claim 18, characterized in that the tube ( 9 ) in a on the metal housing ( 16 ), lockable lock ( 91 ) ends. Device according to claim 19, characterized in that the lock ( 91 ) with a pipe ( 9 ) surrounding block of insulating material ( 92 ) and by means of a lid ( 93 ) is closable.