Reflector for a transmissiometer and transmissiometer equipped with it

Abstract

Retroreflector (26) for a transmissiometer (10) for reflecting a light beam (24), wherein the light beam (24) defines an optical axis (21) and strikes the retroreflector (26) from a light beam incidence direction (21), with - a reflector mount (14), - a plurality of individual reflectors (40) held in the reflector holder (14), each individual reflector (40) having a front interface (42) through which portions of the light beam (26) enter the respective individual reflector (40), - wherein the individual reflectors (40) of a first group are pivoted relative to a plane (56) perpendicular to the direction of incidence of the light ray (21) such that their front boundary surfaces (42-1) lie at first angles (52) greater than 0° relative to the plane (56), - and the individual reflectors (40) of a second group are pivoted relative to the plane (56) such that their front boundary surfaces (42-2) lie at second angles (54) less than 0° relative to the plane (56), - wherein the pivot axes of the individual reflectors (40) of the first group and the second group are parallel.

Description

[0001] The invention relates to a reflector for a transmissiometer and a transmissiometer equipped therewith.

[0002] Transmissiometers are used, for example, to determine the concentration of dust or soot particles in industrial exhaust ducts. They determine particle concentrations using transmission measurement, in which the attenuation of the transmitted light (extinction) is measured in a transmitted light method.

[0003] In a transmission measurement using a transmissiometer of this type, light is emitted from a light source into a measuring section running transversely through the gas duct. After passing through the measuring section once, the light is reflected back into the measuring section by a retroreflector and detected by a receiver after a second passage. The measurement effect of transmissiometers is particularly weak with short measuring sections and the very low dust concentrations common today behind modern filters, so the transmissiometer must be very sensitive and exhibit sufficient zero-point stability. The accuracy of a transmissiometer depends, among other things, on external influences such as optical misalignment, interfering reflections, and thermal distortion.

[0004] To avoid disruptive reflections, optical interfaces are tilted. The angle at which these optical interfaces must be tilted depends on the permissible swivel range of the optical components.

[0005] The "permissible tilt range" refers to the area within which the optical components, including the reflector, may move due to misalignment, thermal distortion, or other external influences without unduly affecting the measurement signal. The tilt angle must be greater than the permissible tilt range. In practice, a tilt of 2° to 5° is common. Optical interfaces also occur on the reflector and must be tilted accordingly. If the reflector consists of multiple individual reflectors, each of these individual reflectors must have its interface tilted accordingly.

[0006] This necessary tilting has the disadvantage that the optical measurement (transmission, opacity, or extinction) is influenced by the reflector's tilt angle. If the reflector's tilt angle increases, the transmission signal decreases. Conversely, if the reflector's tilt angle decreases, the transmission signal increases. Therefore, if the reflector tilts, for example due to thermal distortion of the gas duct, or if the reflector is mounted to the gas duct, the measurement signal will be larger or smaller depending on the direction of the tilt.

[0007] This problem arises particularly when the light-emitting and light-receiving parts of the transmissiometer are located on one side of the gas channel, while the reflector is mechanically separated and located on the opposite side. In this configuration, thermal distortions of the gas channel affect the different components of the transmissiometer independently and therefore usually differently. The measuring beam, i.e., the emitted light beam, must not drift away from the reflector or must only move within the permissible swivel range. The same applies to the light beam reflected by the reflector. Any drift of the measuring beam, i.e., the so-called swivel sensitivity, must remain within a predetermined range. The goal is to keep this swivel sensitivity as low as possible.

[0008] In addition to this prior art, from which the invention is based, retroreflectors are known from EP 1998192 A2 and EP 0342958 A2, which consist of individual triple reflectors, wherein the triple surfaces are formed at special angles to the normal of incidence in order to increase the effective angular range of the retroreflector, i.e. to be able to retroreflect light even when it is incident from a large angle (for example, from an opening angle greater than 80°).

[0009] Based on this state of the art, the object of the invention is to provide an improved retroreflector for a transmissiometer and a transmissiometer equipped therewith, with which the aforementioned disadvantages can be reduced in order to achieve greater independence from external influences and thus higher accuracy in particle concentration measurement.

[0010] This problem is solved by a retroreflector having the features of claim 1 and a transmissiometer having the features of claim 7.

[0011] A retroreflector according to the invention for a transmissiometer for reflecting a light beam, wherein the light beam defines an optical axis and strikes the retroreflector from a direction of incidence of the light beam, comprises: - a reflector mount, - a plurality of individual reflectors held in the reflector holder, each individual reflector having a front interface through which at least parts of the light beam enter the respective individual reflector, - wherein the individual reflectors of a first group are pivoted relative to a plane perpendicular to the direction of incidence of the light ray such that their front interfaces lie at first angles greater than 0° relative to the plane, - and the individual reflectors of a second group are pivoted relative to the plane in such a way that their front boundary surfaces lie at second angles less than 0° relative to the plane, - where the pivot axes of the individual reflectors of the first group and the second group are parallel.

[0012] The invention is based on the realization that a retroreflector is not an ideal retroreflector in reality and, in a slight tilt, which is necessary to avoid disturbing direct reflections at optical interfaces, does not ideally reflect all the light back into itself, but exhibits a slight angular dependence of the light intensity.

[0013] A retroreflector according to the invention has the particular advantage that if the reflector is unintentionally misaligned due to external influences (for example, temperature distortion), the effects of the individual reflectors on the measurement signal are partially compensated by the tilting. Thus, the tilt of one group of individual reflectors increases, while the tilt of another group decreases, so that the opposing effects can cancel each other out.

[0014] A transmissiometer equipped with the retroreflector according to the invention is then less affected by thermal distortion and can better maintain the required accuracies under real-world conditions. Furthermore, a plurality of smaller individual reflectors is more cost-effective than comparable, large-area retroreflectors. The overall goal of reducing the sensitivity to tilting is thus achieved.

[0015] In a further development of the invention, the first angles are all equal to each other, and the second angles are also all equal to each other. This creates symmetry, so that the effect of pivoting in one direction is the same as the effect of pivoting in the opposite direction.

[0016] Since the direction of thermal distortion is usually unpredictable, in a further development of the invention, the first and second angles are advantageously of equal magnitude. Advantageous angle ranges for the tilting are in the range of, in particular, 1° to 2.5°.

[0017] In a further development of the invention, the individual reflectors are designed as triple reflectors, and a plurality of triple reflectors are arranged in the reflector holder, distributed across the cross-section of the light beam. Triple reflectors are effective and cost-efficient.

[0018] In a further development of the invention, it is provided that there are third and fourth groups of individual reflectors, wherein the pivot axes of the third and fourth groups are parallel to each other, but not parallel to the pivot axes of the first and second groups. Since the direction of thermal distortion is not known in advance, it can be advantageous to provide compensation in other directions by means of further groups with a different inclination.

[0019] In order to obtain exact symmetry and to keep the effects of pivoting in opposite directions equal, in a further development of the invention the individual reflectors of the two groups are arranged in pairs symmetrically to the optical axis on the beam cross-section.

[0020] To protect the individual reflectors, the reflector mount can have a front window whose optical interface is arranged at an angle to the direction of incidence of the light beam, the magnitude of which is greater than the first and second angles. The slant of the front window had a negative effect on the sensitivity to swiveling, so the compensating property of the retroreflector according to the invention is particularly important here.

[0021] The problem is also solved by a transmissiometer that uses the retroreflector according to the invention. Such a transmissiometer has a light emitter for emitting a light beam into a measuring volume, a retroreflector according to the invention that reflects the light beam back into the measuring volume, a light receiver for receiving the light passing through the measuring volume and generating received signals, and an evaluation unit for evaluating the received signals and determining the extinction of the light beam in the measuring volume in order to determine the concentration of a fluid in the measuring volume.

[0022] The invention will now be explained in detail using an exemplary embodiment and with reference to the drawing. The drawing shows: Fig. 1 a schematic representation of a transmissiometer according to the invention on a gas channel; Fig. 2 a schematic representation of a top view of a retroreflector according to the invention; Fig. 3 a cross-section of a retroreflector according to the invention; Fig. 4 a representation like Fig. 2 of a further embodiment of the retroreflector according to the invention.

[0023] The transmissiometer 10 comprises a light emitter 22, preferably an LED with a transmitting optic 20, within a transmitter-receiver unit 18. The emitted light beam 24 illuminates a channel 16, e.g., an industrial exhaust duct, and defines an optical axis 21. The emitted light beam 24 is reflected in a retroreflector 26, which is held in a reflector holder 14, and ideally reflected back onto itself. The reflected light strikes a beam splitter 28 in the transmitter-receiver unit 18 and is directed by this mirror onto a light receiver 30. The receiver 30 thus allows the measurement of attenuation (extinction) of the emitted light 24 by particles 32 in the channel 16. For this purpose, the received signals from the light receiver 30, corresponding to the received light, are evaluated in an evaluation unit 34. In this way, transmission measurements are obtained, from which a particle concentration can be determined.

[0024] The core of the invention is the design of the retroreflector 26. The retroreflector 26 according to the invention comprises a reflector holder 14 in which a plurality of individual reflectors are held, which are generally designated by reference numerals 40. When an individual reflector is mentioned alone, it receives the reference numeral 40 plus a consecutive number. A front window 46 protects the retroreflector 26 from the interior of the channel 16 and the gases and particles contained therein. Of the individual reflectors 40, each of which is designed as a retroreflector, preferably as a triple reflector, the following are shown in the top view of the Fig. Six reflectors 40 are shown as examples, numbered 40-1 to 40-6. The arrangement of the individual reflectors 40 relative to each other can, in principle, be arbitrary. However, it is advantageous to use symmetrical arrangements so that the individual reflectors 40 are distributed across the cross-section Q of the light beam 24. One possible symmetrical arrangement is shown in Fig. Figure 2 shows the individual reflectors 40 in pairs arranged symmetrically to a horizontal plane 21-E in which the optical axis 21 is located.

[0025] The individual reflectors 40 are grouped into at least two groups. The first group comprises the individual reflectors 40-1, 40-3, 40-5. The second group comprises at least the individual reflectors 40-2, 40-4, 40-6. In this embodiment, the arrangement is chosen such that the individual reflectors 40 are symmetrically arranged in pairs with respect to the horizontal plane 21-E, and the individual reflectors below the horizontal plane 21-E belong to the first group and the individual reflectors above the horizontal plane 21-E belong to the second group.

[0026] Furthermore, each individual reflector 40 has a front interface 42 through which portions of the light beam 24 enter the respective individual reflector 40. Like the individual reflectors, the front interfaces have a general reference number, here 42, which is assigned a consecutive number when specific front interfaces are referenced. Two of these individual reflectors 40 are in Fig. 3 shown in cross-section, where, as mentioned, the single reflector 40-1 belongs to the first group and the opposite single reflector 40-2 to the second group.

[0027] The individual reflectors 40-1, 40-3, 40-5 of the first group are arranged pivoted relative to a plane 56 perpendicular to the direction of incidence of the light beam (optical axis 21) such that their front boundary surfaces 42-1 lie at first angles 52 greater than 0° relative to the plane 56.

[0028] The individual reflectors 40-2, 40-4, 40-6 of the second group are arranged pivoted relative to the plane 56 such that their front boundary surfaces 42-2 lie at second angles 54 less than 0° relative to the plane 56.

[0029] To fully define the first and second angles 52 and 54 in space, it is still necessary to specify the pivot axes of the individual reflectors. In principle, these are not fixed, but the pivot axes of all individual reflectors of the two groups are parallel and, in the exemplary embodiment, lie according to Fig. 2 in the horizontal plane and perpendicular to the optical axis 21. Other positions of the pivot axes are of course also possible.

[0030] Preferably, the first angles 52, i.e., the inclination of the front interface 42 of the individual reflectors 40 of the first group relative to each other, are all equal, and the second angles 54 are also all equal relative to each other. It is advantageous if the first and second angles are also equal in magnitude, i.e., the inclination of the individual reflectors of the first group is the same as the inclination of the individual reflectors of the second group, but in the opposite direction. Advantageous angle ranges for the inclinations are in the range of, in particular, 1° to 2.5°.

[0031] This will be illustrated again using the Fig. 3 will be described. In the Fig. Figure 3 is a highly schematic cross-section along line III-III of the Fig. Figure 2 shows the retroreflector 26 with its front window 46 and the mounting 14. In this cross-section, the two individual reflectors 40-1 and 40-2 are positioned in the mounting 14 at a specific angle to the plane 56, according to the invention. The front window 46 is inclined relative to the plane 56 by an angle 50°, i.e., not 90° to the optical axis 21, in order to avoid unwanted multiple reflections in a known manner. In the drawing, this angle of inclination is 5°.

[0032] The individual reflector 40-1 is now inclined at the first angle 52 (here 2°) relative to a perpendicular to the optical axis. This corresponds to an angle of 3° between the front window 46 and the front interface 42-1 of the individual reflector 40-1. The individual reflector 40-2 is inclined at the second angle 54 (here -2°) relative to the perpendicular to the optical axis. This corresponds to an angle of 7° between the front window 46 and the front interface 42-2. The pivot axes of the individual reflectors are preferably also parallel to the pivot axis of the front window 46. This arrangement reflects the interfering part of the transmitted light 24, which is reflected at the front window 46, back in a direction that deviates from the optical axis by a sufficiently large angle of 10° (at a 5° tilt angle) and therefore no longer enters the transmission meter, where this reflection would otherwise cause interference.The interfering components of the transmitted light 24, which are reflected at the front interface 42-1 of the single reflector 40-1 of the first group, are reflected at an angle that deviates by +4° from the optical axis due to the 2° inclination of the front interface 42-1. At the front interface 42-2 of the single reflector 40-2 of the second group (inclination -2°), the light is reflected, now deviating by an angle of -4° from the optical axis.

[0033] Due to the opposing tilt of the individual reflectors 40-1 and 40-2, any unintentional swiveling of the retroreflector 26 due to warping or similar causes has an opposing effect on the two individual reflectors 40-1 and 40-2. Any reflection errors of the individual retroreflectors can therefore compensate for each other.

[0034] The arrangement of the individual reflectors 40 is in Fig. 2 is shown only as an example. Other arrangements are possible. For example, the Fig. Figure 4 shows another embodiment of such an arrangement. Here, the individual reflectors are arranged quasi-point-symmetrically with respect to the optical axis 21. Opposite individual reflectors, for example 40-1 and 40-2 or 40-3 and 40-4, etc., belong to different groups.

Claims

[1] Retroreflector (26) for a transmissiometer (10) for reflecting a light beam (24), wherein the light beam (24) defines an optical axis (21) and strikes the retroreflector (26) from a light beam incidence direction (21), with - a reflector mount (14), - a plurality of individual reflectors (40) held in the reflector holder (14), each individual reflector (40) having a front interface (42) through which portions of the light beam (26) enter the respective individual reflector (40), - wherein the individual reflectors (40) of a first group are pivoted relative to a plane (56) perpendicular to the direction of incidence of the light ray (21) such that their front boundary surfaces (42-1) lie at first angles (52) greater than 0° relative to the plane (56), - and the individual reflectors (40) of a second group are pivoted relative to the plane (56) such that their front boundary surfaces (42-2) lie at second angles (54) less than 0° relative to the plane (56), - wherein the pivot axes of the individual reflectors (40) of the first group and the second group are parallel. [2] Retroreflector (26) according to any one of the preceding claims, characterized by , that the first angles (52) are all equal to each other and that the second angles (54) are all equal to each other. [3] Retroreflector (26) according to claim 2, characterized by , that the first (52) and the second angle (54) are equal in magnitude and the magnitude of the angle is in the range of, in particular, 1° to 2.5°. [4] Retroreflector (26) according to any one of the preceding claims, characterized by, that the individual reflectors (40) are designed as triple reflectors and a plurality of triple reflectors (40-1 to 40-6) are arranged distributed over the light beam cross-section in the reflector holder (14). [5] Retroreflector (26) according to any one of the preceding claims, characterized by , that there are third and fourth groups of individual reflectors (40), wherein the pivot axes of the third and fourth group are parallel to each other, but not parallel to the pivot axes of the first and second group. [6] Retroreflector (26) according to claim 5, characterized by , that the individual reflectors (40) of the two groups are arranged in pairs symmetrically to the optical axis (21) on the beam cross-section. [7] Retroreflector (26) according to any one of the preceding claims, characterized by, that the reflector holder (14) has a front window (46) whose optical interface is arranged at an angle to the direction of incidence of the light ray, the angle being larger in magnitude than the first (52) and the second angle (54). [8] Transmissiometer (10) - with a light transmitter (22) for emitting a light beam (24) into a measuring volume, - a retroreflector (26) according to one of the preceding claims for reflecting the light beam (24) back into the measuring volume, - a light receiver (30) for receiving the light passing through the measurement volume and generating received signals - and with an evaluation unit (34) for evaluating the received signals and determining the extinction of the light beam (24) in the measurement volume, in order to be able to determine the substance content of a fluid in the measurement volume.

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