GB2023873A - Optical multiply reflecting arrangement - Google Patents

Abstract

An optical multiply reflecting arrangement, useful for example for a smoke density meter, features a light source or sender 12 at one end of a measuring section 11 and a first retroreflecting device, in the form of a lens 13 in front of a retroreflecting in element 14 which produces sideways deflection of a light beam, at the opposite end of the measuring section 11 for returning the light beam in the general direction of the light source 12. Light deflecting elements such as optical wedges 18, 18' can be moved into the beam path in front of the retroreflecting device and further retroreflecting devices 16, 16' and/or light transmitting elements can be moved into position at the sender side. Depending on the combination of optical elements used at the light source and in front of the first retroreflecting device 13, 14 it is possible to choose between two-fold, four-fold, six-fold or multiple passages of the light beam through the measuring section. Strategically positioned photodetectors 41, 41' can detect the emergent light beam. <IMAGE>

Description

SPECIFICATION Optical multiply reflecting arrangement This invention relates to an optical multiply reflecting arrangement and has particular reference to an arrangement in which light from a light source is multiply reflected to and fro through a measuring section before being received at a light detector.

Optical multiple reflection arrangements of this kind are used where long measurement paths are necessary, for example in order to obtain a distinct reception signal. By multiple reflection along the measurement path the latter can thus be substatially shortened, depending on the number of to-and-fro reflections effected. Examples of application are visibility measuring intruments on airfields and motorways, in which a light transmitter transmits a light beam over a distance of 10 or 30 metres to a reflector, which is preferably of retroreflecting material and which guides the light beam back in itself to the transmitter. Beam splitting is effected in the trasmitter, and the reception light beam is directed onto a photo-receiver. The latter produces an electric signal the amplitude of which varies in dependence on the visibility (from more or less clear visibility, to fog).An eiectronic evaluation device forms from this signal an indication which can be expressed directly on a visibility scale. Another application is for exhaust gas density measuring intruments in tunnels, where the measurement path should, for example, extend over 100 metres. Finally, in this connection mention should also be made of smoke density measuring instruments disposed in chimneys, where the measurement path is limited to the diameter of the chimney.

Although optical multiple-reflection arrangements are of interest for the multiplication of the measurement path in a small space, the adjustment of the transmitter and receiver on the one hand and the reflection device on the other hand is problematical.

The transmitter and receiver arrangement and the reflection device must be absolutely accurately aligned with respect to one another, in which connection a particular problem arises in completely maintaining, over a long period of time, an accurate adjustment once it has been achieved.

This is often extremely difficult, particularly in the case of smoke density measuring instruments, where continuous temperature fluctuations occur.

Several different forms of optical multiple reflection arrangements are known from US-PS 4032 236 which describes a number of distinct arrangements which are respectively capable of allowing a light beam leaving a light source to make a four-fold, six-fold or multiple passages through a measuring section.

A light detector in the form of a photodetector or an optical device for deflecting light two a photodetector is arranged at the light exit point from the measuring section. The significant advantage of the know,n multiply reflecting arrangement is that the reflector at the end of the measuring section only requires to be coarsely adjusted and that a troublefree operation of the apparatus is also ensured when the two parts of the apparatus at the end of the measuring section become somewhat displaced or tilted relative to one another with the elapse of time.

The known arrangement is also extensively insensitive to vibrations.

Whilst however the known optical multiply reflecting arrangements only make possible the four-fold, six-fold or multiply passage of the light beam through the measuring region the principal object of the present invention is to provide an optical multiply reflecting arrangement by means of which the choice between a two-fold, four-fold, six-fold or multiple passge of the light beam through the measuring section can be realized.

According to the present invention there is thus provided an optical multiply reflecting arrangement with reflection devices at both ends of a measuring section and a light source at one end of the measuring section wherein the light source and the reflection devices are so constructed and arranged that a light beam leaving the source passes at least twice through the measuring section before it leaves the measuring section and in which the reflection device at the end of the measuring section remote from the light source comprises a lens with a focal length equal to the spacing from the light source, a retroreflecting element behind the lens which reflects an incident light beam in the same direction but displaced sideways and transparent light deflecting means between the light source and the retroreflecting element which produce a deflection of the incident and/or emergent beams of the retroreflecting element such that the image of the light source formed by the reflection device lies alongside the light source and in which the second reflective device is a retroreflector arranged in the vicinity of, in front of, or behind the image ofthe light source formed by the first reflection device and characterised in that at least one of the following additional features is provided (a) the light beam deflecting means comprises at least one optical element displaceable from its light deflecting position into an inoperative position with reference to the light deflection and (b) the retroreflector arranged alongside the light source is movable from its position in the beam path to a position outside the beam path.

The precise number of to and fro passages can thus be chosen at will depending on which combination of optical elements is chosen.

Thus for example if the single wedge shaped optical element is inserted into a light deflecting position in front of the retroreflecting device at the end of the measuring section remote from the light source then the light beam emergent from the light source can be reflected to a position alongside the light source. This light beam can then either be detected, in which case a simple two-fold passage has taken place or, alternativeiy, a retroreflecting element which produces substantially no sideways deflection can be introduced to return the light beam for a further two-fold passage through the measuring section. Thus a four-fold passage occurs with the emergent light beam passing back through the illuminated slot light source.

As a further alternative a retroreflecting element which also produces sideways deflection can be positioned alongside the light source in which case a six-fold passage results. Removed of the wedge shaped optical element from the beam path will then once more produce only a two-fold passage with the light beam exiting through the illuminated slot.

Thus the use of a retroreflector which does not produce any sideways displacement of the beam enables the choice between a two-fold or four-fold passage of the light beam through the measuring section to be realized. The use of a retroreflector at the sender side which produces a sideways displacement of the beam enables the choice between a two-fold or six-fold passage of the measuring beam through the measuring section to be realized.

If, in accordance with a preferred embodiment, it is arranged that a retroreflector for producing a beam reversal with a sideways displacement of at least the size of the image of the light source is movable into the beam path (in place of the retroreflector which produces no sideways displacement and is moved out of the beam path) then, apart from the two-fold and six-fold passage, the four-fold beam passage can also be selected.

Adjustment of the various optical elements thus enables a number of different configurations to be achieved each of which will produce different num bers of beam passages. A number of individual configurations and their associated beam passages can be seen from the earlier mentioned US-PS 4032 236.

By suitable positioning of the photodetector the emergent beam can in every case be detected.

Alternatively a number of photodetectors can be strategically positioned or can be movable with the individual optical elements so that an appropriate photodetector is aiways correctly positioned for every mode of measurement.

It will be appreciated that arrangements which enable the number of beam passages to be chosen at will without substantially prejudicing the inherent insensitivity of the device to mal-adjustment bring especial advantages. For example for a smoke density meter it may only be practicable for dense smoke to allow two passages of a beam of light through the measuring section. On the contrary when little smoke prevails a much larger number of passages may be required to yield an accurate measurement. Advantageous modifications of the invention are characterized by the subclaims.

The invention will be described in the following by way of example and with reference to the drawings which show: Figure 1 a schematic view of a first embodiment of the optical multiply reflecting arrangement for the selection of a two-fold, four-fold or six-fold passage of the light beam from the light source 12 which enters the measuring section 11, Figure la a plan view of a disk carrying the various interchangeable retroreflectors, Figure 2 another embodiment of the light deflect ing parts of the reflection device of Figure 1 which is remote from the light source, Figure 3 a schematic view of a further embodiment of the multiply reflecting arrangement enabling the choice of a two-fold, six-fold, ten-fold and fourteen-fold passage of the measuring beam, Figure 4 a plan view of the retroreflectors used in the embodiment of Figure 3 on the sender side, Figure 5 a plan view of a further embodiment using a flattened penta prism, Figure 6 a further embodiment of the multiply reflecting arrangement with two movable flattened penta prisms, and Figures 7to 9 a further embodiment of a multiply reflecting arrangement in the positionsfortwo-, four- and six-fold passage of the beam.

In Figures 1,3,5 and 6 the line 12 with small transverse strokes signifies the cross-section of a slot illuminated from the left hand side. The longitudinal direction of the slot thus runs at right angles to the plane of the drawing. The coiled filament of a lamp or a laser light source could be used in place of the illuminated slot 12 and thus the slot 12 will be simply designated the light source for simplifying the subsequent description without any intention of restricting the scope of use of the invention. The illuminated slot thus forms the light source for the purposes of this description. The illuminating beam path can be as shown and explained in relation to Figure 9 of US-PS 4,032,236. A simplified beam path is indicated in the example shown in Figure 1.

In accordance with Figure 1 the slot 11 is illuminated from the left by means of an optical device which is not shown in detaii via the partially transmitting mirror 38. At the other end of the measuring section 11 there is located the reflecting device in accordance with the invention which omprises: a retroreflecting element, in particular in the form of a triple mirror with its apex facing away from the measuring section, a lens 13 arranged in front of the base surface 39 of the triple mirror 14 and a beam deflecting device which is constructed as an optical wedge 18. The apex of the wedge is directed towards the optical axis 19 and the wedge increases in size in the direction away from the optical axis.In place of one optical wedge 18 on one side of the optical axis 19 two wedges 18' with smaller wedge angles can be provided on both sides of the optical axis 19 as is indicated in dotted lines in Figure 1. In Figure 1 the wedges 18 or 18' are so pivotable about axes 18a that in the position shown on the left they are located outside the beam path and in the other position shown on the right they are located in their position which brings about beam deflection in the manner to be described.

Beneath the light source 12 there is located a retroreflector 16 which borders directly on the light source and the base of which remote from its apex 20 has the same width and height as the light source 12. The retroreflector 16 can comprise finely divided retroreflecting material is however preferably formed by a Beck prism. Directly in front of the retroreflector 16 there is located a lens 21 the arrangement and operation of which is the same as is described in US-PS 4,032,236.

In accordance with Figure lathe retro-reflector 16 is located on a disk 1 6a which is rotatable about an axis 16b. The axis 16b extends parallel to the optical axis 19. On the periphery of the disk 16a there is arranged in addition, at various angular spacings a light transmitting opening 41a and a further retroreflector 16' which has a base area twice as long as the retroreflector 16 such that an incident light beam will not only be reflected back on itself but rather also displaced sideways as is explained in relation to Figure 3 of US-PS 4,032,236. The manner of operation of the embodiment of Figures 1 and 1a is as follows: If the wedges 18,18' are swung out of the beam path then the reflected light beam 2 returns directly to the slot 12 where it is deflected via the partially transmitting mirror 38 to the photodetector 41.In this case a two-fold passage through the measuring section 11 is present.

The same result however with decoupled transmitting and receiving beams can be achieved if the wedges 18 or 18' remain in the beam path but however the opening 41a of the disk 16a is swung into the beam path. The reflected beam 2 will in this case be deflected to a photodetector 41' arranged behind the opening 41a.

A four-fold passage is realized by means of the retroreflector 16 brought into the beam path as shown in Figure 1 whilst, on pivoting the retroreflector 16' into the beam path a six-fold beam passage is brought about by the reflection process described in connection with Figure 3 of US-PS 4,032,236.

The displacement of the image 12' of the light source 12 can also be achieved using two half lenses with separate however parallel optical axes or in accordance with Figure 2 by a division of the lens 13 into two lens parts with a separation "a" from one another as an alternative to the wedges 18 or 18' shown in Figure 2. By making the spacing "a" of the optical axes adjustable the image 12', in particular during a test arrangement, can be brought directly to the desired position i.e. in particular directly bordering on the light source 12. In accordance with the invention the lens parts 13' are coupled to displacement devices 13a which for example respectively comprise a sideways displaceable spindle 13cwith a cooperating nut 13b.The range of displacement is so chosen, that the base separation a for the optical axis shown in Figure 2 disappears so that the sideways displacement of the beam is also omitted and solely a two-fold passage through the measuring section 11 can be realized.

Similar arrangement to that of Figure 1 is shown in Figure 5 in which however a flattened penta prism 16' is used in place of the retroreflector 16 which is preferably formed as a Beck prism. The flat central portion and the inclined side parts of the penta prism each have the width of the slot or the light source 12 and the prism has the same length as the light source 12. If a flattened penta prism 16' of this kind is arranged in accordance with Figure 5 symmetric to the optical axis 19 then a light beam 1 leaving a specified point of the light source 12 travels through the measuring section 11 in the illustrated manner and after refraction and reflection in the reflection device 17 becomes the reflected beam 2.Beam 2 once more enters the flattened penta prism 16 ' ' at the illustrated position to the side of the light source 12 and emerges from the symmetrically diametrically opposed side as the reflected beam 3. Finally the beam 3, after further reflection and refraction at 17 returns for a fourth time as reflected beam 4to its starting point from which, similarly to the arrangement of Figure 1, it can once more be deflected to a photodetector via a non-illustrated partially transmitting mirror.

In accordance with the invention the wedge 18 is once more arranged on a shaft 19a which this time extends parallel to the optical axis 19 so that the wedge can be swung out of the beam path by rotation of the shaft 19a. With the wedge 18 swung out of the beam path the measuring beam returns to the slot 12 after a single reflection so that only a two-fold passage is present. The embodiment of Figure 5 thus enables the choice between a two-fold and a four-fold passage through the measuring section 11. When the two-fold passage is realized the penta prism 16'' and, for all embodiments, the lens 21 can also be removed, for example swung, out of the beam path.

By providing several retroreflectors 16'a, 16'b, 16'b... in a row in accordance with Figures 3 and 4 a significantly larger number of passages can be achieved. Each further retoreflector enables four further passages so that in the embodiment of Figures 3 and 4 the light beam first emerges sideways from the device afterfourteeen passages.

Especial significance is attributed to the lens 21 for embodiments with several retroreflectors 16' arranged alongsidefone another as it effectively decouples the two halves of the reflection device 17.

In accordance with Figure 3 and 4 not only is the wedge 18 pivotable about the axis 18a but also the retroreflectors 1 6'a, 1 6'b, 1 6'c can be individually swung outofthe beam path about a shaft 16cwhich extends at right angles to the optical axis. It is for example indicated in Figure 4 that the retroreflectors 16'a and 16'b are located in the beam path whilst the retroreflector 16'c is swung out of the beam path. In this case, in accordance with Figure 3, a ten-fold beam passage would be achieved. On swinging the retroreflector 16b out of the beam path a six-fold passage would be present. If in addition the retroreflector 16'a is swung out of the beam path a two-fold passage would be realized. Preferably the wedge 18 in this embodiment is not pivotable about the axis 1 8a as pivotaing movement is more easily carried out at the sender side and in other respects the pivotability of the retroreflector 16'a simultaneously ensures a decoupling of the transmitted and reflected beams. Beck prisms are particularly well suited for the retroreflectors 16' in the embodiments of Figures 1 to 4 as their base area can be made without further ado equal to the slot area and they can be arranged closely alongside one another as shown in Figures 3 and 4.

Figures 7 to 9 show an example of an embodiment in which the retroreflecting element 14 and the lens 13 are united to form a single optical component; this is possible for all embodiments.

Figure 7 reproduces the simplest case of a twofold beam passage whereby only the lens 13 with the retroreflecting element 14 is necessary apartfrom the front objective 30 which is arranged in the beam path in accordance with Figure 9 of DE-AS 2508860.

In accordance with Figure 8 an accessory 31 with the lens 21 and the flattened penta prism 16' ' (Figure 5) are set in front of the front objective 30 and an accessory 33 containing the wedges 18' is set in front of the lens 13 in order to realize a four-fold beam passage.

Figure 9 finally shows the application of another accessory 32 with the lens 21 and retroreflectors 16' (c.f. Figure 1) for realizing the sixfold beam passage.

The wedge operation of the wedges 18' is reversed in the example of Figures 7 to 9 in relation to the preceding embodiments.

By mounting a series of photodetectors associated with particular numbers of multiple passages through the measuring section at strategic locations (as shown in the accompanying drawings) behind the optical elements on the light exit side a photoelectric detector will always be available for producing the required output signal irrespective of the number of passages selected. As an alternative the photoelectric detectors can be movable with their associated optical elements or one photoelectric detector can be moved to a number of positions. It is however preferred if the photoelectric detectors are movable with their associated reflectors or fixed in position as this ensures accuracy of adjustment.

The optical elements of the apparatus used are preferably movable about respective pivot axes because this also ensures alignment problems are minimised.

It will be understood by those skilled in the art that further modifications can be made to the present arrangement without departing from the scope of the present teaching.

Claims (31)

1. Optical multiply reflecting arrangement with reflection devices at both ends of a measuring section and a light source at one end of the measuring section wherein the light source and the reflection devices are so constructed and arranged that a light beam leaving the source passes at least twice through the measuring section before it leaves the measuring section and in which the reflection device at the end of the measuring section remote from the light source comprises a lens with a focal length equal to the spacing from the light source, a retroreflecting element behind the lens which reflects an incident light beam in the same direction but displaced sideways and transparent light deflecting means between the light source and the retroreflecting element which produce a deflection of the incident and/or emergent beams of the retroreflecting element such that the image of the light source formed by the reflection device lies alongside the light source and in which the second reflection device is a retroreflector arranged in the vicinity of, in front of, or behind the image of the light source formed by the first reflection device and characterized in that at least one of the following additional features is provided a) the light beam deflecting means (13', 18, 18') comprises at least one optical element displaceable from its light deflecting position into an inoperative position with reference to the light deflection and b) the retroreflector (16, 16', 16' ')arranged alongside the light source (12) is movable from its position in the beam path to a position outside the beam path.

2. Arrangement according to claim 1 and characterized in that the beam deflecting means comprise an optical wedge (18) which intercepts one half of the beam path and which is pivotable or dispiaceable out of the beam path.

3. Arrangement according to claim 1 and characterized in that the beam deflecting means comprise two optical wedges (18') which are respectively arranged in one half of the beam path and which are pivotable on displaceable out of the beam path.

4. Arrangement according to claim 1 and characterized in that the beam deflecting means are formed by two lens portions (13') with mutually displaced (a) optical axes which are displaceable to a position with non-displaced optical axes.

5. Arrangement according to one of the preceding claims and characterized in that the retroreflecting element is a triple mirror (14).

6. Arrangement according to one of the preceding claims and characterized in that the location of the image (12') of the light source (12) formed by the first reflection device (17) lies directly adjacent the light source.

7. Arrangement according to one of the preceding claims and characterized in that the retroreflector (16) which forms the second reflection device and produces only a beam reversal without beam displacement is movable out of the beam path and in particular is pivotable or displaceable out of the beam path.

8. Arrangement according to claim 7 and characterized in that the retroreflector (16) comprises a finely divided retroreflecting material.

9. Arrangement according to claim 7 and characterized in that the retroreflector is a reflection prism (pentaprism) (16) the apex edge (20) of which is located at the centre of the image of the light source.

10. Arrangement according to claim 7 and characterized in that the retroreflector is a strip of triples or a single triple mirror (16) the apex (20) of which is located at the centre of the image (12') of the light source.

11. Arrangement according to claim 7 and characterized in that the retroreflector is a Beck-prism (16) the apex (20) of which is located at the centre of the image (12') ofthe light source.

12. Arrangement according to one of the claims 9 to 11 and characterized in that the base of the prism or the base of the triple mirror (20) is of the same size as the image (12') of the light source.

13. Arrangement according to one of the claims 1 to 6 and characterized in that a retroreflector (16') operative to produce beam reversal with a sideways displacement of at least the size of the image (12') of the light source is movable into the beam path, and in particular is pivotable or displaceable into the beam path in place of a retroreflector (16) which produces no beam displacement and which is moved out of the beam path.

14. Arrangement according to claim 13 and characterized in that the two retroreflectors (16,16') and a transmitting aperture (41a) are arranged on a rotatable disk (16a) and are introduceable one after the other in the beam path by rotation of the disk.

15. Arrangement according to claim 13 orto claim 14 and characterized in that the sideways displacement corresponds exactly to the size of the light source (12).

16. Arrangement according to one of the claims 13 to 15 and characterized in that the second retroreflector is a penta prism (16') and the image (12') of the light source (12) is formed on the half of the penta prism which is arranged towards the light source (12).

17. Arrangement according to one of the claims 13 to 15 and characterized in that the retroreflector is a triple mirror (16'), or an arrangement of a plurality of triple mirrors one above the other, wherein the image (12') of the light source (12) is formed on the half or halves of the triple mirrors turned toward the light source (12).

18. Arrangement according to one of the claims 13 to 15 and characterized in that the retroreflector is a Beck prism (16') and the image (12') of the light source (12) is formed on the half of the Beck prism which is turned towards a light source (13).

19. Arrangement according to one of the claims 16to 19 and characterized in that the retroreflector (16') which is located in the beam path directly borders on the light source (12).

20. Arrangement according to one of the claims 13 to 19 and characterized in that the base of the retroreflector (16) is twice as wide as the image (12') of the light source (12).

21. Arrangement according to one of the claim 13 to 19 and characterized in that a further lens (21) is arranged in front of the retroreflector (16') on the optical axis (19) and has a focal length (f) the same as the spacing from the first reflection device (17).

22. Arrangement according to claim 21 and characterized in that several retroreflectors (1 6'a, 1 6'b, 16'c, ...) are arranged directly alongside one another and are movable out of the beam path.

23. Arrangement according to one of the preceding claims and characterized in that the retroreflectors (16, 16') are provided symmetrically in the same arrangement on both sides of the light source and are respectively jointly movable out of the beam path.

24. Arrangement according to claim 13 and characterized in that the sideways displacement in the direction ofthe light source (12) amounts to double the size of the light source.

25. Arrangement according to claim 24 and characterized in that the retroreflector is a flattened penta prism (16") movable out of the beam path.

26. Arrangement according to claim 25 and charaterized in that the flattened central part and the two side disposed inclined parts of the pentaprism (16") are the same size as the light source (12) and the centre of the penta prism lies on the optical axis (19) and wherein the penta prism can be swung out about an axis (19a) extending parallel to the optical axis.

27. Arrangement according to one of the preceding claims and characterized in that the light source is an illuminated slot.

28. Arrangement according to one of the preceding claims and characterized in that at the points behind the second reflection device where light emerges through moving of the retroreflectors (16, 16', 16") there is respectively located a photodetector (41') or an optical device for deflecting light ot the photodetector.

29. Arrangement according to one of the claims 1 to 27 and characterized in that a photodetector (41') or an optical device for deflecting light to a photodetector is located behind the second reflection device and is movable to the-respective light exit point.

30. Arrangement according to one of the preceding claims and characterized in that the optical wedges (18, 18') and/or the retroreflectors (16, 16', 16") are housed together with the additional lens (21) in interchangeable accessories (31,32,33).

31. Optical multiply reflecting arrangement with reflection devices at both ends of a measuring section and a light source at one end of the measuring section substantially as described with reference to Figures 1 and 1 a, Figure 2, Figures 3 and 4, FigureS, Figure 6 or Figures 7 to 9 of the accompanying drawings.