Optical multiple-reflection arrangement

Optical multiple-reflection arrangement comprising a light source, a lens separated from the source by its focal length in front of a retroreflector which returns a laterally offset beam, deflection means to displace the reflected image to one side of the source, and a second retroreflector near the image, whereby the measurement path between the retroreflectors is traversed at least four times by a beam from the source.

The invention relates to an optical multiple-reflection arrangement having 
reflection devices at both ends of a measurement path and having a light 
source or an image of a light source at one end of the measurement path, 
the light source and the reflection devices being so contructed and 
arranged that a light beam starting from the light source passes at least 
four times through the measurement path before it passes out of the 
latter. 
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 substantially shortened, depending on the number of 
to-and-fro reflections effected. Examples of application are visibility 
measuring instruments on airfields and motorways, in which a light 
transmitter transmits a light beam over a distance of 10 or 30 meters 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 transmitter, 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 electronic 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 
instruments in tunnels, where the measurement path should, for example, 
extend over 100 meters. 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 problemmatical. 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. Particularly in the case of smoke density 
measuring instruments, where continuous temperature fluctuations occur, 
this is extremely difficult. 
It is therefore an object of the invention to provide an optical 
multiple-reflection arrangement of the kind first mentioned above, in 
which the light is transmitted and received from one side of the 
measurement and in which a reflection device, which need be only coarsely 
adjusted in relation to the transmitter-receiver, is disposed on the 
opposite side of the measurement path to that where the 
transmitter-receiver is disposed, and wherein certain variations of the 
adjustment during operation can be tolerated without the position and 
intensity of the reception light beam in the transmitter-receiver being 
influenced to such an extent as to impair the accuracy of measurement. 
In order to solve this problem the invention provides for the reflection 
device disposed at the end of the measurement path remote from the light 
source to comprise a lens having a focal length equal to the distance from 
the light source, behind this lens a retroreflecting element which 
reflects an incoming light beam in the same direction but offset 
laterally, and light beam deflection means which impart to the incoming 
and/or outgoing beams of the retroreflecting element a deflection such 
that the image made of the light source by the reflection device lies at 
the side of the light source, and that a second reflection device provided 
is a retroreflector disposed behind the image made of the light source by 
the first reflection device. On the side where the first reflection device 
is disposed there is therefore provided a single retroreflecting element 
which is preferably overshot by the light beam falling on it, so that as a 
whole neither a certain tipping of the reflection device nor lateral 
displacements bring about any change at the reception site or in the 
intensity of reception in the transmitter-receiver. It should be pointed 
out that the light source is preferably a slit illuminated by a lamp, a 
laser, or the like. However, in order to simplify the terminology used in 
this description the term "light source" will always be used even in the 
case of a simple illuminated slit or the like. 
Whereas with a reflection device remote from the light source it is 
required that in addition to reflection in itself the beam should undergo 
lateral displacement, in the case of a retroreflector disposed by the 
light source it is required that the retro-reflector should reflect in the 
same direction the light falling on it. 
In a first embodiment the beam deflection means consists of an optical 
wedge occupying one half of the beam path. The beam deflection means may 
however also consist of two optical wedges, disposed one in each half of 
the beam path. A particularly preferred embodiment is characterized in 
that the beam deflection means are formed by two half-lenses whose optical 
axes are spaced apart from one another in the direction of the offsetting 
of the light source. The optical axes of the half-lenses are offset in 
relation to one another to the extent necessary to project the image of 
the light source to the side of the latter. 
Finally, the beam deflection means may also be formed by grinding one half 
of the lens or of the retro-reflecting element so as to form a deviation 
from the normal shape. The only important point is that the image of the 
light source on the transmitter-receiver side should fall on the 
retroreflector to the side of the light source itself. 
In the embodiment utilising half-lenses, it is expedient for the latter to 
be plano convex and to have their plane surface cemented on the likewise 
plane inlet surface of the retroreflecting element. A particularly compact 
unit not liable to shift is thereby formed. 
The distance between the half-lenses is however variable, particularly for 
test purposes. 
A triple mirror is used with particular advantage for the retroreflecting 
element. 
The first reflection device preferably projects the image of the light 
source so that it directly adjoins the light source itself. Not only is 
optimum use thus made of the available space, but the size of the optical 
parts used is also reduced to a minimum. 
In a first embodiment the retroreflector forming the second reflection 
device effects only a reversal of the beam without offsetting it. In this 
embodiment a fourfold passage of the light beam through the measurement 
path can be achieved. 
In the simplest case the retroreflector consists of finely divided 
retroreflecting material, such as for example "Scotchlite". 
The retroreflector may however also be a roof prism whose roof edge is 
situated in the middle of the image of the light source. In order to 
eliminate a certain sensitivity of height adjustment in this embodiment, 
the retroreflector is preferably a triple mirror, or better still, a Beck 
prism whose apex is situated in the middle of the light source image. A 
Beck prism is practically a disc-shaped portion of a triple mirror. The 
Beck prism is thus particularly suitable when working with an illuminated 
slit because the slit surface can be imaged in the base surface of the 
Beck prism. The base of the prism or triple mirror is advantageously equal 
in size to the image of the light source. 
An embodiment which permits substantially more to-and-fro reflections is 
characterized in that the retroreflector forming the second reflection 
device effects a reversal of the beam with lateral offsetting to the 
extent of at least the size of the image of the light source. In this 
manner at least a sixfold passage of the light beam through the 
measurement path can be achieved. 
In a first embodiment the retroreflector is once again a roof prism and the 
image of the light source is projected onto the half of this prism which 
faces the light source. In this embodiment, however, it is preferable for 
the retroreflector to be a triple mirror or better still a Beck prism. In 
contrast to the embodiment first described, the lateral offsetting is 
achieved by projecting the image of the light source not centrally onto 
the retroreflector but with lateral offsetting. In the preferred case of 
the use of a Beck prism the slit image expediently occupies exactly one 
half of the base surface. On the other half of the base surface the light 
then passes out accordingly. 
In this embodiment also the retroreflector preferably directly adjoins the 
light source. 
Another advantageous embodiment is characterized by the provision on the 
optical axis, in front of the retroreflector, of another lens whose focal 
length is equal to the distance from the first reflection device. The 
purpose of this measure is to ensure that all the light passing out of one 
half of the first reflection device will enter this half again. This is of 
importance because the light beams entering different halves of the 
reflection device are differently deflected. According to the invention, 
therefore, light beams should be clearly allocated to the respective 
halves of the reflection device. 
The number of reflections within the measurement path can be increased by 
disposing a plurality of retroreflectors directly side by side. Each 
additional retroreflector increases the number of passages by four. 
It should however be pointed out that the edge of the additional lens need 
extend only to the outer edge of the last retroreflector, so that the 
outgoing light beam goes past the lens. 
In all embodiments the retroreflectors may be provided symmetrically on 
both sides of the light source, because the deflection of the light beams 
entering the entrance pupil or exit pupil of the first reflection device 
from the light source is effected entirely symmetrically. 
According to another embodiment, when a retroreflector is used which 
reverses the beam and effects lateral offsetting in the direction of the 
light source, the lateral offsetting may amount to twice the size of the 
light source, the retroreflector preferably being a flattened roof prism 
in this case. 
The flattened middle portion and the two lateral inclined portions of the 
roof prism are in this case expediently of the same size as the light 
source, while the centre of the roof prism lies in the optical axis. A 
fourfold passage is achieved with an arrangement of this kind. As compared 
with the embodiment first mentioned the advantage is gained that after the 
fourfold passage the light does not pass out of the measurement path at 
the site of the light source, but laterally offset thereto. 
The use of an illuminated slit as light source is particularly convenient 
because a Beck prism can be disposed close to the slit. 
A particularly suitable device for illuminating the slit is one in which 
the slit is illuminated by the slit of an inclined slit mirror and the 
side portions of the slit mirror receive the light reflected past the Beck 
prism or Beck prisms and guide it onto a photoreceiver. The advantage is 
thus gained that a partially transmitting mirror, which always entails 
losses of light, need not be used. As the result of the arrangement of the 
invention the light losses in the reception of the measurement beam are 
therefore substantially reduced. 
Furthermore, the practical embodiment defined above makes it possible for a 
reference light beam from the main light source to be guided onto the 
photoreceiver through the slot alternately with the light from the 
measurement path.

In FIGS. 1, 3, 4, 5, 6 and 7 the line 12 provided with small cross-lines 
represents the cross-section of a slit which is illuminated from the left. 
The longitudinal direction of the slit thus extends at right angles to the 
plane of the drawing. Instead of the illuminated slit 12 it would also be 
possible to use the coil of a lamp or a laser light source, so that in 
order to simplify the description given below the slit 12 will simply be 
referred to as a light source, without the scope of application of the 
invention being thereby restricted. 
The path of the illuminating beam for the slit 12 will first be explained 
with reference to FIG. 9. 
The coil of an incandescent lamp 26 is imaged by way of a condenser system 
27 into a lens 28 which is disposed directly behind the opening of a 
rotating chopper disc 29. The chopper disc 29 serves to obtain at the 
electrical output of the apparatus an alternating signal which is more 
easily processed. 
The lens 28 is situated at the focal point of an achromat 30, which thus 
transmits a parallel light beam from the left to the slit 12. 
Directly behind the lens 28 the light beam is divided by a deviating mirror 
31, which projects laterally into it, into a measuring beam 32 passing 
straight through and a laterally deflected reference beam 33. The 
reference beam 33 is deflected upwards to another deviating mirror 34, by 
which it is again deflected substantially into the same direction as the 
measuring beam 32. Behind the mirrors 31 and 34 is disposed a segmental 
disc 35 which is rotationally fixed to the chopper disc 29 and which in 
the region of the measuring beam 32 and of the reference beam 33 has 
peripheral slits for the passage of the beams. The peripheral slits are 
however so shaped and offset in relation to one another that only one of 
the two beams 32 or 33 passes at one time through the segmental disc 35. 
After passing through the segmental disc 35 the measuring beam 32 passes 
through the slit 22 (visible in FIG. 10) of a slit mirror 23 which, as 
shown in FIG. 9, is disposed at an angle of 45.degree. to the optical axis 
19. The reference beam is concentrated by way of another deviating mirror 
36 and a concave mirror 37, and through the aforesaid slit 22 and by way 
of a microlens 28', on a photoreceiver 24. The measurement beam 32 and the 
reference beam 33 thus pass through one and the same slit 22 in directions 
substantially at right angles to one another. 
The measurement path 11, which is shown interrupted in FIG. 9, follows the 
slit 12 to the right by way of a lens 21 which will be further described 
below. At the end of the measurement path is situated the reflection 
device 17 according to the invention which throws back incident light, in 
a manner which will be described below, to the transmitter-receiver 
disposed on the left of the measurement path 11. As the result of the 
multiple reflection arrangement according to the invention, which will be 
described below, the reflected light passes in accordance with FIG. 10 at 
12''''' laterally past Beck prisms 16', which will likewise be described 
further on, onto the side parts 23a, 23b, of the slit mirror 23, whence 
the beam is reflected downwards to the photoreceiver 24. 
The transmitter-receiver described in connection with FIGS. 9 and 10 is 
particularly suitable for application in the embodiment shown in FIGS. 3, 
4, 5 and 7. However, FIG. 1, in which a simplified beam path with a 
partially transmitting mirror 38 is used, is more suitable for 
illustrating the basic principle of the present invention. 
According to FIG. 1, the slit 12 is illuminated from the left by way of an 
optical system (not shown in detail) and of the partially transmitting 
mirror 38. At the other end of the measurement path 11 is situated the 
reflection device according to the invention, consisting of a 
retroreflecting element 14, which is more particularly in the form of a 
triple mirror and which has its apex 15 remote from the measurement path, 
a lens 13 disposed in front of the base surface 39 of the triple mirror 
14, and beam deflection means in the form of an optical wedge 18. The tip 
of the wedge points towards the optical axis 19 and widens in the 
direction away from the latter. Instead of an optical wedge 18 on one side 
of the optical axis 19 it is also possible to dispose two wedges 18' 
having a smaller wedge angle, on each side of the optical axis 19, as 
indicated in dashed lines in FIG. 1. 
Below the light source 12 is situated a retroreflector 16 which is directly 
adjacent the light source 12 and whose base remote from the apex 20 has 
the same width and height as the light source 12. The retroreflector 16 
may be made of finely divided retroreflecting material, but is preferably 
in the form of a Beck prism, as shown in FIGS. 8a and 8b. The illustration 
in FIG. 1 corresponds to a section of the Beck prism on the line I-I in 
FIG. 8a. 
The mode of operation of the optical multiple reflection arrangement shown 
in FIG. 1 is as follows: 
From every point of the slit or light source 12 a light beam extends 
through the measurement path 11 to the reflection device 17, this beam 
having, as indicated by dashed lines 43, a solid angle such that it 
overshoots the reflection device 17 on all sides to such an extent that in 
the event of any lateral displacements of the reflection device 17 
relative to the light source 12 all regions of the reflection device 17 
remain within the light beam. Of all the rays of this light beam which 
originate from the centre of the light source 12 only one individual ray 1 
will be considered as an example, this ray entering the upper half of the 
lens 13 and there being directed parallel to the optical axis 19, because 
according to the invention the focal length f of the lens 13 has been 
selected to be equal to the distance between the lens and the light source 
12. The ray 1, directed parallel, enters the triple mirror 14, is offset 
in the latter, and passes out of the triple mirror 14 again, below the 
optical axis 19, parallel to its incoming direction. Because of optical 
laws, without the wedge 18 the reflected, offset ray 2 would pass along 
the dashed line 42 back to the starting point on the light source 12, that 
is to say, the light source 12 would be imaged in itself. 
Because of the optical wedge 18 inserted in accordance with the invention, 
however, the reflected ray 2 is deflected in such a manner that an image 
12' of it is projected directly at the side of the light source 12. Since 
the retroreflector 16 is situated behind the image 12' of the light source 
12, the ray 2 is thrown back in itself as a reflected ray 3. On the wedge 
18 the deflection effected by the latter is cancelled out and, after being 
again reflected and offset in the triple mirror 14, the ray 3 finally 
passes over into the re-reflected light beam 4, which passes to the 
starting point in the light source. The light now passing out through the 
slit 12 can be concentrated by way of the partially transmitting mirror 13 
and by way of a lens 40 onto a photoreceiver 41. The boundary lines of 
each beam coming from the light source 12 are designated 43. As can be 
seen, neither a certain lateral displacement of the reflection device 17 
nor a certain tipping will make any change in the imaging conditions, so 
that even without a rigid connection between the transmitter-receiver on 
the one hand and the reflection device 17 on the other hand perfect 
imaging and intensity conditions will exist in the receiver. 
Because of the conditions of symmetry of the optical multiple reflection 
arrangement shown in FIG. 1, an image 12' of the light source 12 is also 
produced on the other side in the manner illustrated. Through the 
provision of another retroreflector behind this second image 12', the 
light impinging there can also be utilised for the measurement. The beam 
paths to the additional retroreflector 16 extend entirely symmetrically to 
the beam paths shown in the drawing, in which they are not shown simply in 
order not to impair the clarity of the Figure. 
The wedges 18, 18', are shown in FIG. 1 only in order to illustrate the 
principle. In practice the offsetting of the image 12' in relation to the 
light source 12 is best achieved by means of two half-lenses having 
separate but parallel optical axes, or in accordance with FIG. 2 by 
dividing the lens 13 into two half-lenses spaced a distance a apart. By 
making the distance a between the optical axes adjustable, particularly in 
a trial arrangement, the image 12' can be brought exactly to the desired 
position, that is to say in particular directly adjoining the light source 
12. 
In an arrangement which is ready for production, however, the half-lenses 
13' are cemented on the triple mirror 14 in order to avoid separate 
holders for these two parts. 
A similar arrangement to FIG. 1 is shown in FIG. 6, where however the 
retroreflector 16, which is preferably in the form of a Beck prism, is 
replaced by a flattened roof prism 16' whose flat middle portion and 
inclined side portions each have a width corresponding to that of the gap 
or light source 12, the prism 16' having the same length as the light 
source 12. If a flattened roof prism 16' of this kind is disposed 
symmetrically to the optical axis 19, as shown in FIG. 6, a light ray 1 
originating from a determined point of the light source 12 passes through 
the measurement path 11 in the manner illustrated, merges, after 
refraction and reflection in the reflection device 17, into the reflected 
ray 2, which at the point shown enters the flattened roof prism 16" 
again, laterally of the light source 12, and on the symmetrically opposite 
side passes out as a re-reflected ray 3. Finally, after further reflection 
and refraction at 17 the ray 3 returns to the starting point as the ray 4 
reflected for the fourth time, and then similarly to the arrangement shown 
in FIG. 1, is guided again to a photoreceiver by way of a partially 
transmitting mirror (not shown). 
In comparison with that shown in FIG. 1, the arrangement shown in FIG. 6 
provides the advantage that for the utilisation of the light returning on 
both sides of the light source 12 only a single element is necessary, 
namely the flattened roof prism 16". In this arrangement, as in all other 
embodiments, insensitivity to adjustment can also be achieved through the 
fact that each beam originating from the light source 12 overshoots the 
reflection device 17 on all sides. 
If it is not desired that the light beam finally passing out again from the 
multiple reflection arrangement should pass through the light source 12 
itself, it is possible to use the arrangement shown in FIG. 7, in which 
the flattened roof prism 16" is disposed at the side of and immediately 
adjoining the light source 12. With the aid of the reflection device 17 of 
the invention the image 12' of the light source is first produced in the 
position shown, after being reflected once. By reflection within the 
flattened roof prism 16" this image is offset downwards by twice the width 
of the light source, to form the image 12". On renewed passage through the 
optical wedge 18 half of this offsetting is cancelled out, so that finally 
the ray 4 corresponding to the fourth passage passes out through the 
flattened portion of the roof prism 16" in the manner illustrated, at the 
side of the light source 12. As the result of this arrangement, it is not 
necessary to provide a partially transmitting mirror, and thus a better 
light yield is obtained. 
The action of the lens 21, the focal length of which is equal to that of 
the lens 13, is explained below with reference to FIG. 4. 
A particularly preferred embodiment will now be described with reference to 
FIG. 3. This embodiment differs from that shown in FIG. 1 particularly in 
that the retroreflector 16' is relatively twice as wide and that the 
deflection of the beam by means of the wedge 18 is only so great that the 
image 12' after the first reflection lies directly on one half of the 
retroreflector 16'. By reflection within the retroreflector 16' the image 
12' is shifted to the position 12" in the lower half. The beam 3 now 
returning to the reflection device 17 is refracted in the wedge 18 in such 
a manner that the offsetting is cancelled by one light source width and 
the re-reflected beam 4 re-enters the upper portion of the retroreflector 
16'. An image 12''' of the light source is thus again formed at the 
position of the image 12'. The image 12''' is again displaced by the 
retroreflector 16' into the lower half, to the position 12''''. Since the 
returning beam 5 now entering the upper half of the reflection device 17 
is finally again deflected downwards at the wedge 18 in the form of a 
reflected beam 6, the beam 6 passes out of the arrangement, past the 
retroreflector 16'. Thus directly at the side of the retroreflector 16' a 
fifth image 12''''' of the light source 12 is thus formed. 
As the result of the symmetry conditions another retroreflector 16' can 
also be disposed on the other side of the light source 12. A fifth image 
12''''' on the outer side of this retroreflector 16' and a corresponding 
exit beam from the device would then also be obtained. 
In order to ensure in a multiple reflection arrangement according to FIG. 3 
that beams passing out of one half of the reflection device 17 will 
re-enter this half after reflection on the transmitter-receiver side, in 
the particularly preferred embodiment shown in FIG. 4 a lens 21 is 
disposed in front of the light source in the manner illustrated, the focal 
length of this lens being equal to the distance from the reflection device 
17 and thus equal to the focal length of the lens 13. The mode of 
operation of the lens 21 is illustrated by the beams 3 and 5 in FIG. 4. 
Whereas in the embodiment shown in FIG. 3 the reflected beams 3 and 5 
extend parallel to the beams 2 and 4 respectively (so that the danger 
exists that these beams may possibly no longer enter the reflection device 
17 or the correct half of the reflection device 17), the lens 21 ensures 
that the beams 3, 5, will return to the same point of the reflection 
device 17 from which the respective beam 2 or 4 originated. The same also 
applies to all other beams passing out of the reflection device 17 and 
then retroreflected at 16'. For graphic reasons the measurement path 11 in 
all embodiments is shown far too short in relation to the distance between 
the optical elements of the reflection device 17 and to the distance 
between the reflectors 16', 16" and the lens 21. In practice the distance 
between the optical elements on one side of the measurement path in 
relation to the length of the latter can be neglected. 
In FIG. 4 the lens 21 is shown so large that it even projects laterally 
beyond the outgoing beam 6. However, it need extend only to the edge of 
the retroreflectors 16', so that the exit beams 6 pass by the lens 21. A 
construction of the lens 21 of this kind is used in the embodiment which 
will be described below and which is illustrated in FIG. 5. 
The function and construction of the lens 21 according to FIG. 7 are the 
same as described above in connection with FIG. 4. This is also 
illustrated in detail in the embodiment shown in FIG. 7a, where in 
addition the first roof prism 16"a is supplemented by a second, identical 
roof prism 16"b mounted on it and offset downwards by one light source 
width. The number of passages of the beam is thereby increased by 2. A 
further increase can be achieved according to the invention by 
correspondingly adding further prisms 16"c, d, and so forth. 
While the embodiment shown in FIG. 4 permits a sixfold passage of the light 
beam through the measurement path 11, a substantially larger number of 
passages can be achieved by aligning additional retroreflectors 16'a, 
16'b, 16'c, etc., as shown in FIG. 5. Each additional retroreflector 
provides four additional passages, so that in the embodiment shown in FIG. 
5 the light beam passes laterally out of the device only after fourteen 
passages. Particularly in embodiments having a plurality of juxtaposed 
retroreflectors 16' the lens 21 is of special importance for the purpose 
of effectively uncoupling the two halves of the reflection device 17. 
In the embodiments shown in FIGS. 3 to 5, the Beck prisms shown in FIGS. 8a 
and 8b are particularly suitable as retroreflectors 16', because their 
base area can without difficulty be made equal to the area of the slit and 
they can be disposed close side by side as shown in FIG. 5. This can be 
seen particularly clearly in FIG. 10, where the Beck prisms 16' are shown 
directly at the side of the slit 12. Here it can be seen particularly 
clearly that the base surface of the Beck prisms should be selected to be 
twice as wide as the width of the slit, while the length of the base is 
equal to that of the slit. FIG. 10 also shows particularly clearly the 
good light yield which can be achieved with a multiple reflection 
arrangement according to the invention. The entire light intensity passes 
through the slit 22 of the slit mirror 23 to the slit 12. Since the slit 
images 12''''' appear at the side of the Beck prisms 16', the entire light 
intensity of the measuring beams finally passing out of the multiple 
reflection arrangement also passes completely to the side portions 23a, 
23b, of the slit mirror 23. From these side portions they are concentrated 
by the fully reflecting mirror surfaces onto the photoreceiver 24 (FIG. 
9). 
Because of the rotation of the chopper disc 29 and of the segmental disc 
35, not only is alternating light transmitted to the photocell 24, but in 
addition at any one moment of time only either light 25 reflected from the 
measurement path 11 or reference light 33 impinges on the photoreceiver 
14. By comparison of the two signals in a suitable electronic evaluation 
unit it is thus always possible to determine the light intensity of the 
light beams returning from the measurement path 11 in relation to the 
reference beam 33. Electronic evaluation circuit arrangement of this kind 
are known per se for the comparison of two light beams.