DE3629966C2 - - Google Patents

Description

The invention relates to a sensor for converting a physical quantity in an electrical output signal with a light source, from which a bundle of light rays into a first front face a light-guiding body is coupled, the Light rays at an interface of the body depending totally reflected or from the physical size the body and the totally reflected Light rays on a second surface, preferably one of the first rear face opposite second rear face,  fall, and with a plurality of photosensitive Elements that are connected to an evaluation circuit for detecting one of the bundle after Total reflection or decoupling of the angular range.

Such a sensor is from Patents abstracts of Japan, June 21, 1980, 55-53 878 (A).

The known sensor is used to measure the concentration of Lead accumulator battery electrolytes. He has one Light source on, from which a diverging beam through a diaphragm on an oblique side surface of a prismatic, light-guiding body falls. A lower, elongated one Interface of the body borders on the one to be measured Electrolytes. The light rays of the divergent Rays falling on the lower interface are there, depending on their angle of incidence and the density of the electrolyte either totally reflected or but decoupled from the body into the electrolyte. The totally reflected light rays fall on another, also inclined interface of the prismatic body, on which there is a gate of photosensitive elements located. Depending on how large the density of the measured Is electrolyte, the boundary between the shifts still totally reflected and the decoupled light rays of the bundle on the lower interface with the electrolyte and thus also the corresponding boundary line that on the Gate of light-sensitive elements covered by totally reflected Rays of light. Because even when rays of light be coupled out, a partial beam at the lower interface is reflected, results from the length of the gate seen the photosensitive elements, an intensity curve, that of a relatively low signal value  suddenly to a relatively high signal value increases. The known sensor now measures the amplitude of the rays of light falling through the various elements of the gate and pulls the respective position of the jump-like Signal rise on the gate as a measure of the density of the Electrolytes.

However, the known sensor has the disadvantage that it is very high precision must be adjusted, because with the only one Total reflection of the light rays in the prismatic Body already slight misalignments of the incident Bundle of light rays to falsify the Lead measurement result. In addition, the known sensor has the Disadvantage that a measurement of the density of the electrolyte practically only in a point form, namely in one in the Practice very small length section of the interface in which the transition from total reflection to decoupling varies so that the measurement result is only characteristic of the state of the electrolyte in a container, e.g. B. one Accumulator battery, total is when the electrolyte is in the battery has the same density uniformly. This is in practice, however, not always the case, because at an easier one, e.g. B. warmer electrolyte, above in Accumulate accumulator while heavier parts are below discontinue, but on the other hand with accumulators, the movements subject, as is the case for example in motor vehicles the case is also due to the movement of the accumulators spatial density fluctuations occur to a considerable extent can. The known sensor cannot measure the acid density between the plates of a battery, and also installation in pipes is difficult. Another disadvantage of  known sensor is that it is systematic for only one only measurement task, namely the measurement of the density of the the medium adjacent to the prismatic body.

Numerous sensors based on optoelectronic are also known Paths using the refractive properties of a light guide physical quantities in electrical Convert output signals.

From DE-OS 34 33 343 an optical measuring arrangement with fiber optic light transmission known. With this arrangement becomes light from a light source into a light guide coupled in and occurs again at a radial interface out. The one emerging in the form of a conical beam of light Light strikes a mirrored reference surface at a distance to be measured from the end face of the light guide located. That from the reference surface in turn reflected light hits insichreflectors, which cause that the light is guided back in the same way is consequently returned to the end face of the light guide  and after suitable decoupling from the transmission light Light detector is supplied. The measuring effect in this known The arrangement is based on the beam spreading is due to the dependence on the observation angle Light reflection of the insichreflector. The application of the known arrangement is therefore based on the length measurement a configuration of the type described limited.

From DE-OS 34 28 453 is also a sensor device known in which a transparent prism with two 90 ° mutually offset interfaces in a measuring medium immersed. If the measuring medium has a first predetermined density has, then a by means of a first light guide the first interface is a beam of light radiated at an oblique angle totally reflected there, reaches the second interface and is reflected back into a second receiving light guide. Now the density of the measuring medium increases, so the light beam strikes the first one as soon as it occurs Interface of the prism into the medium and on the receiving light guide there is a zero signal. This known sensor device can therefore only be used as a yes / no signal generator, is a continuous digitized measurement not possible.

Another sensor is known from DE-OS 34 03 887, at an elongated, one-sided clamped light guide can be deflected by a gas flow. Towards the free A flat detector is located at the end of the light guide in the form of a luminescent body. Depending on the deflection of the light guide hits the one it radiates Light on another surface area of the flat luminescent body on which on one or on  a detector is arranged at both ends. From the intensity of the detector signal or from the quotient of the intensities of both detector signals can now be the point of impact of the Light and thus indirectly the deflection of the light guide and from it the speed or the amount of deflecting gas can be determined. This sensor too therefore only allows analog measurements and it is open again a special application of measuring gas velocities or gas flow rates limited.

DE-OS 20 34 344 is a device for measurement physical quantities by measuring the intensity of a Light beam known. With this known device one also takes advantage of the fact that Light rays emerge at the interfaces of a light guide or be totally reflected, depending on how the relationship the refractive indices or the densities in the light guide and is in the surrounding medium. This is done at different Arrangements of the effect that exploited by environment a part of the light guide with a dense medium ever increasing amount of light is coupled out of the light guide becomes, whereby measurements, for example a level, possible are. Digital measurement results can through threshold circuits, the photoelectric conversion elements are subordinate, are formed or optically. For this purpose, e.g. B. a number of optical fibers assigned to numbers, the different depths in a container with a solidifying substance immerse, be designed so that the one side of the solidification front directing light guide light to a display on the other Side did not go. The number of the one just controlling light for display The light guide is therefore a measure of the position of the solidification front.

Finally, from DE-OS 21 55 049 there is still an optical one Comparative device with optical fibers known. At this known device is, as already to the next The embodiment described above is explained in the axial  Distance from an end face of one exposed to light Optical fiber one with a reflected surface provided reference plane, the axial distance to be measured from the end face of the light guide. For this purpose, there are several receiving light guides radially next to the light guide arranged, the signal of each individual optical detectors is supplied. These optical detectors are in turn connected to an electronic analog circuit connected. In this known device you do take advantage of the fact that as the distance increases the reflective reference surface from the front surface the edge of the reflected beam of light always radially from the end face of the light guide wanders and successively the faces of the different radially spaced receiving light guide sweeps. As a result, a regionally linear output signal can can only be removed from the receiving light guide, whose face is just reflected from the edge of the Beam is swept. This makes the take advantage of known device by doing the straight "linear" signal to form an overall broader Linear range is used. The well-known Device is therefore only for suitable for a single measurement task and a digital one The output value of the signal is not available here.

The invention is based on the object, one To further develop sensors of the type mentioned in the introduction, that it is suitable for a variety of measuring tasks and The problem with the adjustment is that local Do not make disturbances in the vicinity of the sensor noticeable  and finally, above all, a continuous digital display of the respective output signal is possible.

This object is achieved in that the Body is designed as an elongated light guide, in the light rays are totally reflected several times, that the photosensitive elements at an axial distance of  an end face are arranged and an impact surface for a bundle of light rays emerging from the end face form, and that the evaluation circuit has a counter that counts the number of the bundle illuminated or alternatively the non-illuminated Outputs elements as an output signal in digital form. Below is just the count of the illuminated elements considered in more detail. Should be the number of unlit elements the total number must be counted and evaluated of all elements.

When using an elongated light guide with multiple total reflection become one Adjustment problems avoided because the length of the Seen light guide uniform light beam conditions anyway train, also in the area of the Any existing discontinuities in this way averaged. By guiding the light rays in the light guide can solve numerous different measuring tasks will. For example, like the one described at the beginning known sensor the refractive index and thus also the density of a medium surrounding the light guide is independent from its physical state are measured, there are continuous level measurements possible and there can also have geometric sizes, in particular lengths be measured this way.  

Another important advantage of the invention is that the Angular range of the bundle emerging from the light guide, in other words the so-called "acceptance cone" in digital Form is measured by the number of one certain value of physical size illuminated elements is counted and displayed. Any signs of aging or other drift phenomena have an effect so it does not interfere with the measurement result because the evaluation circuit for each individual photosensitive element only makes a yes / no decision, so that when appropriate set trigger level for the respective element without Significance is how great the intensity of each incident Light beam is or how the conversion factor from incident light beam to emitted voltage in the Element itself changed due to signs of aging Has.

Overall, the invention thus represents a universal usable, robust, reliable and against signs of aging insensitive sensor available.

In a preferred embodiment of the invention photosensitive elements at an axial distance from the second end face arranged.

This measure results in a particularly simple structure of the Light guide, for example in the case of a cylindrical one The light beam guides the beam into one radial end face coupled and the acceptance cone from the opposite radial end face emerging radiation is measured.  

In another embodiment of the invention second end face designed as a reflector and the photosensitive Elements are at an axial distance from the arranged first end face.

This measure has the advantage that with the acceptance of a somewhat more complicated construction of the sensor from only one single side must be accessible, at the same time the light is coupled into the sensor and used to measure the Acceptance cone is coupled out again. So designed The sensor is therefore particularly suitable for measuring tasks difficult to access places, for example to measure the Density or level of a liquid in one closed container.

In a variant of this embodiment, the Light guide divided into two axially spaced sections and the bundle is in the first face of the first axial section coupled, while the photosensitive Elements at an axial distance from the first End face of the second section are arranged.

Another variant serving similar purposes provides that the first end face in two axially spaced stages is formed and that the bundle in the front stage is coupled in while the photosensitive elements spaced axially from the rear step are.

These two variants have the common advantage that a spatial separation of the light source for what is to be irradiated Bundle of photosensitive elements for the  Measurement of the acceptance cone is possible because of these two Operations in axially spaced positions of the light guide occur.

In another embodiment of the invention Impact surface inclined to the axis of the light guide.

This measure has the advantage that when the acceptance cone is varied due to the inclination of the impact surface an enlargement of the area in which the edge of the Acceptance cone fluctuates.

In a preferred embodiment of this variant, the The impact surface is also curved in the specified manner, around any nonlinearities that may exist of the sensor to compensate.

In a first application example of the invention physical quantity the density of a surrounding the light guide Medium and the light guide is only with its Shell surface optically coupled to the medium.  

The medium can make a liquid more variable Density, but it can also be gases of variable pressure be measured because the density of the gas with its pressure varies.  

In embodiments of the invention, the light guide is made made of glass or plastic, e.g. B. polymethyl acrylate (PMMA).

This measure also has the advantage that known materials with also known reproducible properties can be used.

In other variants of the invention, the light guide a translucent tube with a reference medium is filled, the chemical composition of which of the measuring medium surrounding the tube at a predetermined Value corresponds to the physical quantity.

This measure, known per se from DE-PS 34 02 374, has the advantage that the properties of the reference medium and the surrounding measuring medium with variations in the ambient conditions alike change so that too Drift phenomena can be excluded.

In a further variant of the invention, the light guide light-emitting elements after irradiation emit secondary light by means of a primary light.

This measure has the advantage that instead of a diffuse one Light radiation into the light guide also radiation is possible by means of a parallel bundle, in which case the diffuse light rays required for the invention can be represented by the secondary light. For example happen that the primary light on  Impurities (color centers) in the light guide that diffuse reflective interfaces that the primary light hits, be provided or that the light guide with luminescence centers is provided, which in turn generate secondary light.  

Finally, there is another embodiment of the invention preferred, in which the light source on a pulse generator is connected and the evaluation circuit a difference generator has inputs whose measurement values are switched on or switched off light source can be supplied.

This measure has the advantage that measurements in the Impulse pauses measured and thus external light influences can be compensated.

Embodiments of the invention are in the drawing are shown and are described in the following description explained in more detail. It shows

Fig. 1 is a schematic representation of a light guide to illustrate the present invention,

Fig. 2 is an illustration of an optical path, as occurs in the light guide of Fig. 1,

Fig. 3 shows a first embodiment of a planar Lichtmeßanordnung for use in the inventive sensor,

Fig. 4 shows a variant of the embodiment of Fig. 3 with line-shaped Lichtmeßanordnung,

Fig. 5 shows a first variant of an inventive sensor for measuring the density of a medium,

Fig. 6 shows a second variant of this,

Fig. 7 shows a third variant of this,

Fig. 8 shows a first variant of a configuration of a light guide for use in one of the sensors shown in FIGS. 5 to 11,

FIG. 9 shows a second variant of this,

Fig. 10 shows a third variant of this, however, for use in one of the sensors of Figs. 5 to 7,

Fig. 11 is a highly schematic circuit diagram for explaining the connection of a sensor according to the invention,

Fig. 12 shows a further variant, similar to Fig. 1, to increase the sensitivity of a sensor according to the invention.

In Fig. 1, 1 designates a light source from which a diverging ray beam 2 enters off and a cylindrical light guide 10 in a neighboring upper face 3). The point-shaped light source 1 with the diverging beam 2 is only to be understood here as an example, it will be explained further below that parallel beams can also be used, from which diverging secondary light in the interior of the light guide is derived.

In Fig. 1, 11 shows a first, axially directed light beam which passes through the light guide 10 without further deflection or obstruction. With 12 a, 12 b , a second light beam is identified, which strikes a lateral surface 19 of the light guide 10 so flat that it is totally reflected. The light beam 12 a, 12 b thus continues its path through the light guide 10 in the axial direction. 13 a, 13 b , on the other hand, denotes a third light beam which strikes the lateral surface 19 at such a steep angle that it is coupled out of the light guide 10 .

As a result, this means that after several reflection processes in the light guide 10 only light rays 11 or 12 a, 12 b are guided, which are either strictly axially directed or so flat that they are totally reflected on the lateral surface 19 . This creates a so-called “acceptance cone” 14, which is used to denote the shape of a diverging bundle 17 of light rays emerging from a lower end face 16 .

An impact surface 15 is defined at an axial distance h from the lower end face 16 . If one designates the opening angle of the acceptance cone 14 with β , then in the impact surface 15 with a circular lower end face 16 there is a circular light surface with a circumferential ring area of radial width x, which depends on the angle β and the axial distance h .

If, as a result of a change in the refraction conditions in the light guide 10 or in the surrounding medium, the critical angle of the total reflection changes, the angle β and thus the dimension x also change .

FIG. 2 shows the relationships explained for FIG. 1 again for the quantification of the effect that occurs. A fourth light beam 18 is now guided in the light guide 10 , the section 18 a of which strikes the lateral surface 19 just under the critical angle α _T of total reflection. Expressed in graphic terms, this means that all light rays incident in the hatched area in FIG. 2 are totally reflected, while all rays incident more steeply than light beam 18 are coupled out of light guide 10 . The light beam 18 is now (just) still totally reflected in its section 18 a and a reflected section 18 b strikes the lower end face 16 . Provided that the light beam section 18 b occurs outside the total reflection area of the refraction conditions prevailing on the lower end face 16 , a section 18 c of the light beam 18 is coupled out of the lower end face 16 , namely at an angle β which is just the opening angle β of the acceptance cone 14 in Fig. 1 corresponds.

If n _{i denotes} the refractive index of the light guide 10 , n _a the refractive index of the medium surrounding the light guide 10 in the region of its lateral surface 19 and n _st the refractive index of the medium surrounding the light guide 10 on the lower end face 16 , it can be shown that that the following applies to the opening angle β of the acceptance cone 14 :

It goes without saying that n _{i is} greater than n _a and n _st . In the event that the refraction ratios on the lateral surface 19 and on the end surface 16 are the same, ie n _a = n _st , the formula given is simplified accordingly.

It can thus be seen that the width x to be seen in FIG. 1, via the opening angle β and the distance h, is directly a measure of the refraction ratios of the light guide 10 to the medium surrounding it.

The width x is output in digitized form as a measured value.

Fig. 3 shows in this respect an embodiment, in which again the light guide 10 can be seen, from which a light beam in the shape of the acceptance cone 14 emerges below. In addition, an acceptance cone 14 a is shown in broken lines, which is intended to symbolize a second measured value.

Below the light guide 10 , a flat detector array 22 can be seen at an axial distance in the imaginary impact surface, which can be designed, for example, as a charge shift semiconductor component (CCD). The detector array 22 consists of a multiplicity of detector elements 23 distributed in an area, which can be individually controlled and read out. A symbolized data line 24 leads to an evaluation circuit 25 , which essentially contains a digital counter.

In the illustrated example the case of the solid drawn acceptance cone 14, hatched in Fig. 3 8 detector elements are illuminated, so that after counting of these elements of the same is a digital value "8" is output via the data line 24 in the evaluation circuit 25 at the output. Almost any resolution of the measurement result can be achieved by a corresponding number of detector elements 23 and, if this should be of practical importance, a limit value can be specified by setting a certain trigger threshold for only partially illuminated detector elements 23 , from which a detector element 23 is illuminated or counted unlit. Any gray transitions in the edge area of the acceptance cone 14 can also be precisely defined in this way.

It is seen from Fig. 3 without further ado that illuminated at a magnification of the acceptance cone 14 in a cone 14 a (shown in dashed lines) has a correspondingly larger number of detector elements 23 and hence also a correspondingly larger digital value appears at the output of the evaluation circuit 25th

FIG. 4 shows a variant in which the distance x from FIG. 1 is not determined by an area measurement as in FIG. 3 but only by a measurement along a straight line. For this purpose, a linear detector array 26 is provided, for example a linear diode gate or the like. It can be seen from Fig. 4 that in the case of the solid acceptance cone 14 four detector elements 23 are illuminated, which in turn have been hatched in Fig. 4, while when the acceptance cone is opened to a value 14 a in the illustrated example six detector elements are illuminated. In this case too, the number of illuminated detector elements is counted and output in the form of a digital value at the output of the evaluation circuit 25 .

Figs. 5 to 7 show four embodiments in which the sensor of the invention is for measuring the density of a surrounding medium, in particular a liquid, is used.

Fig. 5 shows an embodiment for density measurement, in which an optical fiber 34 is guided through two aligned openings in the walls 31, 32 of the container for the measuring liquid 33 . In the area of the passages, reflectors 30, 30 a are attached to the outer surface 19 , while the outer surface 19 is washed around by the measuring liquid 33 .

In this embodiment, the beam of rays in the form of the acceptance cone 14 emerges from the light guide 24 outside the container and strikes the detector array 22 arranged at an axial distance for the measurement value processing already described.

In the exemplary embodiment according to FIG. 5, the refraction conditions in the area of the lateral surface 19 of the light guide 34 determine the measurement.

Fig. 6 shows a further variant, in which a light guide 35 is divided into two axially offset sections 35 a, 35 b . The upper section 35 a is provided with an upper end face 3 a , into which the beam 2 is coupled. Below the lower end face of the upper section 35 a there is either a flat detector array with an opening aligned with the light guide 35 or laterally two separate detector arrays 22 a, 22 b , as shown in FIG. 6, with a corresponding free space between them leave open, through which the beam 2 ' , which has emerged from the lower portion 35 a below, can pass through.

The lower section 35 b penetrates a bore in the wall 31 of the container for the measuring liquids 33 . Apart from a reflective coating 30 in the area of the passage through the wall 31 , the lower section 35 b is non-reflective in the area of its lateral surface 19 and borders directly on the measuring liquid 33 . However, the lower end face 16 is provided with a mirror 36 or another suitable reflector. This means that a light beam 37 guided in the lower section 35 b is directed upwards again after striking the mirror 36 , so that a beam in the form of the acceptance cone 14 emerges from an upper end face 3 b of the lower section 35 b . The opening angle of the acceptance cone 14 is again measured in the manner described by the detector array 22 a, 22 b .

FIG. 7 shows a variant of the embodiment of FIG. 6.

A light guide 40 , which in turn passes through the upper wall 31 of the container for the measuring liquid 33 , is formed in the area of this throughput as a lower section with a larger diameter, to which an upper section 40 a with a smaller diameter adjoins. The lower section 40 b thereby has an annular end face 41 b , while the upper section 40 a has a circular end face 41 a . In this end face 41 a, the beam hits 2 and passes into the lower portion 40 b, by the downwards of the sensor of FIG. 7 as well as that of FIG. 6 is formed. The light reflected from the lower end of the light guide 40 at its mirror (not shown in FIG. 7) now emerges through the annular end face 41 b of the lower section 40 b and reaches the detector array 22 a, 22 b , which in turn is designed in accordance with FIG. 6 is.

FIGS. 8 to 10 also show some variants of light guides as can be used for some of the exemplary embodiments according to FIGS. 5 to 7.

Fig. 8 shows the variant in which a light conductor 80 consisting essentially of a light-transmissive tube 81, such as a glass tube is filled with a reference media 82. The reference medium 82 is either of the same chemical type as the surrounding medium, for example the measuring liquid 33, or of a defined, different type in order to be able to eliminate disturbance variables in this way.

If, for example, the light guide 80 is used to measure the density of an acid, then this acid can be used as the reference medium 82 , the density of which corresponds to a specific reference value of the acid serving as the measuring liquid 33 . External influences, which then affect the measuring liquid and the reference medium equally, are then no longer included in the measurement result.

FIG. 9 shows a further variant in which a light guide 85 essentially consists of a translucent body 86 made of glass, plastic or the like. However, as shown in the left half of FIG. 14, a luminescent body 87 is arranged at the lower end of the light guide 85 , but, as the right half of FIG. 14 shows, a diffuse reflector 88 can also be arranged there.

The light guide 85 thus makes it possible to apply a parallel beam to the sensor, which at the lower end merges into a diffuse, secondary-emitted or reflected beam which emits light beams of different inclinations upwards again. Such an arrangement can, for example, be used advantageously in the exemplary embodiments of FIGS. 6 and 7.

FIG. 10 shows a variant in which a light guide 90 in turn consists of a translucent body 91 , in which either luminescent elements 92 or diffusion elements, for example color centers or the like, are introduced as a whole.

As the lower half of FIG. 10 shows, this is also possible with liquids, similar to the embodiments according to FIG. 8, in that a glass liquid 93 contains a reference liquid 94 of a predetermined chemical composition, in which suspended particles 95 are contained as a suspension.

In this way, secondary light can also be used in different ways as reflected or generated by secondary emission Light used to diffuse the light Boundaries of the light guide reflected or coupled out to become.

Fig. 11 shows a very schematic circuit diagram of a circuit arrangement for operating a sensor according to the invention.

A pulse generator 100 operates the light source 1 , which emits a clocked beam 2 on a light guide 102 . External light 103 also falls on the light guide 102 . The light guide 102 is connected via suitable detector and evaluation devices, as explained in FIGS. 3 and 4, to an amplifier 104 , which is further acted upon by the output of the pulse generator 100 .

The circuit arrangement according to FIG. 11 has the sense of correcting the influence of the external light 103 . For this purpose, the signal captured by the light guide 102 and originating solely from the external light 103 is determined and stored in the amplifier 104 during the pulse pauses of the pulse generator 100 . During a pulse of the pulse generator 100 , a measured value is again formed, which has arisen in the light guide 102 by the beam 2 of the light source 1 and the influence of the external light 103 and the previously determined measured value of the external light 103 alone is subtracted from this measured value. Since the external light 103 is generally a constant disturbance variable, the influence of the external light 13 can be compensated for in this way.

Finally, FIG. 12 shows a possibility of increasing the width x in FIG. 1 to increase the measuring accuracy by increasing the resolution.

A light guide 106 similar to that in FIG. 1 is located with its lower end at an axial distance from a striking surface 107 , which, however, is inclined to a longitudinal axis 108 of the light guide 106 . In this way, there is an increased width x ' when the bundle of light rays in the form of the acceptance cone 14 falls on the impingement surface 107 .

It is further shown in Fig. 12 with 107 a, 107 b that one can give the impingement surface 107 in addition to or instead of the inclination to the axis 108 also a curved course in order to compensate for or generate certain characteristics, depending on , as is desirable in the specific application.

Claims (14)

1. Sensor for converting a physical quantity into an electrical output signal with a light source ( 1 ), from which a bundle (2) of light beams ( 11, 12, 13; 18; 37 ) in a first front end face ( 3; 41 ) of a light-guiding Body is coupled, the light rays ( 11, 12, 13; 18; 37 ) at an interface ( 16, 19 ) of the body depending on the physical size totally reflected or coupled out of the body and the totally reflected light rays ( 12 b ; 18 b ; 37 ) fall onto a second surface, preferably a second, rear end surface ( 16 ) opposite the first end surface ( 3; 41 ), and with a plurality of light-sensitive elements ( 23 ) which are connected to an evaluation circuit for detection an angular range β ) occupied by the bundle ( 2 ) after total reflection or decoupling, characterized in that the body is an elongated light guide ( 10; 34; 35; 80; 85; 90; 102; 106 ) e is formed in which light rays ( 12 b ; 18 b ; 37 ) are totally reflected several times so that the light-sensitive elements ( 23 ) are arranged at an axial distance (h) from an end face ( 3; 16; 41 ) and a contact surface ( 15; 107 ) for one from the end face ( 3; 16; 41 ) emerging bundle ( 17 ) form light beams, and that the evaluation circuit ( 25 ) has a counter which outputs the number of elements ( 23 ) illuminated by the bundle ( 17 ) or, alternatively, the non-illuminated elements ( 23 ) as an output signal in digital form. 2. Sensor according to claim 1, characterized in that the light-sensitive elements ( 23 ) are arranged at an axial distance h from the second end face ( 16 ). 3. Sensor according to claim 1, characterized in that the second end face ( 16 ) is designed as a reflector and that the light-sensitive elements ( 23 ) are arranged at an axial distance h from the first end face ( 3 ). 3. Sensor according to claim 3, characterized in that the light guide ( 35 ) is divided into two axially spaced sections ( 35 a, 35 b) and that the bundle ( 2 ) in the first end face ( 3 a ) of the first axial section ( 35 a ) is coupled in, while the photosensitive elements are arranged at an axial distance from the first end face ( 3 b ) of the second section ( 35 b ). 5. Sensor according to claim 3, characterized in that the first end face is formed in two axially spaced stages ( 41 a, 41 b ) and that the bundle ( 2 ) is coupled into the front stage ( 41 a ) while the photosensitive elements are arranged at an axial distance from the rear step ( 41 b) . 6. Sensor according to one of claims 1 to 5, characterized in that the impact surface ( 107 ) to the axis ( 108 ) of the light guide ( 106 ) is inclined. 7. Sensor according to one of claims 1 to 6, characterized in that the impact surface ( 107 a , 107 b ) is curved in a predetermined manner. 8. Sensor according to one of claims 1 to 7, characterized in that the physical variable is the density of a medium surrounding the light guide ( 34; 35; 40 ) and that the light guide ( 34; 35; 40 ) only with its outer surface ( 19th ) is optically coupled to the medium. 9. Light guide according to claim 8, characterized in that the medium is a liquid ( 33 ) of variable density. 10. Sensor according to one of claims 8, characterized characterized in that the medium is a gas variable Pressure is.   11. Sensor according to one of claims 1 to 10, characterized in that the light guide ( 10; 29; 34; 35; 40; 50; 52; 59 ) consists of glass or plastic. 12. Sensor according to one of claims 1 to 10, characterized in that the light guide ( 80 ) has a translucent tube ( 81 ) which is filled with a reference medium ( 82 ), the chemical composition of which of the measuring medium surrounding the tube ( 81 ) corresponds to the physical quantity at a predetermined value. 13. Sensor according to one of claims 1 to 12, characterized in that the light guide ( 70; 85; 90 ) has light-emitting elements ( 92; 95 ) which emit secondary light after irradiation by means of a primary light. 14. Sensor according to one of claims 1 to 13, characterized in that the light source ( 1 ) is connected to a pulse generator ( 100 ) and that the evaluation circuit ( 25 ) has a difference former ( 104 ), the inputs of which are the measured values when the or switched off light source ( 1 ) can be fed.