Optical-Pickup Coordinatometer

This invention relates to devices which serve to acquire graphic data either for remote transfer or for utilization by a data-processing machine. This data acquisition operation calls for real-time measurement of the coordinates of a series of points. These points can be marked as dots on a writing table by means of the point of a stylus or selected by sampling of a line plotted on the table.

Data acquisition devices based on the measurement of an electric potential difference are already in existence but have low resolution capability. Higher resolution can be obtained by means of an elastic surface wave coordinatometer. However, this technique entails the use of a table cut in a piezoelectric crystal, thus limiting its surface area. Moreover, the fact of applying one's hand on a surface on which elastic surface waves are traveling is liable to give rise to disturbances and the electrical operation of the associated sensor does not permit of its use as a light pen in conjunction with a data display console.

In order to overcome these disadvantages, the invention utilizes the optical detection of elastic waves which propagate within a refractive medium. Thus the writing table can have a relatively large surface area, the measuring stylus can perform the function of a light pen and the disturbing effect of an object placed on the table becomes insignificant.

The invention is directed to an optical-pickup coordinatometer comprising a writing table provided with electromechanical transducer means for propagating elastic-wave trains in a direction parallel to the table surface, a pickup which is capable of point detection of the passage of elastic waves when it rests on the table and chronometric means for determining each coordinate by measuring the time taken by the elastic-wave trains to travel over the corresponding distance. The invention is distinguished by the fact that the writing table is constituted by a plate of refractive material placed within an optical polarizing cell. Said cell is so arranged as to prevent emergence of incident radiation produced by a source. The pickup mentioned above is an optical device for receiving that portion of the radiation which emerges from the optical cell by reason of the birefringence induced within the plate by the volume elastic-wave trains issuing from the electromechanical transducer means.

A more complete understanding of the invention will be gained from the following description and from the accompanying drawings in which:

FIG. 1 is a plan view of a plate having parallel faces and cut from refractive material;

FIG. 2 is an explanatory diagram;

FIG. 3 illustrates a first form of construction of the coordinatometer according to the invention;

FIG. 4 illustrates a second form of construction of the coordinatometer according to the invention;

FIGS. 5 and 6 are explanatory diagrams.

In FIG. 1, there is shown a plate 1 which has been cut from refractive material. The principal faces of the plate 1 are parallel to the plane of the figure as determined by the x and y axes. The lateral faces of the plate 1 are perpendicular in pairs to the x and y axes. Said faces are adapted to carry electromechanical transducers 2 and 3 which emit volume elastic waves within the plate 1. The waves emitted by the transducers 2 and 3 propagate respectively in the positive directions of the x and y axes until they reach the absorbent coatings 4 and 5 which prevent reflection of said waves.

If it is assumed that no mechanical stress is produced by the transducers 2 and 3 and that the plate 1 is cut from isotropic material such as silica, then the refractive index is the same in all the directions of the space. This state can be represented by drawing a sphere 6 which is known as an index sphere. The result achieved by isotropy of refractive properties is that the state of polarization of an incident optical radiation is maintained as it passes through the thickness of the plate 1.

If it is assumed on the contrary that a mechanical stress is produced, for example, by a longitudinal elastic wave emitted by the transducer 2, then in that case the optical properties of the medium are modified. In a compressed region 7, the index sphere is converted to an ellipsoid 8 whose major axis is parallel to y; and, in an elongated region 9, there is obtained an ellipsoid of the indices 10 in which the major axis is parallel to x. This phenomenon, known as induced birefringence, converts the plate 1 to a birefringent medium in which the x and y axes are the neutral lines. When polarized radiation passes through a birefringent medium, its state of polarization is modified unless the polarization is rectilinear and parallel to one of the neutral lines. In order to visualize the elastic waves propagated within the plate 1, the invention proposes to place said plate within an illuminated polarizing cell in which the elements are so arranged as to prevent any emergence of incident radiation if the plate 1 is not subjected to any stress. It is possible for example to mount the plate 1 between crossed polarizers with directions of polarization P and A which form an angle .alpha. with the neutral lines y and x. An incident polarizing radiation E.sub.y is made up of components E.sub.x and E.sub.y which undergo induced birefringence in the form of a path difference .delta.. The radiation which emerges from the plate 1 exhibits a state of polarization which differs from a rectilinear polarization oriented at right angles to A. Said radiation can therefore pass partially through the analyzer having a direction A which forms the exit face of the polarizing cell.

FIG. 2 is an explanatory diagram which shows the law of variation 11 of the intensity I of the radiation which emerges from a crossed polarizing cell when a mechanical stress produces within the plate 1 a path difference .delta.. Under conditions of zero stress, the path difference .delta. and the intensity I are zero. An undulatory stress as designated by the reference 12 and arising from propagation of an acoustic wave within the plate 1 produces pulsatory emergent radiation 13 which is readily detectable by means of an optical pickup device. The intensity of the emergent radiation varies proportionally to sin.sup.2 2 .alpha. and to sin.sup.2 .pi..delta./.lambda., where .lambda. is the optical wavelength.

The path difference .delta. depends on the width l of the elastic wave beam and on the difference .DELTA.n of the indices n.sub.x and n.sub.y of the medium in the case of luminous vibrations, the vectors of which are respectively parallel to the x and y axes. By choosing .alpha.=.pi./4, the detection effect is at a maximum in the case of waves which propagate in the directions of x and y axes. The induced birefringence measured by the difference .DELTA.n between the indices n.sub.x and n.sub.y depends on the magnitude of the stress C applied to the plate 1 and on the nature of the medium which constitutes this latter.

In the case of a traveling elastic wave having an angular frequency w, the differential phase shift produced by the plate can be written in the form: ##EQU1## where x is the abscissa in the direction of propagation and V is the propagation velocity of the elastic wave.

In more general terms, there exists a birefringence effect which is optically detectable irrespective of the shape of the propagated wave.

In FIG. 3, there is shown a first form of construction of the coordinatometer according to the invention. The writing table comprises a plate 1 of refractive material, looking on the edge of the plate since this figure is a view in elevation. The plate 1 is fitted with an electromechanical transducer 2 which emits the elastic wave trains 12 in the positive direction of the x axis. By reason of the directivity of the transducer 2, the radiated energy is contained within a beam having a small angle of divergence, the external contours of said beam being shown in dashed lines. The elastic wave which has traveled within the plate is collected by a coating 4 and absorbed by this latter in order to prevent reflections. In order to carry out measurements along the y-axis at right angles to the plane of the figure, provision is made for another transducer 3 and for another absorbent coating 5 but these elements are not visible in FIG. 3. Excitation of the volume elastic waves within the plate is produced by an electric generator 14 which delivers a pulse-modulated alternating-current voltage at regular time intervals. Elastic-wave sweeps along the x and y axes take place in alternate sequence. In other words, emission by the transducer 3 takes place after the wave emitted by the transducer 2 has reached the absorbent coating 4 and conversely.

Optical detection of volume elastic waves is produced by means of a light source 15 supplied from an electric generator 16. The value chosen for the emissive surface of the source 15 is small in comparison with the focal distance of the collimating lens 17 in order to illuminate the plate 1 with a substantially parallel light beam. There is shown in FIG. 3 a light beam 18 which illuminates the zone subjected to stress by the elastic pulse 12, the direction of said beam being parallel to the optical axis 19 of the lens 17. In order to ensure that the induced birefringence is optically detectable, the plate 1 is contained in an optical polarizing cell comprising a polarizer P and an analyzer A. Polarization of the incident optical radiation on the plate 1 is rectilinear. The corresponding luminous vibration E.sub.i is therefore contained in the plane of the entrance face of the plate 1 and its orientation is at 45.degree., for example, to the x and y axes. The polarized light radiation which has passed through the plate 1 is analyzed by the analyzer A in such a manner as to prevent transmission of the optical radiation which has not interacted with the elastic wave 12. To this end, the direction of polarization of the analyzer A is rotated through 90.degree. with respect to that of the polarizer P. When seen through the analyzer A, the plate 1 appears to be opaque except within a very narrow band which is rectilinear, for example, and propagates from one edge to the other. Taking into account the high velocity of elastic waves, this luminous strip is not perceptible to the eye but can readily be made visible by means of a low-inertia optical detector.

According to the invention, the optical detection involves the use of a pickup device of the light pen type, the tip of which has been drawn to a considerably larger scale in FIG. 3. Broadly speaking, the optical pickup device is comparable with a pencil which can be used for plotting any desired graphic presentation on the writing table or for marking points thereon.

In order to carry out optical detection which has low sensitivity to localization errors which may result from inclination of the pickup body, the lower end of said pickup body is fitted with a cone-shaped tip 21, the base of which is set in the cylindrical body 20. The extremity 22 of the tip has the shape of a spherical segment. The conical wall of the tip and its base are provided with an opaque coating 23 having both reflecting and light-scattering properties. Said coating 23 is interrupted at the center of the base of the conical tip in order to fit in said base a photoelectric transducer 24 which can advantageously be constituted by a photodiode. The tip 21 is cut from refractive material such as glass.

The radiation which emerges from the analyzer A within the light beam 18 is collected by the rounded extremity 22 of the tip 21. The collected radiation undergoes multiple reflections within the volume delimited by the light-scattering and reflecting coating 23. Since the illumination of the detector 24 results from the integrating action which takes place within the interior of the tip 21, said illumination is therefore proportional to the intensity of the incident radiation and practically does not depend on the angle of inclination of the pickup.

When the elastic wave 12 propagates, the passage of the wave is detected by the pickup which delivers a stair-step or square-wave voltage having a more or less steep edge. Re-shaping of said voltage is carried out by a circuit 25 which can be constituted for example by a threshold transmission circuit. A measuring circuit 26 evaluates the time elapsed between emission of the elastic wave by the transducer 2 and the instant of optical detection which corresponds to emission of a pulse at the output of the re-shaping circuit 25. By way of non-limitative example, the measuring circuit 26 is constituted by a clock which emits isochronous pulses. These pulses are counted by a digital-output counter. Resetting of the counter is initiated by a pulse delivered by the generator 14 at each emission of an elastic wave. The counting operation is stopped by means of the pulse emitted by the re-shaping circuit 25. As can readily be understood, the invention is not limited to this example of operation. It is also possible to perform a measurement of the time of propagation of the elastic wave by resetting and stopping the counting operation by means of re-shaped pulses delivered by two detection channels controlled by two optical pickups. In the case of an elastic wave 12 having a rectangular envelope, the threshold circuit 25 is capable of producing two pulses which correspond to the two threshold transitions. The first of these pulses divides the counting frequency by two and the second pulse initiates stopping of the counter. This technique permits enhanced accuracy since it has the object of determining the peak instant of the signal delivered by the detector 24.

The pickup shown in FIG. 3 reacts to any incident illumination and in particular to ambient light. This influence can be reduced by selecting the pair consisting of radiation source 15 and photodetector 24, and by judiciously interposing an optical filter which achieves efficient coupling within a narrow portion of the spectrum. This filter can be placed in front of the photodetector 24 or else it can be constituted by the coating 21, the reflection of which can be made selective.

By way of example of construction, it is possible to employ a plate 1 of silica which has a square shape and a useful surface area in the vicinity of 100 cm.sup.2. The elastic waves are longitudinal volume waves having a frequency of 40 MHz. The emission is constituted by a train of pulses having a duration of 0.5 .mu.s and a recurrence frequency of the order of 20 kHz in the case of each measuring axis.

The device of FIG. 3 operates by transmission and consequently requires illumination of the entire table surface. In consequence, the greater part of the radiation emitted by the source 15 is not employed at the level of the detector 24. Moreover, the formation of a collimated beam calls for the use of a field lens 17 in which both the dimensions and the focal distance are of appreciable value. The lighting system is therefore of substantial overall size and the device proves costly.

In order to overcome these disadvantages, the invention makes it possible to incorporate both the lighting means and the photodetecting means within the optical pickup unit. The polarizing cell operates on a different principle which entails the use of monochromatic radiation.

In FIG. 4, there is shown a diagram of a coordinatometer which operates on the reflection principle. The same references designate the same elements as in FIG. 3.

The polarizing cell is provided on the side nearest the optical pickup with a polarizer P, the direction of polarization P.sub.1 of which is indicated in the diagram of FIG. 5. The radiation emanating from the optical pickup is reflected back to this latter by means of a mirror M. A quarter-wave birefringent plate 32 has been placed between the polarizer P and the mirror M. The neutral lines N.sub.1 N.sub.1 and N.sub.2 N.sub.2 of the quarter-wave plate 32 are oriented at 45.degree. to the direction of polarization P.sub.1 imposed on the monochromatic incident radiation; this configuration is illustrated in FIG. 5. When no elastic wave is propagated by the refractive plate 1, the radiation reflected by the mirror M has been subjected to a path difference equal to one half-wavelength when it reaches the polarizer P. The polarization is rectilinear but its direction P.sub.2 is in quadrature with the polarization which is allowed to pass through the polarizer P. In consequence, no radiation emerges from the polarizing cell.

On the other hand, if the plate 1 is subjected to an elastic stress, the induced birefringence effect modifies the path difference and a fraction of the incident radiation may emerge from the polarizing cell. The emergent radiation comes from the zone in which the elastic wave 12 is located.

The difference between this optical pickup and the pickup shown in FIG. 3 lies in the presence of a radiation source 30 which is constituted by a photoemissive diode, for example. The source 30 is carried by a transparent support 29 provided with a semi-reflecting facet 31. A collecting lens 28 forms a reduced image of the source at the point C of the exit lens 27. The outer face of the lens 27 consists of a surface which is cut in the shape of a spherical segment having a center C. This arrangement makes it possible to collect radiation which is close to the normal incidence of the mirror while at the same time permitting the possibility of inclining the pickup with respect to the writing table.

FIG. 6 illustrates in greater detail the operation of the reflection pickup of FIG. 4. For the sake of simplicity, the writing table together with all its elements is designated by a single reference 320. The other references of FIG. 6(a) designate the same elements as in FIG. 4, except for an opaque pastille 38 placed at the center of the photodetector 24, a monochromator filter 37, a contact 340 and a control push-button 33. The push-button 33 performs the function of enabling means for the acquisition of data and initiates turn-on of the photoemissive diode 30 by means of the contact 340. The diode 30 emits a wide-angle beam 34 of monochromatic radiation. The collecting lens 28 focuses said beam at the center C of the lens 27. Said lens 27 is a glass ball, for example. The beam which emerges from the tip of the pickup has a wide angular divergence in order to ensure that the writing table 320 can be suitably illuminated by the optical pickup in spite of changes of inclination. In the case of the inclination illustrated in FIG. 6, only the light-ray pencil 35 which has an angle of incidence close to the normal to the table 320 contains radiated energy which will be detected by the detector 24. The reflected beam 36 is directed back to the monochromator filter 37 by the semi-reflecting plate 31. Said beam 36 is received by the detector 24 except at the center where provision has been made for an opaque pastille 38 which performs the function of a spatial filter. In fact, the outer spherical face of the tip of the pickup constitutes a reflecting dioptric element which forms a parasitic image of the source 30 at the center of the detector 24. The pastille 38 blocks said parasitic radiation without preventing detection of the radiation which emerges from the table 320. It is apparent from FIG. 6(a) that the ball 27 penetrates to a slight extent into the refractive material which has a lower degree of hardness and constitutes the top surface of the table 320. The path of the light rays as shown in FIG. 6(a) is the path obtained when the refractive index of the ball 27 is equal to the index of the medium constituting the top surface of the table 320. The zone of contact of the ball does not give rise to any refraction of the rays.

FIG. 6(b) shows the path of the rays in the case of a table 320 whose writing face is formed of material having higher refractivity than the material of the ball 27. The deformed zone behaves as a negative lens and the diametrically opposite zone forms a positive lens. FIG. 6(c) shows the change which has taken place when the ball 27 is held at a short distance above the table. The effect of convergence becomes more marked since the two zones of the ball which transmit the reflected radiation act as positive lenses. In consequence, any loss of contact can be detected spatially at the level of the detector 24. It is in fact necessary to ensure that a line having interruptions provides data which indicate such interruptions. This result can be obtained manually by means of the validation control push-button 33 or automatically by means of the counting indications. The filter 37 serves to protect the system against the influence of ambient light. It is worthy of note that there is nothing to prevent the ball from rotating freely within its housing in order to facilitate sliding motion.