GB2120381A - Sensing eye-movement - Google Patents

Abstract

An oculometer is provided for accurately measuring the sighting direction of an observer. It comprises an optical lens which forms the image of the eye OE in a photodetection plane P, measuring and processing circuits 5 for measuring the centre of the circular image of the pigmented zone of the eye OE, a semi-transparent or dichroic mirror 2 which allows operation of the oculometer and simultaneous viewing of the scene by the observer. An ordered optical fibre bundle may be provided so as to enable the measuring and detection assembly to be positioned at a distance. The eye may be illuminated with infra-red radiation by a light-emitting diode. <IMAGE>

Description

SPECIFICATION An oculometer apparatus for a head eye-piece The present invention relates to an oculometer apparatus, more particularly intended for equipping a head sighting system.

In numerous cases, it is useful to know the direction in which a human operator is looking without inconveniencing him.

The problem arises in ergonomics (measurement of eye movements, measurement of reflexes, of the visual work load etc. .. .) and also in the military field (visual fire control, visual objective designation, etc....).

In the equipment meant for ergonomic laboratories, it is known to use for this purpose the formation of the Purkinje images of an auxiliary pin-point light source on the different dioptres forming the eye (see more particularly "Optique physiologique" vol. 1 by Yves Le Grand, publisher Masson). Measurements of the position, of the size and of the geometrical defects of the image allow theoretically the direction of the principal axis of the eye under experimentation to be known. The equipment is generally sophisticated and application thereof is reserved for the laboratory.

In the military field, it is known to have the operator wear a helmet fitted with an optical apparatus presenting the collimated image, i.e.

projected to infinity, of a symbol (a cross for example) which the operator must bring into coincidence with the object sighted. Thus, the direction given by the collimated image is related to the helmet, and the problem comes down then to measuring the position in space of a direction related to the helmet itself, or to an object fixed on the helmet.

In actual fact, this latter method is only a subterfuge; by forcing the operator to maintain a direction related to his head in the located direction, location of an object related to the reference direction is substituted for location of the direction itself. This method presents two serious drawbacks: ~the operator is forced to cause a direction physically related to his head to coincide with the direction in which he wishes to look. Now, naturally, the movement of the head with respect to the rest of the body constitutes a "coarse" aiming of the glance, the precise aiming being provided by the movement of the eye itself within its socket with respect to the head, which movement is very much more precise than the first one.The coincidence depends then on the accuracy of the coarse positioning; moreover, when the operator has to follow a mobile point, the accuracy of the tracking is limited by the accuracy of the movement of the head, whereas tracking by the combined movement of the head and of the eyes with respect to the head is much more precise; ~the operation for aiming the glance is not natural, and adjustment of the reference direction with respect to the head is fixed once and for all so as to correspond to the normal sighting direction of the operator, i.e. that of the eye in the rest position; now, experience shows that, depending on the vertical height of the object sighted, an operator who is under no constraint due to an optical collimation system does not always sight in this normal sighting direction and that it may deviate therefrom to a greater or lesser degree.

The aim of the invention is to remedy such drawbacks by providing an oculometer, that is to say an apparatus capable of determining the instantaneous sighting direction of the eye by the angular off-aiming thereof with respect to a reference direction.

Summary of the Invention One object of the invention, when it is used in the above-mentioned military field, is to free the operator whose job it is to aim from the constraint of having to make a materially defined line coincide with the direction of the object to be aimed at, while allowing the angular movements of limited amplitude to be measured which result from the movements of rotation of the eyes with respect to the head.

Thus, a complete helmet or head sighting system will comprise, in correspondence with the natural movements of the operator, two pieces of measuring apparatus: - a known direction locating apparatus which measures the instantaneous position of the head of the operator with respect to a fixed reference which represents the earth, or the vehicle, depending on the contemplated use; ~an oculometer apparatus in accordance with the invention for measuring the positional deviation, that is to say the angular off-aiming, of the visual axis of the operator with respect to the first located direction.

The head or helmet sighting system thus equipped therefore no longer requires the provision of an optical collimating device for sighting.

In accordance with the present invention, an oculometer apparatus is provided for measuring the rotational movements of the eye in its socket with respect to the head of the observer. This apparatus allowing the angular off-aiming to be measured presented by the instantaneous sighting direction of the eye with a reference direction corresponding to the normal sighting direction of the observer, comprising an optical lens for forming the image of the eye in an image plane, a semi-transparent mirror placed between the eye and the optical system for allowing both the radiation coming from the outer landscape to be received by the observer and the radiation for forming said image to be transmitted to the optical lens, the mirror being slanted in the reference direction to reflect one of these radiations and to transmit the other, two dimensional photodetection means being disposed in the image plane and processing circuits being provided for reading out the photodetection means and for processing the signals so as to measure the position of the geometrical center of the circular image of the iris.

Brief Description of the Drawings The features of the invention will appear in the following description, given by way of non limiting example, with reference to the accompanying drawings which show: Figure 1, a diagram of an oculometer apparatus in accordance with the invention; Figure 2, a preferred variation of the oculometer apparatus; Figures 3 to 6, diagrams and curves relative to one embodiment in accordance with figure 1, relating respectively to the photodetection means, to the wave forms of the detected signals, to a diagram of the measuring and processing circuits, and to the angular off-aiming measured by the circuits; and Figure 7, a simplified diagram of use of the oculometer apparatus in a helmet sighting eyepiece.

Description of the Preferred Embodiments Referring to figure 1, the oculometer apparatus comprises an optical objective 1, which is shown symbolically, by way of example, by two lenses and which serves for forming the image of the eye in an image plane P. A semi-transparent mirror 2 is disposed between the eye OE and lens 1 so as to allow both reception by the observer of the radiation RP coming from the outer scene and transmission to the lens of radiation RO which comes from the eye and which serves for forming the image of the eye through the lens. Mirror 2 is inclined in the reference direction formed by the normal sighting direction DNV, for reflecting one of its radiations and transmitting the other.The normal sighting direction is, as has been mentioned, the direction in which the eye looks when it is at rest in its socket, that is to say that it is attracted neither to the right, nor to the left, nor upwards nor downwards. In the version shown, the radiation from the observed scene reaches the eye by transmission and the radiation for image formation is reflected by mirror 2; the reverse may be provided, that is to say that the scene may be observed by reflection on the mirror and the image of the eye formed by transmission. Considering the distance as shown between the eye and lens 1, the image of the eye is formed in the image plane P in which two-dimensional photodetection means 3 are disposed which may consist, as shown, of two detection bars or strips 3X and 3Y arranged in two orthogonal directions X and Y.

To remedy ambient lighting which may be insufficient, means 4 for illuminating the eye may be further provided. An advantageous solution consists in using a light-emitting diode in a solid gallium arsenide circuit operating in the 0.8 to 1 micron infrared band, since, on the one hand, this element may be of small size (2 to 3 mm in diameter) and may then be readily incorporated in the structure, for example firmly secured to the lens and, on the other hand, the emitted radiation RI is not visible and so does not disturb the observer. It will be understood that the detection strips 3 are sensitive to this radiation. Diode 4 which emits the infrared radiation RI is supplied with DC from a source not shown.The apparatus further comprises circuits 5 for reading the photodetection device and for processing the detected signals for measuring the off-aiming AD between the instantaneous sighting direction DIV of the eye and the reference direction DNV.

In the presence of an illuminating source 4, the semi-transparent mirror 2 is treated to form a dichroic mirror so as to reflect one radiation and to .transmit the other; one of the radiations is the visible one coming from the outside scene and the other is the infrared radiation produced by the emitting diode 4.

The semi-transparent or dichroic mirror 2 may be formed in a way known per se by the association of two prisms, as shown in figure 2 by elements 7 and 8 joined together by a face treated so as to form mirror 2.

In the variation shown in figure 2, the image formation uses optical fiber transmission allowing the measuring and processing means to be placed at a distance. This transmission comprises a first optical element 9 which forms the image of the eye OE on an endmost face of an ordered optical fiber bundle 10, the real image produced at the other endmost face of this fiber bundle is taken up by a second optical means 11 to be formed, for example, on the screen of a vidicon picture taking tube. Block 12 represents the vidicon tube and associated circuits by means of which the measurement of the off-aiming is effected. In one version of this kind, the measuring and processing circuits may be more elaborate, based on form identification techniques by correlation, video tracking etc.Reference may be made more especially to French patent 2 302 004 (or to the corresponding US patent 3 953 669) which describes a video tracking system. The principle of such signal processing is based on the use of the luminance transitions in the video signal. Once the transitions useful for tracking have been determined, a calculator defines a rectangular socalled window portion of the image where the object to be tracked is located. Then, with this image portion divided into four equal zones by axes parallel to the edge of the window, the calculator will effect weighings for measuring the proportion of the object area included in each of the zones, and inferring therefrom the new position of the barycenter of the image.

For the sake of understanding, the operation has been explained with reference to the following figures 3 to 6, by means of a simplified construction of the detector formed of two strips such as shown in figure 1 and elementary circuits such as shown in figure 5. The photodetection means are determined so as to measure subsequently the position of the geometrical center of the image of the pigmented zone of the eye, that is to say the iris, and which is represented by he circle Cl. It will be readily understood that the detector elements of the strips which are inside the circle Cl receive less light than the elements which are outside this circle since these external cells correspond either to the white of the eye or to the skin of the observer considered lighter than the iris. The circle CI intersects strip 3X at points A and B and strip 3Y at points C and D.Considering what has just been said, the signals S1 and S2 detected respectively by these strips are represented schematically in figure 4. It may be considered that these signals have a mean amplitude V2 outside the pigmented zone of the circle and a lower level, of the order of V1, inside the pigmented zone. The detectors may be formed by charge transfer devices, called CTD. The solid circuit strips of this type may provide fine resolution, comprising up to about 2000 points, i.e. elements, per strip. For the construction of the detector device 3, reference may be made, for example, to French patent 2 450 463 (or to the corresponding US patent 4 315 690).

A clock or time base circuit 13 produces a clock signal H for timing the CTD circuits forming strips 3X and 3Y. The clock circuit is also used for synchronizing the processing circuits shown in figure 5 for one of the channels, channel X, it being of course understood that a similar assembly is provided for channel Y. Signal S1 is applied to the input of a comparator circuit 14 where it is compared with a threshold VS which is predetermined so as to be located between the values of V2 and V1 (figure 4), thus, when the detection signal crosses points A and B corresponding to the limits of circle Cl,there occurs each time switching of the output of comparator 14. This output controls a switch 15 for switching the local clock frequency from the value H to the value H/2 by means of a circuit 16 for dividing by two, incorporated in the corresponding connection.The clock signal of frequency H, or H/2, is applied to a counting circuit 17 followed by a buffer storage 18 which supplies the measurement value XM of point M. A circuit 19 for detecting the trailing edge resets the counter and controls the buffer storage 18.

After each resetting of the system, the clock controls the read-out of the detector cell after cell.

This resetting takes place at the beginning of each read-out. As long as it is the zone corresponding to the white part of the eye which is detected, the signal remains at a high value and exceeds the threshold VS, switch 15 remains in the position such that signal H is transferred to counter 17.

This counter totals the number of clock pulses which was necessary to reach point A, situated on the boundary between the white and pigmented zones of the eye. As soon as the signal becomes low at A, so less than the threshold, the counter has recorded the number NA of clock pulses proportional to the distance FA; switch 15 switches and then transfers the signal H/2 to the counter which totals half of the number of clock pulses required for going from A to B, that is to say (NB - NA)/2. Finally, when the signal rises again at B, the counter has totalled (NA + NB)/2 which represents the abscissa of the mean point of AB, that is the abscissa XM of the center M of the pigmented zoned Cl. This value is transferred to storage 1 8 at the end of this counting cycle and the value previously stored during the preceding cycle is transferred for use. A new counting cycle may begin. The detection of the front corresponding to point B of signal S1 is effected by circuit 19 connected at the output of comparator 14 and which controls the read-out of the counter whose data is stored in storage 18 and resets the system.

A similar processing circuit is associated with the output S2 of the other detector 3Y for providing the ordinate YM of the same point M. The coordinates of M give the position of the center of the pupil of the eye.

Knowing the position of the oculometer with respect to the head (figure 6), which is the same as knowing the position of the detector plane with respect to the center of rotation CR of the eye, the direction of sighting may be determined by relationships giving the angular off-aiming as lateral deflection AG and vertical deflection AS: tan AG = XM/L and tan AS = YM/L, in which values the parameter L represents the distance between the center of rotation CR and the image plane P.

In an oculometer apparatus in accordance with the invention, the image of the eye and the difference in reflectivity of the white of the eye and of the pigmented part is used. Since this pigmented part is circular, its center which is very substantially situated in the center of the pupil may be easily located by means of a twodimensional detection device, or by means of a more elaborate form recognition circuit.

Figure 7 shows a use on a helmet eye-piece.

The helmet sighting system comprises a direction locating apparatus which may be, for example, of the optical type shown with a group of lightemitting diodes 20 mounted on helmet 21, a sensor 22 and a calculator 23 for processing the signals detected at 22 and for measuring the reference direction DR related to the helmet. The successive and sequential supply for the diodes is provided from block 23. The sensor is fixed to the aircraft, mounted inside the cockpit. Solutions of this kind are numerous and reference may be made more especially to French patent 2 399 033 (or to the corresponding US patent 4 1 93 689).

The direction locating device may also be constructed according to a magnetic solution; in this version, a sensor is integral with the helmet and the radiating element is integral with the aircraft, or with the vehicle considered. A measuring device of this kind is described in French patent 2 458 838 (or in the corresponding US patent application US Application Serial NO 156 083), the sensor and the radiator each comprising a three coil assembly defining the axes of a Cartesian trihedron. The helmet sighting system further comprises the oculometer apparatus shown by its mirror 2, lens 9 and fiber 10 which allows the measuring and detection means shown symbolically by block 24 to be placed at a distance and which supply the value of the off-aiming AD to the calculator 23. This value AD represents the additional value to be added in value and sign to that measured by the detection assembly 20, 22 and 23 so as to obtain the real and instantaneous sighting direction of the operator. Mirror 2 is mounted for rotation about a hinge at R so as to be able to be placed in the rest position, shown with a broken line, when the operator does not need to effect a sighting operation. In the version shown, the reference direction DR related to the helmet has been chosen corresponding to the normal sighting direction DNV of the observer.

Claims (9)

1. An oculometer apparatus for measuring the rotational movements of the eye in its socket with respect to the head of the observer, the apparatus allowing the instantaneous angular off-aiming presented by the sighting direction of the eye with a reference direction corresponding to the normal direction of sighting of the observer to be measured, said apparatus comprising:: ~optical means for forming the image of the eye in an image plane, - a semi-transparent mirror placed between the eye and the optical means for allowing both reception by the observer of the radiation coming from the outer scene and transmission to the optical means of the radiation forming said image, the mirror being slanted in the reference direction so as to reflect one of these radiations and to transmit the other, - two-dimensional photodetection means in the image plane, and ~circuits for processing the detected signals for effecting read-out of the detection means and for processing the signals for measuring the position of the geometrical center of the image of the iris of the eye.

2. The oculometer apparatus as claimed in claim 1, wherein there are further provided means for illuminating the eye.

3. The oculometer apparatus as claimed in claim 2, wherein the illumination means consist of a solid circuit light-emitting diode, radiating in the near infrared, and wherein the mirror is treated so as to be dichroic.

4. The oculometer apparatus as claimed in claim 2, wherein the optical image forming means are formed from an ordered optical fiber bundle one end of which is coupled through an optical means to the mirror and the other end of which is coupled through another optical means to the processing and photodetection means.

5. The oculometer apparatus as claimed in claim 4, wherein the mirror of the oculometer apparatus is pivotably mounted so as to be disposed outside the field of vision when it is not in use, and the photodetection and processing means are placed at a distance from the helmet.

6. The oculometer apparatus as claimed in claim 1, wherein the photodetection means comprise two strips disposed in two orthogonal directions, and made of CTD circuit.

7. The oculometer apparatus as claimed in claim 1, wherein the photodetection means use a vidicon tube and the measuring and processing circuits provide form recognition by correlation or video tracking.

8. The oculometer apparatus as claimed in claim 5, combined with a direction locating apparatus of the optical or magnetic type for forming a helmet or head eye-piece.

9. An oculometer apparatus substantially as hereinbefore described with reference to, and as illustrated in the accompanying drawings.