GB2281833A - Fish-eye type optical device for the detection and localization of a radiating source - Google Patents

Abstract

The device for the detection and goniometry of laser illumination for the protection of aircraft, vehicles and buildings, has an optical system combined with a four-quadrant plane detector (5) centered on one and the same optical axis (10). The optical system comprises a front afocal unit (6) with a divergent lens (1) and a convergent lens (2), receiving the light beam coming from the radiating source, an objective formed by at least one convergent lens (3) and a diaphragm (7) whose aperture and position fix the maximum size of a virtual input pupil whose size increases with the angle of incidence of the light beam (61, 62) to keep a sensitivity in detection that is almost constant through the angular field covered by the device. The four-quadrant plane detector (5) is defocused with respect to the focal spot (91, 92) of the objective so that the projection of the light beam (61, 62) on the detector (5) forms a homogeneous spot (91, 92) contained within the sensitive part of the detector (5) for the entire angular field covered by the device. <IMAGE>

Description

FISH-EYE TYPE OPTICAL DEVICE FOR THE DETECTION AND LOCALIZATION OF A RADIATING SOURCE The invention relates to a fish-eye type optical device for the sensing or detection and the localization of a radiating source.

The field of the invention is that of the detection and goniometry of a radiating source, and notably of a laser illumination, applied in earth-based or airborne countermeasures.

With the increasingly frequent use of lasers in weapons systems (for telemeters, illuminators, etc.) it is becoming necessary to give warning to the crew of a vehicle or aircraft if this vehicle or aircraft should be illuminated. Corresponding prior art devices, called laser alert detectors, are designed to detect the presence of this type of threat in wide angular fields around carriers, for example fields of 3600 in bearing or fields of 90" in elevation. The abovementioned detection devices are generally not designed to work in fields of such sizes while at the same time retaining sufficient sensitivity in detection. Their adaptation to these wide fields results in a loss of sensitivity that is incompatible with the detection of the useful signal.Other known types of devices can also be used for an angular localization of the radiating source by a goniometrical operation on the detected illumination, notably on a laser illumination.

Among these devices, one device that is the object of a prior French patent published under No. 2 621 398 and delivered to the present Applicant, entitled "Dispositif de detection et de localisation d'une source rayonnante" (Device for the detection and localization of a radiating source), comprises a fourquadrant plane detector associated with a plane diaphragm, parallel to the plane of the detector centered on the same optical axis, having an aperture and position with respect to the detector that are determined to obtain the highest sensitivity in detection. This four-quadrant detector is associated with a known type of electronic processor that carries out a goniometrical operation based on the relative weightings of the signal coming from each quadrant characterizing the temporal nature of the illumination.

This device has the drawback of requiring a precise opto-mechanical positioning and of showing substantial diminishing of sensitivity in the field, especially at the limit of the angular field covered by the device. This diminishing of sensitivity is proportional to the cosine of the angle formed by the incident light ray received by the device and the optical axis of the device.

The invention is aimed at overcoming the abovementioned drawbacks.

According to this invention there is provided a fisheye type optical device for the detection and localization of a radiating source, of the type comprising an optical system combined with a fourquadrant plane detector centered on one and the same optical axis, characterized in that firstly the optical system comprises, centered on the optical axis, a front afocal unit receiving the light beam from the radiating source, an objective formed by at least one convergent lens and a diaphragm whose aperture and position -fix the maximum size of a virtual input pupil whose size increases with the angle of incidence of the light beam to keep a sensitivity in detection that is almost constant through the angular field covered by the device and, secondly, the four-quadrant plane detector is defocused with respect to the focal spot of the objective so that the projection of the light beam on the detector forms a homogeneous spot contained within the sensitive part of the detector for the entire angular field covered by the device.

The optical device according to the invention has the advantage of enabling a compromise between the compactness of the device and the uniformity of the sensitivity in detection throughout the angular field covered by the device. The device according to the invention makes it possible, by the addition of a very wide-field optical assembly to a defocused fourquadrant detector, to reduce the angles of incidence on the detector and to increase the angular field accepted by the device.

Other features and advantages of the invention shall appear more clearly from the following description, made with reference to the appended drawings, of which: - Figure 1 is an optical diagram describing the principle used by an optical device in which the invention is embodied; - Figure 2 shows a first embodiment of an optical device according to the invention, - Figure 3 shows a second embodiment of an optical device according to the invention, - Figure 4 shows an example of impacts of beams on the defocused four-quadrant detector, - Figure 5 is a one-axis goniometrical curve, - Figure 6 shows a panoramic optical device which embodies the invention, and - Figure 7 gives a view of the fully spread-out field covered by the panoramic optical device.

The optical diagram of Figure 1 illustrates a paraxial layout of a very wide-angle objective, also called a fish-eye objective, using four thin lenses 1 to 4 in a determined optical assembly combined with a defocused four-quadrant detector 5. The principle implemented by this implantation consists in obtaining, as a function of the angle of incidence of an illumination 61, 62, a spot projected on at least one of the four quadrants of the detector 5 at the edge of the field and on at least two of the four quadrants for a smaller field. The defocusing of the detector 5 is adjusted for a determined size of the detector 5 so that the projected spot is tangential to the center of the sensitive surface of the detector 5 for the end of the field of goniometry, this being done in order to optimize the behavior between the angular dynamic range and the angular resolution.This layout includes a front afocal unit 6 comprising a first divergent lens 1 and a second convergent lens 2. This afocal unit 6 is used to reduce the incidence of the light beam reaching a third convergent lens 3 constituting an objective before which there is a diaphragm 7 with a determined aperture on its object focal spot, thus forming a telecentric objective.

Then, at the image focal spot of the objective 3, the optical system has a field unit constituted by a fourth lens 4 that can be used to orient the beam so that the projected spots corresponding to the images of the light beams are included in the diameter of the detector 5 for the entire angular field covered by the device. The four-quadrant detector 5 is defocused with respect to the objective 3. The effect of this is to make the spots homogeneous and improve the sensitivity of the goniometry.

To make it easier to understand the principle of this layout, two incident light beams 61 and 62 are shown in Figure 1. A first light beam 61 reaches the front afocal unit 6 with an incidence of 00. This light beam is demarcated by two parallel lines which reach the divergent lens 1. The beam 61 then reaches the convergent lens 2 which transmits the collimated beam to the objective represented by the convergent lens 3 through the diaphragm positioned at the object focal spot of the lens 3. The convergent lens 4 projects the light beam 61 on the defocused fourquadrant detector 5 to form a first spot 91 centered on the optical axis 10 of the optical system. Similarly, a second light beam 62, also demarcated by two parallel lines, forms an angle of 60 with the optical axis 10.

This second light beam is then directed towards the convergent lens 2 with a smaller angle of incidence.

This convergent lens 2 transmits the beam, which has again become parallel, through the same diaphragm 7 to the objective 3. The objective 3 concentrates the light beam 62 on the lens 4 which projects it on the defocused four-quadrant detector 5 forming a second light spot 92 on the upper hemisphere of the detector 5.

Figure 2 illustrates a first embodiment of an optical device according to the principle of the foregoing layout described in Figure 1. This embodiment is compromised between the paraxial layout described here above and the design constraints of the device.

The divergent lens 1 of Figure 1 is made with a first divergent lens 11 and a second divergent lens 12.

The two divergent lenses 11 and 12 are associated to make it possible to obtain a greater angle of divergence. A virtual input pupil 13, localized between the two divergent lenses 11 and 12, is represented by a straight line segment centered on the optical axis 10 of the optical device. The diameter of the virtual input pupil 13 increases with the angle of incidence of the light beam so as to get rid of the above-mentioned cosine effect.In this embodiment, the diameter of the input pupil 13 is the minimum PE (00) for an incident beam whose angle of incidence is equal to 0", and it is the maximum PE (75.5 ) for a beam forming an angle of incidence of about 75.5'. The angular field accepted by an optical arrangement such as this is therefore of the order of j7501 giving 150".

The convergent lens 2 of Figure 1 is formed by a convergent lens 14 and a diaphragm 15 is formed on the input diopter of the lens 14. The diaphragm 15 is circular. The aperture diameter of the diaphragm 15, setting the condition for the maximum diameter PE (75.5') of the input pupil 13, as well as the distance in relation to the four-quadrant detector 5, are determined so that it is possible to do without a field unit 4 in order to achieve the conditions of projection of the image spots while at the same time making the most efficient use of the height of the spot at the image focal spot of the objective 3. The image height is, for example, 1.8 mm with respect to the optical axis 10 at the edge of the field for a field with an incidence of about 75e and for a four-quadrant detector 5 with a radius of 2 mm.The objective corresponding to the lens 3 of Figure 1 is formed by two convergent lenses 16 and 17. The four-quadrant detector 5 is defocused towards the rear of the focal plane PF1 of the objective 16 and 17, by a distance dl that is determined to make it possible to obtain the desired image height, on a plane that is perpendicular to the optical axis 10 and contains the intersection of the ray, at an angle of 75.5 , at the edge of the field with the optical axis 10. Three light beams having respective values of incidence equal to +60e, Oe and 75.5 are shown in Figure 2. Each light beam incident to the first lens 11 is demarcated by two parallel lines.Each of these beams, after passing through the determined optical assembly constituted by five lenses 11, 12, 14, 16 and 17, forms a spot representing the projection of the incident light beam on the sensitive part of the four-quadrant detector 5.

A defocusing of the four-quadrant detector 5 makes it possible to carry out a goniometrical operation by relative weightings of the signal coming from each quadrant according to a known method described in the above-mentioned patent. It shall therefore not be described again.

For this embodiment, the light flux that re-enters the optical device is conservative, the size of the virtual input pupil 13 increasing with the field. The maximum aperture obtained with a fish-eye lens such as this is F/0.71, where F corresponds to the paraxial focal distance of the objective and 0.71 corresponds to the ratio between the focal distance F and the diameter of the input pupil 13 at the center of the field.

In order to limit the parasitic illumination due to the parasitic reflections of the lenses that could lower the sensitivity of the device, one approach consists in placing one or more ring-shaped spherical mirrors (not shown) oriented towards the detector 5, these mirrors being intended to stop the structural flux. These mirrors should furthermore be designed so as to not to give rise to the reflection, on the detector 5, of a useful flux previously reflected by the detector. A second approach that could be combined with the first one consists in the sending, to the detector 5, of a cold flux coming from a cryostat.

This approach is notably used for detectors sensitive to the infrared spectrum.

Figure 3 shows a second embodiment of an optical device according to the invention. In this second embodiment, the optical device has a smaller aperture than in the first embodiment of Figure 1. For this example, the aperture is F/0.9. This optical device has only four lenses, 18 to 21, and its aperture diaphragm 22 is sent back towards the rear of the structure. The first lens 1 of Figure 1 is formed by the first lens 18 of Figure 3. A virtual input pupil 23 is shown in Figure 3 by a straight line segment.

The afocal lens 6 of Figure 1 is constituted, in this second embodiment, by lenses 19 and 20 forming the lens 2 of Figure 1, combined with the lens 18. A convergent lens 21 forms the objective 3 of Figure 1. The diaphragm 22 is positioned on the output diopter of the objective 21. The four-quadrant detector 5 is defocused towards the front of the structure with respect to the focal plane PF2 of the objective 21 by a determined distance d2 to optimize the sensitivity of the goniometry. The diaphragm 22 can be made in the same way as the diaphragm 15 of Figure 2.

Just as in the case of Figure 2, three light beams respectively having values of incidence +60", Oc and 75.5 are shown in Figure 3.

In this embodiment, the diaphragm 22 is sent back to the rear of the structure and faces the fourquadrant detector 5. Consequently, the output pupil (not shown) is merged with the diaphragm 22. The advantage of this arrangement, as compared with the first embodiment of Figure 2, is that it does away with the parasitic reflections and hence with the need for the mirrors referred to here above. By contrast, the sensitivity is no longer constant in the field and drops slightly at the field limit.

Figure 4 shows an example of three light beam impacts on the sensitive surface of a defocused fourquadrant detector 5 whose diameter, for this example, is equal to four millimeters. A first spot shown by dashes corresponds to the light beam with an incidence of -45 o , a second spot shown by dots and dashes, centered on the horizontal and vertical axes of the detector, represents the spot pertaining to a light beam with zero incidence and third ovoid spot tangential to the center of the detector 5 represents the spot coming from a beam with an incidence of 75.5..

This third spot represents the limit of sensitivity of the optical device according to the invention. The defocusing also has the beneficial effect of making the "image" spots homogeneous.

A defocusing of the four-quadrant detector 5 towards the rear of the focal plane of the objective, for example by 1.6 mm as shown in Figure 2, enables a goniometrical operation in terms of bearing and elevation on a total angular field of about 1500 with a four-quadrant detector having a diameter of 4 mm.

An example of a one-axis angular goniometrical curve obtained with a detector such as this is shown in Figure 5. In this figure, the X-axis represents the angular field 0 graduated from -80" to +80 and the Yaxis represents the value of the one-axis goniometrical curve E(()) graduated from -1 to +1. It goes without saying that if it is only the detection that is being sought, then a smaller defocusing and a detector using one element will suffice. Similarly, if it is only a one-axis goniometrical operation in bearing or elevation that is needed, then a detector with two half-disks will suffice. A device of this type can be used to cover a solid angle of 4.7 steradians.

It is possible to use three identical devices which embody the invention, having their optical axes 101, 102 and 103 positioned in one and the same horizontal plane and at 1200 with respect to one another, thus forming a panoramic optical device. An arrangement such as this is shown in a top view in Figure 6. This arrangement enables a panoramic view of a scene corresponding to a total field coverage of 3600 in terms of elevation and +60 in terms of bearing.

The direction in which the scene is being viewed is represented in the figure by an arrow. Figure 7 illustrates a view of the spread-out field covered by the panoramic optical device. In this figure, the Xaxis and the Y-axis respectively represent the axis in terms of elevation and the axis in terms of bearing.

The two hatched zones correspond to the overlapping regions between each device, the centers of the overlapping zones being spaced out at 120- with respect to one another.

The panoramic zone of coverage, demarcated by dashes in Figure 7, is therefore defined by a band of 360" in length along the elevation axis and +60 in width along the bearing axis.

Claims (7)

1. A fish-eye type optical device for the detection and localization of a radiating source, of the type comprising an optical system combined with a four-quadrant plane detector centered on one and the same optical axis, characterized in that firstly the optical system comprises, centered on the optical axis, a front afocal unit receiving the light beam from the radiating source, an objective formed by at least one convergent lens and a diaphragm whose aperture and position fix the maximum size of a virtual input pupil whose size increases with the angle of incidence of the light beam to keep a sensitivity in detection that is almost constant through the angular field covered by the device and, secondly, the four-quadrant plane detector is defocused with respect to the focal spot of the objective so that the projection of the light beam on the detector forms a homogeneous spot contained within the sensitive part of the detector for the entire angular field covered by the device.

2. An optical device according to claim 1, wherein the afocal group has a first divergent lens and a second divergent lens and a convergent lens, the objective has a first convergent lens and a second convergent lens, the diaphragm is positioned on the input diopter of the convergent lens and the detector is defocused towards the rear of the focal plane of the objective by a distance that is determined to obtained a homogeneous spot on the detector.

3. An optical device according to claim 1, wherein the afocal group has a first divergent lens and a second divergent lens and a convergent lens, the objective has a convergent lens, the diaphragm is positioned on the output diopter of the convergent lens forming the objective and the detector is defocused towards the front of the focal plane of the objective by a distance that is determined to obtain a homogeneous spot on the detector.

4. A panoramic optical device for the detection and localization of a radiating source, comprising three identical devices claimed according to any of the claims 1 to 3, the optical axes of which are positioned along one and the same horizontal plane, each axis respectively forming an angle of 120 with respect to the other.

5. An application of the device according to any of the claims 1 to 4 to the detection and localization of a radiating source.

6. A fish-eye type optical device substantially as described hereinbefore with reference to the accompanying drawings and as illustrated in Figures 1, 2 and 4 or in Figures 1, 3 and 4 of those drawings.

7. A panoramic optical device for the detection and localization of a radiating source substantially as described hereinbefore with reference to the accompanying drawings and as illustrated in Figures 1, 2, 4, 5 and 6 or Figures 1, 3, 4, 5 and 6 of those drawings.