Optical: systems and elements – Lens – With reflecting element
Reexamination Certificate
2000-02-01
2003-01-07
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
With reflecting element
C359S633000, C359S730000
Reexamination Certificate
active
06504658
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical device to correct the image of the pupil of the eye given by a spherical concave mirror. More specifically, a device according to the invention can be used to observe an image that is corrected of distortion due to a spherical or substantially spherical concave mirror that is tilted with respect to the direction at which the eye observes this mirror.
2. Discussion of the Background
The invention can be applied to a helmet visor especially but not exclusively for the pilot of an armed aircraft or helicopter or for the operator of a training simulator.
A helmet visor is an image-presenting device that is integrated into a helmet. The visor enables the wearer of the helmet, for example the pilot of an aircraft in flight, to observe visual information simultaneously with the view of the landscape or of the pilot's cabin, which he perceives most usually through a protective visor.
The presentation of appropriate information, for example in the form of symbols, provides assistance in piloting and navigation. Thus, for armed vehicles, the presentation of a reticule provides assistance in the aiming of a weapon.
The information may also consist of an image of the landscape acquired by sensors other than the eye of the helmet wearer such as infrared sensors or visible light intensifiers to complement or replace direct viewing.
Inside the helmet, an image generator comprises an imager, for example a cathode-ray tube screen or a liquid crystal screen on which an image is formed.
The helmet most usually has a relaying optic to convey this image up a combiner which presents the conveyed image in a state where it is superimposed on the view of the landscape.
In order that the pilot may simultaneously observe the landscape which is viewed directly at infinity and the image from the imager, the latter is also focused at infinity by a collimation optic.
When the combiner is formed by a simple semi-reflective flat plate, the collimation of the image can be achieved by an optic placed between the combiner and the imager. A prior art embodiment of this kind has the major drawback of requiring a collimation optic that takes up far too much space in relation to the restricted field of view that is obtained.
To reduce the space requirement, a combiner with optical power has been proposed. A combiner of this kind provides its user with both the collimation of the image and the superimposition of the collimated image on the view of the landscape.
The prior art has a very rich variety of devices comprising a combiner with optical power. First of all, a concave spherical mirror achieves an average quality collimation of an image placed at a particular point in space located on the axis of the mirror and at a distance from this mirror equal to half of its radius of curvature. By placing an imager at this point, the eye located on the axis of the mirror receives rays coming from the imager after they are reflected on the spherical mirror, these rays being parallel and leading to the perception, by the eye, of a collimated image. If, furthermore, the mirror is semi-reflective, it enables the same eye to observe the landscape by transparency. However, in a device of this kind, the imager is on the axis of the semi-transparent spherical mirror and it conceals the field of view of the user.
To clear the user's view, the spherical mirror may be tilted with respect to the normal to his/her face, and the user's eye is no longer on the axis of the mirror. This tilting has the major drawback of leading to a collimated image that is affected by optical aberrations, especially off-centring aberrations, excessively limiting the use of such a device.
In order not to conceal the field of view of the user while at the same time limiting aberrations, the prior art teaches us the use of a parabolic mirror instead of a spherical mirror. The imager is placed at the focal point of the paraboloid described by the mirror and the eye observes the mirror along a parallel to the axis of revolution of the paraboloid.
The collimated image perceived by the eye has no spherical aberration but remains affected mainly by a coma with highly penalizing effects, the extent of which increases very rapidly with the field. Thus, the imager, while being off the axis of the field of view, remains a hindrance in the field of view.
One improvement consists of the exploitation of a double reflection on the parabolic mirror with an intermediate plane mirror placed at the level of the user's forehead and called an onward reflection mirror. The two reflections are located on either side of the axis of the paraboloid. They make it possible to obtain a collimated image that is free of coma and whose other aberrations remain acceptable for a field of view that is still fairly restricted.
The desire to reduce the hindrance due to the onward reflection mirror has led to a development of the prior art. A device using a parabolic mirror and double reflection exhibiting asymmetry with respect to the axis of this mirror has been described. While this device reduces the hindrance in the field of view, it increases the aberrations, especially astigmatism. The device described comprises lenses that are tilted to reduce astigmatism. It also comprises a field lens to compensate for the field curvature and compensates for the distortion by a deformation of the image during its generation: the image is formed on the screen of the cathode-ray tube of the imager with a distortion that is the reverse of that which it is forced to undergo when crossing the optical device.
Furthermore, the initial idea of collimation by a spherical mirror has undergone new developments. Thus, a device has been described with a semi-transparent spherical mirror having a tilted axis, comprising a prism to compensate for the inevitable aberrations induced.
The prism is placed on the path of the light rays between the imager and the spherical mirror. The aberrations are minimized overall by adapting the tilt and aperture of the prism. And the astigmatism is corrected by an additional optical element that must be cylindrical.
This device is essentially penalized by a small field.
In parallel with this, devices have been made with spherical mirrors having no tilt in relation to the axis of view and with a shift of the imager.
A device of this kind has a semi-reflecting plane mirror placed between the spherical mirror and the user's eye, at the focal point of collimation of the spherical mirror.
From the imager to the user's eye, a light ray follows an optical path where, successively, it strikes the semi-reflecting plane mirror a first time, is reflected from this plane mirror towards the spherical mirror, and is then reflected on this spherical mirror and sent back to the plane mirror, it strikes the plane mirror a second time and goes through it to meet the eye.
The collection of spherical and plane mirrors is transparent for rays emitted by the landscape.
This type of device presents a high quality collimated image.
However, this design which implies a compromise between reflection and transmission by the plane mirror, has the drawback of sending to the eye only a small part of the initial light intensity and of thus too severely limiting the conditions of use of a helmet visor fitted out with this device.
The transmission of the useful image to the eye can be improved by slightly tilting the spherical visor with respect to the axis of view of the user and by subjecting a plane mirror to an anti-reflective treatment that is selective as a function of the angle of incidence of the light rays.
With this geometry, the first and second angles of incidence of one and the same light ray on the plane mirror have distinct angular values whereby the selective anti-reflective treatment, by circumventing the standard compromise between the reflection and the transmission of a ray that penalizes the above device, helps the reflection of the initial ray jointly with the trans
Bignolles Laurent
Potin Laurent
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Sextant Avionique
Sugarman Scott J.
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