Field of the Invention
The present invention relates to a head-up display, also sometimes referred to as a head-up viewer, head-up collimator or head-up visualization system. More particularly, the present invention relates to a compact head-up display having a large exit pupil.
Description of the Related Art
Head-up displays, also known as HUDs, are augmented reality display systems which enable to integrate information on a real scene seen by an observer. In practice, such systems may be placed in the cockpit of a plane or in the interior of a vehicle. They are thus positioned at a distance of a few tens of centimeters from the user's eyes.
FIG. 1 schematically illustrates the operation of such a device.
A beam splitter 10 is placed between the eye of user 12 and a scene to be observed 14. The objects of the scene to be observed are generally located at infinity or at a long distance from the observer. Beam splitter 10 is placed according to an angle relative to the axis between scene 14 and observer 12, for example, 45°, to transmit the information originating from scene 14 to observer 12, without altering this information.
To project an image seen at the same distance as the real image of the scene, and to overlay it thereon, a projection system is provided. This system comprises an image display element 16, for example, a screen, located at the object focal point of an optical system 18. This image displayed on the screen, which is to be projected on the scene, is thus collimated to infinity by the optical system.
The projection system is placed perpendicularly to the axis between the scene and the observer so that the beam originating from optical system 18 reaches beam splitter 10 perpendicularly to this axis. The beam originating from optical system 18 thus reaches beam splitter 10 with a 45° angle relative to its surface.
Beam splitter 10 combines the image of scene 14 and the image originating from projection system 16-18, whereby the observer visualizes, without having to make any accommodative effort, an image comprising the projected image overlaid on the image of scene 14. This enables to limit the visual fatigue of the observer and to improve its decision making rapidity with the projected information.
To visualize the image projected by projection system 16-18, the observer's eye should be placed in the reflection area of the beam originating from optical system 18 on splitter 10. An important constraint to be respected is to take into account the possible motions of the user's head in front of the projector, and thus to provide the largest possible beam at the exit of optical system 18. In other words, an optical system 18 having a large exit pupil, for example in the range from a few centimeters to a few tens of centimeters, should be provided, so that the observer's head motions do not imply a loss of the projected information.
Another constraint of head-up systems is to provide a relatively compact device. Indeed, significant bulk constraints bear on these devices, particularly when they are used in plane cockpits or in the interior of vehicles of limited volume.
FIG. 2 is an enlargement of the projection system of FIG. 1, comprising a screen 16 and an optical system 18. Screen 16 is formed of pixels 20 arranged in an array. The distance separating screen 16 from optical system 18 is equal to object focal distance f of optical system 18.
The angular resolution of the projection system is defined as being the inverse of the tangent of the ratio of the size of a pixel tpix to focal distance f of optical system 18 in the above configuration (as a first approach, the angular resolution is equal to angle θ in FIG. 2). For a proper reading of the information projected by the screen, the angular resolution should be smaller than that of the eye, that is, smaller than 1 mrad. To obtain such an angular resolution, with a pixel size imposed by current technologies (having a lower limit in the range from a few micrometers to a few tens of micrometers), reasonable angular resolutions are obtained for optical system focal distances in the order of a few centimeters.
To further decrease the angular resolution of the screen, the only adjustable variable is the focal distance. However, for compactness reasons, the focal distance cannot be preferably increased beyond a few centimeters.
It is further known that the complexity of an optical system depends on the exit aperture thereof, that is, on the ratio of the diameter of the exit pupil of the device to the object focal distance of the system. More particularly, the larger the aperture of a device, the more complex the device. The more complex the optical system, the larger the number of lenses that it contains, particularly to limit the different geometric aberrations.
The relation between aperture and complexity is described in Warren J. Smith's work entitled “Modern Lens Design” (SPIE Press, 2005, p. 42), having its drawing of page 42 copied in FIG. 3.
FIG. 3 is a graph showing known optical systems classified according to the inverse of their aperture (F/#) and according to the desired full field angle. In this graph, it can be seen that, for a given field angle, the larger the aperture, the more complex the optical system. For example, for small field angles and small apertures, simple parabolic lens systems (“ACHROMATIC DOUBLET”) are adapted. For head-up systems, it is generally desired to obtain a full field angle in the range from 20 to 30°. Current complex optical systems (for example, Petzval or “DOUBLE GAUSS”-type lenses) enable to obtain a maximum aperture in the order of 1.
In head-up displays, a highly compact system is desired to be obtained (object focal distance smaller than a few centimeters) with a system exit pupil of significant size (from a few centimeters to a few tens of centimeters). Such a system thus has a very large aperture, greater than 1. However, as has just been seen with the graph of FIG. 3, such a configuration cannot be obtained, in practice, even by using a very large number of elementary optical systems.
Further, even if optical systems could enable to obtain such an aperture, the forming of such a highly-complex optical system 18, and thus comprising many elementary optical elements, would be incompatible with the requested compactness of a head-up display.
Thus, there currently is no simple device having an exit pupil of significant size while being compact.