Device and method for the laser projection of high-resolution images onto the retina of the eye, superimposed with the image content of the field of vision

[0001] The invention relates to an optical assembly and method for the retinal projection of monochromatic or color video images by a light beam (e.g. composite laser beam) modulated by the brightness of its color components directed by electrically controlling the deflection angle of one or more scanning mirrors over the retina two-dimensionally for crisp sequential pixel imaging. Depending on the retinal projection application this involves either only the video image (head-mounted video display) or the video image superimposed with the image of the field of view of the viewing eye (head-mounted display for pilots). From prior art various optical assemblies are known for achieving this objective, e.g.:

[0002] 1. Helmut-mounted displays and sights Mordekhai Velger, Artech House, Boston/London, 1998

[0003] 2. Der Fernseher in der Brille, Elektronik, pages 18, 20,

[0004] 3. U.S. Pat. No. 6,140,979, Microvision Inc. Oct. 31, 2000

[0005] Because of aberration and the configuration in all of these known display systems the video imaging angle achievable at best is only 20-40 in the image diagonal with unsatisfactory image resolution. The system as it reads from 2. shows e.g. the classic arrangement for retinal projection using a concave mirror the spherical aberration of which dominates in greatly diminishing retinal image resolution and whose optical system substantially differs from the basic arrangement of the present invention (cf. arrangement as shown in FIG. 1).

[0006] The objective of the present invention was thus to define an optical system which achieves an imaging angle of as high as 60 for a video and superimposed field of view in conjunction with an extremely high image resolution, i.e. now limited only by how well the viewing eye can see and by the diffraction limit of the light beam (laser beam). This is achieved by the means as featured by the characterizing clause of claim 1. Advantageous aspects read from the sub-claims and in the following description the invention is detained by way of a few examples whilst its basic optical system, its beam configuration and variants thereof are illustrated in the FIGS. 1 to 3 of the drawings.

[0007] Referring now to FIG. 1 there is illustrated the basic arrangement of the system for high-resolution pixel projection on the retina (11) of the viewing eye, the latter being represented in FIGS. 1 to 3 by an ideal thin lens (10) and a flat retina (11). The light beams belonging to a pixel on the retina (11) (represented in each case by three rays, namely the chief and the two marginal rays) pass converging through the entrance aperture (1) of the system.

[0008] For a viewing eye of normal vision and an infinite virtual objective spacing for an image projection the entrance aperture (1) is located in front of the concave mirror (9) by the optical path length R2f (where ffocal length, Rradius of the concave mirror (9)), i.e. in a virtual spherical centerpoint of the concave mirror (9) generated by the flat beam splitter (8). The light beams converging from the entrance aperture (1) to a pixel on the retina are reflected by the beam splitter (8) to the concave mirror (9), intersect at a point at a distance from the surface of the concave mirror (9) corresponding to the optical path length fR/2, before then being incident divergent at the concave mirror (9). Each chief ray associated with a retinal pixel is incident perpendicular to the surface of the concave mirror (9). The light beams are reflected there as parallel beams, then passing through the beam splitter (8) and the exit aperture (2) located at the spherical centerpoint of the concave mirror (9) and which is the image of the entrance aperture (1) generated by the concave mirror (9) and beam splitter (8).

[0009] Since all light beams associated with a retina pixel are oriented parallel to each other between concave mirror (9) and exit aperture (2) and their chief ray passes through the spherical centerpoint of the concave mirror (9), each light beam of all retina pixels is a paraxial beam of light as regards the concave mirror (9) irrespective of its angle of incidence to the viewing eye. This results in the concave mirror (9) having for the light beams of all retina pixels the same spherical aberration which is very small due to the low f-number (beam diameter/f).

[0010] The beam splitter (8) is located in the parallel beam path and causes only a transversal beam shift but no field curvature aberration or chromatic aberration in the image on the retina (11). The lens (10) is located typically in the exit aperture (2) or slightly in front thereof at a spacing S (S greater than or equal to zero). For a large beam cross-section in the exit aperture (2) and at S>0 retinal projection is also possible even with eye movement without this movement needing to be compensated by the projection system.

[0011] When the image of the retinal projection is to be superimposed with the image of the field of view (pilot's HMD) the concave mirror (9) is configured as a partly transparent mirror.

[0012] To generate a finite virtual object spacing for the retina projection or for correcting deficient eyesight of the viewing eye the entrance aperture (1) is slightly shifted along its optical axis from the virtual spherical centerpoint of the concave mirror (9). This generates for the light beam reflected by the concave mirror (9) the necessary slight divergence or convergnce of the light beam before entering the lens (10).

[0013] Referring now to FIG. 2 there is illustrated a typical assembly for video retinal projection, the retina being identified by the reference number (11). This assumes a monochromatic or color beam of parallel light (e.g. a laser beam mixture) whose color components are modulated by the serial pixel data of a video signal. Further assumed is an electronic signalling device which generates the necessary control signals from the line or image sync signals of the video signal or other pixel coordinate data for steering e.g. a gimballed dual axis scanning mirror (7) so that the light beam is deflected by the necessary azimutal and elevation angle for each pixel of the video signal to make sure that the pixel is projected on the retina (11) of the viewing eye at the desired location. The modulated parallel light beam is directed from the input plane (14), where necessary via an attenuator filter (4), through a single or multiple converging lens system (5) for generating the necessary convergent light beam which is supplied via the deviating mirror (6) to the dual axis scanning mirror (7) arranged in the entrance aperture (1) of the projection system. The scanning mirror (7) as shown in FIG. 2 is simultaneously indicated for three different deflection positions with the corresponding reflected light beams.

[0014] The lens system (5) generates the desired convergence of the light beam, but it is also capable of eliminating minor aberrations of the concave mirror (9) since these aberrations in the assembly of the projection system are the same for all retinal pixels irrespective of the angular position of the scanning mirror (7).

[0015] To nevertheless allow for eye movement for small light beam cross-sections by the lens (10) in video retinal projection an imaging sensor (12) (image or multi-segment sensor) is arranged surrounding the scanning mirror (7) in the plane of the entrance aperture (1). It is on this sensor that the concave mirror (9) images the surface of viewing eye. From signals of the imaging sensor (12) the location of the viewing eye pupil can be determined and from which in turn signals for steering the servo motors for two-dimensional positioners can be generated with which the optical system as a whole, comprising the elements (14), (4), (5), (6), (7), (12), (8), (9) can be shifted horizontally and vertically so that the exit aperture (2) (cf. FIG. 1) follows viewing eye pupil movement. In addition, the illumination level at the surface of the eye generated by the observed field of view by the concave mirror (9) can be measured from the signals of the imaging sensor (12) in adjusting the required brightness of the video retinal display.

[0016] In addition to this, the viewing direction of the viewing eye can be determined from the signals of the imaging sensor (12) as can be exploited for selecting the desired pixel content of the video display by eye-controlled menu toolbar operation.

[0017] To prevent the complete projection system from following eye movements the cross-section of the light beam in the plane of the lens (10) needs to be large. This is achievable either with an expansive deflection mirror (7) as shown in FIG. 2 or, as evident from FIG. 3, by generating a primary image on a scattering screen (3) whose wide scattered light cone emanating from each pixel is focussed by a lens system (13) (e.g. achromatic ball lens) into convergent light beams, the intersections of which locate on a spherical surface having the radius R/2 and which would appear to originate from a virtual aperture of larger cross-section as shown in FIG. 3 as coincident with the entrance aperture (1) of the display system as shown in FIG. 1, in thus enabling the primary image to be imaged on the retina (11). As evident from FIG. 3 it is not every chief ray that is beamed precisely perpendicular to the surface of the concave mirror (9). Such minor deviations are, however, tolerable without any serious loss of focus in the image on the retina (11).

[0018] Generating the primary image on the display screen (3) is possible as follows:

[0019] 1. The primary image is scanned onto the screen (3) by a light beam modulated in intensity with the aid of a dual-axis scanning mirror system.

[0020] 2. The scattering screen (3) is the output surface of a fiber plate on the input side of which the primary image is

[0021] a) scanned by a light beam modulated in intensity with the aid of a dual-axis scanning mirror system,

[0022] b) input by CRT, LCD or plasma display and transferred by the optical fibers to the output side (3) of the fiber plate and emitted scattered.