FIELD OF INVENTION
The present invention relates to a novel projection optics design yielding enhanced performance suitable for use in head mounted display applications.
BACKGROUND OF INVENTION
Projection optics of head mounted display apparatus comprises of three parts viz. a combiner, relay optics and an image source. Combiner provides partial reflectivity at display wavelengths. Relay optics preaberrates the source image and combiner projects this preaberrated image from relay optics at virtual infinity superimposed on the forward field of view of the wearer of the apparatus.
Present invention relates to the design of projection optics based on spherical combiner suitable for use in head mounted display apparatus. The advantages of present design of projection optics based on spherical combiner are contrasted as follows with the prior art designs that are based on toric, plane or aspheric combiner: A toric combiner is used as visor in the projection optics disclosed in the patent US 4761056. Compared to spherical combiners, toric combiners close to the size of hemisphere are difficult to manufacture, unmanageable to maintain their profile when subjected to elevated temperatures experienced during coating and hard to test for any deviation from their profile. Plane combiner based designs suffer from the discomfort caused to the eye of the wearer arising from discontinuity in the field of view at the edges of the plane combiner and from reduced eye relief. By providing gradual transition of coating at the edges of partially reflective area on the spherical combiner this problem of discontinuity in the field of view can be avoided in spherical combiner based designs. Aspheric combiner based designs become extremely sensitive to manufacturing and environmental tolerances of aspheric combiner and hence suffers from performance degradation. This is because the aspherized beam combiner sensitizes the design to its deformation more than a spherical combiner does. Combiner experiences deformation due to integration with helmet structure by hinging at its ends and exposure to environment. This performance degradation is pronounced at extreme fields with increase in field of view, exit pupil size and eye relief. Diffractive plastic elements were used in the projection optics disclosed in the patent US
7450310 B2. The co-efficient of thermal expansions of these plastics are normally higher than glasses, therefore the design suffers from performance degradation when subjected to temperature variations-a qualification requirement military equipment should pass. Where as, the projection optics in the preferred embodiment does not use any plastic elements or difficult to manufacture holographic or diffractive surfaces on these elements.
Prismatic lenses were used for folding and aberration correction in the projection optics disclosed in patent US 5341242. Since these prismatic lenses or prisms with power are difficult to manufacture, assemble, bulky and add mass to the apparatus, plane mirrors are used in the current invention for folding while the desired aberration correction of projection optics is done by means of other components in the relay optics.
OBJECT OF INVENTION
The main objective of the invention is to design a compact lightweight see-through monocular head mounted display apparatus which provides enhanced optical performance by means of a novel sequence and combination of minimum number of components in the relay optics in combination with an off-axis spherical combiner serving as visor.
Yet another objective of invention is to achieve enhanced optical performance in terms of modulation transfer function, eye relief and exit pupil size as well as lower mass by means of minimum number of easy to manufacture and assemble optical components in the relay optics.
Yet another objective of invention is to ensure that relay optics uses no elements interposed between wearer of the apparatus and visor that cause discomfort to the wearer of the apparatus.
Yet another objective of the invention is to improve daytime contrast through use of spherical attenuator which is mounted at same points as visor such that it is substantially concentric with visor and can be pivoted about these points.
Yet another objective of the invention is to improve operational life of elements of relay optics and visor by allowing large margins on radius and thickness of these components through excellent correction of relay optics while achieving desired performance through out life time of these components.
STATEMENT OF INVENTION
A compact and light weight projection optics suitable for use in head mounted display apparatus comprises of an image source, relay optics and off-axis spherical combiner, a partially reflective visor. Relay optics preaberrates the source image and combiner projects this preaberrated image formed by relay optics at virtual infinity superimposed on the forward field of view of the wearer of the apparatus. The projection optics provides enhanced optical performance that is characterized by modulation transfer function which is better than 75% at 30 cycles/mm, eye relief that is larger than 66 mm, exit pupil size that is larger than 14 mm, and field of view which is better than 21°. Compact and light weight projection optics with aforementioned optical performance is based on novel relay optics constructed from: an off-axis aspheric considerably toroidal mirror to form the astigmatically preaberrated image of source image in the focal plane of off-axis spherical combiner, a decentered cylindrical lens near preaberrated image, a doublet with toroidal surface, constructed by cementing a toroidal lens and a singlet, and a decentered even-aspheric lens near image source.
BRIEF DESCRIPTION OF DRAWINGS
Figure FIG 1 shows drawing of cross-section along y-axis of 3D model of optical layout. Since plane mirror (6) is tilted about both x-and y-axes, the z-coordinate of vertex is not zero for components (7),(8) and Image source(9).
The figures FIG 2A, 2B, 2C and 2D show the spot diagram and transverse ray fan plots at various field points for 14mm exit pupil size. In each of the figures, plots to the left are spot diagrams while those two plots to the right are transverse ray aberration plots. Corresponding vertical and horizontal scales of these plots are shown at the bottom of these plots in the figures.
DETAILED DESCRIPTION OF INVENTION WITH REFERENCE TO DRAWINGS FOR THE PREFERRED EMBODIMENT
Description of drawing in FIG 1: Off-axis spherical combiner (1) forms aberrated intermediate image of collimated beam in its focal plane; relay optics forms preaberrated image of image source to match the aberrations of intermediate image in the focal plane of the off-axis spherical combiner. Off-axis aspheric considerably toroidal mirror (7) forms the astigmatically preaberrated image of source image in the focal plane of off-axis spherical combiner (1). A decentered cylindrical lens (3), serving as a field lens, near preaberrated image partly corrects spherical and distortion. A doublet with toroidal surface (5), constructed by cementing a toroidal lens (5a) and a singlet (5b), corrects color and spherical aberrations. A decentered even-aspheric lens (8) corrects field curvature and residual spherical. Plane mirrors (2), (4) and (6) help fold optics to fit head contour. Plane mirrors (2) and (4) are tilted about x-axis while plane mirror (6) is tilted about both x- and y-axes. Anywhere in exit pupil (10) eye receives un-vignetted collimated light. Image source (9) is erected in focal plane of projection optics. A slight de-collimation of display is permitted within the limits of acceptable performance through displacement of image source in focal plane to locate optimum position of better modulation transfer function and less. Here error refers to the maximum angular difference between any projected field ray and a ray corresponding to the same field from infinity. During aberration correction, the ratio of sagittal radius of curvature to tangential radius of curvature (Rx/Ry) of some toroidal components in the design of relay optics should be controlled to contain the image size or focal length. Rx/Ry is about 0.66 for off-axis aspheric considerably toroidal mirror (7) and about 0.78 for toroidal surface on doublet (5). The aspheric toroidal mirror (7) is molded from thin sheet of aluminum and provides enhanced reflectivity at display wavelengths.
Well correction of relay optics and permitting loose tolerances on the radius and thickness of elements of relay optics and spherical combiner, improves operational life of relay optics and spherical combiner beyond two years especially as spherical combiner undergoes warping. Here operation life is defined as the period over which the projection optics provides desired performance. Since the spherical combiner is less expensive to its aspheric or toric counterparts, former can also be replaced with a new one after developing haze or deformation.
The projection optics disclosed in the preferred embodiment provides enhanced optical performance characterized by modulation transfer function which is better than 75% at 30 cycles/mm over entire field of view which is better than 21°, eye relief that is larger than 66 mm, exit pupil size that is larger than 14 mm, parallax error that is less than 1.3 mrad and distortion that is lesser than 1.9%.
Nearly 35 gm mass of the relay optics reduced total mass of HMD while the distribution of mass of optics ensured C.G. of mass of head plus HMD is restored to the CG of head. Plane mirrors (2),(4) and (6) fold the relay optics to follow contour of the head while projecting no parts that may induce injuries to the face of the wearer. Plane mirrors reflect light at display wavelengths.
Day time display contrast improved when a spherical attenuator, considerably concentric with off-axis spherical combiner (1) is mounted and pivoted at the same point of helmet as off-axis spherical combiner.
Drawings in FIG 2A, 2B, 2C and 2D show spot diagrams and the transverse ray aberration plots of the design shown in FIG 1. Transverse ray aberration, defined as difference in the real ray and chief ray intercepts in the image plane along x-axis (Ex) and along y-axis (Ey), is plotted as function of entrance pupil co-ordinates of ray along x-axis or y-axis. The plots correspond to the full pupil size of 14mm. The plots shows that the optics is well corrected for color and that within 85% of the pupil size the design exceeded required performance targets while achieving enhanced performance over the full size of pupil with out vignetting any extreme field rays.

CLAIMS
1. The apparatus in the preferred embodiment is a projection optics suitable for use in head
mounted display comprising of:
-an off-axis spherical combiner wherefrom collimated image is reflected -an image source whereon image is generated
-relay optics that comprises of a cylindrical lens near intermediate image to partly correct spherical aberration and distortion.
2. In apparatus according to claim 1, the relay optics comprises of an aspheric toroidal doublet.
3. The relay optics mentioned in claim 2, comprises of an aspheric considerably toroidal mirror near the image source.
4. The relay optics mentioned in claim 2, comprises of an even aspheric lens disposed between toroidal mirror and image source.
5. In apparatus according to claim 2, the off-axis spherical combiner partially reflects light at display wavelengths.
6. In apparatus according to claim 2, the relay optics does not include any interposing elements between the wearer's eye and the off-axis spherical combiner.
7. In apparatus according to claim 2, the off-axis spherical combiner is a visor, part of the head mounted display.