The disclosure relates generally to methods and systems for a low F/# lens for virtual reality displays and, more specifically according to aspects of certain embodiments, to a lens with an aspheric surface and a Fresnel lens.
By way of example, reference will now be made to the accompanying drawings, which are not to scale.
Those of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons, having the benefit of this disclosure. Reference will now be made in detail to specific implementations of the present invention as illustrated in the accompanying drawings. The same reference numbers will be used throughout the drawings and the following description to refer to the same or like parts.
In certain embodiments, virtual reality headsets use lenses to direct light associated with an image displayed on a panel to the eye. In certain embodiments, a lens with an aspheric front surface and a curved Fresnel lens rear surface may be thinner and lighter than a refracting lens. In addition, it may have a shorter focal length, and may have low spherical aberration, coma, astigmatism, and field curvature using a single element.
It is desirable to produce a lens with one or more of the following attributes: (a) a single-element lens; (b) an imaging lens; (c) a ratio of focal length to lens diameter (f/#) of about 1.0 to about 0.7; (d) a large aperture, which may fall between about 50 mm and about 70 mm; (e) a wide field of view; (f) low spherical aberration and coma; (g) low field curvature; (h) monotonic field curvature for both tangential and sagittal planes; (i) significant but monotonic distortion; (j) a pixel size between about 20 μm to about 100 μm; and (k) an image source with dimensions between about 30 mm×30 mm to about 80 mm×80 mm.
It is difficult to design a lens that has performance attributes (a) through (g) above using only spherical surfaces. Using only spherical surface, there are only two variables with which four aberrations (spherical, coma, field curvature, and astigmatism) need to be managed. The end result can be a spot size that grows rapidly off-axis. The addition of one or more aspheric surfaces according to aspects of the present invention may add flexibility and permit a useful lens to be designed. The aspheric terms may be able to control spherical aberration and coma quite well and have some control over field curvature and astigmatism. In certain embodiments, an imaging lens may be created that includes a monotonic field curvature for both tangential and sagittal planes. The aspheric surfaces may permit tangential field curvature to be non-monotonic.
A variety of design forms may be used to accomplish the foregoing results including without limitation: (1) an aspheric surface with a Fresnel asphere; (2) a Forbes aspheric surface with a Fresnel asphere; (3) an aspheric surface with a curved, 2-figure Fresnel lens; (4) a Forbes aspheric surface with a curved, 2-figure Fresnel lens; and (5) and wide Fresnel zones (e.g., greater than about 500 μm). The lens may be a single element lens or a multi-element system or assembly, which may include for example and without limitation a thin field flattener or a negative lens for lateral color collection.
According to aspects of the present invention, a front aspheric surface may be a Q-type or Forbes asphere. The rear Fresnel surface may have a base curve and an additive curve. The base curve may be fairly shallow and the curve may be preserved. The additional curve may be very strong and may, in certain embodiments be close to a parabola. The slope of the Fresnel surfaces is the sum of the slopes of the base curve and the additional curve.
In certain embodiments, an imaging lens for collimating light in a virtual reality headset is disclosed, including: a front aspheric refractive surface; and a rear Fresnel surface comprising a base curve and an additive curve. In certain embodiments, the imaging lens may collimate the light passing through it (i.e. make the output light more parallel than the input light) to improve the quality of an image viewed by a wearer of a virtual reality headset. The imaging lens may include a ratio of focal length to lens diameter between about 1.2 and about 0.5. The imaging lens may include a ratio of focal length to lens diameter between about 1.0 and about 0.7. The imaging lens may have a field of view with a radius of greater than about 40°. The imaging lens may include a lens diameter between about 40 mm and about 70 mm. The imaging lens may have a distortion greater than about 15% to create stereo overlap. The imaging lens may be configured to image a plurality of pixels between about 20 μm and about 100 μm. The imaging lens may have a maximum field curvature sag of less than 2.0 mm. The front aspheric refractive surface may include a conic. The front aspheric refractive surface may include a conic with aspheric coefficients. The front aspheric refractive surface may include an asphere without a conic. The front aspheric refractive surface may include a Forbes asphere without a conic. The front aspheric refractive surface may include a Forbes asphere and a conic. The rear Fresnel surface comprises a sphere. The rear Fresnel surface may include a conic. The rear Fresnel surface may include a conic with aspheric coefficients. The rear Fresnel surface may include an asphere without a conic. The rear Fresnel surface may include a Forbes asphere without a conic. The rear Fresnel surface may include a Forbes asphere and a conic. The imaging lens may be a single-element lens.
In certain embodiments, a lens assembly is disclosed including: a front aspheric surface; and a rear Fresnel surface comprising a base curve and an additive curve; wherein the lens assembly has a field curvature sag of less than about 1 mm in the field of view and less than about 1/10th wave of spherical aberration. The lens assembly may be used for a virtual reality headset. The lens assembly may have a ratio of focal length to lens diameter between about 1.2 and about 0.5. The lens assembly may have a ratio of focal length to lens diameter between about 1.0 and about 0.7. The lens assembly may include a field of view with a radius greater than about 45° and a maximum field curvature sag of less than about 2.0 mm. The lens assembly may have an aperture between about 50 mm and about 70 mm. The lens assembly may have a monotonic field curvature for tangential and sagittal planes. The lens assembly may have a maximum field curvature sag of less than about 1 mm. The lens assembly may be configured to image a plurality of pixels between about 20 μm and about 100 μm. The lens assembly may include an image source between about 30 mm by 30 mm and about 80 mm by 80 mm. The lens assembly may include a Forbes aspheric surface. The rear Fresnel surface may include a Fresnel aspheric surface. The rear Fresnel surface may include a Fresnel aspheric surface. The rear Fresnel surface may include a two-figure Fresnel lens. The rear Fresnel surface may include a two-figure Fresnel lens. The rear Fresnel surface may include one or more Fresnel zones greater than about 500 μm. The rear Fresnel surface may include a curved Fresnel surface. The lens assembly may further comprise a thin field flattener. The lens assembly may further comprise a negative lens for lateral color correction.
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In certain embodiments, one of the surfaces may be replaced with a Fresnel lens surface. Generally Fresnel lenses have been regarded as not suitable for even modest field of view imaging applications with visible light. In certain embodiments,
In certain embodiments, a Fresnel lens as shown in
In certain embodiments as shown in
The field curvature in certain embodiments as shown in
In certain embodiments as shown in
Many modifications and other embodiments of the invention will come to mind of one skilled in the art having the benefit of the teachings presented in the forgoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included as readily appreciated by those skilled in the art.
While the above description contains many specifics and certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art, as mentioned above. The invention includes any combination or sub-combination of the elements from the different species and/or embodiments disclosed herein.