Two hundred year old lens for virtual reality optics.

[budda]

Hi,

In 1828 an English scientist, W. H. Wallaston devised a simple meniscus lens which provided a means of viewing landscapes with a flatter field of view and better focus than a simple biconvex lens.

This development occured 25 years before the introduction of photography.

Such a lens, like the Wallaston simple miniscus lens, can be adapted for modern use in virtual display optics.

Whilst a single lens has little design freedom to overcome optical aberations, it does illistrate the influence of bending the lens to give a flatter field of view.

The following basic examples show the effect of lens bending on the focal plane.

Plano - Convex Lens and Curved Focal Plane

Wallaston Lens Curved Image 1a.png

Meniscus Convex Lens and Flat Focal Plane

Wallaston Lens Planar Image.png

I have bent a fresnel sheet about the vertical viewing axis and obtained dynamic field curvature reduction in the horizontal direction. Obviously with an axisymmetric lens, bending of the lens can be performed in two axes for uniform field curvature reduction.

Modern single element lenses for virtual display optics need to incorporate extra bending toward the viewers eye to improve the field of view and flatness of view.

The eye is located about the central area where the mechanical stop in the Wallaston lens is located. The pupil of the eye, as it rotates about the eye centre, acts as the aperture stop and thus omits the undesirable aberations which the mechanical stop ordinarily prevents.

The Wallaston meniscus lens suffers from chromatic aberation, and this may need to be corrected for in the display software.

Some interesting history of photographic lenses can be found in this book - A History of the Photographic Lens By Rudolf Kingslake. You can read selected pages on Google Books online.


Thanks.

[geekmaster]

I am near-sighted. I have some 3x reading glasses that have meniscus lenses (surface curved around the eye), that I purchased for HMD experiments.

I also ordered some glasses with 5x, and with 6x magnification.

When wearing two pairs of 3x glasses (stacked one over the other, 6x total, without my normal glasses), my Nexus 7 tablet with 7-inch LCD panel focuses just a couple of inches from the end of my nose. At this distance, the entire screen is in sharp focus, and it all looks flat with no apparent distortion or chromatic aberration.

I also have the recommended 5x aspheric loupe lenses for a DIY Rift clone. Unlike the meniscus lenses in the eyeglasses, they do distort the image and cause some chromatic aberration.

So it depends on what kind of HMD you want to create. In the case of the Rift, we use this aspheric distortion to our advantage, keeping a higher pixel density near the center of the screen. The cost is that objects near the edges have less resolution, but your eye also has less resolution in your peripheral vision. It also requires pre-warp to compensate for the aspheric distortion, and perhaps also to compensate for chromatic aberration.

With meniscus (eyeglass) lenses, there is no significant distortion, so there is even pixel density over the entire screen. This uses pixels less efficiently, but the image has essentially no distortion or chromatic aberration, making it look crisper and sharper.

It all depends on what you need for your application. Until displays get much higher resolution, it probably makes more sense now to use a higher central pixel density by using aspheric lenses instead of meniscus lenses.

[cybereality]

Interesting. So what benefit would these lenses have over, say, the ones in the Rift?

[geekmaster]

Interesting. So what benefit would these lenses have over, say, the ones in the Rift?

They have different uses. It depends on whether you want to focus on a flat or a curved surface, and whether you actually WANT distortion (which is beneficial for the Rift, according to the "more central pixel density" theory). But with a higher resolution display, I would think the high power reading glassed method would be preferred by many test subjects.

I provided links in another post to my large-lens 6x reading glasses from an ebay store for $8.95 shipped, as I recall. I will post photos of my screen shot through them when they arrive.

An HMD with no lenses, shipped with high power (meniscus) reading glasses to provide focus, would be lightweight and simple, with no need for pre-warp. It might even be simply awesome, especially for reading text or watching movies. And you could get prescription high power reading glasses with additional astigmatism correction built in, too.

But I still like the anamorphic lens idea, to map SBS-Half portrait mode images to a landcape mode FoV, for maximum horizontal FoV with minimal wasted pixels. That "larger than Rift" FoV while using a single LCD would be awesome in my book.

[budda]

Hi,

Interesting. So what benefit would these lenses have over, say, the ones in the Rift?

I would say a modern lens, in whatever form it takes for a Rift-like head mounted display, would benefit in every aspect from modern engineering.


To my understanding, there has not been a custom lens specifically designed for Rift-like head mounted displays. The lenses which are being used by makers are appropriated from other uses like magnifying glasses or spectacles.


All I can say, is without the rational design studies being made, the reported experimental efforts so far have achieved very promising results. This can only get better.


And there is one overriding qualification - ' beauty is always in the eye of the beholder '.


Thanks.

[budda]

Hi,

A modern spectacle lens designed for high-quality, wide-angle Head Mounted Displays could be quite expensive. A pair of such lenses could possibly exceed the cost of the remaining VR hardware.

I estimate the cost at $400 or more.

Why ?

It seems a 20 dioptre positive lens is an extreme prescription for hypermetropia (long sightedness). Standard high power spectacle lenses are designed more for myopia (short sightedness) as this seem to be a more common vision problem. These are negative power lenses and are of no use in VR.

A custom spectacle lens will then be required for high end VR.

The single lens is going to be a positive meniscus, aspheric, single point focus and super-high refractive index glass lens (n=1.9). The lens is optically described as a heavy flint, made from lanthanum and doped with other rare earth materials to obtain the high end optical properties.

The advantage of such a prescription lens is that it can be designed to correct for other vision problems so no spectacles are required to be worn, thus preserving the immersion, field of view, flatness and lightness of the VR lens system. The glass lens can be anti-reflection coated to minimise back reflections from the screen for ultimate high contrast ratio viewing.

The design of the lens follows some simple principles.

1. Use a super-high high refractive index material to minimise thickness, reduce depth, reduce overall weight and reduce chromatic aberation compared to a standard 1.5 index glass or polymer material.

2. Bend the positive lens so the tangential astigmatic focus is flattened.

3. Add aspheric curvature to minimise the astignmatic error particularly from the sagittal astigmatism. This also greatly reduces the perception of curvature of field.


Thanks.

[geekmaster]

@budda: Great info. Thanks!

[3dpmaster]

It is very interesting to see the unknown optical quality of a meniscus lens that is focusing on a flat image from middle to edge.

[budda]

Hi,

This information about general opthalmic lens geometry comes from the Zeiss.com website in their Spectacle Lens Compendium . It uses frames so I could not directly link this content.

From Biconvex to Point Focal Lenses

In ca. 1804 the English physician Wollaston discovered that visual acuity decreases when a spectacle wearer looks through the periphery of the biconvex lenses used at that time, and that meniscus-shaped lenses provided a sharper image. After this discovery, repeated attempts were made to improve the imaging properties of lenses. The first endeavours to design meniscus-shaped lenses with reduced peripheral blurring were undertaken by the ophthalmologists Ostwald and Tscherning. However, the results were of no practical significance, as they were only computed for very small viewing angles and for lenses with a theoretical centre thickness of zero.

In 1908 the company Carl Zeiss entrusted a member of its scientific staff, Moritz von Rohr, with the exact computation of spectacle lenses. He succeeded in designing a point focal lens in which peripheral blurring was minimized. These computations of Moritz von Rohr laid the foundations for today’s Punktal lenses from Carl Zeiss.

Point focal lens

Point Focal Lens Zeiss Info.jpg

The way to flatter lenses

Although optimum imaging properties were achieved with the first point focal lenses, the pronounced curvature of the lenses proved to be a major cosmetic drawback, especially for high prescriptions in the plus range.

To make lenses more attractive, so-called periscopic lenses were introduced, in which one surface was almost flat. This also made the lens flatter overall, but considerable aberrations were experienced in the peripheral zones. In higher dioptric powers in particular, these aberrations led to blurred vision.

The solution – a good appearance and good vision with one and the same lens – came in the form of the asphere and the atorus. In 1986 Carl Zeiss was the first manufacturer – and still is the only manufacturer – to offer a single vision lens with an atoroidal front surface: the Hypal lens. This modern surface design permits outstanding image quality over a wide field of view and, at the same time, flat, cosmetically attractive lenses.

Periscopic lens

Perioscopic lens Zeiss Info.jpg

Aspheric (and Atoric lens)

[geekmaster]

More great info! Now you are wanting me to divert my attention to the study of lenses. Yeah, someday RSN I will get around to adding this to my "To Do" list...