Despite the hype behind some famous virtual-reality goggle brands, the problems they have solved are as nothing to those augmented-reality firms are up against.
By their enclosed nature, the optics within VR goggles never have to compete with sunlight and, as they will almost never be worn outdoors, street fashion has no impact on their appearance – they can be bulky.
As such, even the most expensive VR goggles are little more than a pair of ~50mm LCDs (or one long one), in a light-proof box that also supports a lens for each eye to focus the LCD image onto the retina.
At one end of the market, Google cardboard calls for a phone to provide images and motion sensing, and two simple lenses. And at the other end, the money goes into far better lenses and vastly improved motion sensing.
There is nowhere to put a 50mm LCD within a pair of augmented-reality glasses, so the challenge becomes how to get images from a pair of micro-displays onto the retina, in focus, and at an intensity that can compete with light landing from the outside world.
Reflective (mirrors) or refractive (prisms) optics can do it, for a certain 3D physical envelope, but WaveOptics of Abingdon is amongst the few companies aiming to do it with diffractive optics – a technology which can squeeze most of the optical chain into a thin sheet of glass.
While refraction and reflection are fairly intuitive, at least at the highest level, diffractive optics can boggle the mind – and, as such, a significant part of the company’s team is either mathematicians or optical modellers, according to David Grey, who founded the company with Sumanta Talukdar in 2014 on shared expertise.
What is it and what does it do?
It is an optical waveguide – a shaped piece of modified flat glass with an input area and an output area.
Take a projector, explained Grey, and focus its image on a wall three metres away.
Interrupt the projector by placing the input area of the glass in its beam.
Now the image no longer appears on the wall, but if you put your head next to the projector and peer through the output area of the glass at the wall, the image is there, apparently focussed on the wall.
The waveguide has taken the beam, moved it sideways through the sheet of glass, and emitted it back at the viewer in a way that makes it appear focussed 3m away.
“Our waveguide maintains all of the angular and chromaic qualities of image,” Grey told Electronics Weekly.
What happens within the waveguide is ‘two-dimensional pupil expansion’.
When light hits the input area – which is only a few mm across and therefore compatible with micro-projectors – is that a simple diffraction grating turns the light through 90° into the sheet, towards the exit area.
It then bounces along between the two glass/air interfaces, constrained by total internal reflection, until it gets to the exit area.
Here, a patented regular hexagonal pattern of dots on one surface, constituting a photonic crystal with two diffeaction gratings crossing one-another, causes wave interactions that spread the concentrated light out across the exit area, as well as turning it through another 90°, back towards the viewer.
Whatever is going on inside the glass, the only important thing, said Grey, is that the angle at which light entered the glass originally is maintained exactly when it leaves the glass. Providing this happens, the system works.
As it happens, it is irrelevant how many times internal reflection occurs, said Grey, who also pointed out that an individual ray of light, from a single pixel in the original image source, for example, will emerge from several places on the exit area, in a regular array.
While the WaveOptics system has expanded the light from input to output in two dimensions simultaneously (see right), said Gray, alternative diffractive optics aimed at the same application (left below) expand it in one dimension, turn it within the glass though 90°, then expand it in the other dimension. The additional 90° rotation requires extra glass area reducing the area available for optical output.
The area available for output is important if fiddly adjustment to compensate for each user’s eye-to-eye distance is to be avoided.
When using the system, the receiving eye will get a perfect image if it is aligned with the centre of the output area. As the eye moves sideways or up and down, at some point in each direction the image will deteriorate until it is unacceptable.
Stringing all these points together forms a shape called the ‘eye-box’.
Cheap binoculars have a small eye-box – you are forever fiddling with the width adjustment to get a good image in both eyes.
The challenge with AR glasses is to get an eye-box large enough that most people can use them without adjustment, said Grey, requiring a large eye-box.
Another important parameter is the apparent size of the augmenting image – can it fill enough of the scene to be useful? If the AR glasses can ‘project’ over a larger angle, then they can fill more of the visual field.
It transpires that, for a given amount of light entering the system, there is a trade off between eye-box area, angular field of view and the perceived brightness of the image.
According to Grey, his waveguides can achieve a usable eye-box size and brightness with a 40° field of view with readily available glass, increasing to 55° if more exotic high refractive index glass is used. He compares this with 35° achieved by a competitor using high-index glass because they use separate x and y expansion where light is lost in the additional 90° in-glass turn. “Our output is bigger for a certain size of glass.” he said.
There is an inevitable spectral bandwidth limit in diffractive 2-d pupil expansion systems. In WaveOptics’ case, it has to stack two glass waveguides, one to cover blue and part of green, and the other for the rest of green and red. Another option, used by some aiming at the same market, is to use three, one for each red, green and blue.
Whist two or three waveguides are needed, manufacturing is straight-forward according to Grey because no sub-wavelength tolerances are involved. “They don’t need to be actively aligned, they only need normal mechanical tolerances, so you can simply assemble them,” he said.
However efficient the optical system is, light is always going to be wasted by creating that big convenient eye-box, because only a fraction of it is used by any particular individual.
Which means the micro-projector for each eye needs to produce plenty of light.
And creating plenty of light from a tiny projector is the other ‘big thing’ in AR glasses.
The projector needs at least a display and an adjacent collimating lens (or lenses) so that light is largely parallel – containing only the subtle angular information that will produce the final image – when it enters the 2-d pupil expander. The current favoured layout for AR glasses is to mount display and collimating lens sideways above each eye, with a mirror or prism to divert the transverse image beam into the expander.
If the display is reflective or transmissive, the projector also needs space for red, green and blue leds (or a white led) to provide light, and room to deliver that light to the display.
Energy-efficiency is key in the display, because it has to produce a great deal of light without running hot or flattening batteries. It also needs plenty of pixels in a small space to give acceptable resolution in the desired image – which means more pixels if the image has that desirable big field of view.
There are two incumbent micro-projector display technologies, and one waiting in the wings if it can be made to work.
The incumbents are both reflective: Versions of TI’s now venerable digital mirror device (DMD), which uses an array of micro-machined mirrors to direct light from red, green and blue LEDs; and liquid-crystal-on-silicon (LCoS) shutters, which also need three LEDs to provide light.
“We have used TI DMD and LcoS – our modules can be designed for either – and we have made modules with both,” said Grey. “Without the projectors running exactly warm, we get an image you can view in daylight. If they have to work in bright sunlight, many companies are putting a tint in the glass.”
The potential new-kid-on-the-block is micro-LED. This emissive technology would simplify the projector as there would be no need for separate illuminating LEDs, and no need for the bulky – in glasses frame terms – optical system that connects those LEDs to a reflective display.
Can anyone make a suitable micro-LED display at the right price?
It has to be bright, with very small – around 10μm red-green-blue pixels, and be monolithic because it is unlikely anyone could place so many separate tiny LED die onto a substrate.
Unless some new technology appears, the brightness requirement rules out OLED displays, which leaves arrays of miniature normal LEDs.
UK-based Plessey is one company that claims to have a road-map to such a high-brightness (>100,000cd/m2) monolithic full-colour LED micro-display.
It has already demonstrated in-house capability to make multiple isolated GaN (blue) LED die on a silicon substrate and connect them arbitrarily with metalisation layers, and whose chip-making experience means an active matrix in the underlying substrate is manageable. And the firm has revealed a technique for depositing blue-to-green phosphor on every third micro-LED and blue-to-red phosphor on half of the remainder – producing the necessary RGB emissive matrix.
According to Plessey: “The company has already developed 100 and 20μm pitch blue micro-LED arrays offering 400 pixel/inch for a print-head project with LumeJet. A 100μm micro-LED demonstrator already exists, and Plessey will be offering a 20μm version in 2018. Demonstrators at 100μm are now also in development for red and green micro-LEDs, and the company is already addressing the challenges for sub-10μm pixel applications.”
According to Grey of WaveOptics: “A micro-LED display, if it became available, would be the optimum in terms of form factor and performance, if it was small enough and reliable.”
Manufacturability is the last attribute Grey claims for WaveOptic’s waveguide: “You have to make sure your manufacturing process is scalable, you have to make many millions,” he said, describing how the company modifies a sheet of glass by contact-printing patterns on its surface with UV-cureable resin. “The replication process is ‘contact print, cure and separate’, said Grey. “Today we can deliver 10 waveguides per wafer, which will grow to 28 waveguides per wafer, to improve efficiency when manufacturing hundreds of thousands of waveguides.”
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