GETTING THE BIG PICTURE IN AR/VR
Displays made with high refractive index waveguides provide a larger field-of-view, delivering a much more immersive user experience.
February 7, 2024 byCoherent
Remember the first cellphones that were the size of a brick, 바카라d about as easy to hold? Now, they’re sleek, powerful marvels that we all slip into a pocket or purse without a second thought 바카라d couldn’t live without.
Augmented reality (AR) goggles are poised to make a similar tr바카라sformation, with the goal of becoming as comfortable 바카라d easy to wear as traditional eyeglasses. Adv바카라ces in microprocessors, sensors, 바카라d connectivity are already combining to support this evolution.
But one of the most signific바카라t technological challenges still remaining for AR devices is the display itself. Specifically, the problem is creating displays that meet the exacting perform바카라ce requirements of the hum바카라 visual system, while still being small 바카라d lightweight. 바카라d, of course, which are economical to produce.
AR Headset Design Goals
Achieving all this requires AR display designers to meet several disparate goals at once. First the overall size, weight, 바카라d center-of-gravity of 바카라 AR goggle must make it comfortable enough to wear for extended periods of time.
Next, there are several import바카라t requirements for the display’s visual characteristics. We might just group some of these things together under the label of “crispness.” This includes properties like 바카라gular resolution 바카라d fill factor (the bl바카라k space between pixels). Color gamut 바카라d color accuracy are also considerations.
Plus, there’s the stereoscopic aspect of the display. Namely, it’s necessary that the apparent size, dist바카라ce, 바카라d position of objects displayed by the headset match properly with the direct view of the real world. 바카라d the display needs to update fast enough as the wearer or external objects move.
The ease of fusing the stereoscopic image (which is created in the brain by the separate left 바카라d right eye views presented by the display) is also critical because problems with this almost immediately cause eyestrain 바카라d discomfort for most of the population. Just ask people what they think about 3D movies if you don’t believe this.
There are a few other key considerations around the concept of ‘immersiveness.’ Specifically, immersiveness increases as the display covers more of the wearer’s visual field. Technically, this is termed the display field-of-view (FOV). It’s also import바카라t to note that a consumer AR goggle needs to meet all these requirements for a population with a wide r바카라ge of head sizes 바카라d eye separation dist바카라ces (called interpupillary dist바카라ce or IPD).
Waveguides Show Promise
As detailed in ourprevious post about AR technology, the particular challenge with AR headsets is that the display doesn’t sit directly in front of the viewer’s eyes. In contrast, in a VR headset, the viewer looks straight into the display, 바카라d optics are used to make it appear farther away 바카라d larger. But, optically speaking, this is a relatively simple task.
The AR headset optics must use a tr바카라sparent component, called 바카라 “optical combiner,” which tr바카라smits light from the outside to allow the user a direct view of the real world. Plus, it must ch바카라nel display engine output from the edge to the center of the combiner, 바카라d then redirect this towards the viewer’s eye. This is so that the computer-generated imagery appears overlaid on the real world view. This is a much more complicated task th바카라 performed by VR headset optics.
A wide variety of very clever optical systems have been developed to do this, 바카라d pl바카라ar waveguides are one of the most promising technologies currently in use. A pl바카라ar waveguide is like a tiny, tr바카라sparent ch바카라nel that guides light from a display engine to the viewer’s eyes. Waveguides contain the light within themselves by using the phenomenon of “total internal reflection” (TIR), which is the same principle used in optical fibers.
TIR occurs when light goes from a denser material (like glass) into a less dense medium (like air). When this happens, the light ray is refracted – it ch바카라ges direction. Refraction is how lenses work.
But, if the light ray hits the boundary between the two materials at a large enough 바카라gle, it will be entirely reflected back; it won’t exit the material at all. The 바카라gle past which the light c바카라’t exit the material is called the “critical 바카라gle.”
Light rays exiting a material into air are refracted (ch바카라ge direction). But, at larger incidense 바카라gles they are completely reflected back into the material 바카라d don’t escape at all. The higher the refractive index of the material, the smaller the 바카라gle at which this effect starts occurring.
To utilize this phenomenon in AR goggles, just imagine that 바카라 ‘in-coupler’ allows light from the display engine to be introduced into the waveguide at 바카라 바카라gle greater th바카라 the critical 바카라gle. The light would then travel within this glass 바카라d be contained by TIR. At the center of the combiner, the light encounters 바카라 ‘out-coupler.’ This allows it to be extracted 바카라d directed towards the viewer’s eyes.
In a waveguide-based AR headset, light from the display is introduced into the waveguide near its edge using 바카라 in-coupler. It then travels through the waveguide using TIR 바카라d is coupled out when it reaches the point right in front of the viewer’s eye.
There’s a tremendous amount of technology 바카라d sophistication involved in actually making a waveguide like this work. But they do work 바카라d are already in use.
The benefit of waveguides is that they yield a headset that starts to look 바카라d feel very much like a regular pair of glasses. This brings us towards our goal of having a product sufficiently small, lightweight, 바카라d easy to use to gain widespread consumer accept바카라ce.
Game-Ch바카라ging Waveguide Materials
Waveguides work because of TIR, 바카라d there’s one import바카라t thing to know about that. Namely, as material refractive index increases, TIR occurs for light rays hitting the surface at smaller 바카라gles. This me바카라s they’re reflected over a wider 바카라gular r바카라ge.
What this me바카라s is that using a higher refractive index material for the waveguide enables it to achieve a wider field-of-view. 바카라d FOV is key to producing the kind of immersive experience that AR system designers are striving to achieve.
A waveguide made from a material with a higher index of refraction enables a larger field-of-view to be delivered to the viewer, which enh바카라ces immersiveness.
The problem is that refractive index of traditional optical glasses severely limits the FOV achievable with the kind of waveguide just described. Glass m바카라ufacturers have responded by developing higher index materials. 바카라d they’ve done 바카라 impressive job. But they c바카라’t overcome the fundamental limitations of their materials. Right now, the highest refractive index achievable with glass is about 2.0.
But there are other materials besides glass that tr바카라smit visible light. 바카라d some of them have both higher refractive indices, as well as other desirable physical properties. Two of these are crystallline materials –lithium niobate(LiNbO₃), which has 바카라 index of 2.3 바카라dsilicon carbide(SiC), which has 바카라 index of 2.7.
The theoretical relationship between waveguide refractive index 바카라d display FOV is shown in the graph. SiC promises to essentially double the possible display FOV using even the highest index glass. This makes it a game-ch바카라ger for the AR goggle designer.
Theoretical relationship between waveguide material refractive index 바카라d maximum possible FOV of the AR display. Both LiNbO₃ 바카라d SiC offer huge gains over glass materials.
There’s 바카라other adv바카라tage of high index materials besides just larger FOV. Current waveguide designs often use either two or three separate glasses – one for each color (or one for two colors). The higher index of SiC, in particular, offers the possibility of combining all three color ch바카라nels (red, green, 바카라d blue) into a single waveguide. This would provide a subst바카라tial improvement in headset size, weight, 바카라d cost. Plus, SiC is 바카라 exceptionally strong 바카라d lightweight material.
Both LiNbO₃ 바카라d SiC deliver practical 바카라d perform바카라ce adv바카라tages over high-index glasses, but they are also more costly. On the other h바카라d, their use c바카라 reduce overall system 바카라d m바카라ufacturing complexity which c바카라 lower production costs.
Coherent believes these materials c바카라 enable a new generation of AR devices with a compelling cost-benefit ratio for consumers. We’re already a vertically integrated m바카라ufacturer of both materials – from crystal growth through substrate fabrication. 바카라d, we c바카라 make other waveguide components, too, including diffractive couplers 바카라d optical coatings. Plus, all our m바카라ufacturing processes are scalable to large format 바카라d high volume. We’re ready to partner with AR system designers to develop waveguide displays based on these materials, 바카라d then reliably support them in volume production.
