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The difference between pancakes lenses and current fresnel lenses found on VR headsets

VR Expert EM3 Ether Prototype

What do HTC Vive Flow, EM3 Ether, and the upcoming Project Cambria from Meta share? Besides a stylish and thin design, the Pancake lens technology. So what are the differences between the Pancake optics and current virtual reality lenses and what does it signal for the future of VR HMDs?

The current Fresnel lenses

When consumers and enterprises alike, not invested in the VR/AR news streams, think of virtual reality HMDs, they think bulky shoeboxes strapped onto one’s head. This is true, even though design departments have tried to skirt around this barrier of entry, since the resurgence of VR in 2012. Even technology enthusiasts and industry experts know copious details regarding the display in VR headsets: what is the pixel density per inch? Is the refresh rate high? How color-accurate is the screen? And so on. But while the display is vital, the slap of glass, or plastic, is equally critical.

Fresnel lenses work the same as lighthouses in directing and enhancing an existing light source. Its roots date back to the early 1800s by Augustin Fresnel and have been nicknamed: “the invention that saved a million ships”. The lens works like a ring of crystalline prisms arranged in a faceted, ‘beehive-like’ dome, reflecting refracted light.

Crudely put, current virtual reality HMDs are like having a lighthouse-beamed screen strapped to your face. This is something that naturally requires significant space between the display and the Fresnel lens itself. Consequently, this bulks up the headset, adding weight and a large form factor.

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The emergence of the Pancake lens design

Within technology, there is a fact that tech turns more affordable and tries to become more compact. These two natural laws ring true in smartphones, laptops, chips, VR and AR headsets alike.

Augmented reality strives for affordability and thinness, and is driven by iterations of the novel waveguide design. Future virtual reality HMDs will likely be driven by the Pancake lens technology.

The Pancake technology itself is nothing new. The military and scientific community used Pancake optics years before virtual reality adopted it. We first saw a Pancake VR headset prototype in 2015 from eMargin, with Kopin, the leading developer of wearable headset components, following up with their iteration in 2017, called Kopin Elf.

Although recently, HuaweiHTCPico, and EM3, have either released commercial headsets or shown conceptual examples of what is possible. Essentially, Pancake-enabled VR HMDs are pivoting the bulky perception of virtual reality to stylish fashion symbols with a thinner and lighter design, making virtual reality more accessible for early majority consumers.

In effect, expanding the VR market to the benefit of everyone within the industry. Naturally, the bigger the market, the more advanced and innovative VR/AR becomes and better the industry gets.

The differences between Pancake and Fresnel lenses and what it all means

Kopin announced in mid-June 2021 an all-plastic Pancake lens design called P95, explained by Dr. John C.C. Fan, CEO and founder of Kopin Corporation, as being “specifically optimized for use with our 2.6K x 2.6K OLED high-brightness microdisplays. This new technology would be ideal for VR” continuing with that “[t]his major breakthrough represents a foundational milestone for the emerging VR markets. Bulky, heavy headsets have been a major barrier to faster consumer adoption for many years.”

When comparing Fresnel and Pancake, there are a few factors to consider. Where the screen light is originating from and the shape of the lens. The time it takes entering the lens versus exiting. As well as the wavelength (color) of the light itself.

The Fresnel optics features a wide field of view compared to Pancake but is prone to chromatic aberrations (ghosting/overlapping colors). Furthermore, software calibration taking up processing power must account for the Fresnel lens known as ‘pincushion distortion’. This distortion is similar to an image being stretched out in all four corners. The software then must artificially stretch the image further to normalize what is viewed by the wearer, all in real-time.

Pancake, on the other hand, works by folding many lenses together in a curve, bouncing light within the glass or plastic. In effect, slimming the distance needed between the wearer’s eyes and the display. This opens VR HMDs to be thinner and lighter, while it also frees up processing power, as the distortion problem for the Pancake is not present. Lastly, the Pancake design does not have the chromatic aberration present like the Fresnel.

The downside to the Pancake optics, compared to Fresnel, stems from Pancake’s bouncing of light within the lens itself, resulting in low light efficiency. In short, it dims the perceived image for the wearer and is why John C.C. Fan, the founder of Kopin, pointed out that the P95 Pancake lens works well with ‘high-brightness micro displays’. Furthermore, the problem of ghosting is also a scourge for the Pancake lens.

In summary, Pancake features more benefits than disadvantages compared to Fresnel, both in perceived image quality and the significantly slimmer and lighter form factor that Pancake enables.

Conclusion

Like augmented reality, VR is on a path to a slimmer and lighter future. A design philosophy, expanding the VR market, as casual consumers will see interest in adopting a VR HMD. Effectively, this will mature the virtual reality market and expand ancillary services propelling the technology further.

The best indicator of this trend was the released HTC Vive Flow focusing on a stylish design, a broader customer segment, and differentiation on non-tech elements such as meditation.

The enabling technology behind this trend is the Pancake lens design, by itself nothing new but revolutionary within VR HMDs. It will bring on stylish designs that attract a wider audience and will be able to detach itself from external devices such as a capable laptop or smartphone.

2022 and beyond will be the time when virtual reality matures in both hardware and use cases, and the not-talked-about-enough technology, the Pancake optics design, is the enabler of this.

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