What are Pancake Lenses? Let's compare it with a conventional Fresnel lens

The difference between pancakes lenses and current fresnel lenses found on VR headsets - VR Expert Blog (vr-expert.com)

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.

​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 eMagin, with Kopin, the leading developer of wearable headset components, following up with their iteration in 2017, called Kopin Elf.

​Although recently, Huawei, HTC, Pico, 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.

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.

(When viewed in 3D, there is a phenomenon in which the image appears spread, because it is a Fresnel lens. Even if you deliberately make the correction appear spread out in real time in real time, the sense of heterogeneity disappears and the distortion becomes worse)

 

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.

 

https://youtu.be/g2EDMw-4T6I

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.

♬The above article was also published on https://t.me/timesight.