Commentary on All-Plastic Pancake Optics from Kopin

Chris Chinnock, Insight Media

For context, please read this press release

The development of an all-plastic pancake optical solution for a VR headset is a pretty big deal not only for the weight reduction, but as Kopin claims, it actually offers improvements in image quality while lowering costs.

Optical systems with large magnification tend to have large image distortions. In order to suppress the distortion, multiple lenses need to be combined, resulting in a thicker VR optical system. The pancake optical system uses the aspherical concave mirror that generates less aberration, thus reducing the number of lenses required for a VR headset and making it thinner. In order to mount the concave mirror, the Pancake optical system uses a concave mirror as a half-mirror and also uses polarization to fold back the optical path.

The key enabler in the new all-plastic Pancake optical design is a polymer material with no birefringence – something that has never been developed before. While this has now been applied to the Pancake optical design, the potential for the material is huge.  Smartly Kopin has filed three patents related to the all-plastic Pancake optics design, manufacturing and system utilization, and I believe Kopin might have locked up supply of the material to capitalize on this opportunity.

Birefringence is an optical property in certain materials, like polymers, whereby the speed of light in the material is different for the opposite polarization orientations. As a result, the polarization state of light is altered after passing through birefringent materials. In most optical systems, the polarization state of light does not affect the image formation, which explains why polymers, which are light and can make aspherical lenses at low cost, are used in many applications. However, conventional polymers cannot be used in Pancake optics, which is sensitive to polarization disturbance.

Figure 1 shows the configuration of the Pancake optical design and how polarization is used to fold the optical path to thin the design. Circularly polarized light from the OLED display passes through the first lens with half reflection/transmission coating (half-mirror lens). After passing through a quarter wavelength (1/4 l) retarder, the light becomes linearly polarized and is reflected by the reflecting polarizer. After passing through the 1/4 l retarder, reflected by the half mirror lens and passing through the 1/4 l retarder again, the light is linearly polarized but 90° rotated from the first pass and goes through the reflecting polarizer.

Figure 1. Configuration and features of Pancake optics.


The half-mirror lens is an aspherical concave mirror that magnifies the image with low distortion. The second lens accurately corrects any slight image distortion. If the half-mirror lens has birefringence, the polarization state is disturbed after passing through it and some light is transmitted during the first pass. This first-pass transmitted light forms a ghost image which overlaps over the real image. This new polymer material means no ghosting.

One disadvantage of the Pancake optics is low efficiency which is typically around 10%. Figure 2 shows the assembly schematic and pictures of the working all-plastic Pancake optical lenses.

Figure 2. Assembly schematic and pictures of all-plastic Pancake optical lenses.

The pancake optics are a natural fit for a VR headset with micro-OLED displays. It can also be used for a mixed reality headset where a video see-through capability is used, but I don’t see how these optics work for an optical see-through design, unless the micro-OLED display brightness is significantly increased or micro-LED displays are developed.  Panasonic has already shown a VR headset with an earlier Pancake design and Kopin micro-OLED displays, so it would not be surprising they will want to now consider using the all-plastic Pancake optics for their stylish VR glasses.

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