VR Optical Solutions include aspheric lenses, Fresnel lenses, and Pancake. The current industry-recognized trend is Pancake, which can significantly reduce the volume of VR glasses and also noticeably decrease the thickness of the lenses. As shown in the diagram below, compared to the Fresnel optical system, Pancake can reduce the thickness by half, greatly enhancing the comfort of user wear.
The Chinese term "Pancake" refers to a folded optical path. By folding the optical path, it achieves the same total optical path length in a smaller module thickness, thus maintaining an equivalent magnification effect. The specific structure and optical path are as follows:
1. Light emitted from the display screen first passes through a polarizer, transforming into linearly polarized light (polarization direction: p). At this point, brightness experiences a 50% reduction.
2. It transforms into circularly polarized light after passing through the 1/4 wave plate, then goes through the semi-transparent, semi-reflective film, resulting in a second 50% reduction in brightness.
3. After passing through the second 1/4 wave plate, circularly polarized light transforms back into linearly polarized light (polarization direction: s). However, the polarization direction is now perpendicular to the previous direction.
4. Linearly polarized light with polarization direction s is reflected back by the reflective polarizing film, which transmits p and reflects s. Afterward, it passes through the 1/4 wave plate again, transforming into circularly polarized light.
5. It enters lens 1, then is reflected back by the semi-transparent, semi-reflective film, creating a folded optical path. Simultaneously, brightness experiences a third 50% reduction.
6. After passing through the 1/4 wave plate, it forms linearly polarized light with polarization direction p. This light can pass through the reflective polarizing film, go through lens 2, and reach the human eye.
Pancake optics incur significant light loss. LCD inherently comes with a polarizer, so its maximum optical efficiency is 25%, while Micro OLED has a maximum efficiency of 12.5%. However, Micro OLED achieves brightness that is twice that of LCD. Additionally, due to advantages such as pixel density (PPI), response speed, color gamut, etc., the mainstream choice for high-end VR is the Pancake + Micro OLED solution.
In addition to the significant light loss, the Pancake optical solution also faces issues with ghosting and a small Field of View (FOV). Ghosting occurs due to double refraction in the optical path, leading to the formation of false images. The small FOV results from compressing the Total Track Length (TTL).
Therefore, precise geometric positioning of all components in the Pancake optical solution is crucial. Specifically, attention must be paid to the following points:
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Do the paraxial optical axes of all surfaces coincide?
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Do the aspheric axes coincide with the paraxial system axis?
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Are the center thicknesses of the lenses and their distances correct?
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Is the alignment of the individual polarization elements to each other correct?
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Is the required image sharpness achieved?
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Is the exit pupil in the correct position?
During the manufacturing process and quality inspection, manufacturers need to adjust or control the following parameters:
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Paraxial lens centering and tilting
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Tilting and centering of aspherical surfaces
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Center thickness and spacing
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MTF across eyebox and field of view
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Effective focal length
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Distortion
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Relative illumination
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Chromatic aberration
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Ghost & Haze
Source: TRIOPTICS, Core Vision Plus, edited by Aibang Intelligent Manufacturing.
艾邦建有AR/VR产业链微信群，目前有HTC、PICO、创维VR、OPPO、亮亮视野、光粒科技、Rokid、首镜科技、Xreal、佳视、谷东科技、歌尔、立讯精密、多哚（纳立多）、欣旺达、乐镜、耐德佳、联创电子、至格科技、灵犀微光、惠牛科技、舜宇光学、理湃光晶、广景视睿、珑璟光电、水晶光电、京东方、海信视像、科煦智能、阿科玛、金发科技、雅柏工程材料、思立可、新安天玉、四方超轻、大族激光、发那科、承熹设备、康泰达等加入，也欢迎大家长按下方图片识别二维码加入微信群：
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The original article was first published on the WeChat official account (Aibang VR Industry News)："VR Pancake" Optical Principles and Detection Key
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