Reverse-Order Crossed Pancake Lens for HMD Ghost Reduction
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Solution Overview
Problem
Head-mounted displays (HMDs) suffer from reduced contrast and ghost images due to parasitic light generated by reflections between optical elements, particularly with polarizing elements, which degrade the user's experience.
Innovation Solution
A reverse-order crossed (ROC) pancake lens architecture is employed, utilizing a first and second compound retarder with orthogonal optical axes, along with a partial reflector and reflective polarizer, to transform and restore the polarization state of light, minimizing light leakage and ghost images.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If polarizing elements are used in the HMD optical system, then light control and image direction are improved, but parasitic light and ghost images are generated that reduce contrast
Solution Approach 1:
The patent converts the harmful parasitic light generated by reflections off polarizing elements into beneficial reflected light by using a circular polarizer in conjunction with a mirror. The circular polarizer transforms linearly polarized light into circularly polarized light, which then reflects off the mirror and converts back to linearly polarized light, directing the previously lost light back toward the user's eye and improving overall system efficiency
Solution Approach 2:
The patent changes the polarization state parameter of light from linear to circular using a circular polarizer, and then back from circular to linear after reflection. This parameter transformation allows the system to manage light paths more effectively, directing useful light while managing parasitic reflections through the unique properties of circularly polarized light interacting with mirrors
2Reliability
If multiple optical elements are used to direct light to eye boxes, then image delivery is improved, but light reflection between elements generates parasitic light
Solution Approach 1:
The patent captures parasitic light that would otherwise be lost through reflections off polarizing elements and redirects it back to the user's eye using a circular polarizer and mirror combination, converting what was previously harmful waste light into useful image light
Solution Approach 2:
The circular polarizer acts as an intermediary element between the polarizing elements and the mirror, transforming the polarization state to enable effective light redirection while the mirror serves as an intermediary to bounce light back into the optical path, reducing losses from direct parasitic reflections
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The ROC pancake lens effectively reduces parasitic light and enhances image quality by maintaining consistent polarization across a broad spectrum, improving contrast and reducing visual artifacts.
Implementation Method 1
a first and second compound retarder (e.g., including quarter wave retarders) in a reverse order crossed (ROC) configuration
Implementation Method 2
a reflective polarizer
Implementation Method 3
a partial reflector
Data Source
Figure 1
Figure 2
Figure 3A
AI summary
In various embodiments, a pancake lens block may include (i) a first compound retarder comprising a first plurality of retarders oriented to a plurality of first axes of orientation, respectively, where the first compound retarder is configured to selectively transmit a portion of the emitted light, (ii) a partial reflector that receives the portion of emitted light from the first compound retarder and transmits the portion of the emitted light, (iii) a second compound retarder comprising a second plurality of retarders oriented to a plurality of second axes of orientation that are substantially orthogonal to the respective plurality of first axes of orientation, and (iv) a reflective polarizer configured to reflect the portion of the emitted light selectively transmitted by the second compound retarder back to the second compound retarder.