Optical System Ghosting Reduction in HMDs
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Solution Overview
Problem
Head-mounted displays (HMDs) using plastic lenses in ocular optical systems face issues with ghosting due to birefringence, which existing technologies have not adequately addressed, particularly in achieving wide view angles while maintaining image quality.
Innovation Solution
The optical system incorporates a configuration that includes a first phase plate, a semi-transmissive reflective surface with controlled transmittance and reflectance, a lens, and a second phase plate, along with a polarization beam splitter to manage polarization and reduce ghosting, utilizing a half mirror with a silver layer and dielectric multilayer films to optimize light path folding and aberration correction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If a plastic lens is used for weight reduction in the ocular optical system, then the weight of the HMD is reduced, but ghost is generated due to birefringence in the plastic lens
Solution Approach 1:
The patent applies the principle of converting harm into benefit by utilizing the birefringence property of plastic lenses in a controlled manner. By introducing a half mirror with specific optical characteristics (reflectance greater than transmittance) into the optical path, the system converts the harmful ghosting effect caused by unplanned birefringence into a manageable optical parameter. The half mirror's optical characteristics are specifically designed to compensate for the birefringence-induced phase shifts, thereby reducing ghost while maintaining the weight advantage of plastic lenses.
2Object-generated harmful factors
If a semi-transmissive reflective surface with high reflectance is used to reduce ghost, then ghosting is reduced, but image brightness decreases
Solution Approach 1:
The patent applies parameter changes by precisely controlling the optical characteristics of the half mirror. Instead of using a simple high-reflectance mirror that would uniformly reduce both ghost and brightness, the system optimizes the reflectance and transmittance parameters of the half mirror to achieve a balance. The half mirror is designed with reflectance greater than transmittance (e.g., reflectance of 70-85%, transmittance of 15-30%) to preferentially reduce ghost while maintaining sufficient image brightness. This parameter optimization allows the system to achieve the desired ghost reduction without excessive brightness loss.
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
This configuration effectively reduces ghosting intensity, enhances image brightness, and allows for a wider view angle while maintaining a lightweight and compact HMD design, enabling more natural image observation.
Implementation Method 1
a polarization beam splitter configured to reflect a first linearly polarized light and transmit a second linearly polarized light whose polarization direction is orthogonal to that of the first linearly polarized light
Implementation Method 2
a semi-transmissive reflective surface having a transmittance smaller than a reflectance thereof
Implementation Method 3
a first phase plate, a semi-transmissive reflective surface, a lens, a second phase plate
Implementation Method 4
a lens configured to transmit the second linearly polarized light
Data Source
AI summary
The optical system introduces light from a display element displaying an image to an observer. The optical system includes in order from the display element toward the observer, a first phase plate, a semi-transmissive reflective surface, a lens, a second phase plate, and a polarization beam splitter configured to reflect a first linearly polarized light and transmit a second linearly polarized light whose polarization direction is orthogonal to that of the first linearly polarized light. The semi-transmissive reflective surface has a transmittance smaller than a reflectance thereof.