Display Optical System With Low-Reflectance Half-Mirror for Ghost Reduction
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Solution Overview
Problem
Existing display optical systems for head-mounted displays (HMDs) face challenges in reducing external and internal light ghosts, which distract the observer and affect image quality, due to high reflectance of conventional half-mirrors and birefringence of lenses, leading to inefficient light transmission and increased color shifts.
Innovation Solution
The use of a dielectric multilayer film with a reflectance of 35% or less for the half-mirror and controlled birefringence in lenses, along with polarization-based optical path folding, to minimize external and internal light ghosts, while maintaining efficient light transmission and reducing color shifts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a conventional half-mirror with high reflectance is used, then light can be effectively split into reflected and transmitted paths, but external and internal light ghosts increase, reducing image quality
Solution Approach 1:
The patent changes the reflectance parameter of the half-mirror from conventional high values (typically 50% or higher) to a specific low range of 35% or less. This parameter change reduces the intensity of reflected ghost light while maintaining sufficient light splitting functionality for the optical system to operate effectively
Solution Approach 2:
The patent employs a dielectric multilayer film composed of multiple layers with different refractive indices (such as TiO2, SiO2, Nb2O5) to create the half-mirror. This composite structure enables precise control of optical properties including reflectance and polarization characteristics, achieving the target reflectance of 35% or less while maintaining manufacturing feasibility
2Ease of operation
If lenses with high birefringence are used, then optical path folding and polarization control are achieved, but color shifts increase and image clarity decreases
Solution Approach 1:
The patent controls the birefringence parameter of the lenses to fall within a specific range (typically 0.001 to 0.005). This parameter control reduces color shifts caused by excessive birefringence while maintaining sufficient optical path folding capability and polarization control functionality
Solution Approach 2:
The patent applies different optical properties to different components: lenses are designed with controlled low birefringence to minimize color shifts, while the half-mirror is designed with specific reflectance and polarization characteristics. Each component has optimized local properties suited to its specific function in the optical system
3Object-affected harmful factors
If reflectance of the half-mirror is reduced to minimize light ghosts, then image clarity improves, but light transmission efficiency may decrease
Solution Approach 1:
The patent optimizes the reflectance parameter to a specific range (35% or less) that balances ghost reduction with sufficient light transmission. This optimized parameter ensures that enough light is transmitted through the half-mirror to maintain image brightness while reflected ghost light is reduced to acceptable levels
Solution Approach 2:
The patent converts the potentially harmful effect of reflected light (which causes ghosts) into a beneficial controlled reflection that directs light along the intended optical path. By carefully controlling the reflectance at 35% or less, the system utilizes reflection constructively for optical path folding while minimizing harmful ghost effects
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 reduces external light ghosts by half and internal ghosts by a quarter, improving image clarity and contrast, while allowing for a lightweight and compact HMD design with wide viewing angles.
Implementation Method 1
a partially transmissive reflective surface including a dielectric multilayer film including five layers or more and ten layers or less
Implementation Method 2
a polarizing beam splitter and a half mirror, the polarizing beam splitter configured to separate a light flux into a first light flux and a second light flux based on a polarization direction
Implementation Method 3
a first lens and a second lens, each having a positive optical power, are arranged in this order from the display element side, the first lens having a first birefringence and the second lens having a second birefringence
Data Source
AI summary
Provided is a display optical system that guides light from a display element to an observation side, and includes a partially transmissive reflective surface having reflectance of 35% or less for visible light and including a dielectric multilayer film including between five and ten layers, and a polarizing splitting surface.