Multilayer Beam Splitter Stack for Head-Mounted Display Brightness
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Solution Overview
Problem
Head-mounted projection displays (HMPDs) face challenges in achieving a combination of high image quality, compact size, and affordability, often suffering from excessive forward extension, poor brightness, and image quality issues due to the use of 45-degree beam splitters and additional waveplates, which increase cost and complexity.
Innovation Solution
A multilayer stack of a polarizing beam splitter with a phase retarder layer, such as a quarter wave film, is used to optimize the reflection and transmission of light, allowing circularly polarized light to be reflected and then converted to plane polarized light with low loss, while reducing forward extension and improving image quality through adjustments in projector orientation and pixel intensity calibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a 45 degree beam splitter is used to direct projected light to a retroreflective screen, then the HMPD structure is achieved, but significant brightness loss occurs at each pass through the beam splitter
Solution Approach 1:
The patent changes the polarization state parameter of light from linear to circular polarization. By using a quarter-wave plate to convert linearly polarized light from the projector into circularly polarized light, the system achieves low-loss transmission through the beam splitter on the return path, as circularly polarized light can pass through the linear polarizer with minimal attenuation regardless of its orientation.
Solution Approach 2:
The patent employs a composite optical system combining multiple components: a polarizing beam splitter, a quarter-wave plate, and a retroreflective screen. This composite arrangement works together to achieve the desired function of directing light with minimal loss while maintaining image quality and compact form factor.
2Loss of energy
If a quarter waveplate is added to reduce brightness loss, then polarization optimization is achieved, but forward extension increases by 1 cm to 3 cm
Solution Approach 1:
The patent replaces the traditional thick quarter-wave plate with a thin-film quarter-wave retarder deposited directly on the beam splitter surface. This thin-film approach maintains the optical function of converting linear polarization to circular polarization while dramatically reducing the physical thickness and forward extension of the optical assembly.
Solution Approach 2:
The patent merges the quarter-wave plate function with the beam splitter by depositing the retarder film directly onto the beam splitter surface. This integration eliminates the need for separate mounting of the waveplate and reduces the overall optical path length, thereby reducing forward extension while maintaining the polarization conversion function.
3Manufacturing precision
If multiple optical components are used to optimize polarization, then image quality improves, but manufacturing complexity and cost increase
Solution Approach 1:
The patent combines multiple optical functions into a single integrated component. The beam splitter incorporates a polarizing layer and a quarter-wave retarder layer, eliminating the need for separate mounting and alignment of individual components. This integration simplifies manufacturing while maintaining the optical performance required for high image quality.
Solution Approach 2:
The integrated beam splitter component performs multiple functions simultaneously: beam splitting, polarization filtering, and quarter-wave retardation. This multi-functionality reduces the total number of components required, simplifying the manufacturing process and reducing assembly complexity while maintaining image quality.
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 solution enhances brightness and contrast while reducing the forward extension of HMPDs, making them more suitable for consumer applications by minimizing the thickness of optical components and eliminating the need for additional manufacturing steps, thus lowering costs and improving image quality across the field of view.
Implementation Method 1
a polarizing beam splitter with a matching polarization orientation
Implementation Method 2
reflects at low loss off the front surface of a polarizing beam splitter
Implementation Method 3
passes through a quarter waveplate with fast and slow axes set at 45 degrees from the incident light plane polarization axis, thus converting the polarization from a plane polarization to circular polarization
Implementation Method 4
directed projected light out to a retroreflective screen and allow reflected light to be transmitted to a user's eye
Data Source
Figure 1A
Figure 1B
Figure 2A
AI summary
A head mounted projection display includes a polarizing beam splitter stack that includes a waveplate. The polarizing beam splitter stack reduces optical losses and has a low forward extension. The polarizing beam splitter may be used at a 45 degree angle or at a non 45 degree angle relative to the line of view. A correction may be also be performed to the intensity of individual pixels to account for chromatic non-uniformity in the optical response of a retroreflector and other optical components.