Birefringent Phase Compensation for Projector Skew Rays
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Solution Overview
Problem
Current polarization control systems in image display projectors face challenges in maintaining image contrast due to skew rays, which are not adequately compensated by existing quarter-wave plates, leading to reduced contrast and increased manufacturing complexity and cost.
Innovation Solution
A polarization control system comprising a polarizing beam splitter, a reflective type liquid crystal element, a first birefringent element with a slow axis parallel to the reflective surface, and a second birefringent element with an index ellipsoid inclined to the reflective surface, providing a total phase difference of λ/4, effectively compensating for skew rays across a wider area and improving contrast.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single quarter-wave plate is used to compensate for skew rays, then the polarizing direction can be adjusted, but the compensation is only effective for skew rays inclined at 5° or below, which is insufficient for projector skew rays inclined at 10° to 15°
Solution Approach 1:
The single quarter-wave plate is divided into two separate birefringent elements: a first birefringent element with its slow axis parallel to the reflective surface, and a second birefringent element with its index ellipsoid inclined to the reflective surface. This segmentation allows each element to contribute differently to the overall phase compensation, enabling effective compensation for skew rays at larger inclination angles (10° to 15°) while maintaining polarization accuracy.
Solution Approach 2:
Each birefringent element is designed with specific local optical properties: the first element provides phase delay with its slow axis parallel to the surface, while the second element provides additional phase compensation with its inclined index ellipsoid. This local differentiation of optical properties allows the system to effectively handle skew rays across a wider angular range than a uniform quarter-wave plate could achieve.
2Measurement precision
If an O-plate cut obliquely from crystal is used to compensate for skew rays, then the polarizing direction can be adjusted, but it is difficult to shape and requires extra cost
Solution Approach 1:
The complex obliquely-cut O-plate is segmented into two simpler birefringent elements with standard orientations. The first element has its slow axis parallel to the reflective surface, and the second has its index ellipsoid inclined. This segmentation transforms a difficult-to-manufacture single component into two easier-to-produce elements, reducing fabrication complexity and cost while achieving the same optical compensation effect.
Solution Approach 2:
Instead of using expensive, difficult-to-shape obliquely-cut crystal O-plates, the invention employs standard birefringent elements that can be manufactured more cheaply and easily. These standard elements, when combined, provide equivalent or superior compensation performance, making the system more cost-effective and manufacturable.
3Ease of manufacture
If the O-plate is fabricated by oblique deposition or polymerization of liquid crystal molecules, then it can be manufactured, but the main axis is inclined at between 0° and 45° to the surface normal, which is insufficient for quarter-wave plate function requiring 75° to 80° inclination
Solution Approach 1:
The required 75° to 80° main axis inclination for quarter-wave plate function is achieved by combining two elements: the first birefringent element with slow axis parallel to the surface (0° inclination) and the second with inclined index ellipsoid (40° to 70° inclination). This segmentation allows the system to achieve the effective high-angle compensation without requiring any single element to have the difficult-to-fabricate steep inclination.
Solution Approach 2:
Instead of attempting to fabricate a single element with the difficult 75° to 80° inclination angle, the invention changes the approach by using two elements with more manufacturable inclination angles (0° and 40° to 70°). The combined optical effect of these two elements achieves the desired phase compensation for steeply inclined skew rays, making the system both manufacturable and effective.
4Measurement precision
If the O-plate is made with thickness of 1 μm or above to function as quarter-wave plate, then it can provide phase compensation, but such great thickness increases haze and lowers polarization
Solution Approach 1:
The total phase compensation requirement is segmented between two birefringent elements. The first element provides partial phase delay with its slow axis parallel to the surface, and the second element provides additional phase compensation with its inclined index ellipsoid. This segmentation allows each element to be thinner than a single 1 μm+ O-plate, reducing haze and polarization loss while achieving the same total phase difference compensation.
Solution Approach 2:
The invention uses a composite structure of two different birefringent elements with different orientations and properties. This composite approach allows the system to achieve the required phase compensation with thinner individual layers, reducing the harmful effects of haze and polarization loss that occur with thick single-layer O-plates.
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 system provides enhanced image contrast and easier, more cost-effective manufacturing by compensating skew rays across a broader area, resulting in improved projection image quality and simplified production compared to single quarter-wave plate solutions.
Implementation Method 1
the light vertically passing through the first and second birefringent elements causes a total phase difference of substantially λ/4
Implementation Method 2
The polarizing beam splitter has a polarization separating layer to transmit or reflect incident light depending on a polarizing direction
Implementation Method 3
The reflective type liquid crystal element has a reflective surface to reflect the light from the polarizing beam splitter back to the polarizing beam splitter, and controls the polarizing direction of light by the reflective surface
Data Source
AI summary
A phase compensation element is disposed between a reflective type display element and a polarizing beam splitter. Composed of a crystal structure retardation layer functioning as a quarter-wave plate and an inclined-axis retardation layer functioning as an O-plate, the phase compensation element is aligned substantially parallel to a reflective surface of the reflective type display element. The inclined-axis retardation layer is made of inorganic material obliquely deposited on the crystal structure retardation layer. The inclined-axis retardation layer has a principal refractive index axis inclined at between 0° and 45° to a surface normal of the crystal structure retardation layer, and has a thickness not to increase haze of the phase compensation element.