Projection Display Ghost Image Suppression via Wavelength Selective Wave Plates
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Solution Overview
Problem
Existing projection display systems suffer from ghost images due to noise light reentering reflective liquid crystal display elements, causing color nonuniformity and aberration, despite the use of λ/4 wave plates and dichroic mirrors, as the spectral transmissive and reflective characteristics of dichroic films depend on wavelength and angle of incidence.
Innovation Solution
A projection display system that employs wavelength selective wave plates acting as λ/2 wave plates between polarization beam splitters and color combining prisms, converting polarization and preventing noise light from reentering reflective liquid crystal display elements by rotating polarization orientations, eliminating the need for tilted dichroic mirrors and reducing ghost images without introducing color nonuniformity or astigmatism.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a λ/4 wave plate is inserted between the projection optical system and the crossed dichroic prism assembly to rotate the orientation of polarization of noise light, then the noise light is discarded off the optical path, but the spectral transmissive and reflective characteristics of the dichroic film are not uniform about p- and s-polarized light and have dependence on wavelength, causing some p- or s-polarized light of other color light to enter the polarization beam splitters and produce ghost images
Solution Approach 1:
The patent changes the optical path configuration by removing the λ/4 wave plate and dichroic mirrors, and instead positions the polarization beam splitters such that they directly receive light from the light source. This parameter change in the optical system eliminates the polarization-dependent spectral characteristics that cause color nonuniformity while still discarding noise light through the polarization beam splitters' inherent properties.
2Object-affected harmful factors
If dichroic mirrors are disposed at angles to the optical axis to discard noise light, then noise light is reflected off the optical path, but the spectral transmissive and reflective characteristics of the dichroic mirror have dependence on the incidence angle, causing color nonuniformity in the projected image
Solution Approach 1:
The patent extracts and removes the dichroic mirrors from the optical system, replacing them with polarization beam splitters positioned at specific angles. This extraction eliminates the angle-dependent spectral characteristics of dichroic mirrors that cause color nonuniformity, while the polarization beam splitters discard noise light through their polarization-selective properties without introducing color distortion.
3Object-affected harmful factors
If tilted dichroic mirrors are used to prevent noise light from reentering reflective liquid crystal display elements, then ghost images are reduced, but astigmatism is introduced in the projection optical system
Solution Approach 1:
The patent replaces the mechanical arrangement of tilted dichroic mirrors with an optical system using polarization beam splitters positioned at specific angles. This substitution eliminates the astigmatism introduced by tilted mirrors while maintaining the function of preventing noise light from reentering the display elements, as the polarization beam splitters achieve noise light discarding through polarization selection rather than angular reflection.
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
Effectively suppresses ghost images by preventing noise light from reentering reflective liquid crystal display elements, maintaining color uniformity and avoiding astigmatism, through the use of wavelength selective wave plates that rotate polarization orientations, enhancing the quality of projected images.
Implementation Method 1
A wavelength selective wave plate showing a characteristic of a λ/2 wave plate according to wavelength is disposed between an exit face of at least one of the polarization beam splitters and an entrance face of the color combining prism
Implementation Method 2
entering the three color lights on their respective reflective liquid crystal display elements via polarization beam splitters
Implementation Method 3
each reflective liquid crystal display element performs polarization modulation to vary the orientation of polarization
Implementation Method 4
The color lights which have been polarization-modulated are combined into one by crossed dichroic prisms for color combination
Implementation Method 5
a part of the combined light incident on the projection optical system is reflected from lenses or apertures within the projection optical system
Data Source
AI summary
A projection display uses reflective liquid crystal display elements for each color light to prevent ghost image. A λ/4 wave plate is disposed between a crossed dichroic prism assembly for color combination and the projection optical system to convert the state of polarization between linear polarization and circular polarization. Wavelength selective wave plates, each of which transmit the projected color light without varying its orientation of polarization but rotate the orientations of polarization of the other color lights 90 degrees, are disposed between the entrance face of the crossed dichroic prism assembly and each of a polarization beam splitter for blue light, a polarization beam splitter for green light, and a polarization beam splitter for red light.