HUD Combiner Optics Using Cholesteric Reflection to Prevent Double Images
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Solution Overview
Problem
Head-up display systems face challenges with double images due to light reflection on windshield glass, and existing solutions struggle to balance high reflectivity and transmittance while preventing double images, especially when using cholesteric liquid crystal layers and phase difference layers.
Innovation Solution
A member for displaying projected images in head-up display systems is developed, incorporating a cholesteric liquid crystal layer and a λ/2 phase difference layer to suppress reflection from the front surface, ensuring high reflectivity and transmittance without double images, and is optimized for use with polarized sunglasses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If wind shield glass is used as a combiner to display projected video and front scenery simultaneously, then the head up display function is achieved, but double image appears due to reflection of projection light on front surface or rear surface of glass
Solution Approach 1:
The invention converts the harmful reflection from the front surface into a beneficial effect by using a cholesteric liquid crystal layer that selectively reflects only the projected light wavelength while transmitting other wavelengths (front scenery). The Brewster angle design causes p-polarized projection light to be reflected by the front surface, and this reflected light is then selectively reflected back by the cholesteric liquid crystal layer to form a clear virtual image, while the front scenery passes through the layer unchanged.
2Object-generated harmful factors
If Brewster angle is used by allowing p-polarized light to be incident on glass surface to reduce reflection light, then double image is solved, but simultaneously ensuring sufficient light reflectivity and sufficient light transmittance remains problematic
Solution Approach 1:
The invention applies local quality by making the optical properties position-dependent within the glass structure. The cholesteric liquid crystal layer is positioned at a specific depth from the front surface (0.01-0.5mm) and has wavelength-selective optical properties: it reflects projected light (specific wavelength) while transmitting front scenery (other wavelengths). This creates different optical characteristics at different positions and for different wavelengths, solving the contradiction between reflectivity and transmittance.
Solution Approach 2:
The invention changes the optical parameters of the glass system by introducing a cholesteric liquid crystal layer with specific helical pitch and handedness. This layer changes the reflection characteristics from broadband (problematic) to narrowband (selective), allowing high reflectivity for projected light while maintaining high transmittance for visible light and front scenery, thus resolving the contradiction.
3Object-generated harmful factors
If cholesteric liquid crystal layer and λ/4 phase difference layer are combined to convert p-polarized light to circularly polarized light, then selective reflection is achieved, but the projected image amount of light is lower than theoretically predicted
Solution Approach 1:
Instead of using the conventional λ/4 phase difference layer approach that converts linearly polarized light to circularly polarized light (resulting in light loss), the invention inverts the approach by using a cholesteric liquid crystal layer that directly selectively reflects the desired wavelength. This inverted approach eliminates the need for polarization conversion and associated light losses, achieving higher projected image brightness while still preventing double images.
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 solution provides a clear projected image with high reflectivity and transmittance, effectively preventing double images and maintaining image clarity even when observed through polarized sunglasses.
Implementation Method 1
a projected image can be obtained by using selective reflection of a cholesteric liquid crystal layer
Implementation Method 2
a λ/2 phase difference layer is disposed on the incidence ray side with respect to the reflection layer
Implementation Method 3
a double image due to reflection of projection light on a front surface or a rear surface of glass
Data Source
Figure 1
AI summary
The present invention provides a member for displaying a projected image and a projected image display system in which a clear image can be displayed with high reflectivity and high transmittance without a double image, the member for displaying a projected image including a reflection layer, and a λ/2 phase difference layer, in which the reflection layer includes a cholesteric liquid crystal layer having selective reflection in a visible light range, and the projected image display system including the member for displaying a projected image, in which the λ/2 phase difference layer is disposed on an incidence ray side with respect to the reflection layer, and the incidence ray is p-polarized light which vibrates in a direction parallel to an incidence surface.