Projection Unit Volume Gratings for Edge Color Cast Control
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Solution Overview
Problem
Volume holograms in projection units exhibit varying deflection efficiency based on the user's viewing angle and wavelength, leading to undesirable color casts at the edges of the projected image, which are distracting to the viewer.
Innovation Solution
Ensure that the number of interference maxima and refractive index modulation for each exposed volume grating are the same by using separate layers with corresponding thicknesses for each grating, aligned to the respective wavelengths, to achieve identical deflection efficiency profiles across different viewing angles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If volume holograms are used to deflect the projected image, then the projection efficiency is improved, but color casts appear at the edges of the projected image due to varying deflection efficiency across different viewing angles and wavelengths
Solution Approach 1:
The projection unit divides the single volume hologram into multiple separate volume gratings, each optimized for a specific wavelength (red, green, blue). Each grating is exposed in a separate layer with controlled thickness to achieve uniform deflection efficiency across all wavelengths and viewing angles, eliminating the color casts that occur with a single broadband hologram.
Solution Approach 2:
Each volume grating is designed with local optimization for its specific wavelength range. The thickness of each grating layer is precisely controlled to match the optical path requirements for its designated wavelength, ensuring that each color component maintains consistent deflection efficiency across the entire viewing angle range, thereby preventing color distortion at image edges.
2Manufacturing precision
If the number of interference maxima and refractive index modulation are standardized across volume gratings, then color accuracy is improved, but the manufacturing complexity increases due to separate layers with precise thickness requirements
Solution Approach 1:
The system segments the holographic functionality into separate wavelength-specific gratings in individual layers. This segmentation allows each layer to be manufactured with standardized parameters (interference maxima count and refractive index modulation), simplifying the manufacturing process while maintaining high color accuracy through precise thickness control of each layer.
Solution Approach 2:
The invention changes the thickness parameter of each volume grating layer to compensate for wavelength-dependent optical path differences. By adjusting only the thickness parameter while keeping other exposure parameters standardized, the system achieves uniform deflection efficiency across wavelengths without requiring complex manufacturing processes for each layer.
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 approach eliminates undesirable color casts by ensuring consistent color accuracy across the projected image, maintaining a uniform efficiency ratio within a predetermined angular range, thereby enhancing the viewing experience.
Implementation Method 1
a first volume grating for a first wavelength (e.g., 450 nm) and a second volume grating for a second wavelength (e.g., 530 nm), at which the multicolored image is deflected
Implementation Method 2
an interference field or interference volume with the desired number of interference maxima is created over the photosensitive volume holographic material of the layer, thus forming the desired refractive index modulation
Implementation Method 3
a layer (which has a photosensitive volume holographic material or is formed from it) into which the volume grating is to be exposed
Data Source
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Figure 5~6
AI summary
A projection unit (3) is provided, which images a fed multi-color image having a first and a second wavelength into an exit pupil (6) such that an observer can perceive said image as a virtual image when an eye (A) of the observer is positioned in the exit pupil (6) and the observer views the projection unit (3) at a predefined angle of observation (α1). The projection unit (3) has a volume hologram, which deflects the multi-color image, for imaging, into the exit pupil (6). The volume hologram has, for each wavelength of the color partial images, a volume grating applied by exposure. Each of said volume gratings has a deflection efficiency curve dependent on the angle of observation, the deflection efficiency curve being maximal for the predefined angle of observation (α1). The deflection efficiency curves are set equal for a predefined angle range around the predefined angle of observation (α1) by virtue of the fact that, for each of the volume gratings applied by exposure, the number of interference maxima applied by exposure is the same and the refractive index modulation for the corresponding wavelength, which refractive index modulation is applied by exposure, is the same.