Wedge-Shaped Exit Pupil Expander for AR Color Balance

Solution Overview

Problem

Wearable optical devices face challenges in maintaining an adequate eye-box and color space due to size constraints, with conventional exit pupil expanders suffering from diffraction issues that cause varying color balance across the user's field of view.

Innovation Solution

A wedge-shaped exit pupil expander with non-parallel internal reflective surfaces that induce angularly varying total internal reflection, shifting the diffracted light and improving color balance by controlling the wedge shape and angle, thereby maintaining consistent color perception across the field of view.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Area of stationary object

If conventional exit pupil expanders with parallel surfaces are used, then the eye-box size is expanded, but the color balance varies across the user's field of view due to diffraction

Engineering Contradiction:

Improveeye-box sizeVSAvoidcolor balance consistency

Core Design Contradiction:

Area of stationary objectVSManufacturing precision

Solution Approach 1:

The patent applies asymmetry by introducing a wedge angle between the front and back surfaces of the exit pupil expander. This asymmetric configuration causes the light to experience angularly varying total internal reflection, which shifts the diffracted light and compensates for the color variations caused by diffraction in conventional parallel-surface designs.

Inventive Principle:

Principle #4Asymmetry

Solution Approach 2:

The patent changes the geometric parameter of the optical channel by introducing a wedge angle between the front and back surfaces. This parameter change transforms the light propagation path and the total internal reflection angles, thereby shifting the output wavelength to compensate for diffraction-induced color imbalances.

Inventive Principle:

Principle #35Parameter changes

2Area of stationary object

If the optical train size is increased to maintain adequate eye-box, then the color space is improved, but the device size exceeds practical limits for wearable applications

Engineering Contradiction:

Improveeye-boxVSAvoidoptical train size

Core Design Contradiction:

Area of stationary objectVSVolume of moving object

Solution Approach 1:

The patent changes the geometric parameter of the optical channel by introducing a wedge angle between the front and back surfaces. This parameter change transforms the light propagation path and the total internal reflection angles, thereby shifting the output wavelength to compensate for diffraction-induced color imbalances.

Inventive Principle:

Principle #35Parameter changes

3Area of stationary object

If diffractive gratings are used in exit pupil expanders, then the eye-box is expanded, but the diffraction causes varying color balance across the field of view

Engineering Contradiction:

Improveeye-boxVSAvoidcolor space control

Core Design Contradiction:

Area of stationary objectVSManufacturing precision

Solution Approach 1:

The patent converts the harmful effect of diffraction into a beneficial one by using the wedge-shaped configuration to intentionally shift the diffracted light wavelengths. The angularly varying total internal reflection in the wedge shape compensates for the color variations, transforming the diffraction problem into a solution that maintains color balance.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

The patent changes the geometric parameter of the optical channel by introducing a wedge angle between the front and back surfaces. This parameter change transforms the light propagation path and the total internal reflection angles, thereby shifting the output wavelength to compensate for diffraction-induced color imbalances.

Inventive Principle:

Principle #35Parameter changes

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 wedge-shaped design achieves improved color balance and reduced cost by maintaining consistent color perception across the field of view, enhancing the effectiveness of virtual and augmented reality devices without increasing the optical train size.

Implementation Method 1

the light defining a center wavelength that enters the optical channel at the entrance pupil perpendicular to the back surface experiences angularly varying total internal reflection between the front and back surfaces such that the light that exits the optical channel perpendicular to the exit pupil is at a wavelength shifted from the center wavelength

Methodology Applied

Scientific EffectTotal internal reflection: Total Internal Reflection

Data Source

PatentUS20240176148A1Exit pupil expander

Publication Date: 2024.05.30 MAGIC LEAP INC

AI summary

An exit pupil expander (EPE) has entrance and exit pupils, a back surface adjacent to the entrance pupil, and an opposed front surface. In one embodiment the EPE is geometrically configured such that light defining a center wavelength that enters at the entrance pupil perpendicular to the back surface experiences angularly varying total internal reflection between the front and back surfaces such that the light exiting the optical channel perpendicular to the exit pupil is at a wavelength shifted from the center wavelength. In another embodiment a first distance at the entrance pupil between the front and back surfaces is different from a second distance at the exit pupil between the front and back surfaces. The EPE may be deployed in a head-wearable imaging device (e.g., virtual or augmented reality) where the entrance pupil in-couples light from a micro display and the exit pupil out-couples light from the EPE.