Light Flux Expanding Element for HMD Pupil Tracking
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Solution Overview
Problem
Existing head-mounted display technologies face challenges in sufficiently expanding image light, which is often smaller than the pupil diameter, making it difficult to ensure image light input even when the observer's eyes are moving.
Innovation Solution
A light flux diameter expanding element is introduced, comprising a light guiding plate with a diffraction grating on both input and output sides. The diffraction grating on the output side has the same grating period as the input side but with a smaller height, allowing for efficient expansion of the light flux diameter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a laser beam is used to draw an image on the retina, then the device can be compact and suitable for head-mounted display, but the image light diameter is smaller than the pupil diameter making it difficult to ensure image light input when eyes are moving
Solution Approach 1:
The patent divides the single laser beam into multiple beams using a diffraction grating. The light flux diameter expanding element splits the incoming laser beam into several separate beams that travel through the light guiding plate and exit at different positions, thereby expanding the effective light diameter to match the pupil diameter and ensure stable image light input even when the eye moves.
Solution Approach 2:
The patent introduces a light guiding plate as an intermediary component between the diffraction grating and the eye. This plate guides the multiple diffracted beams through its internal structure, maintaining their separation while directing them toward the pupil, thus mediating the transition from expanded beams to stable eye input.
2Area of stationary object
If the diffraction grating on the output side has the same grating period as the input side, then the diffraction angle is controlled, but the light flux diameter expansion is limited
Solution Approach 1:
The patent utilizes the thickness dimension of the light guiding plate to achieve beam separation. By controlling the plate thickness and grating parameters, the diffracted beams are separated in the spatial dimension as they propagate through the plate, allowing the light flux diameter to expand while maintaining controllable beam intervals at the output face.
Solution Approach 2:
The patent optimizes specific parameters including the light guiding plate thickness (0.2-0.8 mm), grating period, and diffraction angle to achieve the desired balance between light flux diameter expansion and beam interval control. By carefully selecting these parameters, the system expands the light diameter to match the pupil while keeping beam intervals within acceptable ranges.
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 configuration effectively increases a single laser beam input to multiple beams while maintaining the input angle, allowing at least one light beam to be input to the observer's pupils even when their eyes are moving, thereby ensuring stable image recognition.
Implementation Method 1
a diffraction grating on an input side which is provided on the light input face
Implementation Method 2
a grating period of the diffraction grating on the input side is a period in which a small diffraction angle in diffraction angles of +1-st order diffracted light and −1-st order diffracted light, which are diffracted in the diffraction grating on the input side, in the light guiding plate becomes larger than a critical angle of the light guiding plate
Implementation Method 3
a diffraction grating on an output side which is provided on the light output face
Data Source
AI summary
A light flux diameter expanding element includes a light guiding plate with a light input face and a light output face, and with a thickness of 0.2 mm to 0.8 mm; a diffraction grating on the input side; and a diffraction grating on the output side, and is provided so as to have the same grating period as that of the diffraction grating on the input side, in which a forming region of the diffraction grating on the input side is smaller than that of the output side, and a grating period of the diffraction grating on the input side is a period in which a small diffraction angle in diffraction angles of +1-st order diffracted light and −1-st order diffracted light, which are diffracted in the diffraction grating on the input side, in the light guiding plate becomes larger than a critical angle of the light guiding plate.