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[SIGGRAPH'22 Review]Holographic Glasses for Virtual Reality

2023/05/03に公開・約6,900字

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*This article was written without using translation apps.

Holographic Glasses for Virtual Reality

SIGGRAPH-2022 conference paper

Author

JONGHYUN KIM, NVIDIA, USA and Stanford University, USA
MANU GOPAKUMAR, SUYEON CHOI, AND YIFAN PENG, Stanford University, USA
WARD LOPES, NVIDIA, USA
GORDON WETZSTEIN, Stanford University, USA

Abstract

In this paper, the authors are presented 3-D based Holographic glasses for virtual reality.
This device features a 2.5mm thin eye-point distance, making it confortable for users to wear.
Previous VR-field research have a bulky glasses due to their thick lenses uses for based on magnifying principle.Furthermore, past researches related to VR-field near-eyepoint preriminary focused on 2 dimension virtual glasses.
In this study, the researchers utilized components such as QWP,GP lens, and SLM(LC lens) to create the hardware structure.
they also optimized the light waveguide with a HOGD-CITL algorithm as part of software architecture.

Important point of this paper

FOV

In this paper, the FOV of Holographic Glasses is 22.8 degrees
　　 This value is smaller than that of commercial FOVs
The FOV depends on the size of the SLM lens and the focal length of the GP lens, but this can be improved by adopting different components
　　 For example, by using two independent GP lenses, the focal length can be halved while maintaining the total thickness (Moon, 2020)
Also, at the prototype level, a FOV of 120 degrees can be achieved with a 2-inch SLM and a 15mm focal length GP lens

Eye box

The size of the eye box is mainly determined by the SLM lens
　 ->From Fig.4(b) to achieve an eye box of 4 to 8mm, a pixel pitch of 1-2um is required considering each $Fc$ (GAEA SLM -> 3.74um)
This requires future technological innovations
Holographic glasses adopt a static eye box, not a dynamic one
　　 ->This allows the moving part to be smaller compared to conventional mechanisms, as the position of the pupil in the viewpoint is accurately centered

Image Quality

Incoherent AR display is used, which allows for the removal of unnecessary interference phases
Further improvement is possible by performing phase mapping using the CITL model while simultaneously processing the excerpt based on illumination intensity
This is useful for both 2D and 3D images (since the Waveguide laws are the same)

Pupil Diameter measurement and control

The pupil-HOGD algorithm presented in this paper requires accurate measurement of the user's pupil size to show the highest resolution images
Usually, users perform calibration in advance, and the pupil size is corrected at this time, but the optimal pupil size varies depending on the scene
　 (For example, if the scene becomes bright after one frame of a dark scene, the calibration cannot keep up)
In simulations, when the pupil correction error is kept within 0.5mm, the Pupil-HOGD algorithm displays higher quality images than the HOGD algorithm

System Integration

In this paper, only monocular results are presented, and real-time phase calculations are not performed
Using a 3-color coherent light source has a high possibility of reducing the size of future VR glasses, but this is beyond the scope of this paper

Below is an excerpt from the paper

Pancake lens [Bang et al,2021; Maimone and Wang;Narasimhan2018]
META CAMBRIA adopts this structures , enabling a 40% size reduction in VR glasses.
Optical design for thin displays form factors
Slim near-eye display using pinhole aperture array[Aksit et al.2015; Lanman and Luebke 2013]

In general, human eye can't focus at a distance as small as 45-50mm. This pinhole can help in this case, as it will bound the cone of rays.

These resaeches have the following problems

They restrict the display to presenting 2D images to each eye
These devices have the potential to cause visual discomfor for users
They offer only limited resolution

Solutions

Holographic Glasses comprise the following components.

SLM Lens ()
To create a small image behind the device.
GP Lens (Geometric Phase Lens)
It functions as a positive lens for a specific input beam polarization, making it thin and lightweight.
QWP (Quarter Wave Plate) Lens
To transform the linearly polarized input light to right-handed circularly polarized (RCP) light required by the GP lens.

Holographic Glasses

The eye box $We$ depends on the diffraction angle $θs$ , $de$ , $θs$ ,and $We$ can be calculated as follows:

The waveguide scrambles the polarization, which can be corrected by the linear polarizer in front of the SLM.

Distinct features

Holographic Glasses have two distinct features compared to conventional VR displays.
1. High Diffraction Orders (HDOs)
2. Pupil replicating waveguide

High-Diffraction Orders, HDOs

This figure indicate that Holographic glasses and human pupil.

$Wh$ :Pupil diameter , $Wp$ :HDOs interval

The periodic structure of SLM pixels creates HDOs
If $Wp>Wh$ is given, HDOs should be considered during the phase caluclation process as high-order gradient descent,HOGD.
When the HDOs are overlapping, then one can use the pupil-HOGD algorithm to create an optimized image.

Pupil Replicating Waveguide

The direction of the entire SLM illumination can be controlled by the input beam direction
With gaze tracker, the system could follow the gaze and move around the center lobe by simply changing the direction of the input beam

Design Trade Off

Left : Eye box size vs SLM pixel pitch
Right : Eye relif point vs converging angle

When SLM pixel pitch gets larger, eye box is smaller
eye box $We$ gets larger when the SLM pitch $Px$ gets smaller
-> Both the FOV and eye box are limited by the characteristic of the SLM lens
To achieve a wearable form, $de$ must be less than 20mm (right)
If the $Wh$ within the pupil diameter range, the HDOs could be perceived by the user based on the pupil diameter.
These conditions can be relaxed with an SLM with a smaller pixel pitch

Discussion

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