Lightguide with Stair Micromirror Structure for Augmented Reality Glasses
Jaeyeol Ryu*
,
**, Nikolay Muravev*, Dmitry Piskunov*, Mikhail Popov*, Natalia Anikanova*,
and Myongjo Choi***
*Samsung R&D Institute Rus, Samsung Electronics, Moscow, Russian Federation
**Moscow Institute of Physics and Technology, Dolgoprudny, Russian Federation
***DMC R&D Center, Samsung Electronics, Republic of Korea
Abstract
The paper presents a lightguide system with stair micromirror
structure for see-though glasses. We use stair micromirror
structure to out-couple virtual image out of the lightguide. It
allows extending eye motion box, where whole virtual image can
be seen by a user. The design principle is described, as well as
main achieved parameters.
Author Keywords
stair micromirror structure; lightguide; augmented reality;
wearable device; see-through
1. Relevance
Currently, it is possible to use augmented reality glasses in
different commercial applications. For wearable device it is
important to decrease size, weight and cost of the product, and to
increase field of view (FoV) and image resolution. However,
ensure of all these parameters leads to contradictory requirements
for optical system due to the fact that improving one characteristic
leads to a deterioration of another.
One of the most common approaches for AR devices is a device
based on lightguide.
Lightguides with single sloped surface, parallel sloped surface
array, diffractive optical element (DOE) or a holographic optical
element (HOE) transmit the virtual image from a projection
system into the human eye, and they have acceptable transparency
for user [1-3]. However, these types of lightguide have the
following significant problems:
EMB of the lightguide with single sloped surface is
proportional to the thickness of the lightguide
The use of the parallel sloped surface array leads to a
low manufacturability and high cost
DOE and HOE cause large chromatic aberration
2. Originality and Research
We propose design of the lightguide with stair micromirror
structure (SMMS) for out-coupling light rays to a user’s eye,
which makes it possible to significantly extend EMB without
increasing the thickness of the lightguide.
SMMS arbitrary consists of the following two surfaces (see.
Fig.3):
Sloped surfaces (marked b), which are disposed at some
angle to the main lightguide surface
Horizontal surfaces (marked a), which are parallel to the
main lightguide surface
Sloped surfaces reflect rays, propagating in the lightguide, toward
the viewer. And horizontal surfaces allow rays to spread further
along the lightguide. Such a structure allows extending EMB
without increasing the lightguide thickness. In contrast to the
above analogues listed above, the suggested design can be applied
for mass-production.
The simplest way to in-couple and out-couple the light from the
lightguide is to use single sloped surface (see. Fig.1). Light in-
couples into the lightguide at an angle
0
on its substrate, falls and
reflects at an angle
1
from the in-coupling element (single sloped
surface) disposed at an angle to substrate of the lightguide. The
reflected rays fall on the substrate of the lightguide at an angle
2
which is larger than angle of total internal reflection (TIR) for the
lightguide material.
Hence, rays (a) propagate along the lightguide, (b) reach an out-
coupling sloped surface disposed at an angle to the substrate of
the lightguide, and (c) reflect on the surface of the out-coupling
element at an angle
3
. After the reflection from the out-coupling
element the light incident on the substrate of the lightguide at an
angle
4
, which is smaller than angle of TIR, therefore it exits
from the lightguide in the direction of the eye.
We represent by the following Equations (1) – (6):
In-coupling part of lightguide:
0 1
(1)
1 2
(2)
TIR
2
(3)
Out-coupling part of lightguide:
2 3
2
(4)
3 4
(5)
) tan(
T
d
coupling out
(6)
Figure 1. In-coupling and out-coupling of ray in the
lightguide with single sloped surfaces
As can be seen from Equation (6), the length of the out-coupling
element is proportional to the thickness of the lightguide. Hence
increasing of the FoV or EMB or eye relief (distance from the
lightguide to the EMB) d
eye
or all the parameters at ones, leads to
increasing of the out-coupling element length d
out-coupling
(see.
Fig.2).
2
tan 2
FoV
d EMB d
eye coupling out
(7)
51-5 / J. Ryu
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