Moverio Optical Technology

Moverio Optical Technology

BT40

Epson's smart glasses in the Moverio series project great-looking images before the wearer's eyes by using an Epson designed and engineered optical engine to deliver content on dual Si-OLED microdisplays. These binocular, see-through smart glasses are compact and lightweight yet deliver perceived big-screen images that are crisp and clear.

Ordinarily, image quality and picture size must be sacrificed for the sake of a design that is compact, lightweight and comfortable, so Epson had to develop numerous technologies to simultaneously achieve both. Read on to learn about the evolving Moverio optical technology and the advances that are bringing amazing perceived big-screen images to lightweight, comfortable glasses with a wide field of view.

The optical engines behind compact Moverio smart glasses

Moverio optical engines consist of three types of components: microdisplays, projection lenses, and light guides. Projection lenses condense the image light from the microdisplays, and the light guides direct it to a place in front of the viewer's eyes. Half mirrors inside the light guides superimpose external light and the image light to enable a see-through display. Epson achieves high display quality by optimizing the design of each component with its unique optical technology.

The BT-100, the first-generation product in the Moverio series, uses a direct-view optical system. This system, which works similarly to a magnifying glass, delivers images of high quality with a relatively simple configuration. On the other hand, the projection lens, which refracts light according to the angle of view incident on the eye, needs to be enlarged as the distance from the eye increases and the angle of view increases. This makes it difficult to reduce the size and weight of the optical unit.

Therefore, in order to make the projection lens smaller in the BT-200 and subsequent generations, we adopted a relay optical system that first combines the light from the microdisplays (intermediate images) and then spreads it near the eyes. This makes it possible to reduce the size of the lens compared to the direct-view optical system but requires a new lens function to widen the intermediate image. Therefore, Epson reduced the weight of the optics by capitalizing on its optical simulation technology and arranging multiple free-form optical surfaces along the light path in the light guides so that they also function as lenses.

Moverio optical engines

Manufacturing technology that enables high image quality

Relay optics became the key technology for scaling down Moverio smart glasses. Epson had to clear numerous hurdles to solve technical issues and establish all-new high-precision optical engine manufacturing technology. The manufacturing technology required for Moverio products was established by drawing on Epson's optical design and analysis technology and on manufacturing expertise developed over many years of projector development.

The first challenge involved the ultra-high precision machining of the free-form optical surfaces inside the light guides. Since the image light from the microdisplays is reflected many times in the light guides, the image quality cannot be maintained without extreme surface accuracy. To achieve the necessary design configuration based on optical simulations, Epson had to fabricate the molds used in the manufacture of light guides with unprecedented precision and accuracy. Epson is able to process the free-form optical surfaces in the light guides by taking exacting three-dimensional measurements of a prototype light guide, detecting minute errors in shape all the way to the edges, and then applying corrections to the molds.

The second challenge was to find a way to accurately and precisely bond the half-mirror surfaces. The front surface of the light guide takes in external light and normally should be flat. However, the half mirrors have to be curved in order to extract the image light. So, to achieve distortion-free see-through performance, The half-mirror part of the light guide and another part are bonded together, and the light guides, which superimpose the external light and image light, are made planar.

Relay optical system

Adhesive is used in bonding, but it took a period of trial and error before Epson was able to establish a bubble-free process. If it is tilted or not level when bonded, the light guide will produce a curved image of the outside world, as glasses with corrective lenses can do. If that happens, see-through images will be blurred or distorted. We have established strict specifications for tilt and level because of this and use a process that ensures that the light guides and half mirrors are precisely and accurately bonded together, ensuring high quality against external light.

The third challenge was to calibrate (align) the left and right optics to achieve natural binocular vision with see-through lenses. If there is a difference in the position, brightness, and color of the displayed images viewed by the left and right eyes, the user may feel uncomfortable, may experience intense fatigue during use, or may be unable to see the images as one. At Epson, we ensure high image quality in the manufacturing process by adjusting parameters such as focus position, convergence distance (the distance to an object), display brightness, and color table for each unit.

Miniaturization achieved
by co-designing Si-OLED microdisplay alignment control
and the optical engine

In general, the light emitted from the displays spreads conically in a direction that is perpendicular to the panels. To display bright images, we had to capture more light in the lenses, so we had to increase the lens diameter by an amount equal to the spread of the display light.

The Si-OLED microdisplays that Epson developed for the Moverio series are customized so that the optical axis of the light emitted from pixels is tilted toward the center of the panel, with the degree of tilt increasing gradually toward the periphery.

A color filter, a transparent sheet that colors the light and that is normally placed directly above the pixels, is shifted by microns at the outer periphery of the display area so that the optical axes in the periphery of the display area are tilted toward the center to condense more display light on the lens surface. In addition, optimizing the design of the optical engine so that the gathered light maximizes performance enabled Epson to further reduce the size of the projection lens.

The bright, clear images of the Moverio series are produced in a compact, lightweight optical engine that was achieved by co-designing the Si-OLED microdisplay's light alignment control and the optical engine.

Moverio optical technology, which required Epson to overcome many technical obstacles, will continue to evolve in order to provide an inspiring visual experience in every aspect of business and life.

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