Merging Epson's Core Technologies
Moverio BT-200 Smart Glasses

Posted on November 2014

See-Through Lenses and Binocular Vision Set these Smart Glasses Apart

Moverio BT-200 Smart Glasses

Released to market in June 2014, Epson's Moverio BT-200 smart glasses feature binocular, see-through lenses. The BT-200 is much smaller and lighter than the BT-100, the first-generation Moverio. In fact, at just 88 grams, the BT-200's is about one-third the weight of its predecessor. Yet the BT-200 still manages to pack a camera, gyroscopic sensor, accelerometer, compass, GPS functions, and more onboard. Offering hands-free convenience and a platform for augmented reality applications, wherein computer-generated text and graphics are superimposed on a person's real-world view, the BT-200 has the potential to change the way we work and play. The BT-200 runs on the Android™ 4.0 operating system, and Epson provides developer support so that external software developers can participate in application development.

Engineer Interview

"A communications tool for sharing"

Moverio developer, HMD Operations Division

*The division name is as of November 2014.

Watch the interview

Merging Epson's Core Technologies

The Moverio BT-200 sports a sleek, light design and true binocular vision on bright see-through displays thanks to a combination of some of Epson's unique core technologies. These core technologies include optical design and production technologies, a miniaturized projection system based on the Ultimicron HTPS panels (high-temperature polysilicon TFT LCD panels) used in Epson's market-leading 3LCD projectors*, and a revolutionary new light guide with multiple free-form optical surfaces.

* Epson has been the global market share leader in the 500-lumen and higher projector category for 15 consecutive years, from 2001 through 2015, per Futuresource Consulting Limited.

Moverio optics

Light from an LED backlight passes through and is modulated by the Ultimicron HTPS panels to form full-color images, and the angle of view (size) of the images is adjusted by the projection lens. The image light enters a light guide, where it is repeatedly transmitted and reflected before being delivered to the eye as a final image by a half mirror in front of the eye. The wearer can also see the outside world beyond the lens screens, so text and images can be superimposed in his or her field of vision for augmented reality applications.

Optics in the BT-100, the first-generation model in the Moverio series

To obtain the desired angle of view with the BT-100, we used larger HTPS panels and a larger-diameter projection lens. In addition, all of the surfaces that reflected the image light inside the light guide were planar, which made the optical path longer and the optics larger.

Comparison between the optical components used in the BT-200 and BT-100

  BT-100 BT-200
Light
guide

61g

33g

Projection
lens

15g

4g

HTPS
panels

0.52 inch

0.42 inch

Free-Form Light Guide Technology in the BT-200

For the BT-200, we solved the issues with the optics used in the BT-100 by using multiple free-form optical surfaces. We were able to reduce the size of the projection lens and shorten the optical path by concentrating the light in the optical path on free-form surfaces and using mainly the center of the lens. This made an optical layout that fits the lines of the face possible and vastly improved the aesthetics and comfort of the product.
Free-form surfaces are used in a total of four locations, one in the projection lens and three in the light guide and, while they are extremely effective in improving performance, a huge number of parameters need to be controlled. Therefore, we created an original algorithm that is compatible with commercially available optical design software and simultaneously developed free-form surface design technology that has extremely wide applications.

Establishing light guide molding technology

Even after clearing the light guide design hurdles on paper, we still faced a huge challenge in figuring out how to produce them. Since they repeatedly transmit and reflect light internally, light guides require extraordinary surface precision and planarity. The light guides in the BT-200 are about 10 mm thick, and conventional injection molds are not accurate enough to produce a light guide with the required optical performance. Therefore, we put together a trans-organizational project to squeeze the most out of the precision machining and processing technologies that we use for Epson's printers and projectors. From material selection to mold fabrication and manufacturing to actual molding, the know-how and technology we had in-house crystallized in the form of a low-cost molded light guide that satisfied the accuracy requirements.

Free-form optical surfaces help solve issues

Free-form optical surfaces played a big role in the development of a small, lightweight overall optical system, but they also helped to solve other issues. They enabled us to use the smaller 0.42-inch Ultimicron HTPS panels instead of the 0.52-inch panels while still obtaining a field of view (FOV) equivalent to that of the BT-100. In addition, the smaller panel has the same resolution as its larger cousin but a lower light transmittance ratio because it has a narrower pixel pitch. A higher-efficiency backlight was thus used to offset the loss of brightness.

Hurdles to achieving slimmer light guides

A light guide that is too thin will not provide the necessary FOV (image size). On the other hand, a thick light guide can increase the FOV (enlarge the size of the image), but this, of course, increases their size and weight.

The half mirror in a light guide is a free-form surface. Its job is to "extract" for the user's eye the image light that is propagated through the light guide. The extracted light is tilted at the prescribed angle for the eye while fulfilling all the reflection conditions. The size of the extracted image is fixed once the maximum FOV is decided.

The thickness of the current light guide was determined by using prototypes to verify the proper balance between weight and the FOV needed for binocular see-through lenses.

Achieving Binocular Vision

What distinguishes Epson's Moverio smart glasses is that both lenses are see-through displays. For binocularity to be satisfactory there can be no differences in focal point, brightness, or color between the two eyes. To achieve this, we apply technology developed for Epson projectors. One technology allows us to adjust the right and left HTPS panels on a pixel level (10 microns or less) and then fix their positions, while we use a circuit system to measure and correct imbalances in image quality between the left and right sides.

Ultimicron Panels for Vastly Superior Image Quality

Epson's Ultimicron HTPS liquid crystal panels are widely used by digital camera manufacturers as electronic viewfinders. The technology for these industry-recognized, ultra-compact, high-resolution HTPS panels is also applied in smart glasses.