Sony Corp unveiled an eyeglass-shaped see-through display that shows full-color video images at a transmissivity of 85% and a luminance of 2500cd/m2 at SID 2008.
With the display, which Sony developed as an MP4 player, users can watch full-color video images overlying the actual view. In the author's interview, engineers formed a line and waited to try out the new display.
Eyeglass-shaped LCD offers bright, see-through display
Sony focused on the following four points: (1) A transmissivity of higher than 80%, which is high enough for use in dark places, (2) a color uniformity of Δu'v' = 0.009 or lower, which is equivalent to that of TVs, and a luminance of higher than 2000cd/m2, which allows the use of see-through mode outdoors, (3) attaching the optical engine on the lateral side (temple) so that the thickness of the front portion (lens) can be reduced to less than 3mm like normal eyeglasses and (4) a weight of less than 80g to make the display comfortable to be worn for at least two hours.
To achieve these goals, Sony made efforts to simplify the device's structure. And it adopted the structure and display method that use a holographic waveguide and an optical engine that is composed of an LED light source, a transmissive LCD panel, etc.
The light modulated by the transmissive LCD panel reaches the eyes via the holographic waveguide. The optical engine can be attached on the lateral side (temple) of the eyeglasses.
Proprietary holographic waveguide
The holographic waveguide is the key component that realized the structure and display method. It is made by attaching holographic films to two places on the surface (outer side) of a glass light guide plate that is 1.4mm thick and 50mm long.
The light coming to the waveguide is reflected (diffracted) by the first holographic film. Then, the total reflection is repeated in the glass plate before the light is finally reflected (diffracted) vertical to the eyes by the second holographic film.
Holographic films reflect (diffract) a certain color of light (wavelength) into a given direction and transmit light of other colors (wavelengths). The color (wavelength) of light to be reflected (diffracted) and the direction of the reflection can be controlled by the holographic pattern.
This time, Sony developed three holographic films that correspond to the RGB primary colors. It isn't necessary for the holographic films to be arranged on an angle to the waveguide. It is because they can reflect the light in any direction even if they are arranged parallel to the waveguide (or attached to the waveguide surface). This enables to reduce the thickness of the entire unit.
Crosstalk countermeasures improved the color uniformity
When the three holographic films are simply stacked on one another, the color uniformity of the image is degraded due to crosstalk. The crosstalk is caused because the directions of reflection vary depending on the color (wavelength).
Thus, Sony made two types of holographic waveguides, one for green (G) and the other for red (R) and blue (B), and stacked the one on the other. Despite the use of the two waveguides, it succeeded in reducing the total thickness to 3mm (1.4mm each, with a 0.2mm gap between them). The display features a gamut of 120% NTSC and a color uniformity of Δu'v' = 0.008.
The prototyped display supports the QVGA resolution, weighs 120g and has a contrast ratio of 50:1. With the use of an anti-reflection film, the display achieved a transmissivity of 85%.
The schedule for commercialization has "yet to be decided," Sony said. As for the technical aspects, however, the company "hopes to promote the development in view of commercialization in about 2010," said Hiroshi Mukawa, senior manager of the Display Division at Sony Information Technologies Laboratories.
Correction Notice: Because of a reporting error, we incorrectly used the name "Hiroshi Takekawa" in the last paragraph of the article, which should in fact be Hiroshi Mukawa.