Head-mountable retinal scanning display relies on MEMS
April 14, 2008
If Brother Industries’ new set of eyeglasses seems a little bizarre at first, it’s doubly so in practice. The main unit installed on the eyeglass frame measures about 20-cm3 and weighs about 25 g. Compared with the prototype unveiled for the first time at the Aichi Expo (Exposition of Global Harmony) in 2005, the size and the weight of the latest model are significantly reduced, in particular, to less than a thousandth of its original bulk.
The retinal scanning display irradiates low-intensity light on the retina and scans it with the light at a high speed. The user perceives the afterimage of the light scanned on the retina as an image. In other words, the retinal scanning display is like a projector that uses the retina as the "screen."
In contrast to the pervasive head-mounted display (HMD) using an LCD panel and so on, the retinal scanning display provides an unobstructed vision. The image can be seen in the actual scenery in an overlapping manner.
The retinal scanning display is mainly composed of three modules: light source, light scanning, and eyepiece modules. The main unit includes the light scanning and eyepiece modules. The company achieved a tremendous reduction in size and weight by re-developing these two modules over the past two years. The combination of aspherical lenses, for example, resulted in a smaller eyepiece module.
Plus, the development of a new mirror module based on microelectromechanical system (MEMS) technology resulted in a smaller light scanning module. The MEMS mirror module measures 12 x 8 x 2 mm. The mirror with a diameter of about 1 mm used in the MEMS module rotates at a high speed while changing its angle to scan the light from the light source.
The optical rotation angle of the mirror is approximately 20°. The mirror module is driven at a drive frequency of about 30 kHz by a piezoelectric element.
In the existing prototype, a modified version of the polygon mirror designed for laser printers is used as a light scanning module. The polygon mirror measures about 80 x 80 x 50 mm.
The light source module is separated from the main unit equipped with the light scanning and eyepiece modules. In the latest prototype, the light source module is connected to the main unit via a fiber optic cable.
The light source module uses laser light beams in three colors, red (R), green (G) and blue (B). A semiconductor laser is used to generate the R and B light beams, while solid laser light is used for the G light. Because the G light is generated from the infrared light by the second-harmonic generation (SHG), the light source box is fairly large. The developers plan to reduce the size of the light source so that the light source module can be incorporated in the main unit, which is equipped with the light scanning and eyepiece modules.
"The challenge is to reduce the size of the G laser light source," the company says.
Brother intends to commercialize the retinal scanning display in fiscal 2010 at the earliest. The power source and the interface circuitry are provided in a compact unit separately from the main unit. The company expects to employ the setup where the video signal is transmitted to the unit via radio communication, memory card, etc.
By leveraging the ability to provide an unobstructed view, the retinal scanning display is expected to be used as an operation monitor, for example. Specifically, it may be used when an operator performs maintenance work on the server while checking a circuit diagram or when a doctor performs an operation while looking at the necessary information.
The company is also expecting other applications for general users, such as photo viewing.
