How the VRD Works

Also see the VRD animations by Quin Smithwick

Using the VRD technology it is possible to build a display with the following characteristics:

  • Very small and lightweight, glasses mountable
  • Large field of view, greater than 120 degrees
  • High resolution, approaching that of human vision
  • Full color with better color resolution than standard displays
  • Brightness sufficient for outdoor use
  • Very low power consumption
  • True stereo display with depth modulation
  • Capable of fully inclusive or see through display modes

In a conventional display a real image is produced. The real image is either viewed directly or projected through an optical system and the resulting virtual image is viewed. With the VRD no real image is ever produced. Instead, an image is formed directly on the retina of the user's eye. A block diagram of the VRD is shown below. To create an image with the VRD a photon source (or three sources in the case of a color display) is used to generate a coherent beam of light. The use of a coherent source (such as a laser diode) allows the system to draw a diffraction limited spot on the retina. The light beam is intensity modulated to match the intensity of the image being rendered. The modulation can be accomplished after the beam is generated or, if the source has enough modulation bandwidth as is the case with a laser diode, the source can be modulated directly.

The resulting modulated beam is then scanned to place each image point, or pixel, at the proper position on the retina. A variety of scan patterns are possible. The scanner could be used in a calligraphic mode, in which the lines that form the image are drawn directly, or in a raster mode, much like standard computer monitors or television. Our development focuses on the raster method of scanning an image and allows the VRD to be driven by standard video sources. To draw the raster, a horizontal scanner moves the beam to draw a row of pixels. The vertical scanner then moves the beam to the next line where another row of pixels is drawn.

In the original prototype the faster horizontal scanning is accomplished with an acousto-optical modulator and the vertical scanning with a galvanometer to produce a 1280 pixel by 1024 line raster that is updated at 72 Hertz. The use of the acousto-optical modulator does, however, come with a number of drawbacks. First, it requires optics to shape the input beam for deflection and then additional optics to reform the output beam to the desired shape. Second, it requires complex drive electronics that operate at a very high frequency. Next, it has a very limited scan angle (4 degrees in our current prototype) such that additional optics are needed to increase the angle to the desired field-of-view. Due to the optical invariant, this optical increase in angle comes with the penalty of decreased beam diameter which leads to a small exit pupil. The small exit pupil necessitates precise alignment with the eye for an image to be visible. Finally, the acousto-optical modulator is expensive and will not, in the foreseeable future, allow us to reach our cost goals for a complete VRD system.

To overcome the limitations of the acousto-optical modulator HITL engineers have developed a proprietary mechanical resonant scanner. This scanner provides both horizontal and vertical scanning, with large scan angles, in a miniaturized package. The estimated recurring cost of this scanner will allow the VRD system to be priced competitively with other displays. A prototype VRD using the new mechanical resonant scanner has been developed and is currently being refined.

After scanning, the optical beam must be properly projected into the eye. The goal is for the exit pupil of the VRD to be coplanar with the entrance pupil of the eye. The lens of the eye will then focus this collimated light beam on the retina to form a spot. The position on the retina where the eye focuses a spot is determined by the angle at which light enters the eye. This angle is determined by the scanners and is constantly varying in a raster pattern. The intensity of the focused spot is determined by the intensity modulation of the light beam. The intensity modulated moving spot focused on the retina forms an image.

The final portion of the system is the drive electronics which must synchronize the scanners with the intensity modulators to form a stable image.

For 3-D viewing an image will be projected into both of the user's eyes. Each image will be created from a slightly different view point to create a stereo pair. With the VRD, it is also possible to vary the focus of each pixel in the image such that a true 3-D image is created. Thus, the VRD has the ability to generate an inclusive, high resolution 3-D visual environment in a device the size of conventional eyeglasses.

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