Neeraj Shukla(B.C.A. III Year, S.J.I.M.S.)
Souvik Ghosh(B.C.A. III Year, S.J.I.M.S.)
ABSTRACT
The Virtual Retinal Display™ (VRD™) technology is a new display technology being developed at Microvision Inc. The displayed image is scanned onto the viewer’s retina using low-power red, green, and blue light sources. Microvision’s proprietary miniaturized scanner designs make VRD system very well suited for head-mounted displays. In this paper we discuss some of the advantages of the VRD technology, various ocular designs for HMD and other applications, and details of constructing a system MTF budget for laser scanning systems that includes electronics, modulators, scanners, and optics.
Keywords: laser scanning, display resolution, Virtual Retinal Display, VRD, head mounted displays, modulation transfer function
1. Introduction
Virtual Retinal Display™ (VRD™) technology is a novel scanning-based display technology where the displayed image is scanned in a raster format onto the viewer’s retina in a raster pattern using low-power red, green, and blue light sources, such as lasers, laser diodes, or LEDs. The VRD systems use two uniaxial scanners or one biaxial scanner to create a raster pattern. The horizontal scanner is a flexure-based mechanical resonant scanner that operates at several KHz and the vertical scanner is also a mechanical scanner operated at a non-resonant mode at the frame rate of the display, which is typically 60Hz
.
The resolution and the modulation transfer function (MTF) are key performance indicators in display systems. Resolution of mechanical scanners is proportional to scan mirror size and scan angle, and inversely proportional to wavelength. High-resolution systems demand horizontal scanners that provide a large mirror-size scan-angle product and high frequency. System resolution issues and scanner design issues are discussed in more detail elsewhere.
This paper is divided into three parts. In the first part, we briefly discuss the VRD system operation and its advantages compared to other display technologies. The second part is dedicated to VRD ocular designs for HMD applications and the MTF performance of different ocular designs. In the last part, we give formulas that are used compute the MTF contributions of electronics, modulators, pixel duty-cycle, and scanner optics, then we illustrate how to construct a system MTF budget for the horizontal and the vertical axes in laser scanning display systems.
VRD Emulator Test System
This project represents a continuing are of interest at the HIT Lab, though it is not currently funded. Virtual Retinal Display (VRD) scans intensity modulated laser light pixels directly onto retina. Each pixel is modulated in short pulses of 30ns to 40ns. The input light is a combined beam from three different wavelengths of laser light, which produces a color gamut exceeding that of a conventional CRT. The area covered by the laser spot on the back of the retina for the duration of a single pixel is called a 'retinel'.
Unlike CRT monitors, the VRD has no phosphor persistence but depends on the light-gathering properties of the photoreceptors and the temporal integration properties of the visual system. Therefore, the interactions between the VRD delivered light and the psychophysical image quality created by the brain are of extreme importance for future enhancements.
The enhanced image quality of the VRD is believed to be the result of several system properties:
1. Small exit pupil which gives large depth of focus.
2. Beam size which minimizes optical aberrations at cornea .
3. Laser light penetrates mild to moderate media opacities with minimal scattering .
4. High luminance and high color contrast .
5. Collimation and coherence of the light.
6. Decreased flicker sensitivity.
7. No perceptual laser speckle.
Unfortunately, the current VRD design does not provide a way to test many of the these system properties. Therefore an Emulator of the VRD has been developed.
The emulator system requires a scanned light sources of the emulator to be modulated by passing the light through a 35mm slide. This provides us with the capability to:
· Vary spot size at the retina.
· Vary the spacing between between horizontal lines
· Vary the spacing between pixels
· Vary scan rates of the system
· Investigate coherent versus incoherent light
· Control light intensity
· Use a common source of modulation
· Match laser lines of VRD (645, 514, 472 nm)and other color combinations
· Characterize optical parameters
· Automate data collection
Our predicted results include:
· Contrast sensitivity at lower spatial frequencies will be higher.
· Possible shift at higher spatial frequencies but not as likely.
· Subject's critical flicker thresholds should be consistently higher with the VRD relative to the CRT.
· Retinels size should be able to approach foveal cone spacing.
· As retinels approach cone spacing line separation will become less tolerant.
· Light coherence will have minimal effect on the enhanced performance. The point and angle of entrance beams at the corneal surface will be identified as major contributors to the enhanced performance.
THE VRD GROUP AND RESEARCH
The Virtual Retinal Display (VRD) team has been focused on developing improvements to the current prototype systems and on creating the parts needed for future prototypes. The VRD, based on the concept of scanning an image directly on the retina of the vi ewer's eye, was invented at the HIT Lab in 1991. The development program began in November 1993 with the goal of producing a full color, wide field-of-view, high resolution, high brightness, and low cost virtual display.
Two prototype systems are currently being demonstrated. The first is a bench-mounted unit that displays a full color, VGA (640 by 480) resolution image updated at 60 Hertz. It operates in either an inclusive or see-through mode. The second is a portable u nit, displaying a monochrome, VGA resolution image. The portable system is housed in a briefcase allowing for system demonstrations at remote locations (see photo above).
Techniques that expand the system's exit pupil, making it easier to align one's eye to the display, have been developed and demonstrated for a monochrome display. Work is continuing to optimize the design and to develop components that will expand the pupil of the color system.
The largest component in the portable system is the commercially purchased vertical (60 Hertz) scanner. A new vertical scanner is being designed that should significantly decrease devices size and cost. Once this design is complete a head-mounted demonstr ation prototype will be assembled.
Commercial applications of the VRD are being developed at Microvision Inc.
HOW THE VRD WORKS
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 acoustic-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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