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Virtual Retinal Display, Ask Latest information, Abstract, Report, Presentation (pdf ,doc,ppt),Virtual Retinal Display technology discussion,Virtual Retinal Display. Download your Reports for Virtual Retinal Display. The Virtual Retinal Display ( VRD) is a personal display device under development at the University of. A ppt on Virtual Retinal Display (VRD) technology explaining how The also ppt shows the difference between virtual retinal display and pixel based displays and other key features. Download Virtual Retinal Display ppt.
The VRD scans light directly onto the viewer's retina. The viewer perceives a wide field of view image. The development began in November The aim was to produce a full color, wide field-of-view, high resolution, high brightness, low cost virtual display. Microvision Inc. The VRD projects a modulated beam of light from an electronic source directly onto the retina of the eye producing a rasterized image.
State of the art scanners can scan over a lines per frame which are comparable to HDTV. Brightness, Perceived brightness is only limited by the power of the light source. SLD sources can provide very good brightness levels even for the see-through mode in daylight.
See through mode systems have it slightly over 40 degrees. These figures are far better than existing HMD systems. Stereoscopic display, Supports stereoscopic display as both eyes can be separately addressed.
Thus provides a good approximation to natural vision. The MRS is the heart of the system.
It is a lightweight device approximately 2 cm X 1 cm X 1cm in size and consists of a polished mirror on the amount. The mirror oscillates in response to pulsed magnetic fields produced by coils on the system mounting.
It oscillates at 15 KHz and rotates through an angle of 12 degrees.
The high frequency of scanning allows the fine resolution in the images produced. As the MRS mirror moves, the light is scanned in the horizontal direction. Because the mirror of the MRS oscillates sinusoidally, the scanning in the horizontal direction has been arranged for both the forward and reverse direction of the oscillation. The scanned light is then passed to a mirror galvanometer or second MRS which then scans the light in the vertical direction.
The current generations of personal displays do not perform well in high illumination environments. This can cause significant problems when the system is to be used by a soldier outdoors or by a doctor in a well lit operating room. Unfortunately, this does not work well when a see through mode is required. The VRD creates an image by scanning a light source directly on the retina. The perceived brightness is only limited by the power of the light source.
Through experimentation it has been determined that a bright image can be created with under one microwatt of laser light. Laser diodes in the several milliwatt range are common. As a result, systems created with laser diode sources will operate at low laser output levels or with significant beam attenuation. The exit pupil of the system can be made relatively small allowing most of the generated light to enter the eye. In addition, the scanning is done with a resonant device which is operating with a high figure of merit, or Q, and is also very efficient.
The result is a system that needs very little power to operate.
A True Stereoscopic Display The traditional head-mounted display used for creating three dimensional views projects different images into each of the viewer's eyes.
Each image is created from a slightly different view point creating a stereo pair. This method allows one important depth cue to be used, but also creates a conflict. The human uses many different cues to perceive depth. In addition to stereo vision, accommodation is an important element in judging depth. Accommodation refers to the distance at which the eye is focused to see a clear image. The virtual imaging optics used in current head-mounted displays place the image at a comfortable, and fixed, focal distance.
As the image originates from a flat screen, everything in the virtual image, in terms of accommodation, is located at the same focal distance. Therefore, while the stereo cues tell the viewer an object is positioned at one distance, the accommodation cue indicates it is positioned at a different distance.
Virtual Retinal Display Seminar Report Government Engineering College, Thrissur 17 With the VRD it is theoretically this is currently in the development stage possible to generate a more natural three dimensional image.
The VRD has an individual wavefront generated for each pixel. It is possible to vary the curvature of the wavefronts. Note that it is the wavefront curvature which determines the focus depth. This variation of the image focus distance on a pixel by pixel basis, combined with the projection of stereo images, allows for the creation of a more natural three-dimensional environment.
Inclusive and See Through Systems have been produced that operate in both an inclusive and a see through mode. The see through mode is generally a more difficult system to build as most displays are not bright enough to work in a see through mode when used in a medium to high illumination environment where the luminance can reach ten thousand candela per meter squared.
As discussed above, this is not a problem with the VRD. In the VRD a light source is modulated with image information, either by direct power "internal" modulation or by an external modulator. The light is passed through an x-y scanning system, currently the MRS and a galvanometer.
Light from the scanner pair enters an optical system, which in present implementations of the VRD forms an aerial image and then uses and eyepiece to magnify and relay this image to infinity. Virtual Retinal Display Seminar Report Government Engineering College, Thrissur 18 Components of the Virtual Retinal Display Video Electronics In its current form, the video electronics of the VRD controls the light intensity modulation, scanner deflection, and the synchronization between modulation and scanning.
The horizontal and vertical synchronization signals in the video signal are used to determine scanner synchronization. A user selectable delay of up to one full line is incorporated into the video electronics to allow for phase difference between the horizontal scanner position and the modulation timing.
Also, the respective drive levels for intensity modulation of each light source are output from the electronics. The drive electronics control the acousto-optic modulators that encode the image data into the pulse stream. The color combiner multiplexes the individually-modulated red, green, and blue beams to produce a serial stream of pixels, which is launched into a single mode optical fiber to propagate to the scanner assembly.
The drive electronics receive and process an incoming video signal, provide image compensation, and control image display. Light Sources and Modulators The light sources for the VRD generate the photons which eventually enter the eye and stimulate the photo receptors in the retina. The modulation of the light source determines the intensity of each picture element.
The size of the scanning spot and the rate at which it can be modulated determine the effective size of each picture element on the retina. As the light is scanned across the retina, the intensity is synchronized with the instantaneous position of the spot thereby producing a two dimensional pattern of modulated light that is perceived as a picture. Virtual Retinal Display Seminar Report Government Engineering College, Thrissur 19 According to conventional additive color theory, any color can be represented as a mixture of three appropriately chosen primaries.
The three ideal VRD light sources would be monochromatic for maximum possible color saturation.. Spatial coherence is also important - larger source spots will correspond to larger spots on the retina, decreasing resolution. The primary cause of the real if sometimes exaggerated hazards of laser light are the result of spatially coherent light focusing to a small area on the retina, causing highly localized heating and ablation of tissue.
In the VRD the spot is traveling in two directions and even when stationary is not at a power level that would cause damage. We are working with ophthalmologists and will publish a definitive article on this in the near future.
Incidentally, polychromatic sources can be shown to form spots comparable to monochromatic ones of the same spatial extent. Therefore spatial coherence is responsible for the small spot size which leads to both high resolution and given enough power retinal hazard.
To achieve the desired resolution, all current VRD prototypes have used lasers for their superior spatial coherence characteristics. In order to use a point source such as an LED, the image of the source should be smaller than the diffraction limit of the scanner.
Using the lens magnification, one can determine the maximum source size that can be used before degrading the diffraction limited spot size at the image plane. The angular divergence of the source is effectively limited by treating the scanner as a stop. Light which does not hit the mirror does not contribute to the image plane spot size. From this geometric argument we can derive an equivalent point source size between 4 and 5 microns for a VGA resolution image in our current system.
For a system where the scanner is illuminated with a collimated Gaussian beam, similar arguments can be made to determine the required divergence and beam waist from the equations for image plane spot size.
The light source module contains laser light sources, acousto-optic modulators to create the pulse stream, and a color combiner that multiplexes the pulse streams. To provide sufficient brightness, full-color displays suitable for outdoor, daylight applications incorporate red diode lasers nm , green solid-state lasers nm , and blue solid-state or argon gas lasers nm range.
Systems designed Virtual Retinal Display Seminar Report Government Engineering College, Thrissur 20 for indoor use can incorporate LEDs; red, blue, and green devices currently under development for such systems are being tested. Generally, the energy levels are on the order of nanowatts to milliwatts, depending on display requirements. The levels of light involved are well within laser safety standards for viewing, as confirmed by analysis.
Generally two types of intensity modulation of lasers are done in existing designs. They are Laser diode modulation and acousto-optical modulation. The laser diode modulation is generally used for red laser. The small rise time of the solid state diode laser device allows high bandwidth up to [MHz] analog modulation.
The video electronics regulate the voltage seen by the laser current driver and it controls the current passing through the laser which in turn controls the light output power from the laser. The laser diode is operated between amplitudes of 0. Acousto-optic A-O modulators intensity modulate the green and blue laser beams. Acousto-optic modulators create a sound wave grating in a crystal through which a light beam passes.
The sound wave creates alternate regions of compression and rarefaction inside the crystal. These alternating regions locally change the refractive index of the material. Areas of compression correspond to higher refractive indices and areas of rarefaction correspond to lower refractive indices. The alternating areas of refractive index act as a grating and diffract the light. As the sound wave traverses the light beam, the diffracted beam is intensity modulated according to the amplitude modulated envelope on the carrier signal.
Scanners The scanners of the VRD scan the raster pattern on the retina. The angular deviation of the horizontal scanner combined with the angular magnification of the imaging optics determines the horizontal field of view. The angular deviation of the vertical scanner combined with the angular magnification of the imaging optics determines the vertical field of view.
The scanning device consists of a mechanical resonant scanner and galvanometer mirror configuration. The horizontal scanner is the mechanical resonant scanner MRS ]. The MRS has a flux circuit induced by coils which are beneath a spring plate.
The flux circuit runs through the coils and the spring plate and alternately attracts opposite sides of the spring plate and thereby moves the scanner mirror through an angle over time. In a design developed at the HITL the vertical deflection mirror was chosen as the galvanometer mirror. The galvanometer deflection can be selected according to the aspect ratio of the display and a typical ratio of can be chosen.
The galvanometer frequency is controlled by the video electronics to match the video frame rate. The galvanometer and horizontal scanner are arranged in what is believed to be a novel configuration such that the horizontal scan is multiplied. The scanners are arranged, as shown in the following figures. Such that the beam entering the scanner assembly first strikes the horizontal scanner then strikes the vertical scanner.
The beam is reflected by the vertical scanner back to the horizontal scanner before exiting the scanner assembly. The beam therefore strikes the horizontal scanner twice before exiting the scanner Virtual Retinal Display Seminar Report Government Engineering College, Thrissur 22 configuration.
In such an arrangement, the first scan corresponding to the first bounce or reflection is doubled by the second scan corresponding to the second bounce or reflection. Virtual Retinal Display Seminar Report Government Engineering College, Thrissur 23 The result of arranging the scanners as in the above figures is a doubling of the horizontal optical scan angle. Other configurations have been applied to this approach to achieve a tripling in the horizontal direction and simultaneously a doubling in the vertical direction.
For more compact designs, techniques from micro electro-mechanical systems maybe utilized in the fabrication of scanners. The electrostatic actuation of a MEMS scanner had been developed. By etching thin layers from a sliver of silicon, the researchers were able to build a scanner that weighs a mere 5 grams and measures less than 1 square centimeter. The mirror, too, is much smaller at 1 millimeter across and is mounted on the end of a thin, flexible, bar which is anchored to the silicon.
The mirror is turned into one plate of a capacitor, with the other plate formed by a small area of silicon beneath it. Put a rapidly varying voltage across the two plates and then the mirror will be first repelled and then attracted. The mirror can move up or down more than 30, times each second. The electronics are fabricated using integrated circuit IC process sequences, while the micromechanical components are fabricated using compatible "micromachining" processes that selectively etch away parts of the Virtual Retinal Display Seminar Report Government Engineering College, Thrissur 24 silicon wafer or add new structural layers to form the mechanical and electromechanical devices.
The electromagnetic actuation of the scanners yields more life to the system and imparts more torque. Such designs have also been developed for retinal scanning displays. Pupil expander Nominally the entire image would be contained in an area of 2 mm2. The exitpupil expander is an optical device that increases the natural output angle of the image and enlarges it up to 18 mm on a side for ease of viewing.
The raster image created by the horizontal and vertical scanners passes through the pupil expander and on to the viewer optics. For applications in which the scanned-beam display is to be worn on the head or held closely to the eye, we need to deliver the light beam into what is basically a moving target: the human eye.
Constantly darting around in its socket, the eye has a range of motion that covers some 10 to 15 mm. One way to hit this target is to focus the scanned beam onto exit pupil expander.
When light from the expander is collected by a lens, and guided by a mirror and a see-through monocle to the eye, it covers the entire area over which the pupil may roam. For applications that require better image quality using less power, we can dispense with the exit pupil expander altogether either by using a larger scan mirror to make a larger exit pupil or by actively tracking the pupil to steer light into it.
Viewer optics The viewer optics relay the scanned raster image to the oculars worn by the user. The optical system varies according to the application. In the case of military applications such as helmet mounted or head mounted display optics, the system incorporates glass and or plastic components; for medical applications such as image-guided surgery, headmounted plastic optics are used.
The viewing optics, or the optics through which the user sees the intended image, are diagrammed in the following figure. The convergent tri-color beams emanating from the scanner pass partially through a beamsplitter. The mirror is actually a rectangular section of a spherical mirror with radius of curvature [mm]. The negative sign denotes concavity. It can be likened to looking through a pair of high- magnification binoculars that one must line his eyes precisely with the beam or the image disappears.
Since we rarely fix our eyes on a single point for more than few seconds, using VRD becomes difficult. So en eye-tracking system that follows the movements of the pupil by monitoring the reflections from the cornea had to be developed.
The tracker calculates where the eye is looking and moves the laser around to compensate. But this system is complex and expensive. A better solution may lie with a special kind of lens known as a holographic optical element. An HOE is actually a diffraction grating made by recording a hologram inside a thin layer of polymer. It works by converting a single beam of laser into a circular array of 15 bright spots. Place the HOE between the scanning mirrors and the eye, and the array of beams that forms will illuminate the region round your pupil.
Move your eyes slightly and one of the beams will still strike the cornea and be focused to form an image on the retina. HOEs have a big advantage over eye tracking systems: because they are made from a thin layer of polymer, they weigh next to nothing.
All of the action takes place in a layer just a fraction of millimeter thick, says a researcher. Virtual Retinal Display Seminar Report Government Engineering College, Thrissur 27 Estimated Retinal Illuminance The relationship between estimated retinal illuminance and scene luminance is important in understanding the display operating on this principle.
As the display in this thesis contains no screen or real object, it is impossible to discuss the brightness of the display in terms of luminance. In terms of brightness, estimated retinal illuminance is a common denominator, so to speak, of screen based display systems and retinal scanning displays systems.
As shown by dimensional analysis on the equation for I , trolands reduce effectively to the units of optical power per unit steradian. The Stiles-Crawford effect describes the contribution to brightness sensation of light entering different points of the pupil i.
Some standard scene luminance values, L, and their corresponding Stiles-Crawford corrected estimated retinal illuminance values, I, are given in Table II. Standard scene luminance values and corresponding estimated retinal illuminance values. Transmission Characteristics of the Ocular Media Transmission losses in the eye result from scattering and absorption in the cornea, lens, aqueous humor, and vitreous humor. The transmittance of the ocular media is a function of the wavelength of the light traveling through the media.
Figure 2. Transmittance of the ocular media vs. Virtual Retinal Display Seminar Report Government Engineering College, Thrissur 29 Image Quality as Related to the Eye Introduction Measurements of display image quality depend heavily on two display characteristics, resolution and "contrast" see subsequent sections.
It is virtually fruitless to discuss image quality in terms of either resolution or "contrast" without including the other. Definitions for display resolution, contrast, contrast ratio, and modulation contrast are given in the following discussion.
Whenever possible, the meanings of the terms are related to the effect or result at the retina. Display Resolution and the Eye The resolution of a display can be defined as the angle subtended by each display resolution element. For the VRD, the angular extent of each spot on the retina dictates the system resolution. Display resolution is often measured in cycles per degree for periodic gratings such as bar patterns or sinusoidal gratings. Display Contrast and the Eye The contrast, C, of a display is the ratio of the difference between the maximum display intensity and the minimum display intensity divided by the maximum.
In other words, the values of IDmax and IDmin correspond to the estimated retinal illuminance values of displays with luminance values of LDmax and LDmin respectively. In the case of a retinal scanning display, as in this thesis, estimated retinal illuminance is a preferable measure of display brightness as there is no screen in the system. Display Contrast Ratio and the Eye The contrast ratio, CR, of a display is the ratio of the maximum display intensity to the minimum display intensity.
The values of IDmax and IDmin correspond to the estimated retinal illuminance values for displays with luminance values of LDmax and LDmin respectively. Display Modulation Contrast and the Eye The modulation contrast, CM, of a display is the ratio of the difference between the maximum display intensity and the minimum display intensity divided by the sum of the minimum and maximum intensities. Stereographic Displays using VRD As discussed previously while treating the possibility of three-dimensional imaging systems using VRD there are two cues by which the human beings perceive the real world namely the accommodation cue and the stereo cue.
There is a mismatch of the information conveyed by the two cues in projection systems so that prolonged viewing can lead to some sort of psychological disorientation.
In VRD we can generate individual wavefronts for each pixel and hence it is possible to vary the curvature of individual wavefronts which determines the focal depth, so what we get is a true stereographic view.
By integrating a deformable mirror into the VRD, the wavefront of light being scanned onto the retina can be changed and various fixation planes created depending on the divergence of the light entering the eye. Previous embodiments of 3D displays allowing for natural accommodation and vergence responses include the use of a varifocal mylar mirror and the use of a liquid-crystal varifocal lens. In the former, a reflective mylar surface was deformed by air pressure Virtual Retinal Display Seminar Report Government Engineering College, Thrissur 32 using a loudspeaker behind the mylar mirror frame.
A CRT screen was positioned so that the viewers saw the reflection of the CRT in the mirror at various virtual image depths. In the latter, an electrically-controllable liquid-crystal varifocal lens was synchronized with a 2-D display to provide a 3D image with a display range of 1.
Although these systems provided for a 3D volumetric image allowing for natural human eye response, they are large and cumbersome benchtop systems. The mirror is bulk micromachined and consists of a thin, circular membrane of silicon nitride coated with aluminum and suspended over an electrode.
When a voltage is applied to the electrode, the mirror membrane surface deforms in a parabolic manner above the electrode. The wavefront of a beam of light hitting the mirror membrane surface can be changed by varying the voltage applied to the electrode.
With no voltage applied, the mirror membrane surface remains flat. With a certain amount of voltage applied, the reflecting beam will be made more converging. By integrating the deformable mirror into the VRD scanning system, a three-dimensional picture can be created by quickly changing the scanned beams degree of collimation entering the eye.
Optical Design The HeNe laser beam is spatially filtered and expanded before striking the deformable mirror. When the mirror is grounded, the beam is at maximal divergence when entering the eye.
Conversely when the mirror voltage is at maximum, the resultant beam is collimated when entering the eye. The beam is reflected off a scanning galvanometer and through an ocular lens to form a viewing exit pupil. A viewer putting his eye at the exit pupil would see a 1-D image at a focal plane determined by the amount of beam divergence. With no voltage on the mirror this image is located at close range; with maximum voltage on the mirror the image is at optical infinity.
In this way the optical setup provides a range of focal planes from near to far which can be manipulated by changing the voltage on the mirror. As a result of the work, a patent application was filed and the technology licensed to a Seattle based start up company, Micro Vision, Inc. This development work began in November Prototype 1 The original prototype had very low effective resolution, a small field of view, limited gray scale, and was difficult to align with the eye.
One objective of the current development effort was to quickly produce a bench-mounted system with improved performance. Prototype 1 uses a directly modulated red laser diode at a wave length of nanometers as the light source. The required horizontal scanning rate of 73, Hertz could not be accomplished with a simple galvanometer or similar commercially available moving mirror scanner.
The use of a rotating polygon was deemed impractical because of the polygon size and rotational velocity required. It was thus decided to perform the horizontal scan with an acousto-optical scanner. The vertical scanning rate of 72 Hertz is within the range of commercially available moving mirrors and is accomplished with a galvanometer. Thus, 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. Prototype 2 To overcome the limitations of the acousto-optical scanner, HITL engineers have developed a miniature mechanical resonant scanner. This scanner, in conjunction with a conventional galvanometer, provides both horizontal and vertical scanning with large scan angles, in a compact package. The estimated recurring cost of this scanner will allow the VRD system to be priced competitively with other displays.
Prototype 2 of the VRD uses the mechanical resonant scanner.. The system was built and demonstrated during the summer of The VGA resolution images produced are sharp and spatially stable. The mechanical resonant scanner is used in conjunction with a conventional galvanometer in a combination which allows for an increase in the optical scan angle. When the mirrors of the two scanners are arranged in such a manner that a light beam undergoes multiple reflections off the mirrors, then the optical scan is multiplied by the number of reflections off that mirror.
Optical scan multiplication factors of 2X, 3X and 4X have been realized. Prototype 2 uses a system with 2X scan multiplication in the horizontal axis. Prototype 3 The third prototype system developed uses the same scanning hardware as Prototype 2 but uses three light sources to produce a full color image.
In addition the eyepiece optics have been modified to allow for see through operation. In the see through mode the image produced by the VRD is overlaid on the external world. Such units are largely used by the production units of many industries, most of them automobile manufacturers. Like a high-tech monocle, a clear, flat window angled in front of the technician's eye reflects scanned laser light to the eye.
That lets the user view automobile diagnostics, as well as repair, service, and assembly instructions superimposed onto the field of vision. The information that the device displays comes from an automaker's service-information Web site through a computer running Microsoft Windows Server in the dealership or repair shop. The data gets to the display via an ordinary IEEE Typical MEMS scanner today measures about 5 mm across, with a 1. Such a scanner provides SVGA by equivalent resolution at a hertz refresh rate and is now in production and in products.
In addition, multiple scanners could provide higher-resolution images by each providing full detail in a tiled subarea. Eventually, costs will become low enough to make this practical, allowing the scannedbeam approach to surpass the equivalent pixel count of any other display technology. With green laser diodes, it will be possible to build bright, full-color see-through displays.
Microvision uses laser light sources in many of its see-through products because our customers' applications demand display performances with color-gamut and brightness levels far exceeding the capabilities of flat panel displays, notebook displays, and even higher-end desktop displays.
For today's commercial products, only red laser diodes are small enough, efficient enough, and cheap enough to use in such see-through mobile devices as Nomad. Blue and green diode-pumped solid-state lasers are still too expensive for bright, full-color, head-up or projection displays for mainstream markets, but that could change soon. When these designs and materials are extended to green laser diodes, it will be possible to build bright, full-color see-through displays.
As an alternative, small green laser are now being produced which use a crystal to frequency double a neodymium YAG laser.
These devices are larger than desired and are not directly modulatable at the required frequency. They do however, offer a short term solution. In the HITL researchers are investigating a number of alternatives to blue and green laser diodes.
One frequency doubling technique being researched uses rare earth doped fibers as the doubling medium. A second technique uses wave guides placed in a lithium niobate substrate for the doubling. The above methods all utilize a laser as the light source. Additional work is directed at using non-lazing, light-emitting diodes LEDs as the light source.
In order for this to be successful two primary issues are being addressed. The first issue is how to focus the LED output to the desired spot size. The second issue is the development of fabrication techniques that will allow us to directly modulate the LEDs at the desired frequency.
Enter the edge-emitting LED.