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The Environment, Optics and the display

The Environment, Optics and the display. Dr. Yan Liu Department of Biomedical, Industrial and Human Factors Engineering Wright State University . Visible Light.

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The Environment, Optics and the display

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  1. The Environment, Optics and the display Dr. Yan Liu Department of Biomedical, Industrial and Human Factors Engineering Wright State University

  2. Visible Light • Visible light waves are the only electromagnetic waves humans can see, which accounts for a tiny part of the electromagnetic spectrum of radiation • Visible light waves can be seen as the colors of rainbow Electromagnetic Spectrum Visual Light Spectrum

  3. The Human Eye • Cornea • A tough and transparent membrane that forms the front surface of the eye • Iris • The aperture stop for the eye • D=2mm in bright light, D=8mm in dim light • Crystalline Lens • Bi-convex structure about 9mm in diameter and 4mm in thick • Pliable and changes shapes to fine tune vision

  4. The Human Eye (Cont.) • Retina • A light sensitive layer at the back of the eye that covers about 65% of its interior surface • Rods: about 120 million; mainly responsible for night vision; not sensitive to colors • Cones: about 6 ~ 7 million; mainly responsible for daylight vision; sensitive to colors • Blind spot • The region of the retina that has no light sensitive rods or cones • Fovea • Located at the center of the macula region (an oval yellow spot) of the retina; responsible for sharp central vision • Eye muscles • Enable eye movement

  5. Visual Angle • Visual angle is defined as the angle subtended by an object at the eye of an observer • In the units of degrees, minutes and seconds of arc (1 degree = 60 minutes = 3600 seconds) • Useful Rules of Thumb • A thumbnail held at arm’s length subtends about 1 degree of visual angle • A 1cm object viewed at 57cm has a visual angle of approximately 1 degree. • 57cm is a reasonable approximation to the distance at which we view a • computer monitor

  6. Nodal point (optical center) Compound lens (Eq. 2) f3: is the focal length of the compound lens as the result of combining lens f1 and f2 Compound Lens • Two Key Components • Curved front surface of the cornea: the main focusing • Crystalline lens: less focusing power Simple lens (Eq. 1) f: focal length of the lens d: the distance from the nodal point to the object that is imaged r: the distance from the nodal point to the image that is formed

  7. Power of Lens • The reciprocal of the focal length in the unit of diopters • A 1-diopter lens has a focal length of 1m • About 40 diopters comes from the front surface of the cornea and the rest from the crystalline lens • The eye becomes less flexible with age, at roughly the rate of 2 diopters per decade • Young children are capable of adjusting over a range of 12 diopters or more (focus on an object as close as 8cm) • By the age of 60, the lens is almost completely rigid

  8. Depth-of-Focus • The range over which objects are in focus when the eye is adjusted for a particular distance • Varies with the size of the pupil • Assuming a 3mm pupil and a human eye focused at infinity, objects between 3m and infinity are in focus • Described in terms of the power change that takes place without the image becoming significantly blurred (about 1/3 diopter assuming a 3mm pupil) • Assuming the 1/3-diopter depth-of-focus value and the eye focused at distance d (in meters), objects in the range [3d/(d+3), -3d/(d-3)] will be in focus

  9. Depth-of-focus at various viewing distances (assuming a 3mm pupil)

  10. Augmented-Reality (AR) Systems • What are Augmented-Reality Systems • Involve superimposing visual imagery on the real world so that people can see a computer graphics-enhanced view of the world • e.g. Automobile in which the technician sees instructions and structural diagrams superimposed on the actual machine when performance maintenance and checkup (http://www.youtube.com/watch?v=P9KPJlA5yds&feature=related) Simulated augmented reality medical image

  11. AR Systems (Cont.) • Head-Mounted Display (HMD) • A display device, worn on the head or as part of a helmet, that has a small display optic in front of one (monocular HMD) or each eye (binocular HMD) • Some HMDs display only a computer-generated image (CGI; virtual environment) • See-through HMDs • Allow superimposing a computer-generated image upon a real-world view (AR environment) • Optical see-through HMDs • Combine real-world view with CGI by projecting the CGI through a partially reflective mirror and viewing the real world directly • Video see-through HMDs • Combine real-world view with CGI electronically by accepting video from a camera and mixing it electronically with CGI

  12. AR Systems (Cont.) • Some Issues • Only one depth at which the computer-generated imagery and the real-world imagery are in focus • Desirable if the real-world and virtual-world scenes need to be perceived together • Undesirable if the computer imagery should remain perceptually distinct from the real-world image • Problems with see-through HMDs • HMDs do not allow for the redirection of the gaze with head movements and thus cause discomfort to observers when looking sideway in a large angle • Effect of binocular rivalry when HMDs are worn over one eye • Two images are not seen superimposed; one image is seen for a few moments, then the other, then the first, and so on

  13. Optics in Virtual-Reality Displays • Virtual-Reality Displays • Block out the real-world, only concerned with computer-generated imagery • Correct depth-of-focus is important to help differentiating the objects in fixation from other objects in the environment • Simulating Depth-of-Focus with a Flat-Screen • Simulating optical blur • Simulating the optical distance of the virtual object • Knowing what the user is looking at • Object of attention can be made sharp while other objects are displayed as though out of focus

  14. Using an eye tracker to present correct depth-of-focus information on a virtual-reality display

  15. Chromatic Aberration • Chromatic Aberration in the Human Eye • Different wavelengths of light are focused at different distances within the eye • Short-wavelength light is refracted more than long-wavelength light • A lens of power of 1.5 diopters is need to make blue and red focus at the same depth • Implications • Pure blue text on a black background can be almost unreadable if there is white or red text nearby to attract the focusing mechanism • Strong illusory depth effects can be induced in a screen which contains blue and red texts and has a black background • For about 60% of observers, the red appears closer; • 30% see the reverse; 10% see the colors lying in the same plane

  16. Point acuity(1 minute of arc): the ability to resolve two distinct point targets Grating acuity(1-2 minutes of arc): the ability to distinguish a pattern of bright and dark bars from a uniform gray patch Letter acuity(5 minutes of arc): the ability to resolve letters Stereo acuity(10 seconds of arc): the ability to resolve objects in depth Vernier acuity(10 seconds of arc): the ability to see if two line segments are collinear Visual Acuities • Visual Acuities • Measurements of our ability to see details • We can resolve things down to about 1 minute of arc, which is in rough agreement with the spacing of receptors in the center of fovea • Superacuities • The ability to perceive visual properties to a greater precision than could be achieved based on a simple receptor model

  17. Visual Acuities (Cont.) • Ways to Improve Acuities • Binocular viewing improves acuity by 7% as compared with monocular viewing • Use the ability of eye to integrate information over space and time to allow perception of higher-resolution information • e.g. A sequence of video frames seems of higher quality than any single frame

  18. Acuity Distribution and the Visual Field Region of binocular overlap (both eyes receiving input) Irregular boundaries are caused by facial features The visual field of view (FOV) for a person gazing straight ahead (FOV is the angular extent of the world seen at a moment)

  19. Acuity Distribution and the Visual Field (Cont.) Distribution of the acuity as a function of the distance from fovea

  20. Brain Pixels and the Optimal Screen • Brain Pixels • The image units used by the brain to process space • Non-uniformly distributed in the brain • Visual Efficiency of A Display Screen • What screen size provides the best match of screen pixels to brain pixels • Model how many brain pixels are stimulated by different screens with different sizes yet the same number of pixels

  21. Brain pixels (BP) Screen pixels (SP) a & b. At the fovea, there are many brain pixels for each screen pixel; therefore, having higher-resolution screens would help foveal vision c & d. In the periphery of the visual field, there are more screen pixels than brain pixels; therefore, it is unnecessary to have many screen pixels for the side vision Illustration of how computer screens of different sizes but with the same number of pixels match human visual acuity

  22. Visual Efficiency of A Display Screen TBP = Total number of brain pixels stimulated by the display USBP = Number of uniquely stimulated brain pixels USBP = TBP – redundant brain pixels Display efficiency (DE): a measure of how efficiently a display is being used Visual efficiency (VE): the proportion of brain pixels in the screen area that are getting unique information DE = USBP/SP VE = USBP/TBP

  23. The normal monitor can stimulate about 45% of brain pixels, and the wide screen stimulate about 60% of brain pixels • USBPs peak at a width close to the normal monitor and declines somewhat as the screen gets wider • The normal monitor is about the right size for most tasks A numerical simulation with one-million pixel screen to show the number of stimulated brain pixels and display efficiency as functions of the display width

  24. To increase visual efficiency, we should use small and high-resolution screens A numerical simulation with a one-million pixel screen to show the visual efficiency as a function of the display width

  25. Visual Efficiency of A Display Screen (Cont.) • A display with more than one resolution can increase visual efficiency • Low-resolution background display for each eye • High-resolution insert coupled to the user’s eye position via an eye-tracking system • Provides computer graphics imagery to less than half of the total visual field • The region of binocular overlap is less than 15% of that available under real-world viewing conditions CAE fiber-optic helmet-mounted display (one of widest-field displays made and designed for helicopter simulators)

  26. Spatial Contrast Sensitivity Function • Spatial Contrast • A measure of the range of luminances of objects in a visual image • Spatial Modulation Sensitivity Function • Measure of the sensitivity of the eye/brain system to the lowest contrast (C) that can be detected and how it varies with spatial frequency • Spatial frequency is a measure of how often a structure repeats per unit of distance • A sine wave rating can be used Contrast (Eq. 3) Grating Luminance (Eq. 4) c: the contrast ω: the wavelength Φ: the phase angle x: the position on the screen

  27. Parameters that can be varied: • Spatial frequency (the number of bars of the grating per degree of visual angle) • Orientation • Contrast (the amplitude of the sine wave) • Phase angle (the lateral displacement of the pattern) • Visual area covered by the grating pattern Lmax Lmin Sine wave grating

  28. Contrast sensitivity falls off at both high and low spatial frequency values • we can be insensitive to both slow and rapid changes in light patterns • Most sensitive to the frequency of about 2 or 3 cycles per degree • Aging decreases the sensitivity to higher spatial frequency Contrast sensitivity as a function of spatial frequency

  29. Spatiotemporal Contrast Sensitivity • Making patterns flicker at 7 or 8Hz would be the most effective in detecting them • Interaction effects between spatial frequency and temporal frequency • When a pattern is flickering at between 5 and 10Hz, fall off in sensitivity is much less under the low spatial frequency than under the high spatial frequency Spatiotemporal contrast sensitivity of luminance perception

  30. Visual Stress • Stripped patterns of about 3 cycles per degree and flicker rates of about 20Hz are most likely to induce visual stress (e.g. eyestrain, headaches and migraines) in most people A Pattern that is designed to be visually stressful (If it is viewed from 40cm, the spacing of the stripes is about 3 cycles per degree)

  31. The Optimal Display • Some Statistics • A modern high-resolution monitor has about 35 pixels per cm, or 40 cycles per degree at normal viewing distances • Given that the human eye has receptors packed into the fovea at roughly 180 per degree of visual angle, we are about a factor of four from having monitors that match the resolving power of the human retina in each direction • Without considering Superacuities, a 4000×4000-pixel (16 million) resolution monitor should be adequate for any conceivable visual task • Humans can resolve a grating of approximately 50 cycles per degree; therefore, we need more than 100 pixels per degree according to the sampling theory

  32. Aliasing • The distortion or artifact that occurs when a regular pattern is sampled by another regular pattern at a different frequency • Aliasing can be useful • e.g. It is much easier to judge whether a line is perfectly horizontal on the screen with aliasing than without Original pattern Aliasing Antialiasing A stripped pattern is sampled by pixels whose spacing is slightly wider than the wavelength of the pattern, resulting in a pattern that has a much wider spacing Aliasing artifacts, with antialiasing as a solution

  33. Superacuities and Displays • We may wish to have high-resolution displays due to superacuities • With proper antialiasing, superacuity performance to better-than-pixel resolution can be achieved • e.g. 7.5-sec vernier acuity threshold was achieved for antialiased lines compared to 15-sec threshold for aliased lines

  34. Temporal Requirements of the Perfect Display • 50-75Hz refresh rate of the typical monitor would seem to be adequate • Temporal aliasing artifacts are common in computer graphics and movies, especially when the image update rate is low • Temporal antialiasing techniques can be employed, but they can be computationally expensive in practice

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