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					Head-Mounted
Display



  C Babu, Hodges 2008   1
Visually Coupled Systems
A system that integrates the natural visual and
  motor skills of an operator into the system he is
  controlling.
Basic Components
 An immersive visual display (HMD, large screen
  projection (CAVE), dome projection)
 A means of tracking head and/or eye motion
 A source of visual information that is dependent
      on the user's head/eye motion.


                     C Babu, Hodges 2008              2
Head-Mounted Displays
 Optical System
 Image Source (CRT or Flat Panel (LCD))
 See–Through or Non–See–Through
 Mounting Apparatus
 Earphones
 Tracker (Pos & Ori)



                 C Babu, Hodges 2008       3
 Field of View

Monocular FOV is the angular subtense
(usually expressed in degrees) of the
displayed image as measured from the
pupil of one eye.
Total FOV is the total angular size of the displayed image
visible to both eyes.
                 Binocular(or stereoscopic) FOV refers to
                 the part of the displayed image visible to
                 both eyes at the same time.
               FOV may be measured horizontally,
               vertically or diagonally.
                        C Babu, Hodges 2008               4
Focal Length & Diopter
Focal Length - The distance from the
  surface of a lens (or mirror) at which rays
  of light converge.
Diopter - The power of a lens is measured in
  diopters, where the number of diopters is
  equal to 1/(focal length of the lens
  measured in meters).

                  C Babu, Hodges 2008       5
Ocularity and IPD
Ocularity                Interpupillary Distance
 Monocular - HMD          (IPD)
  image goes to only one  IPD is the horizontal
  eye.                     distance between a
 Biocular - Same HMD      user's eyes.
  image to both eyes.     IPD is the distance
 Binocular                between the two
  (stereoscopic) -         optical axes in a
  Different but matched    binocular view system.
  images to each eye.
                    C Babu, Hodges 2008         6
Vignetting and Eye Relief
Vignetting
The blocking or redirecting of light rays as
 they pass through the optical system.
Eye Relief Distance
   Distance from the last optical surface in
    the HMD optical system to the front
    surface of the eye.

                    C Babu, Hodges 2008         7
Basic Eye


 Cornea
            Crystalline
            Lens
                               Fovea
                                       Optic
                      Retina           Nerve


                C Babu, Hodges 2008            8
The Eye
   Accommodation - Term used to describe the
    altering of the curvature of the crystalline lens by
    means of the ciliary muscles. Expressed in
    diopters.
   Retina - The sensory membrane that lines the
    back of the eye and receives the image formed
    by the lens of the eye.
   Fovea - The part of the human retina that
    possesses the best spatial resolution or visual
    acuity.
                       C Babu, Hodges 2008             9
Properties of the Eye
   Approximate Field of View
     120 degrees vertical
     150 degrees horizontal (one eye)
     200 degrees horizontal (both eyes)

   Acuity
     30   cycles per degree (20/20 Snellen acuity).



                       C Babu, Hodges 2008             10
Simple Formulas
 Visual Resolution in Cycles per degree
  (Vres) = Number of pixels /2(FoV in
  degrees)
Example: (1024 pixels per line)/(2*40
  degrees) = Horizontal resolution of 12.8
  cycles per degree
 To convert to Snellen acuity (as in 20/xx)
Vres = 600/xx (20/47)
                  C Babu, Hodges 2008          11
Optical System
 Move image to a distance that can be
  easily accommodated by the eye.
 Magnify the image




                 C Babu, Hodges 2008     12
Simple Magnifier HMD Design
                                            q
                               p


 Eye                              f             Image



           Eyepiece
       (one or more lenses)        Display
                               (Image Source)



                      C Babu, Hodges 2008               13
Thin Lens Equation
1/p + 1/q = 1/f where
    p = object distance (distance from image source to eyepiece)
    q = image distance (distance of image from the lens)
    f = focal length of the lens
Conventions:
   If the incident light comes from the object, we say it is a real object, and define the
    distance from the lens to it as positive. Otherwise, it is virtual and the distance is
    negative.
   If the emergent light goes toward the image, we say it is a real image, and define the
    distance from the lens to it as positive.
   f = positive for a converging lens
   A light ray through the center of the lens is undeflected.




                                         C Babu, Hodges 2008                                  14
Virtual Image


                                    Virtual
                                    Image




      Lens   Display


              C Babu, Hodges 2008       15
LEEP Optics
 Large Expanse Extra Perspective
 Give very wide field of view for
  stereoscopic images
 Higher resolution (more pixels) in the
  middle of the field of view, lower resolution
  on the periphery
 Pincushion distortion


                   C Babu, Hodges 2008        16
Fresnel Lens
 A lens that has a surface consisting of a
  concentric series of simple lens sections
  so that a thin lens with a short focal length
  and large diameter is possible
 More even resolution distribution
 Less distortion




                   C Babu, Hodges 2008        17
Relationship between angle and
screen distance
                   80.00
                   70.00
  Distance in mm




                   60.00
                   50.00
                                                    Leep
                   40.00
                                                    Fresnel
                   30.00
                   20.00
                   10.00
                    0.00
                               00

                               00

                                0

                                0

                                0

                                0

                                0

                                0

                                0
                       R




                             .0

                             .0

                             .0

                             .0

                             .0

                             .0

                             .0
                            2.

                            6.
                           10

                           14

                           18

                           22

                           26

                           30

                           34
                             Angle in Radians


                              C Babu, Hodges 2008             18
Distortion in LEEP Optics




   A rectangle                         Maps to this

                 C Babu, Hodges 2008                  19
To correct for distortion
                             Must predistort image
                             This is a pixel-based
                              distortion
                             Graphics rendering
                              uses linear
                              interpolation!
                             Too slow on most
                              systems

              C Babu, Hodges 2008                 20
Distorted Field of View
 Your computational model (computer
  graphics) assumes some field of view.
 Scan converter may over or underscan,
  not all of your graphics image may appear
  on the screen.
 Are the display screens aligned
  perpendicular to your optical axis?

                 C Babu, Hodges 2008      21
 Distorted FoV (cont.)



Distance along
    z-axis




                 C Babu, Hodges 2008   22
Collimated: p=f

 1/p + 1/q = 1/f     q = , if p=f
 If the image source is placed at the focal
  point of the lens, then the virtual image
  appears at optical infinity.
                   f




                  C Babu, Hodges 2008          23
Compound Microscope HMD
Design
Relay lens produces a real image of the display image
  source (screen) at some intermediate location in the
  optical train. The eyepiece is then used to produce an
  observable virtual image of this intermediate image.
     Exit
     Pupil
                 Intermediate                      Image
                 Real Image


                                      Relay Lens
          Eyepiece


                        C Babu, Hodges 2008                24
Exit Pupil
 The area in back of the optics from which
  the entire image can be seen. Important if
  IPD not adjustable, mount not secure.
 Compound microscope optical systems
  have a real exit pupil.
 Simple magnifier optical systems do not
  have an exit pupil.

                  C Babu, Hodges 2008      25
Virtual Research V6 HMD
   Display
     Dual 1.3 diagonal Active Matrix Liquid Crystal
      Displays
     Resolution per eye: 640 x 480 (307,200 color
      elements)
   Optical
     Field   of view: 60° diagonal


                        C Babu, Hodges 2008        26
What is the horizontal resolution in
cycles per degree?
 Horizontal FoV?
 Equivalent to how many RGB pixels of
  horizontal resolution?
 (Horizontal Resolution) / 2*Horizontal FoV
 3.85 Cycles per degree or 20/156




                  C Babu, Hodges 2008      27
Characteristics of HMDs
   Immersive
     You are inside the computer world
     Can interact with real world (mouse,
      keyboard, people)
 Ergonomics
 Resolution and field of view
 Tethered


                     C Babu, Hodges 2008     28
Assignment – Due next class
period
 Go to http://www.virtualresearch.com/
 Figure out for the VR1280 HMD
     Vertical and Horizontal Resolution
     Vertical and Horizontal FoV
     Vertical and Horizontal Equivalent Snellen
      acuity



                     C Babu, Hodges 2008           29

				
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