Optical see-trough 
Optical see-trough HMDs work by placing optical combiners in 
front of the user's eyes. These combiners are partially 
transmissive, so that the user can look directly through them to 
see the real world. The combiners are also partially reflective, 
so that user sees virtual images bounced off the combiners from 
head-mounted monitors. Fig. 2 shows a conceptual diagram of 
an optical see-trough HMD. Fig. 3 shows a optical see-trough 
HMD made by Kaiser Electro-Optics Inc. 
  
A Head Graphic 
Head Tracker images 
locations 
  
    
Scene 
generator 
  
  
  
  
P" Real 
world 
Optical 
combiners 
Figure 2. Conceptual diagram of an optical see-trough HMD 
(Azuma, 2001) 
  
  
Figure 3. Optical see-trough HMD: ProView XL40/50 STm 
Kaiser Electro-Optics Inc. 
Video see-trough 
Video see-trough HMDs work by combining a closed view 
HMD with one or two head-mounted video cameras. The video 
cameras provide the users view of the real world. Video from 
these cameras is combined with the graphic images created by 
the scene generator, blending the real and virtual images. The 
result is sent to the monitors in front of the users eyes in the 
closed-view HMD. Fig. 4 shows a conceptual diagram of a 
video see-trough HMD. Fig. 5 shows a video see-trough HMD 
made by SDT. (Azuma, 2001) 
  
  
  
   
   
  
Head 
    
  
  
  
  
Head 
locations Tracker 
loca \ Video cameras 
Video : > , à \ Real 
of + #, 97 ^ World 
real / j % 
world Scene 
generator |] 
Graphic \ | Monitors 
images a 
  
  
  
Video compositor 
    
  
  
Combined video 
Figure 4. Conceptual diagram of a video see-trough HMD 
(Azuma, 2001) 
  
Figure 5. Video see-trough HMD: SDT HMD 800 
3.1.2 Virtual Retinal Systems 
Virtual Retinal display (VRD) is a visual display device that 
uses scanned light beams. Instead of viewing a screen, the user 
gets the image scanned directly into the eye. A very small spot 
is focused onto the retina and is swept over it in a raster pattern. 
The VRD uses very low power and yet can be very bright. Fig. 
6 shows a block diagram of VRD systems. Fig. 7 shows a VRD 
made by Microvision (Viirre, 1998). 
Horizontal 
Scanner f 
    
    
rr 
  
  
  
  
  
Modulator a | 
Laser ; a À S 
sim 8 Delivery 
V^ Vertical Optics 
Scanner 
  
  
  
  
VGA NE Controlling Electronics 
Input | 
  
  
Figure 6. Block diagram of a Virtual Retinal Display 
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