mechanical structure of the optical setup and by shadow 
effects. A synchronized geometry provides a way to alleviate 
these tradeoffs. Rioux, 1984, introduced a synchronized 
scanning scheme, with which large fields of view with small 
triangulation angles can be obtained without sacrificing 
precision. With smaller triangulation angles, a reduction of 
shadow effects is inherently achieved. The intent is to 
synchronize the projection of the laser spot with its 
detection. As depicted in Figure 3, the instantaneous field of 
view of the position detector, defined by P and f, follows 
AZ : 
| Object (X,Z) 
RM een Eee > X 
| Lens 
       
<::: Position 
Detector 
rice prep 
| Laser Fon 
Figure 3: Synchronized scanner approach 
the spot as it scans the scene. The focal length of the lens is 
therefore related to the desired depth of field or measurement 
range and not to the field of view. Implementation of this 
triangulation technique by an auto-synchronized scanner 
approach allows a considerable reduction in the optical head 
size compared to conventional triangulation methods. Figure 
4 displays schematically the basic components of a dual-axis 
auto-synchronized camera. A 3-D surface map is obtained by 
(1) scanning a laser beam onto a scene with two oscillating 
mirrors mounted orthogonally from one another, (2) 
collecting the light that is scattered by the scene in 
synchronism with the projection mirrors, and (3) focusing 
this light onto a linear position-sensitive photo-detector. 
Beraldin et al., 1993, give the functions of 3-D coordinates 
computation for this implementation. 
  
  
  
  
Laser 
| 
Y-Axis 1 
\ Scanner 
ST — 
= 
> 
X-Axis 
Scanner —— Collecting 
Lens 
MM co 
05m «e, Position 
Detector 
Figure 4: Auto-synchronized scanner approach: dual-axis 
synchronized scanner. 
142 
2.2. The BIRIS Camera 
The BIRIS range camera was developed at NRC to work in 
difficult environments where reliability, robustness, and 
ease of maintenance are important. The optical principle of 
BIRIS is shown in Figure 5. The main components are a 
mask with two apertures, a camera lens, and a standard CCD 
camera. In a practical implementation, the double aperture 
mask replaces the iris of a standard camera lens (hence the 
name bi-iris). A laser line, produced by a solid state laser 
diode and a cylindrical lens, is projected on the object and a 
double image of the line is measured on the CCD camera. 
The separation between the two imaged lines is 
proportional to the distance between the object and the 
camera and provides direct information about the shape and 
dimensions of the object. For example, in Figure 5, the line 
separations bl and b2 represent the ranges Z1 and Z2 
respectively. Details of the mathematical model and the 
calibration can be found in Blais et al, 1992. 
Object 
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BIRIS Image Equivalent Range Image 
Figure 5: The BIRIS range camera 
3. CALIBRATION 
Rigorous calibration is needed for both internal and 
external camera parameters, and, when multi-sensors are 
used, for the registration of the data acquired by the various 
sensors. Specifically, it must recover: 
- interior sensor parameters, including distortion parameters, 
- position and orientation of all sensors (figure 6-b). 
The calibration requires points of precisely known positions 
in the object space coordinate system. For the set up shown 
in figure 6-a, which is designed for environment modeling, a 
set of well-defined targets mounted at various heights on 
three orthogonal sides, is employed. The targets are centered 
on blocks with flat surfaces. The main requirements for this 
field of control point is the dimensional stability and the 
high accuracy of measuring the target locations after they 
have been built. This accuracy must be significantly higher 
than the expected accuracy of the system. Therefore, care 
must be exercised in measuring these targets with manual 
surveying equipment or close-range digital photogrammetry. 
The number and distribution of these targets must be 
designed to completely cover the expected viewing volume 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996 
  
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