the accuracy 
>S compared to 
ler accuracy is 
tical axes which 
camera) than in 
oerpendicular to 
tion of camera 
e and geometry 
lies, the sensor 
of 0.015 pixels. 
th six LEDs and 
depth (z) of 200 
sults for different 
    
o Z [mm] 
    
  
n] 
0.05 
0.18 
0.41 
0.79 
1.25 
2.65 
   
    
    
    
    
    
ults for SCS 
perimental mea- 
shows the results 
amera to object 
  
  
  
  
  
  
nm] | e Z [mm] 
5 0.05 
6 0.13 
5 0.26 
38 0.54 
58 0.87 
  
  
  
results for SCS 
‘simulated and the 
ion of one of the 
perimental results 
This indicates that 
3} on the sensor 
ditions. 
st was done to find 
its. 50 points were 
| better than 0.001 
optical axis of the 
ely parallel with the 
were fitted to a 
mathematical plane and the measurement standard 
deviation was estimated based on the residuals. For a 
.camera-to-object distance of 2500 mm, the accuracy was 
found to be (1*o level); 
  
Distance [mm] | o, [mm] 
2500 0.010 
  
  
  
  
  
Table 3: Plane measurement results for SCS 
It is clearly shown that the accuracy in the directions 
normal to the line of sight (X and Y direction in the simula- 
tions and experiments) is superior to the accuracy parallel 
with the line of sight (Z direction). With other words, the 
SCS is a metrology system where the angular 
measurement accuracy is superior to the distance mea- 
surement accuracy. 
More extensive tests, like standardized CMM accuracy 
tests, have not yet been done. Nevertheless, both the 
simulations and the experimental results so far indicate 
that the SCS meets the accuracy requirements for a 
variety of industrial metrology applications. 
5. Post Processing of SCS Measurements 
If on-line measurements are not of paramount importance, 
accuracy can be significantly improved by repeating mea- 
surements of the same object points using different 
camera positions, and subsequently entering the 
observations into a post processing module. The accuracy 
improvement achieved by the post processing approach 
is based on utilizing the high angular accuracy of the SCS 
System. The post processing approach is ideal for the 
establishment of high precision reference networks. 
As explained in Section 3, the SCS software is based on 
estimating the position of the Light Pen tip as referenced 
to the camera coordinate system. When the Pen tip is 
pointing to a measurement point, the point of contact is 
mathematically back-projected on to the sensor, based on 
the estimated relationship between the coordinate system 
of the Light Pen and the camera coordinate system (see 
Figure 1). Equation 1 is used for this perspective 
transformation. The back-projected point constitutes the 
"fictitious" observation that is recorded and subsequently 
input to the post processing bundle adjustment. For every 
camera position, the same object points (triangulation 
points) are touched with the Light Pen tip, and the 
"fictitious" sensor observation is calculated for each of the 
object points. The accuracy of the back projected point is 
determined by the angular measurement accuracy of the 
SCS. 
In the post processing bundle adjustment, the 3D 
coordinates of the triangulation points are estimated. The 
accuracy of the triangulation points is dependent on the 
geometry of the photogrammetric network. Network 
geometry is characterized by the number and orientation 
of camera stations, and number and position of 
triangulation points. Due to the approach of "indirect" 
measurement of triangulation points using the Light Pen, 
the points are observable from "all" directions. Therefore 
it is easy to achieve a favorable network geometry when 
operating the SCS. 
The full 3D measurement capability of SCS provides 
approximate coordinate values for all the unknown 
triangulation points, which is needed for the post 
processing. To achieve a high accuracy, the post 
processing is a free network bundle adjustment only 
constraining given distances between some of the 
triangulation points. 
6. Applications 
To obtain a favorable result for SCS applications, the 
accuracy characteristics of the SCS must be taken into 
account. Two main application groups can be defined: 
1. Applications where full 3D measurement 
capabilities are not required, like 
straightness measurements. 
2. Applications with low accuracy requirements, 
where the moderate length measurement 
accuracy of SCS is satisfactory. 
Some examples include: 
= Measurement of the straightness of airplane 
fuselages. If the line of sight is approximately 
parallel with the airplane fuselage when 
doing the measurements, the high angular 
accuracy will provide a high accuracy of the 
straightness measurements. 
= Measurement of wing contour. If the line of 
sight is parallel to the wing surface, high 
accuracy is achieved. 
= Measurement of flush and gap on nacelle 
(aircraft engine cover). Flush is measured 
with the camera aimed in the length direction 
of the nacelle, while gap is measured with 
the camera pointing perpendicular to the 
length direction. 
= Measurements on collision tested cars. This 
is an application where the accuracy 
requirements usually are moderate. 
Measurements inside the crash tested car 
are particularly easy to perform. 
= Establishment of high precision reference 
networks by SCS in combination with post 
processing. 
References 
[1] Alf Pettersen. "Metrology Norway System - 
Optimum Accuracy based on CCD 
Cameras". International Archives of 
Photogrammetry and Remote Sensing, 
Vol.XXIX, ISPRS 1992, Commission V.