METROLOGY NORWAY SYSTEM - OPTIMUM ACCURACY BASED ON CCD 
    
CAMERAS 
Alf Pettersen, Tech. Mgr., Metronor AS 
(P.O.Box 238, N-1360 Nesbru, Norway) 
Commission V 
SUMMARY: 
Metrology Norway System is an on-line industrial photogrammetry system based on high resolution 
CCD cameras measuring coordinates of laser spots or light emitting diodes. A patented camera calib- 
ration technique, as well as high performance signal analysis ensures accuracy of 0.1 mm within a 
measurement volume of 1.0 x 1.0 x 1.0 m?. The Light Pen, a mechanical probe, turns the system into 
a "Hand-Held Coordinate Measurement Machine", and allows the system accuracy to be verified 
according to standards for Coordinate Measurement Machines. The paper discusses accuracy limi- 
tations, verifications, and the result of accuracy tests. 
KEYWORDS: 
1 INTRODUCTION 
The use of CCD camera based photogrammetry systems 
for industrial applications has so far been limited by low 
accuracy. As an example the automotive industry has a 
general accuracy requirement of 0.1 mm for car body 
measurements. 
The Metrology Norway System (MNS) presents a totally 
new concept for industrial metrology. It is the first, and so 
far the only, fully on-line photogrammetric system having 
the necessary accuracy to be accepted in the automotive 
and aerospace industry. 
This paper focuses on the accuracy of the system, and 
the reliability of the measurements. A discussion of the 
measurement concept as well as applications is given by 
Pettersen (1992). 
2 SYSTEM DESCRIPTION 
21 Cameras and calibration 
MNS is based on high resolution CCD cameras (Videk 
MegaPlus). MNS measures the spatial (XYZ) coordinates 
of special infrared Light Emitting Diodes (IR LEDs), or 
alternatively, points of reflected IR laser light. 
To obtain optimum accuracy, and to make the system 
easy operational, the cameras are factory calibrated. This 
one-time calibration process which turns the camera into 
an "ideal" photocrammetric camera involves a complete 
mapping of the entire sensor, giving a detailed descrip- 
tion of lens distortions, sensor geometry and sensor 
defects. The calibration method is a unique, proprietary 
technique developed and patented by Metronor, and will 
not be described in further detail in this paper. 
22 Exterior orientation 
The exterior orientation of the two cameras of the stan- 
dard MNS, i.e. their relative position and orientation, is 
Accuracy, Calibration, Close-range, Industrial, 3-D 
determined by the use of a Reference Bar as illustrated in 
figure 1. The Reference Bar contains several infrared (IR) 
LEDs embedded in a bar made of the alloy Invar. The 
Reference Bar is aimed so that both cameras are able to 
view the LEDs. The Reference Bar LED positions are 
measured for several bar locations throughout the entire 
intended measurement volume. 
The camera positions and orientations are calculated 
using the method of bundle adjustments. 
  
  
  
  
  
Fig.-1. Camera Setup and Exterior Orientation 
2.2 The Light Pen 
The Light Pen as shown in figure 2 is a mechanical device 
containing three LEDs whose coordinates in the Light 
Pen's local coordinate system are well-known. With this 
knowledge of the Light Pen's geometry, MNS easily 
calculates the spatial coordinates of the Pen's stylus tip 
by measuring the spatial coordinates of the three LEDs 
on the Pen. This tool makes MNS into a transportable or 
"hand held" Coordinate Measurement Machine (CMM). 
The Light Pen enables accuracy verifications according to 
CMM standards as described in chapter 6.3.