A emt M 
Ww I 2 
7.1 Resolution — sub-pixel accuracy 
A light emitting diode (LED) is mounted to a 3-axis 
translation stage in the center of the measurement 
volume. Three series of measurement were made, in each 
serie the LED is moved stepwise along one of the axes; 
steplength 0.05 mm, total translation 10 mm. At each step 
the position of the LED is measured. The measured data 
are fitted to a straight line of unity slope, and the devia- 
tions from linearity are calculated (Ugg): 
X-axis (depth) 31 um 
Y-axis (width) 22 um 
Z-axis (height) 20 um 
7.2 Repeatability of Light Pen measurements 
The Light Pen is placed in the center of the measurement 
volume, and the repeatability of the Pen tip coordinates is 
checked by making 50 measurements in the same loca- 
tion. The Light Pen is hand held in its vertical orientation. 
In the camera plane (horizontally): 0.077 mm 
Vertically: 0.020 mm 
The values for the camera plane are larger than the single 
LED resolution due to lack of redundant information from 
a Light Pen having only 3 LEDs. To overcome this 
problem, new Light Pens having 5 or 6 LEDs are develo- 
ped. 
7.3 Length measurements — geometric accuracy 
A certified step gauge is placed along all four diagonals 
of the measurement volume as illustrated in figure 4. 
Maximum length: 1000 mm. 
Accuracy: 0.05 mm 
Uncertainty of measurements: 0.12 mm 
The length measurements include statistical spread of 
single measurements at both ends of bar. 
Compared to the results of chapter 7.2, the major contri- 
bution to the Uncertainty of Measurements is the Light 
Pen repeatability. 
233 
8 CONCLUSIONS 
MNS is the only on-line photogrammetry based coor- 
dinate measurement system fulfilling the accuracy spe- 
cifications given by the automotive industry for car body 
measurements. This high accuracy, as well as the por- 
tability of the system, makes it into a unique tool for a 
number of applications beyond the reach of present 
coordinate measurement systems. 
Further improvements will be achieved by the new, 
improved Light Pen design. 
References 
Pettersen, A., Metrology Norway System - An on-line 
industrial photogrammetry system, Presented paper, 
Comission V, XVII Congress of ISPRS, Washington 1992.