Mw 
— 
= UU UV << Ww 
  
Tip Selector Button 
    
Indicator LEDs 
Exposure Butto 
Twist Lever 
  
  
  
Fig. 2. Light Pen 
3 RESOLUTION 
The MegaPlus sensor resolution is 1350 x 1035 pixels, 
having 100 96 fill ratio. The pixel size is 6.8 x 6.8 um. 
By the use of a thorough statistical analysis of the point 
image, a subpixel resolution of close to 0.01 pixel (10) is 
obtained. This high quality subpixel resolution is a result 
of: 
-sensor and camera electronics quality, 
—optical design, i.e. spot size optimization, 
-signal to noise ratio optimization. 
The reliability of the measurements at the sub-pixel level 
is strongly related to the use of unique and well defined 
light emitting diodes and laser sources. This overcomes 
the weakness of conventional techniques based on 
targets or object features, which may be very dependent 
on ambient light conditions. 
The sensor resolution of 0.01 pixels projects into the 
object space to e.g. 0.01 mm in a field of view of 
1.0.x 1.0 n°. 
4 ABSOLUTE, GEOMETRIC ACCURACY 
Absolute geometric accuracy is ensured by the camera 
calibration which allows the camera to be considered as 
an "ideal" photogrammetric camera, i.e. the inner orienta- 
tion of the camera is determined once and for all. 
Untill now, no performance degradation has been ob- 
served. The longest periode of operation of a camera has 
been one year, including rough treatment in industrial 
environments. The validity of the camera calibration is 
checked at each new setup as explained in chapter 6.2. 
5 APPLICATION DEPENDENCIES 
5.1 Setup geometry 
The resolution as measured in the object space is 
proportional to the dimensions of the measurement 
volume. 
Generally, a two camera system always have a better 
accuracy vertically due to redundancy. In the horizontal 
plane the error distribution between width and depth 
depends on intersection angle as shown in figure 3. 
Reducing the intersection angles improves the width 
accuracy, but reduces the depth accuracy. This has to be 
taken into account when designing the camera setup. For 
alignment purposes (e.g. to check the straightness of an 
aircraft fuselage) the cameras should be mounted close 
together. 
  
  
  
  
  
  
  
  
  
  
  
  
  
b: 60 deg 
  
Fig. 3 Distribution of error in the horizontal plane. 
Introducing a third camera could significantly improve the 
accuracy. 
5.2 Light pen 
The accuracy depends on: 
— the size and shape of the Light Pen relative to the 
measurement volume, 
— the distance from the pen tip to the light sources. 
The standard MNS Light Pen has so far been T-shaped, 
having three light emitting diodes (LEDs). 
A new light pen is developed, having a new geometry 
and 5 or 6 LEDs. To optimize the Light Pen to the appli- 
cation, Light Pens of different sizes should be used. 
5.3 Laser applications 
To achieve optimum accuracy for laser applications, the 
laser is focused to eliminate speckle problems. The accu- 
racy may depend on the material, as diffuse reflection is 
required. Accurate measurements are demonstrated for 
difficult materials as unpainted steel, aluminum and black 
plastics.