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2. THE MULTISENSOR SYSTEM 
The layout of the multisensor system is shown in Fig. 1. The system includes two main sub-systems: (i) a whole field 
macro profilometer and (ii) a point-by-point scanning distance meter. The macro profilometer uses a liquid crystal 
projector combined with a CCD camera. The scanning distance meter is based on a double-heterodyne absolute 
interferometer (DHI) and on a galvanometric scanner which enables to cover the whole working space. The 
measurements from the scanning device are combined with those from the macro profilometer in order to enhance the 
overall performance in terms of ease of system calibration and of good profile reconstruction, also in the presence of 
profile discontinuities such as fast slope changes. The two sub-systems are described in the following sections. 
  
  
  
  
  
OBJECT 
  
OBJECT UNDER REFERENCE 
MEASUREMENT y 
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Fig. 1. Layout of the combined set-up for 3-D Fig. 2. Schematic layout of the whole field profilometer 
profilometric measurements. DHI: absolute double 
heterodyne interferometer; SC: optical X-Y scanner; 
CCD: CCD-camera; LCP: liquid crystal projector. 
2.1. The whole field profilometer 
In Fig. 2 a schematic layout of the profilometer is presented. The system is based on the projection of a Ronchi grating 
onto the object and on the observation, from a different perspective, of the pattern deformed according to the shape of 
the object. Both the CCD camera and the projector are at distance L from plane FH, on which the object to be measured 
is placed. This plane represents a reference for the profile measurement of the object. The projection and imaging units 
are at distance d from each other, and W is the width of the illuminated area. The image of the deformed pattern is 
acquired by a black and white video-camera and stored into a personal computer equipped with an advanced frame 
grabber. The image resolution is 512x512 pixels with 256 gray levels. The height evaluation procedure of the 
profilometer is based on two subsequent steps: in the first step, the grating deformed by the object and the reference 
grating are both elaborated: a suitable algorithm (Tang et al., 1990) allows to evaluate the two corresponding phase 
maps, which are then subtracted from each other, resulting in the relative phase map Ag(x,y). In the second step, the 
phase Ag(x,y) is converted into the height z(x,y) by means of triangulation, based on the following equation: 
Lpo(x, y) (1) 
Ay 2nd-«po(x y) 
2.2. The absolute distance meter 
The absolute (i.e. non-incremental) distance meter is shown in Fig. 3. In contrast to a conventional interferometer, 
which performs only displacement measurements, this set-up can be used to perform absolute distance 
measurements. The system is based on a double heterodyne interferometer (DHI) using a pair of tunable 
diode-pumped Nd:YAG lasers. As detailed in a previous paper, (Gelmini et al., 1993), by applying a suitable electronic 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop “From Pixels to Sequences”, Zurich, March 22-24 1995