The tool is placed on a rotating table so that by turning
the cutting edges can be put in a position parallel to the
reseau grids and the cameras. Otherwise some edges could
be hidden by the tool. As measurements with an accuracy
of a few microns in the object space are influenced by
a variation in temperature a device for the measuring of
the temperature with six single sensors was also installed.
Fig.2 shows the laboratory layout (without the PC).
Fig.2: photograph of the laboratory layout
The arrangement of the beam-splitters and the course of
the light rays is explained in Fig.5 and Fig.6.
3. Orientation of the Cameras
The layout shown in Fig.2 was mainly chosen because of
the very small angular aperture of the CCD-cameras. The
combination of a 1/2" -CCD-array with a principal distance
of 120 mm gives an angular aperture of only 4 gons. Under
these conditions the orientation parameters are highly cor-
related and the orientation is unstable. So the original idea
of placing the cameras between the tool and the reseau
grid as shown in Fig.3 had to be rejected.
tool cameras reseau
Fig.3 : rejected layout
The position of the cameras can only be computed with
an accuracy of a few millimeters which does not matter, if
the unknown object points lie in the object space surroun-
ded by control points. But if there is an extrapolation like
in Fig. 3 the accuracy of the projection centres is essen-
tial and the computation of the object point coordinates
difficult.
Therefore several different configurations were tested. Two
of them are presented with theoretical and empirical re-
sults.
In order to render the orientation parameters more sta-
ble, the angular aperture of the cameras was artificially
widened by adding another beam splitter to each camera
pointing to another reseau grid. Fig. 4 and Fig. 5 show
these layouts, in one case the tool opposite to the reseau
grids and in the other case the tool between them. Fig. 5
corresponds to the finally chosen layout of Fig. 2.
tool cameras reseau grids
Fig.4 : layout 1
1
reseau grids + tool
cameras
Fig.5 : layout 2