Proceedings of Topical Meeting on Optoinformatics (pdf-format, 1.21 ...

SAINT-PETERSBURG, October 17 – 20, 2005 21
3-D MEASUREMENT OF AUTOMOTIVE GLASS BY USING A
REFLECTIVE FRINGE TECHNIQUE
Oleksandr A. Skydan, Michael J. Lalor and David R. Burton
General Engineering Research Institute, Liverpool John Moores University,
James Parsons Building, Byrom Street, Liverpool, L3 3AF, England, UK
There has been much interest in the automotive industry in developing non-contact
techniques for measurement <strong>of</strong> reflective surfaces to provide in-line glass shape quality
control system. This presentation describes a technique for the measurement <strong>of</strong> non fullfield
reflective surfaces <strong>of</strong> automotive glass by using a reflective fringe technique. The
theoretical principles <strong>of</strong> phase demodulation using a basic four-steps algorithm and further
3-D height reconstruction procedures in the case <strong>of</strong> measuring surfaces with specular
reflective properties like curved glass are explained.
Physical properties <strong>of</strong> the measurement surfaces do not allow us to apply optical
geometries used in existing techniques for surface measurement based upon direct fringe
pattern illumination. However, this property <strong>of</strong> surface reflectivity can be used to
implement similar ideas from existing techniques in a new improved method. In other
words the reflective surface can be used as a mirror to reflect illuminated fringe patterns
onto a screen behind. It has been found that in the case <strong>of</strong> implementing the reflective
fringe technique, the phase shift distribution depends not only on the height <strong>of</strong> the object
but also on the slope in each measurement point. This requires the solving <strong>of</strong> differential
equations to find the surface slope and height distributions in the x and y directions and
development <strong>of</strong> the additional height reconstruction algorithms.
The main focus has been made on developing a mathematical model <strong>of</strong> the optical
sub-system and discussing ways for its practical implementation including calibration
routines, and possible problems which may arise during real measurement processes.
Figure 1 shows the optical system used in the reflected-fringe technique.
Camera
Screen
x
Projector A 2
z
Surface element
y
H
Figure 1. Fringe reflection optical system
A surface point A is observed by the CCD camera. The beam from a point on the
screen A 1 reflects via surface point A to the CCD camera. By tilting the surface element at
a different angle α and changing the surface element height the surface point A will reflect
point A 2 from the screen onto the CCD camera. This means that the shift between the
reference point and the measured point <strong>of</strong> the object is proportional to the surface slope
and its height. When a fringe pattern is illuminated by the LCD projector, for example, the
fringes are distorted in accordance with the slope and height <strong>of</strong> the measurement object in
the x and y directions.
h
A
R
θ
α
z y
A 1