Aspherical lenses are usually generated by a multi-axis computer numerically controlled machine and axis guidance
errors as well as wear and environmental influences lead to unavoidable form deviations. Therefore, the manufacturing
process is often performed iteratively with intermitting measurement steps outside of the manufacturing machine and
repositioning the sample into the machine, which is causing additional errors.
We present a new deflectometric sensor designed for the machine integration, so that the form measurement is done
inside of the manufacturing machine and errors due to the sample removal are avoided.
The compact and robust sensor is based on the deflectometry principle. It detects the deflection angle of a focused laser
beam on the surface under test and measures the local slope angle of the surface in 2D. By scanning the specimen's
surface using the machine axes and integration of the slope angles, the topography can be calculated. The angular
measurement range of +/-9.5° permits the measurement of highly aspheric surfaces, e.g. at a clear aperture of 8 mm a
maximum deviation of more than 500 μm can be measured at a resolution on the nanometer scale.
Image processing and thermography for its own are very versatile and established measurement techniques for many
years. However, the combination of these two measurement technologies opens a new field of applications. The online
monitoring of the laser-brazing of titanium overlap joints is such a new application. The laser brazing process for overlap
joining of formed titanium sheets for the production of heat exchangers is presently being investigated at the Fraunhofer
IPT. In comparison to conventional furnace brazing the laser brazing technology decreases substantially the heat impact
and thus reduces the thermal material damage in the parts due to local selective heating in a laser beam focal spot. Even
though the process is stable, errors in the brazing seam such as pores or unacceptable material oxidation can occur. To
ensure a high quality an online process monitoring or even process control is necessary. But since the surface remains
unchanged during this brazing process no geometrical inspection of the surface can be conducted. Therefore today's
quality assurance performs x-ray or destructive testing. This paper demonstrates how the use of thermography in
combination with image processing allows a machine integrated online monitoring of the laser brazing process. First the
basic principals are presented which cover the fields of heat coupling, heat transmission and heat distribution as well as
the temperature emission of light and the spectral properties of the laser beam shaping optic and so lead to the optical
set-up. Then analysis algorithms are derived which characterize the process, detect process failures and make a seam
tracking possible.
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