The measurement system in use. The complete surface is checked on the production line for defects.  (Source: BMW)

The identification of surface structures and defects plays an important role in process and quality controls. The projection of a stripe system on a reflective surface shows up form deviations and faults in the reflected light. The analysis of the reflections enables the safe detection of form deviations and defects on the surface, right down into the nanometer area.

Faults on the surface of reflective objects are noticed by the eye and reduce the quality of the product. The analysis of errors is often difficult to perform because it is partially manual and is not easy to automate. Deflectometry is an automatic optical procedure for the industrial error analysis of reflective surfaces, which allows qualitative and quantitative conclusions on forms, surface structures and defects to be made quickly and reliably.

The measuring principle generates a system of straight stripes with a sine-shaped contrast distribution, which is projected onto the object to be tested. The stripe system is, for example, generated on a screen with white light diodes. Curvatures on the surface deform the linear alignment of the stripes and provide a more or less deformed image. The reflected stripes are recorded with a camera. The angle of the surface normals to the local curvature of the reflected stripes is analyzed as measured data. These data are processed in a computer with special software.

With the analysis of the image data, different errors are definitively identified and clearly differentiated. While a dent causes a small change in the surface curvature over a comparably large area, the curvature change with an inclusion is very big over a small area. The algorithms used for the automatic error analysis and classification are therefore very complex.

Depending on how much an object is reflected, various black-white contrasts are required. With use on lenses with greater focal lengths in combination with adjusted stripe patterns, it is possible to achieve resolutions down into the area ranging from tenths of a micrometer to the nanometer area. Depending on the display size and the resolution of the camera, only a limited surface area of the object can be examined in a measurement process. A complete surface check may therefore be necessary with bigger objects.

The RC-Robotic system from Micro-Epsilon (picture) is an example of objects as big as car bodies. The optical part of the measurement system is used here as a sensor on a robot. A measurement process covers an area of 80 x 60 cm2. The robot moves the measurement system to various positions around the object to be inspected. The full inspection of a car body is completed on an industrial production line in cycle time with four RC-Robotics within 60 seconds. The defects found are then shown on the object by a marking robot.

Deflectometry is generally a good solution for measuring all reflective surfaces, e.g. in the automotive industry, the aviation industry, the painting industry, and for solar cells and liquids. Several research institutes, such as the Bremer Institut für angewandte Strahltechnik - BIAS (Bremen Institute for Applied Laser Technology), the Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Fraunhofer Institute for Applied Optics and Fine Mechanics) and the Ravensburg-Weingarten University are currently working in this area. BIAS was awarded the "Innovationspreis Lasertechnik 2007" (innovation award for laser technology) for its "Zebra" stripe analysis software. This very reasonably priced contact-less optical measurement technology is also used in inspecting round and cylindrical objects.