Carl Zeiss Optotechnik GmbH - Optische Messtechnik
Tuesday, 10 March 2015 08:55

Intelligent 3D digitization

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Fringe projection systems have become a well-accepted measurement tool in industrial metrology, as they are capable of generating large numbers of points in just a short time, in contrast to tactile methods. The limits of these systems are still tested, however, when different areas of the surface to be digitized exhibit stark differences in reflexive properties, and as a result, the dynamic range detected by the camera is no longer adequate. The COMET 6 sensor from Steinbichler Optotechnik GmbH takes a new approach in this regard by automatically adapting the amount of light projected based on the local reflexive properties of the measured object.

Authors: Christian Thamm & Sebastian Oberndorfner, Development 3D Digitization, Steinbichler Optotechnik GmbH

Compared with conventional coordinate measurement methods, fringe projection systems have certain undisputed advantages. Instead of tactile scanning of a few selected points, these systems allow contact-free, full-surface measurement of complex object surfaces with high point densities. Once the measurement data have been acquired and saved, features previously not taken into account can be evaluated subsequently, thus allowing for retrospective evaluation of the production process. Moreover, due to their compact size and light weight, the sensors can be transported and used in a variety of applications. Typical applications include quality control, reverse engineering and the design process, for instance.

The COMET 6 is the new high-end fringe projection system in the product portfolio of Steinbichler Optotechnik GmbH. Thanks to the system's modular design, the measurement field can quickly be modifiedto suit the measurement job at hand, with small operating distances to ensure maximum flexibility when large measurement fields are involved. The 16 mega-pixel camera provides resolutions previously unattained in fringe-projection applications.

On the projector side, tried-and-tested blue LED technology allows measurements to be carried out independently of ambient light. At the heart of blue LED technology is a high-performance LED which operates in the blue spectral range. Combined with a specially developed filter element, unwanted light effects such as daylight or ceiling lighting can be minimized, thereby significantly increasing the quality of the measured data. Constant LED output and maintenance-free operation are further advantages of blue LED technology.

Successful measurement with all fringe projection systems has previously hinged on an adequately diffuse reflexion of the projected light onto the surface to be digitized. This is due to the fact, amongst others, that directional reflexions result in excessive outshining or undercut effects of the fringe pattern in the camera image, which in turn prevents measured data being generated in the appropriate places. Thus when digitizing shiny objects, users are frequently obliged to prepare the object surface in advance with chalk spray in order to ensure a diffuse reflexion of the fringe pattern. But because this type of pretreatment of the object always entails extra work and in many cases is undesired by the user, often the only remaining option is the addition of further measurements with modified exposure times. This increases the total measurement time, making it more difficult to use fringe projection systems particularly in industrial applications.

The COMET 6 sensor meets this challenge with the brand-new Intelligent Light Control (ILC) technology, which adapts the intensity of the projected light to the reflexive properties at specific points, irrespective of the camera exposure. This is possible because the projection display allows pixellated modulation of the light intensity. Since the amount of detected light depends in particular on the orientation of the camera with respect to the measured object and the projector unit, such a method can only be used with a single-camera system.

Figure 1: Fringe pattern on a sheet metal panel.    
Outshining camera pixels are coloured red.
Figure 2: Fringe pattern on a sheet metal panel
with activated Intelligent Light Control.
The camera pixels are homogeneously balanced.

Each time a measurement begins, a brief series of different brightnesses is first projected onto the object area to be digitized in order to determine the ideal projection intensity individually for each camera pixel. A correspondence search also determines which part of the projection display is illuminating the portion of object surface observed by the respective camera pixel. This enables measurements with locally adapted light intensity and ensures homogeneous lighting of the measured object, thus ensuring optimal utilization of the camera's dynamic range.

For automated applications with a repeated accuracy of sensor positioning, through the use of an industrial robot for example, the defined correspondences and intensities can also be saved once and retrieved each time the measurement program commences, helping further decrease measurement times.

Shiny measured objects such as unmachined sheet metal panels, whose surface primarily reflects incidental light in a directed manner, previously placed high demands on fringe projecting systems. Intelligent Light Control in the new COMET 6 sensor is an innovative method for digitizing such measured objects more efficiently in future.

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