Precise 3D surface inspection and defect detection

Glenn Wedgbrow looks at a sensor solution for surface inspection and defect detection on highly reflective and transparent surfaces. Due to their phase-measuring deflectometry, the sensors can be used for full-surface measurements and deliver precise measurements

The reflectCONTROL series of 3D sensors from Micro-Epsilon use phase-measuring deflectometry to precisely analyse reflective surfaces. A striped pattern is projected onto the surface and its reflection is captured by two cameras. The sensor calculates a 3D point cloud of the surface structure from the images, making unevenness, scratches and other defects visible. The sensor can be integrated in a stationary manner or moved over the target on a robot. Localised abnormalities or defects are evaluated and displayed in the CAD data.

The RCS130-160 3D HLP sensor from Micro-Epsilon is specially designed for measurement and inspection tasks in production lines and has a GigE Vision interface. This provides GenICam-compliant data, enabling seamless integration into existing image processing systems.

Due to the improved camera arrangement, the sensor delivers sharper 2D images than its predecessor model as well as a 3D representation of highly reflective surfaces. It can be used to determine the topology of the components (e.g. flatness, deflection and curvature). The high z-axis resolution in the nanometre range and excellent repeatability of <1µm, in combination with its large measuring range of approximately 170mm x 160mm, enable the sensor to record up to five million data points of the surface to be measured within a few seconds. A conventional point sensor with an X-Y scan would need at least half an hour to do this.

Semiconductor, automotive and glass

reflectCONTROL sensors can be used in semiconductor production for example, where they can detect the exact shape of a wafer. The sensors are also ideal for the automotive industry: even the smallest surface defects of less than 1µm can be detected precisely and reliably during the surface inspection of painted attachments. The sensors are also used in the quality inspection of mirrors, flat glass (e.g. smartphone production) and in the surface inspection of head-up displays.

Furthermore, reflectCONTROL sensors can be used together with Micro-Epsilon’s 3DInspect software, which transmits measurement data from the sensor to a PC via Ethernet and visualises it in 3D. This 3D data is processed further, evaluated and assessed with 3DInspect and, if necessary, logged and transmitted to a control unit. The 3D data can also be saved.

Measuring principle in detail

The reflectCONTROL sensor works according to the principle of phase-measuring deflectometry. The method is particularly suitable for high precision defect detection and three-dimensional geometrical detection of flat reflective objects. With deflectometry, the surface itself is not examined but its optically distorted or intensity weakening effect, which shows itself in the mirror image of a pattern. In the process, a sine pattern is shown on a display and then the reflection of this pattern is recorded by cameras. A few images are taken with CCD cameras with a phase shift of the displayed pattern. The data obtained is processed using special algorithms to determine curvatures and intensity amplitudes over the entire surface.

Deflectometry systems are superior to other optical methods in that the required local curvature, intensity and amplitude data are available over the entire area captured by the camera. The advantage of the stereo measuring system lies in the simultaneous image acquisition of two cameras from different directions. The combined evaluation of the data from both cameras enables a 3D reconstruction of the measuring object with nanometer precision.

Glenn Wedgbrow, Business Development Manager at Micro-Epsilon UK.