## 3 Selected Results of Optical Metrology Techniques
### Techniques Based on Geometrical Optics Suitable for In-Process and In Situ Applications
#### 3.2.1 Measurement of Wing Shapes Using Fringe Projection  
The demand for a reduction of the fuel consumption and therefore \(CO_{2}\)-emission of aircrafts requires new technologies. One of them is to minimize turbulences on wings by manufacturing the surface with high quality. In the framework of the EU-Project _CleanSky_, we have developed a metrology technique that allows for precise form measurement during the wing production process [43]. Within the FP7 context of the European Union's CleanSky program, carried out as BLADE (Breakthrough Laminar Aircraft Demonstrator in Europe) under management of Airbus between 2011 and 2017, technology was being developed to build airplane wings for passenger planes that allow Natural Laminar Flow (NLF) under cruise conditions. \"Natural\" in this context means unsupported wings by extra flow sheets through micro-bores. The manufacturing of the surface requires tight tolerances on wing waviness and roughness. During manufacturing of NFL surfaces, an adequate quality control is therefore mandatory. At the start of BLADE, no measurement system existed that was capable of measuring large areas of wing surfaces with high lateral and depth resolution. For the purpose of the project, CleanSky's BLADE flight lab, an Airbus A 340-300, was used. It is the first aircraft in the world combining a transonic laminar wing profile with a standard aircraft internal primary structure. Our institute, together with the VEW Corp. Bremen, supported the BLADE demonstrator with the development and application of an innovative metrology system to measure and evaluate profile, waviness, steps, gaps, 3D disturbances, and surface roughness of the wings from the beginning of their manufacturing up to the final assembly phase [43; 44]. The measurement system is an FP-based system, custom built for the specific requirements of wing metrology. The nominal field of view is 1300 \(\times\) 800 mm\({}^{2}\), making it perfectly suited for measurements in inter-rib spaces, the nose and on the upper cover of the BLADE wings with high accuracy. It can be adjusted to any orientation desired, and its position can be registered with laser tracker assistance. This makes it possible to locate unconnected surface segments correctly in 3D space, thus eliminating the need for large overlaps between surface sub-areas. To prevent problems of specular reflection of a glossy finish, the system can be equipped with three or more recording cameras. The depth resolution, in particular, the ability of the system to retrieve waviness, has been tested with a reference artifact that was designed in collaboration between Airbus and the National Physical Laboratory (NPL)of the UK and certified at the NPL. The artifact contains numerous features that are relevant to the BLADE metrology program. The measurement system is required to detect waves of different lengths and amplitudes. The smallest wave present on the test artifact has an amplitude of 100 \(\upmu\)m, and the smallest steps are 50 \(\upmu\)m high. As an example, we report on measurements of an Invar (nickel-iron alloy) Outer Mold Line (OML) tool (size 9 \(\times\) 2.1 m), which will serve as the exterior form of the experimental wing cover, and provide indications of the achievable 3D resolution in individual and combined measurements, as well as estimates on the match of the tooling surface against the CAD model [45]. In Figure 8, the measurement system is presented while measuring above the OML tool. In this case, a stereo-camera setup with an LED projector is used. For complete coverage of the tooling surface, over 50 individual measurements were necessary, generating a very large collection of data amounting to over 170 million points. Figure 9 combines the measured point cloud and the nominal CAD data. Blue and red marked areas are slightly over the defined specifications. The surface was brought into the tolerance band over its entire surface by tweaking the frame support.