White-Light Interferometry

The measurement principle of the White-Light Interferometer makes use of the constructive and destructive interferences of light beams of the same wavelength and phase. In general, a light beam which is emitted from an integrated light source is separated into two beams by a beam splitter. One beam is directed onto the device under test (DUT) and is partially reflected while the other beam is reflected by a reference mirror at a known distance. Both beams are then superposed and interfere with each other. The resulting intensity is detected by a photo element. The important characteristic of the white light interferometer is the broad wave length spectrum which leads to a very short coherence length, so that the constructive interference maximum can be unambiguously detected when the distance to the reference mirror is exactly the same as the distance to the DUT. Theoretically, a vertical resolution of 0.5 µm is attainable.

The WLI Technology we use at 3D.aero has many special features that differentiate the process from other surface data measurement methods like camera-based systems. For one, the White Light Interferometry is not influenced by any reflective surface that might disturb visual measurements. The Smart Pixel Technology allows the inspection of nearly any surface condition with no preparation required. Therefore, almost any workpiece can be measured without having any trouble of disturbances. The WLI Technology also has the feature that the measurements happen in the direction, the measuring head is facing. This coaxial measurement technique allows for deeper geometrical insights into drill holes for example, which gives far more reliable information in comparison to only visual surface measuring methods. At last, the incredible high resolution of the WLI process is one of the most impressive features. The system delivers a resolution of about 0.2 μm in the x- and y-axis. The z-axis has a resolution of up to 100 nm in total. With this high resolution, even μm-scratches from the workpiece production can be made visible. Especially the function that the lateral sensor resolution is variable, so it can be defined based on customer requirements (typical lateral resolution 5-20µm), is a highly use-case oriented function.

Conclusion

In conclusion, the White-Light Interferometry Technology delivers incredibly high resolution surface data that can be used for any surface examination. The system can measure deep insights into drill holes and even create a full 3D data set for the whole workpiece. The measurement of one 250 x 250 pixel section only takes about 400-600 ms, which shortens the complete measurement process compared to other methods by far. In general, the technology can be applied to any surface examination that comes to mind.

Learn more about the products that use the WLI Technology:

AutoInspect
Cooling Hole Inspection