The scattering of light from periodic micro-structures on the surface of plastic films is the basis of the optical appearance found in, for example, premium gift-wrap and anti-counterfeit film on high-value toiletries. These optical grating structures are typically produced in high volume using roll to roll processes such as thermo-mechanical embossing.
Adaptable, high-speed and quantitative verification of such optical grating structures over vast regions is both slow and prohibitively expensive to achieve with conventional instruments due to the high dynamic range challenge.
Currently, a sparsely sampled global inspection ‘by eye’ is employed, which is highly subjective and suffers from poor repeatability.
To reimagine the metrology solution, the National Physical Laboratory (NPL) and Fraunhofer are developing and applying a variety of Fourier imaging techniques for the affordable inspection of these optical gratings.
Key Innovative Features
The systems exploit the optical Fourier transform to probe key spatial frequencies in the light scattered from the grating structure in order to deduce key functional and geometrical information.
Coupled with correlated defect information captured at the product design stage, the Fourier imaging techniques can also be used to efficiently detect and locate the signature of key defects. A number of reduced-cost inspection systems have been realised from the Fourier imaging concept, including:
- Fourier space imaging: A scanning Fourier point probe that interrogates the distribution of spatial frequencies present in local regions of the structure.
- Fourier filtered imaging: A full field-of-view mapping system that interrogates application specific spatial frequencies in a single-shot global measurement.
The Fourier imaging systems can be tailored toward specific performance, cost and compatibility requirements, by the substitution of key components. The systems are easily scalable for monitoring larger substrates in-situ, via parallelisation or cross-web scanning. Alternatively, the systems may be implemented in the user’s offline inspection.
Image processing algorithms are employed to interpret the acquired data based on surface structure models, yielding user-relevant information such as:
- Grating period and orientation (Fidelity of pattern reproduction)
- Diffraction grating efficiency (Brightness and contrast of pattern)
- Sparse defect distribution (Clarity and homogeneity of aesthetic function)
The systems can also be configured to monitor the surface of the embossing roller surface for wear.
Markets and Applications:
Structured surfaces are increasingly being used as a cost-effective way to impart both aesthetic (attractive appearance and tactile qualities) and functional properties
(integrated security, self cleaning and safety features) to products and their packaging.
The NanoMend partners are keen to collaborate with companies to apply Fourier imaging techniques for quantitative inline quality control, and to develop metrology best practice both for users’ offline product development and for inline routine inspection.
The technology is aimed at the in-line inspection of optical gratings produced on nominally planar surfaces. However, the technology can be applied to other well defined nanostructured surfaces, or even to smooth, featureless surfaces for general defect detection and classification (e.g. particulate contamination, inclusions and scratches).