Chromatic Aberration
Optical design for precision surface metrology exploits the longitudinal dispersion of light to map physical distances. A hyper-chromatic lens deliberately amplifies chromatic aberration rather than correcting it, spreading the focal points of different wavelengths along the optical axis. This dispersion creates a continuous sequence of focal planes corresponding to specific wavelengths.
The system determines the distance to the substrate by identifying which wavelength is in sharpest focus on the surface.
Surface Profiling
The measurement of paper roughness and coating thickness relies on the high resolution of the returned spectral signals. Light reflected from the surface of the paperboard passes back through the hyper-chromatic lens to a spectrometer. Because each wavelength corresponds to a unique distance, the peak intensity in the spectrum reveals the precise height of the surface at that point.
This non-contact method avoids damaging delicate coatings or wet inks during high-speed production runs. It provides real-time topography data that traditional mechanical styluses cannot capture. Variations in paper gloss or absorption do not affect the height calculation because the measurement relies entirely on spectral wavelength rather than signal amplitude.
Metrology Application
Inline quality control of barrier coatings requires sub-micron accuracy to ensure barrier properties are uniform across the entire web. The dispersion range of the lens defines the total measurement window. A lens with a large dispersion range covers thicker coatings but reduces the axial resolution.
Therefore, the optical configuration must balance the required measurement range against the precision needed for the specific substrate.
