Materials characterisation by light scattering and reflectometry

 


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Imaging Reflectometer principles

 

 

Light directed at a surface is reflected and scattered according to the angle of incidence, wavelength and properties of the surface. The two key parameters defining the reflection and scattering are refractive index and surface roughness.

 

The refractive index is related to the composition and physical structure of the material surface. For a material like coated paper, it is strongly related to the surface void fraction (i.e. surface porosity).

 

 

Roughness may occur on various scales and in various forms, but many surfaces of industrial interest are characterised by random roughness. The exact roughness distribution may be rather complicated, but a two parameter model comprising “microroughness” (less than the wavelength of light) and “macroroughness” (much greater than the wavelength of light) is useful for describing many surfaces of industrial importance.

 

 

 

 


 


In the Imaging Reflectometer, collimated laser light is directed at the surface at a fixed angle of incidence (75°). At the surface, the light is scattered and the angular intensity distribution of the scattered light measured with a fixed imaging detector a limited angular range of about 18°.

 

The scattered light forms a 2-D scattering pattern (2-D reflectogram) which may be displayed in various forms. Usually the data is displayed as slices (1-D reflectograms) in the in-plane and out-of-plane directions.

 

Materials like coated or printed paper usually produce a fairly broad reflectogram (like that illustrated). In this case, the shape of the scattered light distribution is mostly determined by roughness features greater than the wavelength of light and can be expressed as an orientation distribution of surface slopes (or facets). Roughness at this scale is often termed macroroughness. The integrated intensity contained within some specified acceptance angle can be used to calculate gloss (by reference to a suitable standard) or other reflectance parameters.

 

 


 

 

 

 


The scattering pattern produced by macroroughness does not depend significantly on wavelength or angle of incidence. However, roughness features on a scale around or less than the wavelength of light (often called microroughness) cause scattering which is highly dependent on angle of incidence and wavelength. This is useful, because, by comparing reflectance properties at two wavelengths it is possible to obtain a quantitative measure of microroughness.

 

 


Further information can be extracted from the surface by using polarised light. The reflectance of light of different polarisation states is described by the Fresnel equations and depends upon the effective refractive index of the surface (and the angle of incidence). By comparing reflected intensities for light polarised normal and parallel to the surface, it is possible to derive the effective refractive index.

 

Thus the Imaging Reflectometer uses collimated laser light at two wavelengths and two polarisation states to determine a unique combination of useful data in a single measurement: refractive index, macroroughness, microroughness plus gloss and other measures of reflectance.

 

For further details on what the data mean, limitations in use and background theory, please follow the links in the Technical pages.

 


 

 

 

 

 

 


The instrument itself employs the latest opto-electronics: components with a strong reputation for robustness and longevity. The only moving part in the  basic instrument is the device to switch between standard and mapping illumination beams (large and small analysis spots). Optical alignment is fixed and no user adjustment is required.

 

The basic instrument layout is shown in the figure below:

 

 

 

 

 

 

In practice all quantitative measurements are referred to a highly polished glass standard of known refractive index. A CCD camera provides a colour view of the specimen, useful when setting maps or measuring specific sample features.

 

 

 

 

 

 

 

 

© Dayta Systems Ltd 2011