One of the major drawbacks of the Torrance-Sparrow model, and similar physical-optics approaches of specular reflection from rough surfaces derives right from this fact, i.e. In this model the reflectance is based on geometrical optics, hence it is applicable only when the surface is rough (has sufficiently large facets) in comparison to the wavelength of the incident radiation. One of the first, and probably most popular models is the Torrance-Sparrow model, which provides a direct physical connection between a rough metal surface and its visual appearance. There are theoretical models predicting this fact. Variable surface roughness affects the relative ratio of specular to diffuse reflectance, and can hence be observed as contrast change in the appearance of the surface, if viewed in a particular direction. The latter ones are the result of diffuse reflection, which can arise from multiple surface reflections in a rough, facetted surface as well as diffraction scattering from the microtopography of the top surface. A rough metal surface has multiple reflection lobes: a directional specular beam with superimposed multi-directional lobes. Specular reflection is the mirror-like reflection at the air-surface interface, and it occurs when the incident light is reflected from a smooth surface. Reflection from a surface is a combination of both diffuse and specular reflection components. Heikkilä, in 4M 2006 - Second International Conference on Multi-Material Micro Manufacture, 2006 3 Light scattering and reflectance For a comprehensive and critical discussion of this theory the reader is referred to the Further reading section.Ī. This mathematical procedure is generally performed according to the Kubelka–Munk theory. The resulting diffuse reflection spectrum then has to be corrected in order to correspond to the absorbance of a transmission spectrum. The disturbance by specular reflection may be reduced considerably by technical means (trapping) on the reflection attachment. d̄ is found to be inversely proportional to the absorption coefficient α, thus leading to a certain leveling of the band intensities. Nevertheless, DRIFT spectroscopy is confronted with two intrinsic problems: (i) the superposition of diffuse and specular reflected light (see Figure 2), which may lead to distorted line shapes, and (ii) the dependence of the mean penetration depth d̄ on the absorption coefficient. A number of high-performance reflection accessories are available from different manufacturers (see below), allowing the detection of quantities down to the nanogram region. the depth of light penetration into the surface layer which results in an intensity decrease by a factor of 1/e, and α denotes the napierian absorption coefficient.ĭiffuse reflectance infrared Fourier transform spectroscopy (DRIFT) has become a frequently used technique to obtain IR spectra from materials intractable by transmission spectroscopy. Where d̄ is the mean penetrated layer thickness, i.e. The situation is further complicated when light penetrates inside the matter, is partially absorbed, and then scattered back to the medium and mixed with diffused reflection from the rough surface. In each case, the law of reflection is followed, resulting in five diffused reflected rays (labelled A /, B /, C /, D /, and E /). The normal line (approximated) at each point of incidence is shown without direction sign. In the figure, five incident rays (labelled A, B, C, D, and E) approach a surface. The result is that the concentrated bundle of rays of light that is incident upon the rough surface is reflected diffusely. Subsequently, when the individual rays reflect off the rough surface according to the law of reflection, they scatter in different directions. The normal line at the point of incidence is different for different rays. However, the roughness of the material means that each individual ray meets a surface that has a different orientation. The reflected light from each individual incident ray follows the law of reflection. Asim Kumar Roy Choudhury, in Principles of Colour and Appearance Measurement, 2014 Rough surfacesįigure 3.1c shows diffuse reflection from a rough surface.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |