True specular reflectance is a surface measurement technique that works on the principle of reflective efficiencies. This principle states that every sample has a refractive index that varies with the frequency of light to which it is exposed. Instead of examining the energy that passes through the sample, true specular reflectance measures the energy that is reflected off the surface of a sample, or its refractive index. By examining the frequency bands in which the rate of change in the refractive index is high, users can make assumptions regarding the absorbency of the sample. The true specular reflectance technique provides excellent qualitative data.
Reflection-absorption works on the same principle, but due to sample properties, some of the energy passes through the surface layer, is absorbed into the bulk of the sample, and then reflects off a substrate below the surface layer. A combination of true specular reflectance and reflection-absorption can occur when criteria for both techniques are met. If a qualitative comparison to transmission spectra is desired, users can apply the Kramers-Kronig correction to the data to remove the effects of dispersion.
Specular reflectance is commonly used for the analysis of both organic and inorganic samples having large, flat, reflective surfaces. Reflection-absorption can occur when one of the above criteria is compromised and the sample has a reflective substrate present just below the surface. This type of analysis is commonly used for:
- Metallic surfaces
- Thin films on reflective substrates
- Silicon wafers
- Laminated materials on metals
- Sensitivity to monolayer samples – can detect Angstrom thick coatings on metal substrates
- Nondestructive analysis – no contact or sample damage during analysis
- Wide range of accessories available – can utilize main spectrometer and microscope accessories depending on the size of the sample and the thickness of surface layer
Dr. Michael Bradley received his BS degree in chemistry from the University of South Carolina and his PhD in physical chemistry from the University of Illinois, and also completed his MBA in management. He taught graduate and undergraduate chemistry for 15 years, prior to becoming a field applications scientist with Thermo Nicolet, subsequently Thermo Fisher Scientific, in 2002.
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