Reflectance and Transmittance

The human eye needs reflected light to see the color or texture of objects as well as to identify shapes and patterns and to sense the distance of an object. To a spectrophotometer, reflection is simply the fraction of light reflected from a surface as a function of wavelength. Spectral reflectance can gather much of the same information as the eye, but it does much more quantitatively and objectively. Reflectance measurements can identify the color and the texture of a sample surface as well as examine differences between objects for sorting or quality control. It derives information about the material by measuring the light that is absorbed instead of reflected by a sample. The measurement can be repeated endlessly without altering the sample. 

The Science

Reflection is a special case of diffuse backscatter and refers to the directionally reflected light returned back from low-scatter, smooth surfaces. In addition to the sensor, as spectrometer system requires a light source with a known spectral quality. Reflectivity is strongly dependent on the angle α of the light source. The most simple setup for measurements at α= 0° can be obtained using a special light guide which supplies the light and directs it to the detector. Many reflection spectra do not display any marked structures. For this reason, high absolute wavelength accuracy is considerably more important in many cases than good spectral resolution. 

Diffuse reflection measurement

The diffuse reflection of scattered light from rough surfaces supplies information on the color of the surface. The position of the light source is important for this procedure and in most cases, a light source with a wide-band emission, e.g. a halogen lamp, is used. It is often sufficient to bring the input of the cross section converter close to the colored surface to be measured without using additional optics. There are spectrometer modules that have been specially designed for color measurement. Their high repeatability and light intensity, combined with moderate spectral resolution, exactly meet the demands for sample readings made in this field. 

 

Examples of use

  • Color measurement of different surfaces or materials
  • Condition and presence of coatings
  • Antireflection coatings of surfaces using metals or dielectric coatings
  • Ellipsometry
  • Determining the fat content of meat and sausages
  • Determining the humidity content in cereals, food and cellulose
  • Plastic identification for recycling and disposal
  • Analysis of paper quality
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