Combination of the well-tried measuring principle of the integrating sphere with the radiative transfer theory: precise method for the determination of optical properties

For the determining of optical properties with an integrating sphere, a sample layer is placed at an opening of an integrating a port and illuminated once through the sphere and once again from the outside in a punctiform manner. The total diffuse reflectance and transmittance of the sample is measured spectrally resolved with a spectrometer. From these two measured quantities, the absorption and effective scattering coefficients for each wavelength can be determined by comparison with the solution of the radiative transfer equation.

can be determined (see figure below left). At the ILM, the method was optimized taking into account all possible sphere errors (e.g. loss due to open ports, direct illumination of the detector) and validated on a large number of phantom measurements. With an optimized integrating sphere was created using modern 3D printing and professionally coated on the inside. In general, the integrating sphere is often regarded as a reference method for determining optical properties.

 

Advantages of separating absorption and effective scattering coefficient:

  • The determined absorption coefficient is identical to the absorption coefficient measured on clear samples with an absorption photometer
  • The determined absorption coefficient shows a direct correlation with the concentrations of the ingredients
  • A change in the microstructure of the sample is imaged in the effective scattering coefficient and has no effect on the absorption coefficient

[1] Foschum, Florian, Florian Bergmann, and Alwin Kienle. "Precise determination of the optical properties of turbid media using an optimized integrating sphere and advanced Monte Carlo simulations. Part 1: Theory." Applied Optics 59 (2020): 3203-3215.

[2] Bergmann, Florian, et al. "Precise determination of the optical properties of turbid media using an optimized integrating sphere and advanced Monte Carlo simulations. Part 2: experiments." Applied Optics 59 (2020): 3216-3226.

Contact persons

Dr. Florian Foschum

Group Leader Calibration-Free Sensors

Tel: +49 (0)731 / 1429 779

Prof. Dr. Alwin Kienle

Head of Materials Optics & Imaging

Tel: +49 (0)731 / 1429 224

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