In simple terms, refractive index (RI) is a dimensionless number that represents the measurement of the bend of a ray of light while passing through a material. It is calculated from the ratio of the speed of light in a vacuum to that of a second medium (solid, liquid or gas). Knowing and using the correct refractive index of a suspension and particle is critical to generating representative particle size distributions using light scattering techniques such as laser diffraction.
Laser diffraction generally uses either Mie theory or Fraunhofer Approximation to calculate the particle size distribution (on a volume basis) from a captured scattering matrix. If using Mie theory, knowledge of the optical properties of the material being measured (real refractive index and imaginary component) and the dispersant (real refractive index only) should be known. The real refractive index of common materials can often be found from published data.
The International Organization for Standardization (ISO)13320:2009 (Particle Size Analysis – Laser Diffraction) defines the complex refractive index of a particle as consisting of both a real and an imaginary (absorption) component. For particles where the real refractive index is not known, they can be measured directly using a Becke line test (if applicable), for example. For uncommon dispersants without known literature values, the real refractive index can be determined by using a refractometer.
The imaginary or absorption component of a particle represents the reduction of a light beam's intensity that is not due to scattering. Particle properties, such as surface roughness, texture, and reflectivity; internal reflection or refraction; or color and opacity (i.e. actual absorption) can affect the beam intensity, thereby influencing the absorption value. An appropriate imaginary value can be estimated by evaluating the fit between modeled and actual data determined by laser diffraction analysis.