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Refractive Index (RI)

Refractive index (RI) determination at PTL is primarily used as a supportive technique to provide more accurate particle size distributions from light scattering techniques (such as laser diffraction).  These techniques are dependent on the refractive index values of both the particle and carrier medium (e.g. liquid or air). PTL uses two approaches when determining the refractive index of particles and/or liquids.

Liquid Refractive Index Determination

The most common way to determine the refractive index of a liquid is an automated refractometer. In this technique, the liquid is placed on a measuring prism and illuminated by an LED. The incident beam reflected from the sample is detected by a sensor and the angle of total reflection is calculated and used to determine the refractive index value. In simple terms, refractive index (RI) is a dimensionless number which represents the bend of a ray of light while passing through a material. In some cases, solid particles can be dissolved into liquid across a range of concentrations and a series of refractometer tests on the solution can determine the RI of the solid particle.   Specific particle properties are required for this to be successful, please contact PTL to discuss if this approach is applicable for your material

Particle Refractive Index Determination

In other cases, the refractive index of a crystalline solid material can be determined using Becke Line testing. This technique is an optical microscopy test that determines the relative refractive index of a specimen in comparison to its surrounding medium, typically a certified oil of known refractive index. By adjusting the focus of the microscope, a bright line called the Becke Line appears at the boundary between the particle and the surround medium. Its movement into or away from the particle indicates whether the specimen has a higher or lower refractive index in comparison to the surrounding medium.  The particles are evaluated across a series of known refractive index oils until the particle appears to vanish or nearly disappear.   At this point, the particle and oil are matched in refractive index.

PTL’s approaches for refractive index determination are intended to support particle sizing techniques as previously mentioned.  Most instrument software which use these values for result calculation do not require high resolution values.  As a result, the refractive index values provided by PTL for these techniques may not be as sensitive compared to more advanced technologies used in other fields.

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  • Automated refractometer is temperature controlled and performed at 589.3 nm wavelength
  • Values provided are well suited for common light scattering particle sizing techniques such as laser diffraction and dynamic light scattering
  • Can report refractive index values from approximately 1.30 to 1.70


  • Testing for Becke Line requires transparent crystalline material > 40 microns
  • Particle refractive index values are reported to a maximum of two significant digits
  • PTL is not able to determine the particle’s absorption (imaginary value) directly

Sample Requirements

Ideal amount of sample is 2 grams for solid samples, and 20 ml for liquids

The above sample quantities are preferred. If sample availability is further limited, please contact us to share specifics about your sample and options for suitable sample quantities.

Detection Range

PTL can report refractive index values for the Becke Line method from 1.40 to 1.70 using Cargille oils.  This range is typical for many organic materials such as pharmaceuticals, polymers and food.

Data Reported