The actual measurement is of The critical angle of total reflection. Rays having an angle lower than the critical angle are reflected from the interface with the liquid, rays with a steeper angle are lost into the liquid. An optical image is projected on a linear digital sensor, with a shadow edge indicating the critical angle.

Note: The rays measured by the digital sensor do not pass through the liquid. Particles, crystals, bubbles or color do not influence the measurement.

Note: The measurement is directly digital. The measurement signal is absolutely stable.

To get a reading expressed in percentage of the dissolved material, we need also the temperature of the liquid and a refractive index table of the specific material.

Note: An accurate measurement result requires a good temperature measurement. A probe inserted into the liquid is the only proper way to measure temperature.

Note: Refractive index measurement has been a standard laboratory practice for more than a century. Precise refractive index tables are available for any dissolved material.

In PolyChrome, the different measurement angles are color coded with the help of two diffraction gratings attached to the prism.. The optical image is transmitted through a single optical fiber. It means that the probe can have any length. A further diffraction grating recreates the optical image.