The
optical course of a light ray when it crosses the interface between two
different materials, is deverted due to refraction phenomenon.
An extreme condition of refraction (total reflection) is that in which
the ray does not penetrate into the second material but is reflected as
if the ray had encountered a mirror. The incidence angle of the ray in
which this phenomenon occurs is called the "critical angle".
The magnitude of the critical angle is strictly tied to the refraction
indices of the two neighboring materials. If the refraction index of one
of the two materials is known, the measurement of the critical angle allows
to determine the refraction index of the second material. The refraction
index is a physical characteristic specific to each material.
In case of liquid's analysis, it depends on the concentration of the solute.
The Maselli analysis system measures a liquid's refraction index by measuring the critical angle, taking advantage of the total reflection phenomenon, and it is composed of the following elements:
- light source
- biconvex lens that captures the beam from the source
- input lens
- measuring prism with known refraction index
- reflex beam focusing lens
- optical sensor
The total reflection phenomenon causes the original beam to be partially reflected by the interface and creates an image divided into two parts: one luminous (reflected rays) and one dark (refracted rays). The position of the separation line between light and dark is dependent on the critical angle and therefore of the nd of the liquid being analyzed. Using a CCD optical sensor, the magnitude of the critical angle is detected and a microprocessor processes the data and calculates the nd of the product being analyzed.