Vaisala PR-23 Series User Manual

Appendix B. Principle of
measur
ement
The Vaisala K‑PATENTSâ in‑line refractometer determines the refractive index n
D
of the
process solution. It measures the critical angle of refraction using a yellow LED light source
with the same wavelength (589 nm) as the sodium D line (hence n
D
). Light from the light
source (L) in the figure below is directed to the interface between the prism (P) and the
process medium (S). Two of the prism surfaces (M) act as mirrors bending the light rays so
that they meet the interface at dierent angles.
L
P
MM
S
A C B
Figure 141 Refractometer principle
The
reflected rays of light form an image (ACB), where (C) is the position of the critical
angle ray. The rays at (A) are totally internally reflected at the process interface, the rays at
(B) are partially reflected and partially refracted into the process solution. In this way the
optical image is divided into a light area (A) and a dark area (B). The position of the shadow
edge (C) indicates the value of the critical angle. The refractive index n
D
can then be
determined from this position.
PR-23 Series User Guide
IM-EN-PR23-E
258

Vaisala PR-23 Series Specifications

General

Brand	Vaisala
Model	PR-23 Series
Category	Measuring Instruments
Language	English