Appendix B: Principle Of Measurement; Figure 141 Refractometer Principle - Vaisala PR-23 Series User Manual

PR-23 Series User Guide IM-EN-PR23-E
Appendix B. Principle of
measurement
The Vaisala K‑PATENTSâ in‑line refractometer determines the refractive index n of the
D
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 ). Light from the light
D
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 different angles.
A C
B
L
P
M M
S
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 can then be
D
determined from this position.
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