UMPIOXR400V2-7-1EN, 2012-07-01
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2 Measuring Principle of PIOX R400
2 Measuring Principle of PIOX R400
The refractive index n of a solution will be determined by means of transmitted light refractometry. A light beam
propagates through the solution and will be refracted at the interface to a prism. The angle of refraction will be
measured by a detector. The refractive index n of the solution can be calculated from Snell’s law of refraction:
n
i
.
sin
i
= n
t
.
sin
t
with
n
i
refractive index of fluid
i
angle of incidence
n
t
refractive index of prism
t
angle of refraction
Fig. 2.1: Path of light beam through the sensor
The light source is a special LED with a wave length
= 590 nm (sodium D-line) (see Fig. 2.1). The light passes an
aperture, will be parallelized by a lens and reversed by a Dove prism. Then it enters the fluid through a window in
the sensor head. When the light beam re-enters the sensor, it will be split at the vertex of a biprism and refracted at
its inclined edges.
The two resulting measuring beams will be focussed by a lens projecting sharp images of the aperture on the
detector, a CCD sensor with 2048 pixels.
The angle of refraction will be determined from the difference between the two measuring beam projections. The
zero point will be calculated continuously, thus eliminating influences of process pressure and temperature.
Dove prism
pipe
biprism
fluid
window
temperature probe
LED
CCD image
CCD sensor
sensor
