3 General principles
PIOX R721
3.1 Measurement principle
11
UMPIOX_F72xV1-3EN, 2021-09-01
3
General principles
3.1
Measurement principle
Terms and abbreviations
Determination of the refractive index
The refractive index of a fluid is determined using transmitted light refractometry. A light beam propagates through the
fluid and is refracted at the interface of a measuring prism. The angle of refraction is measured and is used for the
calculation of the refractive index.
The refractive index is calculated using Snell’s law of refraction:
where
An LED with a wave length λ = 590 nm (sodium D-line) is used as the light source. The light passes through a slit, is
parallelized by a lens and reversed by a deviating prism.
Then the light enters the fluid through a window and hits a measuring prism. The light beam is split and refracted at the
lateral surfaces of the measuring prism.
The resulting light rays are focused in a lens and reproduced on the image sensor creating 2 slit images. The angle of
refraction is determined from the distance between the 2 slit images. Influences of pressure and temperature are
compensated.
term/abbreviation
explanation
analyte
components of a mixture which are to be chemically analyzed
matrix
components of a mixture which are not to be chemically analyzed
fluid data set
data set containing fluid and its properties
• standard fluid data set
data set of a standard fluid (one o the most important watery solutions)
• customized fluid data set
data set of a customized fluid
scale
conversation to a scale value for an arbitrary or unknown material system (e.g., Brix scale)
n
i
· sinθ
i
= n
t
· sinθ
t
n
i
– refractive index of the fluid
n
t
– refractive index of the measuring prism
θ
i
– angle of incidence
θ
t
– angle of refraction
