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4. Detection Principle
4-1 Optical System
In a refractive index detector of the deflection type, the light beam undergoes
a deflection in proportion to a difference in refractive index between one liquid and
the other as it passes through the two compartments of the flow cell, each being
filled with a different liquid. The deflection of the light beam is taken as
a displacement on the light-receiving element and is displayed as a difference of
the refractive index.
Fig. 4.1 Principle of detection of the deflection type
A variety of ingenious devices are provided in the optical system to enhance its
operational stability and make the detector more compact. The light beam from
the light source passes the flow cell through condenser lens, slit No.1,
collimator lens and slit No.2. The light is reflected by the mirror which exists just
behind the flow cell and it forms an image of slit No.1 onto the photo sensor
through the flow cell, slit No.2, collimator lens and null glass.
The photo sensor has two light receivers placed side by side.
1
2
n
s
1
n
r
3
1 : Light beam
2 : Sample cell
3 : Reference cell
4 : Light beam at the time of n
5 : Light beam at the time of n
6 : Displacement
7 : Light-receiving element
2
3
8
9
1 : Light source
2 : Condenser lens
3 : Slit No.1
Fig. 4.2 Optical System
- 4 -
4
6
7
5
n
: Refractive index of liquid
s
in sample cell
> n
,
n
: Refractive index of liquid
s
r
r
in reference cell
= n
,
s
r
4
4 : Collimator lens
5 : Slit No.2
6 : Flow cell
5
6
7
7 : Mirror
8 : Null glass
9 : Photo sensor
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