8
MA881 Digital Invert Sugar Refractometer
4. PRINCIPLE OF OPERATION
The Invert sugar determination is made by measuring the refractive
index of a solution. Refractive Index is an optical characteristic of a
substance and the number of dissolved particles in it. Refractive
Index is defined as the ratio of the speed of light in empty space
to the speed of light in the substance. A result of this property is
that light will “bend”, or change direction, when it travels through
a substance of different refractive index. This is called refraction.
When passing from a material with a higher to lower refractive
index, there is a critical angle at which an incoming beam of
light can no longer refract, but will instead be reflected off the
interface. The critical angle can be used to easily calculate the
refractive index according to the equation:
sin (
θ
critical
) = n
2
/ n
1
Where n
2
is the refractive index of the lower-density medium; n
1
is
the refractive index of the higher-density medium.
i
r
n1
n2
= angle of incidence
= angle of refraction
n1 , n2 -
refractive index
Refraction
Critical Angle
Total Internal Reflection
r
i
i
critical
i
r
n2
n1
n2
i
i
critical
i
critical
<
>
=
n1
θ
θ
θ
θ
θ
θ
θ
θ
θ
θ
θ
θ
θ
In the
MA881
, light from an LED passes through a prism in contact
with the sample. An image sensor determines the critical angle
at which the light is no longer refracted through the sample. The
MA881
automatically applies temperature compensation to the
measurement and converts the refractive index of the sample to
Invert sugar concentration in units of percent (by weight).
Prism
Lens
Linear Image Sensor
Sample
Shadow
Led
Sample Well
