38
12. Finding average dispersion value and Abbe number
2. Finding average dispersion
value
Dispersion value (nF-nC) can be found
according to the following equation.
nF
㸫
nC = A + B
Ș
In the above equation, values A and B
can simply be found out of Table 1 on
page
40
by
checking
up
values
corresponding to a measured Refractive
Index on the table after measuring the
sample.
To obtain a value
Ș
, achromatize
the boundary line by turning the color
compensator knob and read the scale
indication (Z) on the knob at the time.
A value
Ș
can simply be found out of
Table 2 on page 41 by checking up a
value
Ș
corresponding to the value Z
on the table.
Abbe number can be obtained by
calculation
with
the
following
equation.
3. Finding Abbe number
Ȥ
=
nD
㸫
1
nF
㸫
nC
Ȥ
= Abbe number
nD = Refractive Index
nF
㸫
nC = Dispersion
value
where,
4. Example of measurement
Refractive Index (nD)=1.4827
Reading of dispersion scale
Left
Right
42.3
42.2
42.2
42.2
42.2
42.1
42.3
42.2
42.2
42.2
Average reading for right and
left (Z)=42.2
A=0.02336
nF
㸫
nC=A+B
Ș
=0.00683
B
Ș
=
㸫
0.01653
Ș
=
㸫
0.596
B=0.02774
Z=42.2
Ȥ
=
nD
㸫
1
nF
㸫
nC
=
0.4827
0.00683
=70.7
5. Finding values A and B
more accurately
Assuming that nD (Refractive
Index) = 1.4827;
look up in the column A for a value
corresponding to 1.48 in the nD column
of Table 1, and there is 0.02337 found as
the objective value.
0.02337...................................................(a)
In the same manner,
㸫
2 is found in the
Difference column.
When (a) and
㸫
2 are applied to the
following equation, calculation is carried
as:
27
(
㸫
2)
10
㸫
7
=
㸫
0.0000054............(b)
Therefore,
A = a + b
Ҹ
0.02336
Carry out the same calculation for a
value of B.
[Example]
When measuring a sample to calculate the Abbe value, be sure to use the natural light
(white light) instead of the accompanying illuminator lamp (orange light).
Note:
