2.1 Refractive Index
The refractive indices
listed in this catalog were determined to the fifth decimal place for the
following 20 lines of the spectrum. The refractive indices for dline
(587.56 nm) and eline (546.07 nm) were determined to the sixth decimal
place.
Table1
Spectral Line 




t 
Light Source 
Hg 
Hg 
Hg 
Hg 
Hg 
Wavelength (nm) 
2325.42 
1970.09 
1529.58 
1128.64 
1013.98 
Spectral Line 
s 
A' 
r 
C 
C' 
Light Source 
Cs 
K 
He 
H 
Cd 
Wavelength (nm) 
852.11 
768.19 
706.52 
656.27 
643.85 
Spectral Line 
HeNe 
D 
d 
e 
F 
Light Source 
Laser 
Na 
He 
Hg 
H 
Wavelength (nm) 
632.8 
589.29 
587.56 
546.07 
486.13 
Spectral Line 
F' 
HeCd 
g 
h 
i 
Light Source 
Cd 
Laser 
Hg 
Hg 
Hg 
Wavelength (nm) 
479.99 
441.57 
435.835 
404.656 
365.015 
On the catalog
pages, the wavelengths of each line are given in um units in
parentheses under each spectrum line symbol.
2.2 Dispersion
We have indicated
(n F n C ) and (n F´ n C´ ) as the main dispersions. Abbe
numbers were determined from the following vd and ve formula and
calculated to the second decimal place:
vd = (n d 1)/(n F n C )
ve =(n e 1)/(n F' n C' )
We have also listed 12 partial
dispersions (n x n y ), 8 relative partial dispersions for the main
dispersion (n F n C ) and 4 for (n F´ n C´ ). To make
achromatization effective for more than two wavelengths, glasses
which have favorable relationships between vd and the relative
partial dispersion ө x,y for the wavelengths x and y are required.
These may be defined as follows:
θ (x,y ) = ( n x n y ) / (n F n C )
2.3 Dispersion
Formula
The refractive indices
for wavelengths other than those listed in this catalog can be
computed from a dispersion formula. As a practical dispersion
formula, we have adopted the use of the Sellmeier formula shown
below.
n² 1={A^{1} Λ² /(Λ² B^{1} )} + {A² Λ² /(Λ² B² )} + {A^{3 }Λ² /(Λ² B^{3} )}
The constants A1 ,A2 , A3 , B1 , B2 , B3 were computed
by the method of least squares on the basis of refractive indices at
standard wavelengths which were measured accurately from several
melt samples. By using this formula, refractive indices for any
wavelength between 365 and 2325nm can be calculated to have an
accuracy of around ± 5 ×10^{6}.
These constants A1 ,A2 , A3 , B1 , B2 , B3 are listed on the left side of the individual
catalog pages. However in some glass types, not all refractive
indices in the standard spectral range are listed on the data sheet.
In such cases, the applicable scope of this dispersion formula is
limited to the scope where refractive indices are given. When
calculating a respective refractive index, please bear in mind that
each wavelength is expressed in µm units.
2.4 Effect of Temperature on Refractive
Index(dn/dt)
Refractive index
is affected by changes in glass temperature. This can be ascertained
through the temperature coefficient of refractive index. The
temperature coefficient of refractive index is defined as dn/dt from
the curve showing the relationship between glass temperature and
refractive index. The temperature coefficient of refractive index
(for light of a given wavelength) changes with wavelength and
temperature.
Therefore, the
Abbe number also changes with temperature. There are two ways of
showing the temperature coefficient of refractive index. One is the
absolute coefficient (dn/dt absolute ) measured under vacuum and the
other is the relative coefficient (dn/dt relative ) measured at
ambient air (101.3 kPa {760 torr} dry air).
In this catalog, figures of the relative
coefficients are listed. The temperature coefficients of refractive
index dn/dt were determined by measuring the refractive index from 
40C to + 80C at wavelengths of 1,013.98nm (t), 643.85nm (C'),
632.8nm (HeNe laser), 589.29nm (D), 546.07nm (e), 479.99nm (F') and
435.835nm (g).
These measurements are shown in the
temperature range from  40C to + 80C in 20C intervals, and are
listed in the lower part of each catalog page. The absolute
temperature coefficient of refractive index (dn/dt absolute ) can be
calculated by the following formula:
dn/dt absolute = dn/dt relative + n
·(dn air /dt)
dn air /dt is the
temperature coefficient of refractive index of air listed in Table
2.
Table2
Temperature range
(degC) 
dnair/dt (106/K) 
t 
C' 
HeNe 
D 
e 
F' 
g 
40~20 
1.34 
1.35 
1.36 
1.36 
1.36 
1.37 
1.38 
20~0 
1.15 
1.16 
1.16 
1.16 
1.16 
1.17 
1.17 
0~+20 
0.99 
1 
1 
1 
1 
1.01 
1.01 
+20~+40 
0.86 
0.87 
0.87 
0.87 
0.87 
0.88 
0.88 
+40~+60 
0.763 
0.77 
0.77 
0.77 
0.77 
0.77 
0.78 
+60~+80 
0.67 
0.68 
0.68 
0.68 
0.68 
0.69 
0.69 
2.5 The refractive indices in
Ultraviolet and the Infrared Range
The refractive
indices in the ultraviolet and the infrared can be measured down to
157 nm in the ultraviolet and up to 2,325.42 nm in the infrared.
2.6 Internal Transmittance
Most types of Ohara optical glass are
transparent and colorless because they are made of very pure
materials. However, some optical glasses show remarkable absorption
of light near the ultraviolet spectral range. For certain glasses
with extreme optical properties, such as high refractive index,
absorption extends to the visible range.
This not only depends on the chemical
composition, but also on unavoidable impurities. In this catalog the
internal transmittance is given  i.e., reflection losses are
eliminated. Glass varies slightly from melt to melt and, therefore,
listed are typical values of internal transmittance obtained on 10
mm thick samples chosen from many melts, measured from 280 nm to
2400 nm.
