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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 d-line (587.56 nm) and e-line (546.07 nm) were determined to the sixth decimal place.

 
Table1
Spectral Line     t
Light Source HgHgHgHgHg
Wavelength (nm) 2325.421970.091529.581128.641013.98
Spectral Line sA'rCC'
Light Source CsKHeHCd
Wavelength (nm) 852.11768.19706.52656.27643.85
Spectral Line He-NeDdeF
Light Source LaserNaHeHgH
Wavelength (nm) 632.8589.29587.56546.07486.13
Spectral LineF'He-Cdghi
Light SourceCdLaserHgHgHg
Wavelength (nm)479.99441.57435.835404.656365.015

On the catalog pages, the wavelengths of each line are given in im units in parentheses under each spectrum line symbol.

 

Dispersion*

We have indicated (nF -nC) and (n -n) as the main dispersion. Abbe numbers were determined from the following νdand νe formula and calculated to the second decimal place:


νd=(nd-1)/(nF-nC)   νe=(ne-1)/(nF'-nC')

We have also listed 12 partial dispersions (nx -ny ), 8 relative partial dispersions for the main dispersion (nF -nC) and 4 for (n -n). To make achromatization effective for more than two wavelengths, glasses which have favora-ble relationships between νd and the relative partial dispersion e x,y for the wavelengths x and y are required. These may be defined as follows:

 

Constants of 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={A1λ2/(λ2-B1)}+{A2λ2/(λ2-B2)}+{A3λ2/(λ2-B3)}

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.

 

Thermal coefficient of refractive indices* (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 .

There-fore, 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 (He-Ne 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/dTabsolute=dn/dTrelative+n · (dnair/dT)


dn air /dT is the temperature coefficient of refractive index of air listed in Table 2.


Table2
Temperaturerange(°C)dnair/dT(10-6/K)
tC'He-NeDeF'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~+20-0.99-1.00-1.00-1.00-1.00-1.01-1.01
+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
 

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.

 

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.

 

Coloring*

Coloring can be determined by measuring spectral transmission, including reflection losses with test pieces of 10 mm in thickness. The wavelengths corresponding to 80% transmission and 5% transmission are indicated as 405/355 in units of 5nm, after rounding off 2 and rounding up 3 or more and rounding off 7 and rounding up 8 or more.
For glass types with a high refractive index of ne≥1.85, the wavelength showing transmittance of 70% was used instead of 80%, because the reflection loss is large. The indication has been made as (415).

 

Internal Transmittance λ80/λ5

As the simple indicator of the coloring, the wavelengths that the internal transmittance is 80% and 5% are given.

 

CCI (Colour Contribution Index)

ISO Colour Contribution Index (ISO/CCI): The color characteristic index calculated by the method prescribed in JIS B7097-1986.
CCI is an index indicating "the amount of colour variation caused by the spectral characteristics of lens" comparing with the original color when photographed.
In this catalog, in order to refer to the influences caused by the glass material itself, the index is listed.
For more information, please refer to JIS 7097-1986 "Determination of ISO Colour Contribution Index (ISO/CCI) of Camera Lenses".

 
* indicates measurement by the methods stipulated by Japan Optical Glass Manufacturers' Association
** indicates a test by methods stipulated by ASTM
***indicates a test by methods stipulated by International Organization for Standardization (ISO)

 
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