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Refractive Index and Abbe Number

Refractive index and Abbe number values of our fine annealed products vary from catalog value by:

nd:±0.0003   νd:±0.5%

Upon request, we will supply blanks of optical glass to the following tolerances: Refractive index :

nd:±0.0002   νd:±0.3%

We normally send certification (melt data) of refractive indices to the 5th decimal place (six significant digits) measured at each spectral line: C, d, F, g and νd to the1st decimal place, calculated from nd.
On special request, we can supply refractive indices measured at other spectral lines.
The following is the accuracy of standard measurements of refractive index and dispersion for raw glass and normal pressed blanks:

Refractive index = ±0.00003   Dispersion = ±0.00002

On request, we shall provide precision measurements of refractive index and dispersion :

Refractive index = ±0.00001   Dispersion = ±0.000003

We will report the environmental temperature, humidity and atmospheric pressure of the room where the precision measurement was undertaken. For, "ultraprecision measurements" and measurements at spectral line not described in this catalog, please contact us.

Refractive index:±0.0003    Dispersion = ±0.00002

On request, we shall provide precision measurements of refractive index and dispersion:

Refractive index:±0.00001    Dispersion = ±0.000003

We will report the environmental temperature, humidity and atmospheric pressure of the room where the precision measurement was undertaken. For, "ultra-precision measurements" and measurements at spectral line not described in this catalog, please contact us.

 

Homogeneity

It is sometimes necessary to measure the index variation across a blank. In such cases, Ohara firmly controls each process and can supply high homogeneity blanks.
Please note that we indicate Δn in the sixth decimal place. The anneal required must also be specified in terms of birefringence (nm/cm). Generally, low Δn also implies low birefringence from precision annealing. Using phase measuring interferometers, we measure transmitted wavefront of each test piece. Interferograms are supplled for each blanks that is ordered with Grade Special A5 or higher homogeneity.


Table1
ClassificationHomogeneity (Δn)
Grade Special A1±1×10-6
Grade Special A2±2×10-6
Grade Special A5±5×10-6
Grade Special A20±20×10-6
 

Stress Birefringence*

Depending on the annealing condition, optical glass retains slight residual strain in most cases. This can be observed as optical birefringence, and measured by optical path differences and specified in nm/cm.
Stress birefringence of a rectangular plate is measured at the middle of the long side where maximum values occur at a point 5% of the width from the edge. A disc is measured at 4 points located 5% from the edge of the diameter.
The maximum value of the 4 points is shown as the Birefringence value.
On request, we shall supply birefringence data for precision annealed blanks in the form of a "BMC" (Birefringence Measurement Chart).


Table 2
Class1234
Birefringence(nm/cm) <5 ≥5 
<10
≥10 
<20
≥20
 

Striae*

Striae are thread-like veins or cords which can be considered to be parts of different chemical components, of the molten glass.
Striae in glass are detected by means of a striaescope, which consists of a point source of light and a collimating lens. Polished samples are examined from several different angles in the striaescope. They are then compared with the standards and graded as in Table 3.


Table 3
Striae GradeStriae Grade Striae Content Using Striaescope
ANo visible striae
BStriae is light and scattered
CStriae is heavier than Grade B
 

Bubble & Inclusion

It is most desirable to manufacture bubble-free optical glass, but the existence of bubbles to some extent is inevitable. Bubbles in optical glass vary in size and number from one glass to another due to the many different compositions and production methods. The classifi cation of bubble content is established by specifying in mm2 the total bubble cross sections existing in 100cm3 of glass volume. Inclusions such as small stones or crystals are treated as bubbles.
The bubble classes are shown in Table 4. The classifi cation includes all bubbles and inclusions measuring larger than 0.03mm.

Table 4
Class12345
The total cross section
of bubbles (mm2/100cm3)
<0.03 ≥0.03
<0.1
≥0.1
<0.25
≥0.25
<0.5
≥0.5
 

Coloring

Variation of coloring between melts is generally within ±10 nm. On special request, we shall report the coloring or the transmission, including reflection losses, of the melt to be supplied by measuring spectral transmission.

 

Other

We showed each properties as representative value except for 7.1~7.6. Please contact us when you want to know the other value. In addition, please let us know your preferred specification when you place the orders.

 
* 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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