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Internal Quality of ProductsList of Glass Types

1. Tolerance of Refractive Index and Abbe-number

Since the listed refractive indices and Abbe-number are the mean of several melts, those for an individual melt will differ slightly from the mean. The tolerances are generally as follows:

[Optical glass for lens polishing]
Refractive index nd:±30×10-5  Abbe-number νd:±0.5%
For H, S, glass upon special request, you can select nd:±20×10-5 and νd : ±0.3%.。

[Heat Process PF, Cold Process PF]
Refractive index nd:±30×10-5  Abbe-number νd:±0.5%

[Glass Molded Lenses]
Refractive index nd:±50×10-5  Abbe-number νd:±0.8%

The standard measurements have a precision of ±1×10-5 for the refractive index, and ±1×10-5 for dispersion. Please contact us if you require more precise measurements.

2. Coloration Code

The extent of coloration varies slightly from one melt to another. Therefore the coloration code and the internal transmittance listed in this catalog is the mean of several melts. Coloration between lots is controlled within ±10nm of the listed nominal values. Transmittance data can be furnished for delivered products upon request. Upon special request, we can deliver the products with a tighter tolerance. Please contact us when a tighter tolerance is required.

3. Thermal Properties

The thermal properties vary slightly from one melt to another. Therefore the thermal properties listed in this catalog are the mean of several melts. Transformation Temperature (Tg), Sag Temperature (Ts) and Softening Point T107.6 between lots are controlled within ±5°C.

4. Optical Homogeneity

For glass blanks used in extremely high-precision optical systems, variations in the refractive index within a single piece must be controlled within very narrow limits. Glass blanks with a tight index control or very high optical homogeneity control are manufactured by special manufacturing processes and inspected by an interferometer. Please contact us when glass blanks with a tight index control are required.

Table 1:Optical Homogeneity

Grade Variation of nd
H1 ±2×10-5
H2 ±5×10-6
H3 ±2×10-6
H4 ±1×10-6

There could be a case where the variation (Δn) in the refractive index within a single piece of optical glass will be over ±2 × 10-5depending on the dimension and the shape.

5. Stress Birefringence

Stress birefringence means the grade of birefringence (double refraction) caused by the residual stress within the glass. Excessively large stress birefringence may affect optical performance or cause breakage. Generally in the case of optical glass, fine annealing will be provided to limit the distortion amount to a low level. For disc-shaped products, the stress birefringence is measured at a distance 5% of the diameter from the circumference, and for rectangular plates, at a distance 5% of its width from the edge in the middle of the longer side. Stress birefringence is graded as shown in Table 2.

Table 2 : Grades of Stress Birefringence

Grade Stress Birefringence (nm/cm)
1 < 5
2 ≥ 5 - < 10
3 ≥ 10 - < 20
4 ≥ 20

For articles with shapes other than rectangles or discs, birefringence measurements at significantly meaningful locations may be arranged upon request

6. Striae

Striae refers to the linear or layered portion of glass material of which the refractive index differs from the host glass.
Striae are inspected by a striae-scope that is equipped with a point light source and an optical lens system.
For inspection, striae are first identified in a selected direction that facilitates good viewing and then rated in one of HOYA's own striae grades. With respect to standard reference samples, the JOGIS striae grade is compared to HOYA's own grade, as shown in Table 3.

Table 3 Grades of Striae

Striae HOYA striae grade
1 No Striae Visible
2 Thin Separation Striae,Visible Limits
3 A Few Parallel Striae 

7. Bubbles and Inclusions

Although bubbles and inclusions in our glasses are not entirely absent, they are very scarce due to our well-developed melting methods. The size and number of bubbles varies with the glass composition and melting conditions.
Bubbles are counted to obtain the total cross sectional area (mm2) of bubbles present in every 100ml of glass.

Table 4 Classes of Bubbles and Inclusions

Class Total cross sectional area per 100ml of glass (mm2)
1 < 0.03
2 ≥0.03 - < 0.10
3 ≥0.10 - < 0.25
4 ≥0.25 - < 0.50
5 ≥0.50

Inclusions such as small stones or crystals are treated together with bubbles. The total cross-sectional area of bubbles and inclusions with diameters greater than 0.03mm is measured. This measurement is used to classify the glass according to Table 4.
The permissible number of bubbles and inclusions with diameters or maximum dimensions less than 0.03mm is set per unit volume or mass according to the diameter of babbles and inclusions.

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