7. Guarantee of Quality

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

Refractive index (nd): +/-0.0003
Abbe number (vd): +/-0.5%

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

Refractive index (nd): +/-0.0002
Abbe number (vd): +/-0.3%

When special demand exists for specifications with other optical constants than the above, please consult us. We urge our customers to enjoy the cost savings and benefits of our close index control, melt to melt, over long periods of production. Usually this is done at no extra cost. We normally send certification (melt data) of refractive indices measured at the spectral lines: C, d, F, g and vd . 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, “ultra-precision measurements” and measurements at spectral lines not described in this catalog, please contact us.

7.2 Homogeneity
It is sometimes necessary to measure the index variation across a blank. In such cases, Ohara pays special attention to each process and can supply high homogeneity “Grade Special A” blanks. Grade Special A is our term for high homogeneity (Low Δn) optical glasses. Our Grade Special A glasses are available in the following homogeneity levels:

Table

Classification Homogeneity ( Δn)
Grade Special A1   ±1X10-6

Grade Special

A2

±2X10-6

Grade Special

A5

  ±5X10-6
Grade Special A20 ±20X10-6

Please note that the Grade Special A number indicates n 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 wave front of each test piece. Interferograms can be supplied for high homogeneity blanks upon customer request.

7.3 Stress Birefringence
Depending on the annealing condition, optical glass retains slight residual strain in most cases. This can be observed as optical birefringence, 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 disk 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. We guarantee the strain according to the grade of anneal as follows:

Table 2

Class 1 (Precision) 2 (Fine) 3 (Course)

4 (Very

Course)

Birefringence

(nm/cm)

<5 <10 <20 >20

Birefringence Measurement Chart (BMC) can be supplied upon customer request.

7.4 Striae
Striae are thread-like veins or cords which are visual indications of abruptly varying density. Striae can also be considered to be a lack of homogeneity caused by incomplete stirring of the molten glass. Some glasses contain components that evaporate during melting, causing layers of varying density, and therefore parallel striae appear. 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 at several different angles in the striaescope. They are then compared with the standards and graded. These established standard glasses are of a high order of quality and are certified to U.S. military specification MIL-G-174B.

Table 3

Striae Grade

Striae Grade Striae Content

Using Striaescope

A

B

C

No visible striae

Striae is light and scattered

Striae is heavier than Grade B

7.5 Bubbles
Bubble content is determined by taking a sample of glass from each melt. The total bubble cross-section per 100ml of glass volume is measured. Please refer to 6.1 . On request, we shall undertake bubble examination with the method and procedures of MIL-G-174B or the customer’s own specifications.

7.6 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.

7.7 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.

OPTICAL PROPERTIES

2.5 Temperature Coefficient of Refractive Index

Temperature coefficient of refractive index 〔Δn relT

The refractive index of glass changes with temperature. The amount of change in the refractive index due to temperature changes is expressed as the temperature coefficient of the refractive index, and is defined by Δn / ΔT from the curve showing the relationship between the glass temperature and the refractive index. Δn / ΔT changes depending on the measurement wavelength and temperature range, so the Abbe number also changes with temperature.
There are two ways of showing the temperature coefficient of refractive index; one is the relative coefficient, Δnrel/ΔT (10-6 K-1) measured in dry air (101.3 kPa) at same temperature as the glass, and the other is the absolute coefficient ,Δnabs/ΔT (10-6 K-1) measured under vacuum.

The temperature coefficient of refractive index of each glass type is measured as Δnabs/ΔT according to ISO 6760-1 and from this value the Δnrel/ΔT value normally used in optical design is calculated. The relationship between Δn abs/ΔT and Δn rel/ΔT is given by the following formula.

Formula for temperature coefficient of refractive index of glass

n :Refractive index of glass sample (in air, 25 ° C)

OPTICAL PROPERTIES

2.7 Internal Transmittance

Internal transmittance 〔 τi(10 mm)〕

“Internal transmittance” refers to the spectral transmittance of the glass itself, not including reflection losses at the optical glass-air interface; it indicates the transparency of the glass. Most optical glasses absorb a substantial amount of light in the near-ultraviolet region. For some glasses, especially those with a high refractive index, this absorption range also extends into the visible range. This absorption is not only caused by the composition of the glass; it is also affected by impurities in the glass, and varies slightly from melt to melt.

The spectral transmittance (including reflection loss) is measured based on the JOGIS-17 standard at wavelengths from 280 nm to 2400 nm in a pair of glass samples with different distances through which transmitted light passes. Then, the internal transmittance 〔τ<sub>i</sub>(10 mm)〕 at a glass sample thickness of 10 mm is calculated from the measurement data.

OPTICAL PROPERTIES

2.10 CCI (Color Contribution Index)

CCI

CCI (Color Contribution Index) is an index for predicting how much the color of a photograph taken using a certain lens system changes compared to the original color, due to the spectral characteristics of the lens. It is indicated by a set of 3 numbers for blue (B) / green (G) / red (R). Ohara uses this index to predict how much the color will change as a single glass element. For the measurement method, refer to JIS B 7097 “How to express the color characteristics of a photographic lens by the ISO color characteristic index (ISO / CCI)”. The numbers shown are calculated using the sum of the values of the internal transmittance of the glass sample every 10 nm and the average color film weighted spectral sensitivity, described in JIS. For example, B / G / R of 0/3/5, is shown in Fig. 4 in trilinear coordinates.

CCIE
OPTICAL PROPERTIES

2.2 Dispersion and Abbe Number

Dispersion and Abbe Number

Dispersion refers to the phenomenon arising from a variation in the refractive index depending on the wavelength. Here, nF-nC and nF’-nC’are displayed as the main dispersion. The Abbe number is an index of the magnitude of the variance and is also called the inverse dispersion rate. The larger the variance, the smaller the Abbe number.

Abbe Numbers Calcuation

The glass type data sheet indicates the dispersion, calculated from the refractive index to six decimal places . Abbe number is indicated to two decimal places, this is the result of the calculation from nd to six decimal places and the principal dispersion to six decimal places .

Two decimal places: This is the result of calculation from nd to six decimal places (with seven effective digits) and the principal dispersion to six decimal places (with four or more effective digits).

OPTICAL PROPERTIES

2.6 Relational Constant for Temperature Coefficient of the Refractive Index

Relational constant for temperature coefficient of the refractive index

The temperature coefficient of the absolute refractive index of glass for wavelengths not listed in the data sheet can be calculated as a function of wavelength and temperature. Ohara uses the following equation.

Equation for Temperature Coefficient of absolute refractive index of glass
(λ,T0) Refractive index at reference temperature
0 Reference temperature (°C) (Ohara defines this as 25°C)
T: Target temperature (°C)
λ: Vacuum wavelength (μm)
D0D1 D2E0 E1、λTK Constant (listed in the data sheet)

To determine the temperature coefficient of the relative refractive index, refer to the equation given in the previous section, “Temperature coefficient of the refractive index”.

OPTICAL PROPERTIES

2.1 Refractive Index

Refractive Index

When light enters the glass, it slows down inversely proportional to the refractive index compared to in a vacuum or in air. The refractive index of optical glass is usually expressed as the speed ratio of light in the air to themedium (glass sample).

The refractive index is measured by sending a predetermined wavelength of light into the sample and measuring theminimum deviation angle of the emitted light bent by refraction, according to JIS B 7071-1. For the 20 spectral lines shown in the table below, numerical values are shown to five decimal places. The refractive indices (principal refractive indices) for d-line (587.56 nm) and e-line (546.07 nm) are also shown to six decimal places.

Spectral Line Symbol t
Light Source Hg Hg Hg Hg Hg
Wavelength (nm) 2325.42 1970.09 1529.58 1128.64 1013.98
Spectral Line Symbol 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 Symbol He-Ne D d e F
Light Source レーザー Na He Hg H
Wavelength (nm) 852.11 589.29 587.56 546.07 486.13
Spectral Line Symbol F′ He-Cd g h i
Light Source Cd レーザー Hg Hg Hg
Wavelength (nm) 479.99 441.57 435.835 404.656 365.015
OPTICAL PROPERTIES

2.4 Disperson Formula Constant

The refractive index for wavelengths not listed in the data sheet can be calculated using the dispersion formula. The Sellmeier equation is used as a practical dispersion formula, as detailed below.

Sellmeier Equation
n : Refractive index to be calculated
λ : Arbitrary wavelength (μm)
A1、A2、A3、B1、B2、B3 Constant (listed in the data sheet)

Using this dispersion formula and the constants for each glass type, the refractive index of any wavelength in the standard measurement wavelength range (365 to 2325 nm) can be calculated with a calculation accuracy of ±5×10<sup>-6</sup>. However, for glass types for which the refractive indices for the entire standard measurement wavelength range are not listed in the data sheet, the applicable wavelength range of the dispersion formula is limited to the refractive index range listed in the data sheet.

OPTICAL PROPERTIES

2.8 Coloring

Coloring

Coloring refers to the degree of coloration of the optical glass and is determined by measuring the spectral transmittance, including reflection losses, for a glass sample with a thickness of 10 mm, according to JOGIS-02. From the spectral transmittance curve (Fig. 3), the wavelengths showing the transmittance of 80% and 5%, respectively, are rounded and displayed in 5 nm units. We use this rounding method: the range 0 nm to 2 nm counts as 0 nm, the range 3 nm to 7 nm counts as 5 nm, the range 8 nm to 10 nm counts as 10 nm . For example, if the wavelength with 80% transmittance is 403 nm and the wavelength with 5% transmittance is 357 nm, the coloring is shown as 405/355.

Optical Glass Coloring

For glass types with a high refractive index, nd ≥ 1.84, the reflection loss is large, so the wavelength showing transmittance of 70 % is used, instead of 80 %, and the value is shown in paranethesis. For example, (415).

OPTICAL PROPERTIES

2.9 Internal Transparency

Internal transparency〔λ0.800.05

As a simplified indicator of coloring, the wavelength values in nm at which
the internal transmittance of a 10 mm thick glass sample is 0.80 and 0.05
are indicated.

OPTICAL PROPERTIES

2.3 Partial dispersion ratio and anomalous dispersion

Partial dispersion ratio 〔θx, y〕 and anomalous dispersion 〔Δθx, y
Anomalous dispersion refers to how far away a glass is from the trend line between the partial dispersion ratio θx, y = (nx-ny) / (nF-nC) for wavelengths x and y and the Abbe number νd. In optical design, glass with anomalous dispersion is required to enable color correction of the secondary spectrum.
Therefore, we have released the θg, Fd diagram and the θC, td diagram as means to show the relationship between θx, y and νd of each glass type. In order to numerically express the anomalous dispersibility, 511605 (NSL 7) and 620363 (PBM 2) are used as reference glasses, and the straight line connecting these two glass types is considered the “normal” line. The difference between the “normal” line and the vertical coordinates θx, y of each glass type is calculated as anomalous dispersion Δθx, y (Fig. 2). In this catalog, the partial dispersion ratio is θg, F and θC, t, and the anomalous dispersion is Δθg, F and ΔθC, t.

Although NSL 7 and PBM 2 are not currently produced by Ohara, the conventional NSL 7 and PBM 2 values ​​(Table 2) are used as the reference values.

Reference Values

θc,t
θC,A'
θg,d
θg,F
θi,g
vd
NSL 7
0.8305
0.3492
1.2391
0.5436
1.2185
60.49
PBM 2
0.7168
0.3198
1.2894
0.5828
1.4214
36.26

g,Fd図とΔθg,F

2.3 Chart