OHARA INC.

Each optical glass has its own properties which are closely connected to the key chemical elements contained therein. In OHARA’s glass type designation system the first or second characters include the atomic symbols of one or two important chemical elements for that glass type. The third letter of the glass type designation refers to the refractive index of each glass type within its glass group: H, M or L for high, middle, or low index . Lastly we assign a one or two digit number to each glass type within a given glass family. Thus each glass type is typically represented by three letters plus a one or two digit number. The prefix “S-“ stands for environmentally safe and the prefix  “L-“ is used for low transformation temperature (Tg) glass types. The suffix “Y” is used for i-Line-glass types and the suffix “W” is used for glasses with improved transmittance. For example, the glass type S-BSL 7 is environmentally safe (S-), contains Boron (B) and Silicon (S), shows a low index (L) and is the seventh (7) glass within this BS glass family . Along with OHARA’s glass type designation, each single glass type is identified by a six digit code. The first three digits represent the refractive index at the helium line (n_d) and the last three digits represent the Abbe number (ν_d). This six digit code is internationally recognized within the optical community.  

Refractive index

When light enters the glass, it slows down 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 the medium (glass sample). The refractive index is measured by sending a predetermined wavelength of light into the sample and measuring the angle of the emitted light bent by refraction, according to JIS B 7071-1. In this catalog, the values up to 5 digits after the decimal point are displayed for the 10 spectral lines shown in Table 1.

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)	632.8	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

 

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. The Abbe numbers ν_d and ν_e are calculated using the following formulas, respectively:

Partial dispersion ratio 〔θ_{x, y}〕 and anomalous dispersion 〔Δθ_{x, y}〕

Anomalous dispersion refers to how far away a glass is from the trend line between the partial dispersion ratio θ_{x, y} = (n_x-n_y) / (n_F-n_C) for wavelengths x and y and the Abbe number ν_d. In optical design, glass with anomalous dispersion is required to enable color correction of the secondary spectrum.

Therefore, we have released the θ_{g, F}-ν_d diagram and the θ_{C, t}-ν_d diagram as means to show the relationship between θ_{x, y} and ν_d 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 θ_{x, y} of each glass type is calculated as anomalous dispersion Δθ_{x, y} (Fig. 2). In this catalog, the partial dispersion ratio is θ_{g, F} and θ_{C, t}, and the anomalous dispersion is Δθ_{g, F} and Δθ_{C, 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

 

Dispersion 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.

   

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 ±5x10^-6. 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.  

Temperature coefficient of refractive index 〔Δn _rel/ΔT〕

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, Δn_rel / ΔT (10^-6 K^-1) measured in dry air (1013 hPa) at same temperature as the glass, and the other is the absolute coefficient , Δn_abs/ΔT (10^-6 K^-1) measured under vacuum. For each glass type the figures of the relative coefficients [Δn_rel / ΔT ], measured according to method JOGIS-18, are disclosed. In this catalog, the values for temperature range of +40 to + 60 ° C for the wavelength of D line (589.29 nm) are displayed. The temperature coefficient of absolute refractive index (10^-6 K^-1) can be roughly calculated using the following formula.  

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. The wavelength range that can be used is 0.436 to 1.014 μm, and the temperature range is -40°C to +80°C.      

Internal transmittance [τ_10mm]

  "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_{10 mm}) at a glass sample thickness of 10 mm is calculated from the measurement data.

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.  

Internal transparency〔λ_0.80/λ_0.05〕 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.

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. trilinear coordinates

Strain point [StP]

The strain point is the maximum temperature at which the viscous flow of glass is not possible, and at which the internal strain (residual stress) cannot be removed even by annealing (at or below the strain point). This is the temperature where the viscosity is 10^14.5 dPa·s {poise}. The fiber elongation method is used for measurement, with reference to JIS R 3103-2.

Annealing point 〔AP〕

The annealing point is the temperature at which the internal strain of the glass is substantially eliminated in 15 minutes, which corresponds to the upper limit temperature in the slow cooling region.  The viscosity of the glass is 10¹³ dPa · s {poise} at this temperature. The annealing point is measured by the Fiber Elongation Method in JIS-R3103.

The transformation point is one of the state changes of glass, and refers to the temperature at which the glass transforms from an amorphous "solid" to a supercooled "liquid", and the expansion characteristics change at this temperature.

The measurement method reads the following temperatures from the thermal expansion curve (figure below) obtained by heating up at a constant rate of 4K per minute as specified in JOGIS-08.

The yield point is the temperature at which the glass sample softens and begins to deform due to the applied load, and the expansion characteristics change at this temperature.

The transition point Tg is the temperature corresponding to the intersection of the extension lines of the two straight lines before and after the curve slope changes significantly, and the yield point At is the temperature at which the elongation (ΔL) of the sample is maximized.

Note: The measurement equipment is being updated. Therefore, values obtained with the new equipment are marked with an asterisk (*) on the right.
In the new equipment, a glass sample of 25 mm in length and 4.5 mm in diameter is measured by heating up at a constant rate of 4 K per minute over the entire temperature range.

Softening point 〔SP〕

The softening point is the temperature at which glass deforms under its own weight and is the lower limit temperature for glass molding and pressing. The glass viscosity is 10^7.65 dPa · s {poise} at this temperature. The softening point is measured by the Fiber Elongation Method referred to JIS-R3103. This method measures the temperature at which the upper part of the glass fiber sample, heated in an electric furnace, grows at a rate of 1 mm / min under its own weight.

The average coefficient of linear expansion is the rate of elongation of a glass sample between two specified temperatures per 1 K. The average coefficient of linear expansion is measured via the method referred to in JOGIS-16 and JOGIS-08. The thermal expansion curve is obtained by measuring a well annealed glass sample, size 4.5 mm diameter by 50 mm long, heated at a rate of 2 K / minute in the low temperature range and at a rate of 4 K / minute in the high temperature range. From the temperature and elongation of the glass sample, the mean linear coefficient of thermal expansion between – 30 °C to + 70 °C and + 100 °C to + 300 °C is determined and shown up to the first integer in units of 10^-7 / K.   Note: The measurement equipment is being updated. Therefore, values obtained with the new equipment are marked with an asterisk (*) on the right. In the new equipment, a glass sample of 25 mm in length and 4.5 mm in diameter is measured by heating up at a constant rate of 4 K per minute over the entire temperature range.

Thermal conductivity〔λ〕

Thermal conductivity is a numerical value that indicates the ease with which heat can be transferred within a single substance.
It is the amount of heat that flows in a unit time, through the unit area of the plate, when there is a temperature difference between both ends of the plate.
The measurement method uses the transient hot wire method, and the value of the glass sample at 35 ℃ is displayed to three significant digits.

Water resistance 〔RW _(P)〕 and acid resistance  〔RA _(P)〕 by powder method

Water resistance RW _(P) is a measure of the likelihood of "white discoloration", and acid resistance RA _(P)  is a measure of the likelihood of "blue discoloration", occuring on the surface of the glass. These tests are conducted in accordance with the methods in JOGIS-06. The glass to be tested is crushed to 425 μm ~ 600 μm grains. A sample of this powder equivalent to the specific gravity in grams is placed on a platinum basket . This is put in a flask of silica glass containing the reagent and boiled for 60 minutes . The sample is then carefully dried and re-weighed to determine the loss of weight (percent) and classified as per Table 3 and 4

※ Water resistance RW(P) ･･･	Pure water (pH6.5～7.5)
Acid resistance RA(P) ･･･	0.01 N aqueous nitric acid solution

 

RW_(P)

Class	1	2	3	4	5	6
Weight loss(%)	＜0.05	≧0.05 ＜0.10	≧0.10 ＜0.25	≧0.25 ＜0.60	≧0.60 ＜1.10	≧1.10

Class	1	2	3	4	5	6
Weight loss(%)	＜0.20	≧0.20 ＜0.35	≧0.35 ＜0.65	≧0.65 ＜1.20	≧1.20 ＜2.20	≧2.20

Weathering Resistance [W_(s)] (Surface Method)

Surface method weather resistance W _(S) is a measure of the likelihood of "white discoloration" occuring on the surface of the glass. The test method used is as follows: expose a glass sample size 30 x 30 x 3 mm polished on two sides, in a temperature and humidity chamber at 60 ° C and 95% relative humidity for 24 hours, and then observe the polished surface with a 50x microscope. Glass types showing discoloration when observed at both 1500 lux and 100 lux illuminance are tested again, using a new glass sample for 6 hours, and then same observed and graded. The classes are shown in Table 5.

Acid resistance 〔SR〕

Acid-resistant SR is a measure of the likelihood of "blue discoloration" or elution occuring on the surface of the glass using an acidic solution. The test method is based on ISO 8424. The glass samples size  30 x 30 x 3 mm with 6 faces polished, are hung into nitric acid solution (pH 0 .3) or acetic acid buffer solution (pH 4 .6) at 25 °C for specified time (10 minutes, 100 minutes, 16 hours, 100 hours).  

t0.1 :	the time (h) decreases a surface layer at a depth of 0.1μm
te :	the time (h) for attack in the experiment
d :	the specific gravity of the sample(g/cm3)
S :	the surface area (cm2) of the sample
m1, m2 :	the mass (mg) of the sample before and after the test

The calculation is carried out by use of the value of the loss of mass which is observed by the minimum test condition (i.e.,test solution and test time) for obtaining a loss of mass greater than 1mg / sample. If the loss of mass is less than 1mg / sample after 100 hours exposure to pH 0.3, this value shall be accepted. The acid resistance class SR is obtained by comparison of the pH of the erosion fluid and the time required for the attack to a depth of 0.1μm (h) with time scales given in the classification Table 6.  

Acid resistance class SR	1	2	3	4	5		51	52	53
attacking solution	nitric acid solution (pH 0.3)					acetic acid buffer solution (pH 4.6)
The time(h) decreases a surface layer at a depth of 0.1μm	＞100	≦100 ≧10	＜10 ≧1	＜1 ≧0.1	＜0.1	＞10	≦10 ≧1	＜1 ≧0.1	＜0.1

In addition, changes in the surface of the sample following the treatment are qualitatively evaluated with the naked eye. Additional classification numbers are given according to Table 7.

 

Phosphate resistance 〔PR〕

Detergent resistance PR is a measure of the ease of the surface of glass to influenced by the detergent used in the cleaning process. The test method is based on ISO 9689. A 30 x 30 x 3 mm glass sample with 6 polished surfaces is hung in a 0.01 mol / l purified Na5P3O10 aqueous solution * at a temperature of 50 ° C for the specified times (15 minutes, 1 hour, 4 hours or 16 hours). After this treatment, the sample is weighed and the mass loss is determined. We calculate the time (min) required to erode the 0.1 μm thick glass layer from the formula used for acid resistance. * Na₅P₃O₁₀ (sodium tripolyphosphate) is one of the inorganic materials (cleaning aids) contained in detergents, and it can  produce a chemical reaction on the surface of the glass. For this calculation, the value obtained under the minimum test conditions where the mass loss per sample is 1 mg or more is used. Detergent resistance class PR is determined according to Table 8 from the time (minute) required for 0.1 μm erosion. As with acid resistance SR test, changes in the conditions of polished surface of the sample are observed and numbered according to the classification in Table 7.

Phosphate resistance class PR	1	2	3	4
The time decreases a surface layer at a depth of 0.1μm(min)	＞240	≦240 ≧ 60	＜60 ≧15	＜15

 

Elastic modulus

An object that changes shape when a force is applied is called an elastic body. The elastic modulus is the ratio of the force applied to an elastic body to the strain (amount of deformation). Elastic modulus is measured using the ultrasonic pulse-echo-superposition method, which measures the sound velocity of longitudinal and transverse waves at room temperature in a sufficiently slow-cooled glass sample of 100 x 10 x 10 mm. Young's modulus (E; measured in GPa) and shear modulus (G; measured in GPa) are calculated using the following equations. The Poisson's ratio (σ) is calculated from the calculated Young's modulus and shear modulus using the following equation.    

Knoop hardness 〔Hk〕

Knoop hardness is an index that indicates the indentation hardness of an optical glass and is determined using a micro hardness tester. Knoop hardness is measured via the method shown in JOGIS-09. We press the diamond square cone indenters (opposite edges 172.5º and 130º) against the flat polished surface of the glass sample for 15 seconds with a load of 0.98 N using a microhardness meter. The length of the longer diagonal of the permanent dent generated at this time is measured, and the Knoop hardness is calculated using the following formula:   In this catalog, the measured values are rounded to the first integer and the grades classified according to Table 9.

Class	1	2	3	4	5	6	7
Knoop Hardness	＜150	≧150 ＜250	≧250 ＜350	≧350 ＜450	≧450 ＜550	≧550 ＜650	≧650

 

Abrasion (Aa)

Abrasion is an indicator of the ease of removing material when processing glass. In accordance to JOGIS-10, a sample of size 30 x 30 x 10 mm is lapped on a 250 mm diameter cast iron flat, rotating at 60 rpm. The test piece is located 80 mm from the center of the flat and is under a 9.8 N load.  20 ml of water containing 10 g of aluminous abrasive as the lapping material, with mean grain size 20 μm (# 800), is supplied evenly to the test piece for 5 minutes. The weight loss of the test piece is then measured. The known weight loss of the standard glass is compared according to the formula below. The values of abrasion are calculated and shown to the nearest integer as the abrasion data.

Photoelastic constant 〔β〕

Optical glass is usually isotropic, with its optical properties (refractive indices, scattering, transmission, etc.) independent of its orientation. However, when mechanical or thermal stress is exerted upon it, distortion or stress occurs in the glass and it shows birefringence. Stress F [Pa], optical path difference σ [nm] and thickness of glass d [cm] have the following relationship:     The proportionality constant β is called the photoelastic constant, and in this catalog, the optical path length difference is measured by the e-line (546.07 nm) and displayed in units of (nm / (cm ･ 10⁵Pa)). The photoelastic constant is different for each glass type and it is possible to calculate the optical path difference from the given stress and the internal stress.

Specific gravity〔d〕

Specific gravity d is the density value of well annealed glass referenced against pure water at 1 atm at 4 ° C. The measurement method is in accordance with JIS Z 8807. We weigh a glass sample in air and water, and divide the density obtained by the density of water to calculate the specific gravity. The density for pure water at 1 atm 4 ° C is 0.999975 g / cm³, therefore density = specific gravity. The values are shown to two decimal places.

Refractive Index and Abbe Number

Refractive indices and Abbe number, which are given in this catalog, are the representative value of plural melting lots. The values of the melting lot delivered will be within the tolerances specified below: [ Optical Glass for Polished Lenses ] n_d : ±30×10^-5 ν_d : ± 0.5% (rounded up to the guaranteed number of digits) Upon special request, and depending upon the glass types, the following tolerances can be accepted : n_d : ±20×10^-5 ν_d : ±0.3% (rounded up to the guaranteed number of digits) In the supply of our standard products, Melt Data is attached and includes the following data : - Refractive indices: the measurement values of each spectral lines (C, d, F, g ). These are shown to 5 decimal places. (effective number: six digits) - Abbe numberνd: the calculated value by the measured refractive indices. These are shown to 1 or 2 decimal places. e.g. S-BSL7 n_d : 1.51633 ± 0.00030 ν_d : 64.14 ± 0.33 For special requirements, other than what is mentioned above, please contact us. [ Optical Glass for Glass Mold Lenses ] RC value : Standard RC value± 30 (Raw material control value) ν_d : ± 0.5% (rounded to the guaranteed number of digits) When products are supplied, the RC value is given to Melt No.. This RC value is Δnd at 5 decimal places concerning the material that underwent OHARA’s standard annealing (-600°C per day). The RC value is rounded to 4 decimal places and shows with a unit of 10^-5. And it comes with a report of theνd value to 1 or 2 decimal places concerning the product after annealing as specified in catalog annealing (-200°C per day). e.g. L-BSL7 RC value : -30 ± 30 ν_d : 64.06 ± 0.33 The standard measurement accuracy for refractive index and dispersion is as follows : Refractive index : ±3×10^-5 Dispersion : ±2×10^-5 On request, we shall measure at precision accuracy as follows : Refractive index : ±1×10^-5 Dispersion : ±3×10^-6 For ultra-precision measurements and reading at spectral lines not described in this catalog, please contact us.

Homogeneity

For glass used in very precise lens systems it is sometimes necessary to measure the refractive index variation across a blank. In such cases, Ohara pays special attention to each process and can supply high homogeneity glass. The homogeneity is measured by calculating PV value of refractive index fluctuation width (excluding linear change component of refractive index) using computer analysis of the transmitted wavefront of a planeparallel glass sample with the phase-measuring interferometer. In case that the value of the change amount (PV) must be guaranteed within 4×10^-5, measurement of an individual product is conducted and the value of the change amount is guaranteed at six decimal places (10^-6).
Note: The homogeneity of our general glass products could exceed PV (Range) 4×10^-5, depending on glass type, dimensions, shape, etc.

Stress Birefringence

In case that glass retains residual strain, this can be observed as optical birefringence.
The amount of stress is measured by optical path differences and specified in nm/cm.
On special request, when we measure stress birefringence in detail, a rectangular plate is measured 4 points located 5% from the edge at the middle of the each side.
A disc is measured at 4 points located 5% fromthe edge of the diameter.
The maximum value of the 4 points is shown as the birefringence value and is categorized based on Table 11.
For other special forms and shapes the points to be measured will be determined. Table 11

Class	1	2	3	4
Birefringence(nm/cm)	＜5	≧5 ＜10	≧10 ＜20	≧20

 

Striae

In case that glass retains residual strain, this can be observed as optical birefringence. The amount of stress is measured by optical path differences and specified in nm/cm. On special request, when we measure stress birefringence in detail, a rectangular plate is measured 4 points loacated 5% from the edge at the middle of the each side. 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 and is categorized based on Table 11. For other special forms and shapes the points to be measured will be determined.

Table 12

Striae Class	Striae Content Using Striaescope
1	No visible striae
2	Striae is light and scattered
3	Striae is heavier than Class 2

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 classification 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 13. The classification includes all bubbles and inclusions measuring larger than 0.03 mm.

Table 13

Class	1	2	3	4	5
100cm3The 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 melting lots is generally within ±10 nm. On special request, we shall report the coloring or the transmission, of the melt to be supplied by measuring spectral transmission.

Others

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.