Optical Glass Properties
The properties and characteristics of Ohara optical glass necessary for glass handling.
The properties and characteristics of Ohara optical glass necessary for glass handling.
Ohara Corporation Western Office
23141 Arroyo Vista, Suite 200
Rancho Santa Margarita, CA 92688
TEL: (949) 858-5700
FAX: (949) 585-5455
Ohara Corporation Eastern Office
50 Columbia Road, Branchburg
New Jersey 08876-3519
TEL: (908) 218-0100
FAX: (908) 218-1685
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 (nd) and the last three digits represent the Abbe number (νd). This six-digit code is internationally recognized within the optical community.
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 |
| 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.
“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.
Therefore, we have released the θg, F-νd diagram and the θC, t-νd 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 |
>θg,F-νd図とΔθg,F
| ｎ(λ,T0)： | Refractive index at reference temperature |
| Ｔ0： | Reference temperature (°C) (Ohara defines this as 25°C) |
| Ｔ： | Target temperature (°C) |
| λ： | Vacuum wavelength (μm) |
| D0、D1、 D2、E0、 E1、λTK： | Constant (listed in the data sheet) |
Internal transparency〔λ0.80/λ0.05〕
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.
Internal transparency〔λ0.80/λ0.05〕
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.
Internal transparency〔λ0.80/λ0.05〕
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.
Internal transparency〔λ0.80/λ0.05〕
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.
Internal transparency〔λ0.80/λ0.05〕
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.
Internal transparency〔λ0.80/λ0.05〕
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.
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.
| ｎ(λ,T0)： | Refractive index at reference temperature |
| Ｔ0： | Reference temperature (°C) (Ohara defines this as 25°C) |
| Ｔ： | Target temperature (°C) |
| λ： | Vacuum wavelength (μm) |
| D0、D1、 D2、E0、 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”.
