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.
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
2.1 Refractive Index
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, 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
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.
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”.
OPTICAL PROPERTIES
2.1 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.