2.4 Effect of Temperature on Refractive Index (dn/dt)
Refractive Index is affected by changes in glass temperature (see Fig. 1). This can be ascertained through the temperature coefficient of refractive index. The temperature coefficient of refractive index is defined as dn/dt from the curve showing the relationship between glass temperature and refractive index. The temperature coefficient of refractive index (for light of a given wavelength) changes with wavelength and temperature. Therefore, the Abbe number also changes with temperature.
There are two ways of showing the temperature coefficient of refractive index. One is the absolute coefficient (dn/dt absolute ) measured under vacuum and the other is the relative coefficient (dn/dt relativ ) measured at ambient air (101.3 kPa {760 torr} dry air).
Figures of the relative coefficients are listed in this catalogue.
The temperature coefficients of refractive index dn/dt were determined by measuring the refractive index from -40°C to +80°C at wavelengths of 1,013.98 nm (t), 643.85 nm (C'), 632.8 nm (He-Ne laser), 589.29 nm (D), 546.07 nm (e), 479.99 nm (F') and 435.835 nm (g). These measurements are shown in the temperature range from -40°C to + 80°C in 20°C intervals, and are listed in the lower part of each catalogue page.
The absolute temperature coefficient of refractive index (dn/dt absolute ) can be calculated using the following formula:
dnair /dt is the temperature coefficient of refractive index of air listed in table 2.
| Temperature Range °C |
dnair /dt (x 10-6/°C) | ||||||
| t | C' | He-Ne | D | e | F' | g | |
| -40 ~ -20 | -1.34 | -1.35 | -1.36 | -1.36 | -1.36 | -1.37 | -1.38 |
| -20 ~ 0 | -1.15 | -1.16 | -1.16 | -1.16 | -1.16 | -1.17 | -1.17 |
| 0 ~ +20 | -0.99 | -1.00 | -1.00 | -1.00 | -1.00 | -1.01 | -1.01 |
| +20 ~ +40 | -0.86 | -0.87 | -0.87 | -0.87 | -0.87 | -0.88 | -0.88 |
| +40 ~ +60 | -0.76 | -0.77 | -0.77 | -0.77 | -0.77 | -0.77 | -0.78 |
| +60 ~ +80 | -0.67 | -0.68 | -0.68 | -0.68 | -0.68 | -0.69 | -0.69 |
| Table 2 |