Infrared (IR) optical materials are primarily used for transmitting, reflecting, or modulating infrared light (typically in the wavelength range of 0.75 μm to 1000 μm). Different IR bands—such as near-infrared (NIR), mid-wave infrared (MWIR), and long-wave infrared (LWIR)—require different materials because their transmission, refractive index, thermal stability, and other properties affect the performance of optical systems.
| Material | Refractive Index (n) | Dispersion Coefficient (v) | Density (g/cm³) | CTE (×10⁻⁶/°C) | dn/dt (×10⁻⁴/°C) | Knoop Hardness | Properties / Typical Applications |
| Calcium Fluoride (CaF₂) | 1.434 | 95.1 | 3.18 | 18.85 | -10.6 | 158.3 | Low absorption; high refractive index uniformity; used in spectroscopy, semiconductor processing, and cooled thermal imaging |
| Fused Silica Windows (FS) | 1.458 | 67.7 | 2.2 | 0.55 | 11.9 | 500 | Low CTE; excellent IR transmission; used in interferometry, laser instruments, spectroscopy |
| Germanium (Ge) | 4.003 | N/A | 5.33 | 6.1 | 396 | 780 | High refractive index; high Knoop hardness; excellent transmission in MWIR to LWIR; used in thermal imaging and harsh-environment IR imaging |
| Magnesium Fluoride (MgF₂) | 1.413 | 106.2 | 3.18 | 13.7 | 1.7 | 415 | Broad transmission range; resistant to high-energy radiation; used in windows, lenses, and polarizers without AR coatings |
| Potassium Bromide (KBr) | 1.527 | 33.6 | 2.75 | 43 | -40.8 | 7 | Mechanically shock-resistant; water-soluble; broad transmission range; used in FTIR spectroscopy |
| Sapphire | 1.768 | 72.2 | 3.97 | 5.3 | 13.1 | 2200 | Extremely durable; good IR transmission; used in IR laser systems, spectroscopy, and harsh-environment equipment |
| Silicon (Si) | 3.422 | N/A | 2.33 | 2.55 | 1.6 | 1150 | Low-cost; lightweight; used in optics, MWIR laser systems, THz imaging |
| Sodium Chloride (NaCl) | 1.491 | 42.9 | 2.17 | 44 | -40.8 | 18.2 | Water-soluble; low-cost; outstanding transmission from 250nm to 16μm; sensitive to thermal shock; used in FTIR spectroscopy |
| Zinc Selenide (ZnSe) | 2.403 | N/A | 5.27 | 7.1 | 61 | 120 | Low absorption; resistant to thermal shock; used in CO₂ laser systems and thermal imaging |
| Zinc Sulfide (ZnS) | 2.631 | N/A | 5.27 | 7.6 | 38.7 | 120 | Excellent transmission in visible and IR; harder and more chemically resistant than ZnSe; used in thermal imaging |
The selection of IR optical materials depends on factors such as transmission range, refractive index, thermal stability, mechanical strength, and environmental adaptability. Germanium (Ge), zinc selenide window (ZnSe), and zinc sulfide (ZnS) are commonly used for MWIR/LWIR applications, while silicon (Si) and fused silica (FS) are suitable for NIR. Hygroscopic materials (e.g., KBr) require special handling, whereas sapphire and diamond are ideal for extreme environments.
