Sapphire, as an artificially synthesized optical material, occupies a significant position in the field of optics due to its exceptional physical and chemical properties. Notably, its superior infrared transmission performance has led to widespread application in fields that impose stringent requirements on infrared optical capabilities. The article will analyse the characteristics, influencing factors and practical applications of infrared transmission of sapphire lenses, thus elucidating the performance advantages of this unique optical material for readers.

Sapphire (Al2O3) is a single-crystal alumina material, and its crystal structure determines its unique optical properties. Sapphire displays exceptional transmission performance within the infrared wavelength range. Sapphire lens with a thickness of 1mm exhibits a transmittance of over 85% in the mid-infrared wavelength range of 1-5μm. Even in far-infrared wavelength range of 8-12μm, it can maintain a transmittance of approximately 60%. The excellent infrared transmission performance of sapphire lens arises from the strong bond energy of aluminium oxide bonds in its crystal structure, resulting in an extremely low absorption coefficient in the infrared range. Sapphire demonstrates significant advantages over standard optical glass in terms of transmittance within the infrared spectrum. This is particularly notable within the two atmospheric window wavelength ranges of 3-5μm and 8-12μm.

The key factors influencing the infrared transmission performance of sapphire lenses encompass three aspects: crystal quality, surface processing accuracy and coating process. In terms of crystal quality, impurity content and defect density are key indicators. High-purity sapphire crystals (with impurity content below 10ppm) have been shown to significantly reduce the absorption loss of light in the infrared wavelength range. Surface processing accuracy has a direct impact on light scatter loss. It is generally required that surface roughness of sapphire lenses is controlled at the nanometer level (Ra<1nm). The coating process of sapphire lenses can be tailored to specific application requirements by designing multi-layer anti-reflective film systems to enhance their transmittance in specific wavelength ranges. For instance, an optimized anti-reflective film can increase peak transmittance of sapphire lenses by over 95% within the wavelength range of 3-5 μm.

In terms of practical applications, the infrared performance advantages of sapphire lenses are demonstrated in a number of areas. Firstly, its high light transmission across a wide range of wavelengths enables it to meet the imaging requirements of both the visible and infrared light range simultaneously. Secondly, it boasts excellent environmental stability. Sapphire has a Mohs hardness of 9 and is far more scratch-resistant than other optical materials. Additionally, sapphire has high thermal conductivity (approximately 40 W/(m·K)) and a low coefficient of thermal expansion (5.3×10⁻⁶/K). This enables it to maintain stable optical properties even in environments with drastic temperature changes.

Industrial inspection is another important application field for sapphire infrared lenses. In semiconductor manufacturing, sapphire lenses are widely used in high-temperature process monitoring systems and can withstand working temperatures above 1000°C while maintaining optimal performance. On the glass production line, infrared thermometers with sapphire lenses can accurately measure the temperature of molten glass. In laser processing equipment, sapphire protective lenses can withstand the irradiation of high-power lasers without interfering with infrared monitoring during processing.

Sapphire lenses are assuming an increasingly prominent role in the optical domain, owing to their infrared transmission capabilities, environmental durability, and advancements in manufacturing technologies. As technological progress continues and the benefits of large-scale production become evident, sapphire is poised to make significant contributions to the broader domain of infrared optics, offering robust technical support for humanity's exploration of the invisible spectrum.