Properties and Selection Methods of Optical Materials Commonly Used in Optical Lenses

K9

K9(H-K9L, N-BK7) is the most commonly used optical material, from visible to near infrared (350-2000nm) has excellent transmittance, has a wide range of applications in telescopes, lasers and other fields. H-K9L (N-BK7) is the most commonly used optical glass for the preparation of high-quality optical elements, and is generally chosen when the additional advantages of ultraviolet fused quartz (good transmittance and low coefficient of thermal expansion in the ultraviolet band) are not required.

Ultraviolet fused quartz

Ultraviolet fused quartz (JGS1, F-SILICA) has a high transmittance from the ultraviolet to the near-infrared band (185-2100nm), and has a high transmittance in the deep ultraviolet region, making it widely used in ultraviolet lasers. In addition, UV-grade fused quartz has better uniformity and lower coefficient of thermal expansion than H-K9L (N-BK7), making it particularly suitable for applications in ultraviolet to near-infrared bands, high-power lasers and imaging fields.

Calcium fluoride

Due to the high transmittance of calcium fluoride (CaF2) in the wavelength of 180nm-sum (especially in the 350nm-7um band transmittance of more than 90%), low refractive index (for the operating wavelength range of 180nm to 8.0um, its refractive index change range of 1.35 to 1.51), so even without coating has a high transmission. It is often used as a spectrometer window and lens, and is also used in thermal imaging systems. In addition, because of its high laser damage threshold, it has a good application in excimer lasers. Calcium fluoride has higher hardness than similar substances such as barium fluoride and magnesium fluoride.

Barium fluoride

Barium fluoride material has a high transmittance from the 200nm-11um region. Although this property is similar to that of calcium fluoride, barium fluoride still has better penetration after 10.Oum, while calcium fluoride has a plummeting decline. And barium fluoride is more resistant to high energy radiation. However, barium fluoride has the disadvantage of poor water resistance. When exposed to water, the performance deteriorates significantly at 500 ° C, but in dry environments, it can be used for applications up to 800 ° C. At the same time, barium fluoride has excellent scintillation performance and can be made into various optical components such as infrared and ultraviolet. It should be noted that when handling optical elements made of barium fluoride, gloves must be worn at all times and hands must be thoroughly washed after handling.

Magnesium fluoride

Magnesium fluoride is popular in many ultraviolet and infrared applications and is ideal for applications in the 200nm-6um wavelength range. Compared to other materials, magnesium fluoride is particularly durable in the deep ultraviolet and far infrared wavelength ranges. Magnesium fluoride is a strong material that can be used to resist chemical corrosion, laser damage, mechanical shock and thermal shock. The material is harder than calcium fluoride crystals, but relatively softer than fused quartz and has a slight hydrolysis. It has a Knoop hardness of 415 and a refractive index of 1.38.

Zinc selenide

Zinc selenide has a high transmittance in the 600nm-16um band and is commonly used in thermal imaging, infrared imaging, and medical systems. And because of the low absorption rate of zinc selenide, it is especially suitable for high-power CO2 laser. It should be noted that zinc selenide material is relatively soft (Knox hardness of 120), easy to wipe, it is not recommended for harsh environments. With zinc selenide lens, be careful when holding and cleaning, pinch or wipe with even force, it is best to wear gloves or rubber finger covers to prevent contamination. Cannot be held with tweezers or other tools.

Silicon

Silicon is suitable for the near infrared band in the 1.2-8um region. Because the silicon material has the characteristics of low density (its density is half of the germanium material or zinc selenide material), it is particularly suitable for some occasions that are sensitive to weight requirements, especially in the 3-5um band applications. Silicon lens has a Knopf hardness of 1150, which is harder than germanium and less brittle than germanium. However, because it has a strong absorption band at 9um, it is not suitable for transmission applications in CO2 lasers.

Germanium

Germanium is suitable for the near infrared band in the 2-16um region and is suitable for infrared lasers. Because germanium has the characteristics of high refractive index, minimum surface curvature and small color difference, correction is usually not required in low-power imaging systems. However, germanium is more seriously affected by temperature, and the transmittance decreases with the increase of temperature, so it can only be applied below 100℃. The density of germanium (5.33g/cm) should be considered when designing systems with strict weight requirements. Germanium plano-convex lenses are turned on precision diamond lathes, a feature that makes them ideal for a variety of infrared applications, including thermal imaging systems, infrared spectroscopes, telemetry and FLIR.