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ANTIREFLECTION COATINGS Antireflection Application notes COATINGS Optical coatings are used to vary the reflectance and transmittance of an optical component. Typical coating examples include: antireflection coatings which reduce the amount of light reflected at a given wavelength or wavelength range; Filters which allow only a specific range of wavelengths to transmit; and High Reflective Coatings, used in the production of mirrors. Coatings are designed for a specific incident angle of light and for a specific polarization of light such as S-polarized, P-polarized, or randomly polarized. If a coating is designed for light at a 0° angle of incidence, but is used with light at a 45° angle of incidence, the coating will not perform at the stated transmission or reflection specifications. Similarly, coatings are generally designed for randomly polarized light so using S- or P-polarized light with a coating designed for randomly polarized light will again fail to produce the stated specifications. Coatings are created by depositing dielectric and metallic materials such as SiO2, TiO2, or Al, in thin layers that are typically equal in thickness to one-quarter the wavelength of the light used in the application. These thin films alternate between layers of high index of refraction, and low index of refraction, thereby inducing the interference effects needed. Most of our optics have some form of coating and we also design and apply custom coating solutions. antireflection coatings Edmund Optics® offers all TECHSPEC® lenses with a single layer dielectric, antireflection (AR) coating to reduce surface reflections. An AR coating can VIS 0, 45 vastly improve the efficiency of an optic by increasing transmission, enhanc¹⁄₄ Wave MgF ing contrast and eliminating ghost images. The simplest AR coating is a ¹⁄₄ VIS-NIR wave MgF2 centered at 550nm (with an index of refraction of 1.38 at 550nm). UV-VIS Telecom-NIR The MgF2 coating is ideal for broadband use though it gives varied results deUV-AR pending upon the glass type involved. The VIS 0° (for 0° angle of incidence) NIR I NIR II and VIS 45° (for 45° angle of incidence) provide optimized transmission for 400-700nm, reducing average reflection to 0.4% and 0.75% respectively. The 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 UV Visible Near-IR VIS 0° AR coating is preferred over MgF2 for visible applications. Other anWavelength (nm) tireflection coatings include our visible/near-infrared (VIS-NIR) broadband antireflection coating specially optimized to minimize relections for visible and near-infrared wavelengths. Special care has been taken between 870 and 900nm to provide less than 0.2% reflection per surface. We also offer specialized broadband AR coatings for popular telecommunications wavelengths (Telecom-NIR), on our drum lenses and micro PCX lenses. Ultraviolet (UV-AR) coatings are applied to our UV fused silica lenses and windows, as well as a UV-VIS coating for applications using UV and visible wavelengths. Two additional near-infrared coatings are offered NIR I and NIR II. Custom single-layer, multi-layer, V, and 2V coatings are available for both our off-the-shelf and large volume custom orders. AR Coating 2 Antireflective MgF Coating Properties As light passes through an uncoated glass substrate, approximately 4% will be reflected at each surface. This results in a total transmission of only 92% of the incident light through a single optic. Antireflection coatings are especially important if the system contains many transmitting optical elements. Coating each component will increase the throughput of the system and reduce complications caused by back reflections (ghost images). Many low-light systems incorporate AR coated optics to allow for an efficient use of the light. This curve demonstrates the difference between an uncoated and coated single surface BK7 substrate. The coating is a ¹⁄₄λ of MgF2 centered at 550nm. Substrate: BK7 2 6.0 Reflectance % 5.0 Uncoated (Avg.=4.25%) 4.0 3.0 2.0 1.0 400 Coated (Avg.=1.5%) 450 500 550 600 Wavelength (nm) 650 700 750 101 East Gloucester Pike | Barrington, NJ USA 08007-1380 | 1-800-363-1992 | www.edmundoptics.com ® COPYRIGHT 2008 EDMUND OPTICS, INC. ALL RIGHTS RESERVED why do i need an antireflection coating? Antireflection Application notes COATINGS how does an ar coating work? The transmission properties of a coating are dependent upon the wavelength of light being used, the index of refraction of the substrate, the index of refraction of the coating, the thickness of the coating, and the angle of the incident light. The coating is designed so that the relative phase shift between the beam reflected at the upper and lower boundary of the thin film is 180°. Destructive interference between the two reflected beams occurs, cancelling both beams before they exit the surface. The optical thickness of the coating must be an odd number of quarter wavelengths (l/4, where l is the design wavelength or wavelength which is being optimized for peak performance), in order to achieve the desired path difference of one half wavelength between the reflected beams, which leads to their cancellation. The equation for determining the index of refraction of the thin film needed for complete cancellation of the two beams is: nf = (no x ns)Z\x where: nf is the index of refraction of thin film no is the index of refraction of air (or incident material) ns is the index of refraction of the substrate Air no Thickness, t Optical Thickness = n f * t Thin Film = + nf Substrate ns ar coatings for the Ultraviolet ar coatings for the near-infrared 5 5 4.5 4.5 4 UV-AR on Fused Silica 3 Percent Reflection Percent Reflection 4 3.5 UV-VIS on Fused Silica 2.5 2 1.5 Telecom-NIR on LaSFN9 NIR II on LaKN22 NIR I on LaSFN9 3.5 3 2.5 2 1.5 1 0.5 1 0 0.5 200 250 300 350 400 500 450 550 0 600 Wavelength (nm) 700 800 900 1000 1100 1200 1300 1400 1500 1600 Wavelength (nm) ar coatings for the visible Percent Reflection 4 3 2 VIS 0° on BK7 MgF2 on BK7 MgF2 on SF5 1 VIS-NIR on BK7 0 200 400 600 800 1000 1200 1400 1600 Wavelength (nm) Note: All graphs and values are typical and for reference only. 101 East Gloucester Pike | Barrington, NJ USA 08007-1380 | 1-800-363-1992 | www.edmundoptics.com ® COPYRIGHT 2008 EDMUND OPTICS, INC. ALL RIGHTS RESERVED 5 Antireflection COATINGS AntiReflection Coatings Catalog optics are available off-the-shelf with a variety of Antireflection coating options. Coating is applied to both sides of the optical component. Custom coatings are also available. Coating Type Wavelength Range Specifications Z\v Wave MgF2 @ 550nm VIS 0° VIS 45° VIS 425-675nm 425-675nm UV-AR 250-450nm UV-VIS 250-700nm VIS-NIR 400-1000nm NIR I 600-1050nm NIR II 750-1550nm Telecom-NIR 1200-1600nm Rave ≤ 1.75% 400 – 700nm (BK7) Rave ≤ 0.4% 425 – 675nm Rave ≤ 0.75% 425 – 675nm Rabs ≤ 1.0% 250 – 425nm Rave ≤ 0.75% 250 – 425nm Rave ≤ 0.5% 370 – 420nm Rabs ≤ 1.0% 350 – 450nm Rave ≤ 1.5% 250 – 700nm Rabs ≤ 0.25% @ 880nm Rave ≤ 1.25% 400 – 870nm Rave ≤ 1.25% 890 – 1000nm Rave ≤ 0.5% 600 – 1050nm Rabs < 1.5% 750 – 800nm Rabs < 1.0% 800 – 1550nm Rave < 0.7% 750 – 1550nm Rabs ≤ 0.25% 1295 – 1325nm Rabs ≤ 0.25% 1535 – 1565nm Rave ≤ 0.25% 1200 – 1600nm Available off-the-shelf with AR Coating options Singlets Windows Achromatic Lenses UV and IR Lenses 800.363.1992 www.edmundoptics.com 101 East Gloucester Pike | Barrington, NJ USA 08007-1380 | 1-800-363-1992 | www.edmundoptics.com ® COPYRIGHT 2008 EDMUND OPTICS, INC. ALL RIGHTS RESERVED Request a FREE catalog!
