Infrared Window Coatings For Sensor Systems

Infrared window coatings for sensor systems W. HasanThe Perkin-Elmer Corporation Main Avenue, Norwalk, CT 06856AbstractInfrared coatings are required on both surfaces of infrared windows to increase transmission and thus enhance system performance. For certain applications, one of the surface coatings is required to have deicing, defogging, and EMI (Electro-Magnetic Interference) attenuation capabilities.This paper deals with the transparent infrared coatings, including antireflection, conductive and induced transmission coatings for application in sensor systems. The wavelength ranges covered are 3.0 to 5.0 and 8.0 to 12.0 microns. Theoretical aspects of the design techniques are discussed in detail. The experimental spectral data is presented for various coatings on various substrates. In addition to the spectral data, data on the mechanical and environmental durability of the coatings is also presented. The mechanical and environmental durabilities of the coating are very important for military systems such as electro-optical sensor systems for use on aircraft, submarines, tanks, and ships.The transparent antireflection coatings are sometimes also required to have rain erosion resistance. Data on such coatings is also presented.IntroductionAntireflection coatings (AR) are used today on the surfaces of the windows, domes, and lenses of almost all infrared optical equipment. There are two primary reasons for using antireflection coatings on the surfaces of these elements. First, most of the light reflected at each surface is lost to the transmitted beam, and if there are either many elements or elements of high refractive indices, this loss may be intolerable. Secondly, the multiple reflections between the surfaces of the elements cause unwanted light to fall on the image plane, thereby reducing the contrast and definition of the image.In the last decade, considerable interest in the development of IR transmitting materials capable of performing in an adverse environment has grown due to the development of imaging electro-optical sensors for use on high-performance aircraft, submarines, land vehicles, and ships. This increased development of IR transmitting materials has stimulated a large amount of theoretical and experimental work in the field of coatings. The coatings not only have to be efficient, but also need to survive harsh operational environments, such as that of high humidity, salt fog, missile exhaust, sand, dust, and rains.Windows are sometimes also required to have electrical conductive (EC) coating for deicing, defogging, and, most importantly, attenuating Electromagnetic interference (EMI) while permitting a reasonable .transmission for the infrared region. These conductive coatings generally have high reflectance in infrared, and induced transmission (IT) coatings are required to reduce the losses due to reflection so that reasonable transmission can be obtained. Both EC and IT Coatings are discussed in this paper, along with antireflection coatings. The wavelength ranges considered are three to five microns and eight to twelve microns.Theoretical considerations AR coatingsAntireflection coatings can be designed using either a single-layer, double-layer, triple-layer, four-layer, or multilayer coating design. The selection of a design technique depends upon the wavelength bandwidth and the reflectance values requirements. The discussion in this paper will be limited to single- and double-layer coatings primarily due to high durability requirements. Higher layer film systems have more stress and, in general, tend to fail in harsh environments. A detailed discussion of various design techniques for AR Coatings can be found in Reference 1.