<title>Applications of Electrically Conductive Coatings In Infrared Instruments</title>

Abstract

AbstractOptical thin films which are electrically conductive, (EC), are finding increasing usein infrared instruments. This paper describes three application examples in which threedifferent functions are performed by the EC coating. One example describes an electro-magnetic shielding application of an EC coating on IR sensor windows. Example two, dis- cusses a deicing /defogging application on the XM -1 tank laser (1.06 pm) range finder window. The third example describes an unusual EC coating application as a 655 nm /10.6 pm beamsplitter for an advanced CO,, laser range finder system.IntroductionAlthough optical thin films which are electrically conductive (EC) have been knownsince the early 1900's, practical applications began in the 1940's. Windshield deicing inaircraft became the first successful application of EC coatings. Most applications sincethat time have also used only the visible transmitting properties of EC coatings. How- ever, recently the spectral characteristics of EC coatings at other than visible wave-lengths have been applied in problem solving. Three examples of applications using theIR properties of EC coatings are described in the following sections.Transparent conductive coatings can be divided into two general categories; semi -conductive metal oxides and conductive thin metal films. The most common representativesof these two categories are Sn02 and Au. Some of the differences and similarities betweenthese two classes of EC coatings will be described with emphasis on their characteristicsin infrared instruments.EC coating fundamentalsBefore attempting to understand a specific EC coating application, a review of somepertinent fundamentals and nomenclatures is appropriate.The conductivity of a coating is usually described by the inverse characteristic,resistivity. Conductive thin films are typically characterized in terms of surface re-sistivity (sheet resistance), r, in ohms per square rather than volume resistivity, p,