Browse Prior Art Database

METHOD TO DISGUISE INFRARED TRANSMITTING WINDOWS

IP.com Disclosure Number: IPCOM000005698D
Original Publication Date: 1988-Oct-01
Included in the Prior Art Database: 2001-Oct-29
Document File: 1 page(s) / 72K

Publishing Venue

Motorola

Related People

Mark Mills: AUTHOR

Abstract

It is sometimes desirable to disguise infrared transmitting windows. Since these windows typically are made of exotic materials such as germanium, silicon, orzinc selenide, and often have adistinctive anti-reflection coating on them, they can tip off a knowledgeable observer about the operating spectrum of the equipment. They can also attract attention to a high-tech device, as in covert surveillance gear or in IR thermometers that measure bearing temperatures of railroad cars as they pass. In these applications, a high-tech looking window could invite theft or vandalism of the device.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 50% of the total text.

Page 1 of 1

0 M

MOTOROLA Technical Developments Volume 8 October 1988

METHOD TO DISGUISE INFRARED TRANSMITTING WINDOWS

by Mark Mills

   It is sometimes desirable to disguise infrared transmitting windows. Since these windows typically are made of exotic materials such as germanium, silicon, orzinc selenide, and often have adistinctive anti-reflection coating on them, they can tip off a knowledgeable observer about the operating spectrum of the equipment. They can also attract attention to a high-tech device, as in covert surveillance gear or in IR thermometers that measure bearing temperatures of railroad cars as they pass. In these applications, a high-tech looking window could invite theft or vandalism of the device.

   This method involves applying a coating to the surface of the window that has a diffusely scattering prop- erty (like flat or matte paint) in the visible region, but is transparent in the infrared. This coating can be white or shades of gray, yellow, or red, and thus can be disguised by merging it with the surrounding design. This method can be very effective since it is difficult for most people to recognize the fact that a diffuse surface can be transparent in the infrared.

   To accomplish this, the surface of an IR window is coated with a fine powder of more or less uniform parti- cle size, consisting of one or more materials that transmit infrared radiation when in bulk form. Suitable materials include MgFz, ZnO, CaF2, MgO, Ge, and Si. Light scatters from small particles with a very strong dependence on wavelength. In one approximation, this scattering is proportional to 01 'I h' where a is the particle diameter and X is the wavelength of the light. This effect is sometimes used to make fairly sharp long-pass filters in the infrared.

The wa...