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Specifications and Standards for Optical Coating Durability
Author(s): Michael Hausner
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Book Description

This Spotlight gives a general overview and useful information about the durability of optical coatings, and greater information about different durability tests for the applicable civilian and military standards and specifications. It allows for the quick detection of coating testing durability requirements and test conditions in MIL-Specs, military standards (MIL-STDs), ISO standards, and other standards or specifications according to the requirements defined in the relevant drawings or coating specifications.

Book Details

Date Published: 13 August 2019
Pages: 48
ISBN:
Volume: SL51

Table of Contents
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1 Purpose

2 Glossary

3 Environmental Durability of Optical Coatings
3.1 Common durability requirements for optical coatings
     3.1.1 According to MIL-Specs
     3.1.2 According to ISO 9211 and ISO 9022
3.2 Laser damage threshold durability for optical coating
3.3 Windscreen wiper test
3.4 Rain erosion test
3.5 Optical coating durability in industrial oils and fuels
3.6 Durability testing of coating on actual components: when?
3.7 Visual inspection/examination

4 Characterization of Optical Coatings
4.1 Functionality of the coating
4.2 Spectral region of the coating

5 Factors Impacting the Quality of the Coating
5.1 Coating material composition
5.2 Raw material of the element's substrate
5.3 Optical performance
5.4 Environmental durability for which the coated element should be resistant
5.5 Size and shape of the element
5.6 Cleaning of the optical surfaces
5.7 Additional factors

6 Environmental Durability: Equipment, Tools, and Materials for Testing
6.1 Equipment and tools
6.2 Materials

7 Quality and Safety Aspects
7.1 Quality aspects
7.2 Safety aspects

8 Environmental Durability Requirements and Test Conditions According to Accepted Standards and Specifications
8.1 MIL-M-13508C (1973): Military Specification - Mirror, Front Surfaced Aluminized for Optical Elements
8.2 MIL-F-48616 (1977): Military Specification - Filter (Coatings), Infrared Interference: General Specification for
8.3 MIL-C-48497A (1980): Military Specification - Coating Single or Multilayer, Interference Durability Requirements for
8.4 MIL-C-14806A (1969): Military Specification - Coating Reflection Reducing for Instrument Cover Glasses and Lightning Wedges
8.5 MIL-C-675C (1980): Military Specification - Coating of Glass Optical Elements (Anti-Reflection)
8.6 MIL-PRF-13830B (1997): Optical Components for Fire Control Instruments; General Specification Governing the Manufacture, Assembly, and Inspection of
8.7 MIL-STD-810G (2008): Department of Defense Test Method Standard: Environmental Engineering Considerations and Laboratory Tests
     8.7.1 MIL-STD-810C (1975)
     8.7.2 MIL-STD-810D (1983)
     8.7.3 MIL-STD-810E (1989)
     8.7.4 MIL-STD-810G
8.8 ISO 9211-3 (2010): Optics and Optical Instruments - Optical Coatings - Part 3: Environmental Durability
     8.8.1 Application categories and recommended tests for environmental durability
     8.8.2 Types of tests according to the standard
8.9 TS-1888 (1979): Technical Specification for Infrared Optical Coatings

9 Examples of Failed Coatings on Witness Samples or Elements during Environmental Durability Tests

10 Summary

11 Appendix: List of Publications

Preface

Most optical components assembled in optical military and civilian systems need to have coatings. The type of coating and its composition are determined according to its intended use and the system's region. In addition, several issues such as optical and mechanical properties, environmental durability requirements, and the optical element's material type are taken into consideration. Coating characteristics and functionality requirements are defined by the optical system designer and are specified in the relevant drawing or coating specification. The coating specifications include optical mode (e.g., transmission or reflection), environmental durability requirements, laser-induced damage threshold (LIDT)/laser damage threshold (LDT), and the windscreen wiper test.

Environmental durability of the coating is defined and tested normally according to American military specifications and sometimes according to civilian standards [International Organization of Standardization (ISO) and American National Standards Institute (ANSI)]. These specifications or standards contain requirements and test conditions for the environmental durability of optical coatings. The optical designer may, of course, set additional requirements according to the needs of the system.

The responsibility for environmental durability is the coating manufacturer's, but the customer must also ensure, through control and inspection, that the coating manufacturer's statement meets the durability requirements.

In the optical coatings industry, the environmental durability of the coating layer is a critical performance feature. Environmental durability and spectral requirements are defined by the optical designer and brought to the attention of the coating manufacturer.

Coating layer components are defined for optics performance depending on their refraction and absorption indices. Evaporation coating is done in a vacuum, and the behavior of the coating materials during this process induces certain restrictions that influence the coating layer's ability to withstand the environmental conditions previously defined.

After the evaporation process, the coatings should withstand assembly into the mechanical system, as well as the operational environmental conditions of this system. These conditions can be as simple as wiping an outer window, cleaning an internal component with a soft wet cloth, or rigorous exposure to extreme heat or cold conditions, a desert sandstorm, or a salty environment.

The coated optical element should be exposed to the environmental conditions of the system where they are assembled. For example, a coated element placed within a sealed assembly and protected from the outside environment should not be as durable as an external coated element (lens, window, or dome) exposed to severe environmental conditions. When determining the durability requirements of an optical coating, the designer should take into account cases of long-period storage under uncontrolled conditions (humidity, temperature, or packaging), cleaning processes during assembly, and the intended use of the system. An external surface that is exposed to high humidity or a salty atmosphere for a long period of time (marine environment) or has to be cleaned frequently to remove insects (airborne systems) should have a harder and more durable coating.

Quality assurance of optical coatings should include specifications of optical requirements (transmission, reflection, and others) and the environmental durability of the coating. This specification can be included in a separate document to be referred to the optical component's drawing or they can be an integral part of the drawing itself.

Environmental tests are usually accelerated tests. In a short-time test, they should represent long-time stability. It is important to select an appropriate environmental specification or standard with appropriate tests that give good indications of long-time stability.

The structure of the report was prepared in a way that enables the user a convenient and effective use of it. The page size is A4, each section starts on a new page, and every paragraph ends on its page and doesn't split to another one. From my long experience in writing and reading different kinds of reports, the structure of this report is the preferred way of using it.

Michael Hausner
July 2019


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