A common question is "how do you test optical coatings"? The answer will always have some variables depending on the coating type, substrate material, application and complexity of testing requirements (mechanical and optical). This quick article should help provide an overview of the methods used in the coatings industry and help you make an informed decision about which tests are beneficial for your project.
Abrasion Test
Abrasion resistance testing is specified when the coating is exposed to the environment and the intact optic is cleaned/handled on a regular basis. Two tests are usually specified; moderate wear and heavy wear. The moderate abrasion test is suitable for applications where the coating can withstand cleaning and handling under controlled or "best practice" conditions. Severe abrasion testing is specified when coatings are subjected to harsh conditions with less control over cleaning and handling methods. Two specific MIL-C-48497 tests are performed as follows:
Moderate Abrasion - Apply 50 strokes to entire surface with a 1/4 inch thick pad of clean, dry cheesecloth. During the test, a minimum load bearing force of 1 lb. was applied to the mat. Coatings that achieve a "pass" mark for this test do not show any signs of deterioration, such as streaks or scratches, after testing.
Severe Abrasion - Apply 20 strokes to coating over entire surface with a standard eraser conforming to MIL-E-12397 (rubber pumice stone). During the test, a 2 to 2.5 lb. load is applied to the eraser. Coatings that achieve a "pass" mark in this test show no signs of deterioration after testing.
Adhesion test
Adhesion testing is performed to verify the effectiveness of the performed process and to ensure that the end-use film has achieved the correct adhesion to the optical substrate. The test applies to coatings that will be processed and cleaned on finished optical components. Again, a commonly used adhesion test is MIL-C-48497. The test is performed as follows: A piece of 1/2 inch wide cellophane tape meeting FED specifications. LT-90, Type 1 is firmly pressed against the coated surface of the component. Quickly remove tape at an angle perpendicular to the coated surface. Acceptable assemblies show no evidence of coating removal in the test area. The removed film has a higher reflectivity or multi-colored sheen than the clear cellulose tape surface.
Environmental Exposure Testing
Environmental testing provides data that can be used to determine whether a product meets specified benchmarks. Using this empirical data, manufacturers can estimate the life cycle of their optical products. Procedures outlined in specifications including MIL-C-675, MIL-M-13508, MIL-C48497, and MIL-F-48616 can help you verify that your product currently meets specified requirements. The procedures outlined in MIL-STD-810E specifically allow manufacturers to determine storage and use of products in specific geographic/climatic regions of the world. This standard defines eight discrete climate zones. The tests defined in MIL-STD-810E simulate the temperature and humidity conditions and diurnal cycles to which a product is exposed when used in a specific region. Manufacturers can choose appropriate test parameters to qualify a product based on where it may be used geographically. MIL-STD-810E has an intensive test protocol for temperature and humidity exposure that allows optical device manufacturers to perform accelerated life testing. The importance of these protocols for comparative testing cannot be overemphasized. All simulated life cycle tests are useful tools, but should only be considered theoretical rather than deterministic as to the actual real world life cycle possible for an optical system. These protocols are important for comparative testing. All simulated life cycle tests are useful tools, but should only be considered theoretical rather than deterministic as to the actual real world life cycle possible for an optical system. These protocols are important for comparative testing. All simulated life cycle tests are useful tools, but should only be considered theoretical rather than deterministic as to the actual real world life cycle possible for an optical system.
Temperature Exposure Test
Temperature exposure testing helps manufacturers understand the stability of a coating or component to the temperature changes an optical system sees during use.
