Technical Note:
Optical Filter Reliability Testing

The complexity of optical coating manufacturing has encouraged a dependence upon well controlled processes to insure constant and consistent optical filter product output. True control of processes depends upon assessment of the process output and appropriate adjustment of the process when results indicate a deviation from desirable /expected results. Reliability testing provides the statistical data necessary to monitor and control process variables which can contribute to product durability and ultimate lifetime.

Reliability testing is the only valid method of verifying whether a product meets all of its specified requirements at the time of manufacture and whether it is likely to achieve the useful lifetime for which it was designed and manufactured.

Data produced from reliability testing also provides the necessary information to allow statistically based product capability assessments to be made relative to customer specified lifetime requirements.

To support the critical need for statistically sound product durability and reliability data, we maintain a continuous reliability testing program utilizing comprehensive in-house environmental test chambers and test equipment. This program provides information for production quality control, engineering development, and long term product reliability programs. The tests explained below are the most commonly used and the most appropriate testing procedures for the optical coating and optical assembly industry.

Abrasion Resistance Testing

Abrasion resistance testing is appropriate for coatings which will be exposed on the completed component. Two tests are detailed in MIL-C-48497; Moderate Abrasion and Severe Abrasion. The moderate abrasion test is utilized to determine if a coating will withstand cleaning and handling in the semi-controlled environment typically present in opto-electronic assembly facilities. The severe abrasion test is appropriate for coatings which will be exposed to uncontrolled conditions such as may be encountered by surveying equipment at a construction site. The tests are conducted as follows:

Moderate Abrasion - The coating is subjected to 50 strokes across the surface with a 1/4" thick pad of clean, dry cheesecloth. A minimum bearing force of 1 pound is applied to the pad during the test. An acceptable coating will show no signs of deterioration such as streaks or scratches after exposure.

Severe Abrasion - The coating is subjected to 20 strokes across the surface with a standard eraser conforming to MIL-E-12397. A bearing force of 2 to 2.5 pounds is applied to the eraser during the test. An acceptable coating will show no signs of deterioration such as streaks or scratches after exposure.

Adhesion Testing

Adhesion testing is performed to verify the effectively of processes, performed to insure proper adherence of coatings to substrates. Testing is appropriate for coatings which will be exposed on the completed component. The Adhesion test detailed in MIL-C-48497 is performed as follows:

A piece of 1/2" wide cellophane tape conforming to FED Spec. L-T-90, Type 1 is pressed firmly onto the coated surface of the component. The tape is quickly removed at an angle normal to the coated surface. An acceptable component will exhibit no signs of coating removal in the tested area

Environmental Exposure Testing

Environmental testing provides data from which a determination can be made that a product meets a specified benchmark, and a projection can be made regarding the anticipated lifetime of the product.

Testing procedures prescribed in military specifications including MIL-C-675, MIL-M-13508, MIL-C-48497, and MIL-F-48616 are useful for validating that a product meets a specified minimum requirement, but have limited applicability to lifetime simulation applications.

The testing protocols set forth in MIL-STD-810E were developed specifically to provide manufacturers with test methods to qualify products for storage and use in specific geographical/climatic regions of the world. Environmental conditions are defined for eight discrete climatic regions in this standard. The tests defined in the standard attempt to simulate the extreme conditions of temperature and humidity as well as the diurnal cycles which would be experienced by a product used in a particular region. Having knowledge of the expected geographical deployment of a product, the manufacturer can select appropriate testing procedures to qualify a product for use in those geographical areas.

In addition, MIL-STD-810E also prescribes an aggravated test protocol for temperature/humidity exposure which was developed to provide a means of conducting accelerated lifetime testing. This protocol is the principal temperature/humidity test used by us for all product comparison testing.

All testing conducted to simulate lifetime conditions and to project useful life of a product can only be considered as informational in nature and therefore non-conclusive as to the actual lifetime that a product may achieve. We attempt to verify the adequacy of all environmental testing of this type through actual operational data, wherever possible.

Temperature Exposure Testing

Temperature exposure testing is performed to determine the resiliency of the coatings and assemblies to changes in temperature and to insure that no damage will result from exposing the coating or assembly to the temperature extremes to which they may be exposed during use. We routinely perform temperature testing under the guidelines of two military specifications; MIL-C-48497, and MIL-STD-810E

Temperature MIL-C-48497

This test is a standardized test which is useful for routine verification of resiliency of a component to temperature variation. The coating or assembly is exposed to temperatures of -80 +/-2deg.F and 160+/-2deg.F for a period of 2 hours at each temperature. The rate of change between the 2 temperatures can not exceed 4deg.F per minute. After exposure, an acceptable component will exhibit no signs of flaking, peeling, cracking, or blistering of the coating and no signs of separation or softening of the cement, if assembled.

High Temperature MIL-STD-810E, Method 501.3

This test is performed to determine the effects of storage of coated components for prolonged periods of time in high temperature areas. Testing can be modified to simulate ambient hot(91 to 110deg.F), induced hot(91 to 145deg.F) and extreme hot(91 to 185deg.F). The component is exposed to seven 24hour cycles(168hours total) over the specified temperature range. After exposure, an acceptable component will exhibit no signs of flaking, peeling, cracking, or blistering of the coating and no signs of separation or softening of the cement, if assembled.

Low Temperature MIL-STD-810E, Method 502.3

This test is performed to determine the effects of storage of coated components for prolonged periods of time in cold temperature areas. Testing can be modified to simulate mild cold(-6deg.F), basic cold(-27deg.F), cold(-51deg.F), and severe cold(-60deg.F). The component is subjected to a 24 or 72 hour exposure(depending on construction). After exposure, an acceptable component will exhibit no signs of flaking, peeling, cracking, or blistering of the coating and no signs of separation or softening of the cement, if assembled.

Humidity Testing

Humidity testing is performed to determine the resistance of the coatings and assemblies to degradation from exposure to high relative humidity. Two types of test are typically performed. The humidity test prescribed in MIL-C-48497 is useful to determine whether a coating or assembly has a "basic" resistance to humidity. The humidity test prescribed in MIL-STD-810E is used to simulate the effects of prolonged exposure to climatic conditions to which the coating or assembly may or will be exposed during use.

We continually conduct testing to both MIL-C-48497 and MIL-STD-810E humidity specifications as required by customer specifications. When testing per MIL-STD-810E is specified, testing will be performed per Procedure III (modified to the test duration as appropriate for the item under test).

Humidity - MIL-C-48497

The component is placed in an environmentally controlled test chamber and exposed to a temperature of 120 deg.F and 95-100% relative humidity. After exposure, an acceptable component will exhibit no signs of flaking, peeling, cracking, or blistering of the coating and no signs of separation, moisture penetration, or softening of the cement, if assembled.

Humidity MIL-STD-810E, Method 507.3

Procedure I - Natural, simulates natural environmental cycles and is appropriate for items which are open to the environment. Three different cycle profiles are available for natural testing, each of which represents the natural climate found in a particular geographical region.

Procedure II - Induced, simulates unventilated situations which may occur during transit or storage. Two different cycle profiles are available for induced testing.

Procedure III - Aggravated, exposes the test item to more extreme temperature and humidity levels than those found in nature but for shorter periods of time. It is used to reduce the time and cost of testing.

Temperature is cycled from 86 to 140 to 86deg.F and relative humidity from 95 to 85 to 95%RH over 24 hours. Normal test duration: 10 days.

After exposure, an acceptable component will exhibit no signs of flaking, peeling, cracking, or blistering of the coating and no signs of separation, moisture penetration, or softening of the cement, if assembled. In addition to these physical signs of degradation, no reduction of spectral performance will be evident.