Technical Note:
Optical Filter Lifetime

All products have certain inherent weaknesses dictated by the product and it's intended use. Automobiles manufactured for use in the northern climates must be resistant to road salt. House paint must be resistant to sunlight, heat, cold, and humidity. In each of these examples, the manufacturer of the product is well aware of the threats to prolonged product lifetime. Having this information, manufacturers seeking to provide a high level of product reliability and product life will make every effort to minimize the effects which the threat will have over the expected product life.

Over the past thirty years, we have developed an extensive working knowledge of optical coating and optical assembly technology. During this time, we have conducted comprehensive development programs to investigate and understand the causes of product failure. As a result of these efforts, we have made design improvements to eliminate premature product failure and have developed families of products, supported by substantiating data, that endure every type of environment. A brief summary of the principal causes for product failure is provided below.

Laminated Products - Physical Separation

The principle cause for failure of all laminated optical assemblies is the failure of the bonding between assembly elements. Bondline failure initially appears as "feathering" at the point of moisture penetration. The feathered appearance is the result of uneven separation of the bonding material from the substrate creating color variation. Over a considerable period of time, the separation process can continue to extend over the entire substrate area allowing the substrate to become detached from the rest of the assembly. Physical separation however, does not imply spectral failure. Many optical assemblies are manufactured from stable, hard coatings which are impervious to change from moisture. These types of assembly can continue to achieve spectral performance close to the original level even after complete separation(as long as the elements are mechanically captured within the light path). The process of physical separation can result from a number of factors.

Process Factors

  • Substrate cleaning
  • Bonding process temperature
  • Bonding material mixing
  • Curing process temperature
  • Bond line thickness control

Material Factors

  • Material moisture resistance - cured
  • Material chemical stability - un-cured
  • Material shelf life control

Environmental Factors

  • Exposure to cyclic high humidity at high temperatures
  • Prolonged exposure to solvents and water

Of the above mentioned factors, all can be controlled by the manufacturer except the environmental factors. Lacking the ability to control environmental factors, we have maintained continuous programs to develop new materials and processes to improve environmental durability, and to establish statistically sound reliability profiles for manufactured products. These programs insure that customers are receiving the latest technology and accurate information regarding lifetime.

Film Degradation -Moisture Induced

The majority of the evaporation chemicals employed in Soft Coating technology exhibit some susceptibility to decomposition from exposure to water. The degree of susceptibility varies widely from the high level found in silver to the low level of zinc sulfide. Therefore, coatings manufactured from these materials require some degree of protection from moisture penetration if maximum lifetime of the coating is to be achieved. Film degradation in laminated optical components occurs when moisture comes in contact with a coating material susceptible to moisture damage. As moisture penetrates into the coating material, the coating begins to discolor, typically changing from a highly reflective to a cloudy appearance. As physical deterioration of the film proceeds, the spectral performance will begin to degrade as well but at a much slower pace. In accelerated lifetime testing, films have been observed which showed gross physical deterioration but which showed no degradation in spectral performance. Moisture induced film degradation typically results from one of two causes, either design constraints that restrict the installation of a moisture barrier, or physical separation that allows moisture to reach a film which was previously protected by a moisture barrier.

Wherever possible, our soft-coated products are manufactured with scribed borders which provide the best seal against moisture penetration. However, in some situations, scribed borders can not be incorporated into the design either due to size constraints or extreme integrated blocking requirements. In these cases, we will design the product either using less moisture sensitive soft coatings or with an appropriate hard coating films, depending upon the specified lifetime requirements and cost objectives

Epoxy Solarization

Certain epoxy resin systems exhibit relatively low resistance to degradation attributable to prolonged exposure to intense ultraviolet energy. This type of degradation is typically found only on components manufactured using ultraviolet transmitting substrates, such as ultraviolet filters. Photochemical changes in these epoxies result in a reduction in the ultraviolet transmittance of the resin thereby causing a reduction of filter assembly transmittance. We have developed product designs to overcome this problem which include the use of improved stability, ultraviolet transmitting epoxies and our Argon-Gap construction method

Physical Deterioration - Hard Coatings

The majority of hard coatings are designed and manufactured to be resistant to damage from handling during assembly and use and to withstand prolonged exposure to adverse environmental conditions. The absence of protection for these coatings creates conditions wherein coating failure usually occurs in a short period of time and is catastrophic in nature.

Typical forms of failure include cracking, crazing, peeling, and blistering of the coating. Weak coatings can exhibit poor adhesion or poor abrasion resistance. Hard coating technology is highly process dependent. Any change in any of the multitude of processes utilized to manufacture a hard coating can result in a catastrophic coating failure. Fortunately, these types of failure will generally become evident upon completion of environmental reliability testing before the product is shipped.

Some hard coatings may be laminated but in these cases, the lamination process is generally being utilized either to form an assembly or to provide spectral stability rather than to provide protection for the coatings.

All of our hard coatings are subjected to environmental reliability testing prior to shipment to insure that the products will achieve their designed lifetime. In addition, we maintain continuous process control data to insure that all hard coating processes remain within statistical control thereby significantly reducing the probability of catastrophic failure.