Technical Note:
Optical Filter Construction

The diverse nature of optical elements provides the designer with significant flexibility in the development of an optical system design. The basic functions which optical elements perform include protecting the system from damage, defining the image intensity and quality of the optical system, and filtering the total system energy to isolate the desired spectral signature.

Optical Filters can be manufactured in many physical forms. Filtering coatings can be deposited on exterior windows of optical systems or on geometrical optics within the optical system. Complex filter coatings can be deposited on a single substrate or deposited on several separate substrates and laminated together to form a single element.

The process of filtering may be accomplished by one or more of the following methods; absorption, reflection, or transmission of light energy.

The choice of the method of filtering, the physical form of the filter, the method of assembly of the filter, and the coating technology to be employed to coat the optical filter substrates, will depend upon the constraints imposed by the specifications for the filter. Design engineers at our Corion coating facility will always seek to provide the most economical construction which will satisfy all of the specified requirements for the filter. The construction methods that follow are commonly utilized by our design engineers to create a wide range of custom thin-film filters and coatings.

Soft Coating - Scribed & Laminated

Scribed & Laminated Soft Coating construction is a commonly used construction for bandpass and edge filters. Filter components (narrow dielectric bandpasses, induced transmission filters (ITF), broad dielectric bandpasses, reflectors) are coated onto separate substrate plates. These plates are then scribed to remove the film symmetrically around the required specified active film areas of the optical filters to be produced from the plate. The scribed plates are subsequently laminated together with a moisture resistant epoxy. A glass-epoxy-glass seal is formed in the scribed areas forming the best possible barrier to moisture penetration. Filters are cut through the scribed area to their finished size using diamond cutting tools. Additional moisture barriers may be applied to the cut filters if necessary to achieve extended life requirements.

Soft Coating - Laminated

This construction is typically used for bandpass filters requiring extreme signal to noise ratios coupled with size restrictions prohibiting a scribed border. Filter components (narrow dielectric bandpasses, induced transmission filters (ITF), broad dielectric bandpasses, reflectors) are coated onto separate substrate plates. Designers will attempt to design the filter components using soft coating chemicals with the best resistance to moisture degradation when using this construction method. These plates are then laminated together with a moisture resistant epoxy. Optical filters are cut to their finished size using diamond cutting tools. Additional moisture barriers may be applied to the cut filters if necessary to achieve extended life requirements.

Soft Coating - Argon Gap

This construction is typically used for filters which will be exposed to high levels of solar or ultra-violet radiation. Filter components (narrow dielectric bandpasses, induced transmission filters (ITF), broad dielectric bandpasses, reflectors, metal-dielectric-metal filters(MDM)) are coated onto separate substrates. These filter components are then laminated into specially designed holders with a moisture resistant bond line extending around the edges of the component, only. No epoxy is present in the light path. An additional moisture barrier is applied to the assembly to achieve maximum product life.

Hard Coating - Single Element - Spectrally Dynamic

Optical filters constructed using this method consist of conventionally deposited hard filter coatings, on a single substrate. Coatings may either be deposited on finished substrates such as lenses or precision windows, or on large plates which will be cut to finished size after coating. These optical filters will exhibit spectral shifting as ambient relative humidity changes but will be designed and verified to meet all spectral requirements at the specified operating conditions

Hard Coating - Single Element - Spectrally Stable

Optical filters constructed using this method consist of Stabilife® hard filter coatings, on a single substrate. Coatings may either be deposited on finished substrates such as lenses or precision windows, or on large plates which will be cut to finished size after coating. These filters exhibit no change in spectral location as ambient relative humidity changes.

Hard Coating - Multi-Element - Laminated to Stabilize/Assemble

Optical filters constructed using this method consist of conventionally deposited or IAD hard filter coatings, on one or more substrates. Coatings may either be deposited on finished substrates such as lenses or precision windows, or on large plates which will be cut to finished size after coating. Lamination with a moisture resistant epoxy after coating may serve two purposes in this construction method. In all cases, lamination will eliminate the short and/or long term humidity induced spectral shifting inherent in these types of coatings. In cases where the filter will consist of a number of different filter elements, lamination serves two functions; stabilization of the coatings and bonding of the completed assembly.

Hard Coating - Multi-Element - Laminated to Assemble

Optical filters constructed using this method consist of Stabilife® hard filter coatings, on one or more substrates. Coatings may either be deposited on finished substrates such as lenses or precision windows, or on large plates which will be cut to finished size after coating. In this construction method, lamination with a moisture resistant epoxy after coating is utilized solely for the purpose of assembling multiple optical filter elements together to form the completed assembly

Hard Coating - Multi-Element - Mechanical Assembly

Optical filters constructed using this method consist of Stabilife® hard filter coatings, on one or more substrates. Coatings may either be deposited on finished substrates such as lenses or precision windows, or on large plates which will be cut to finished size after coating. In this construction method, filter components are assembled together to form a completed filter assembly using machined holders and mechanical retaining hardware.