What are Sealed Optics?
Sealed optics designs consider seal integrity and optical performance equally critical.
Applications for sealed optics span the entire electromagnetic spectrum. System requirements likewise span wide and various ranges of pressure and levels of required hermeticity. Sealed Optics is our name. It was coined with our design engineering philosophy in mind.
The term sealed optics proclaims our view that optical performance and seal integrity are equally important and necessarily must be considered simultaneously to achieve optimum sealed optic performance. Unique design challenges result from the combination of seal and optical parameters; the following examples are but a small set.
Transmission - Opaque Boundaries
The purpose of a sight glass window is to transmit light across an otherwise opaque boundary. Consider, for example, transmission of visible light through an ordinary glass bottle. If instead of visible inspection, it was desirable to analyze information stored in the infrared spectrum of its contents; at least a small opening in the glass wall would be necessary to transmit “beyond the visible” into the infrared.
If spoilage or spillage was likewise important, this opening would need to be reliably re-sealed with an infrared transparent cover. The bottle would need to have a sealed optical window for the infrared. In this common application of fluid containment, it is also important that all container materials be physically and chemically compatible with its contents.
This represents the full concept of a sealed optic design – consideration of optical, mechanical, chemical and physical performance, simultaneously.
Vacuum System Example - Viewport
Sealed systems abound which require optical windows. Most sealed chambers or vessels are comprised of more solid stuff than glass – i.e. metal. Metals won’t pass any but the most energetic forms of radiation – i.e. X-rays or gamma rays.
Chambers such as ultrahigh vacuum (UHV) vessels require uhv viewport windows, or more simply uhv viewports, for viewing of the interior vacuum-scape. For laser processing or detection within such a vessel, laser vacuum viewports with image quality optics and high-performance coatings are essential.
In these applications, the level of hermeticity is a key consideration. Avoiding contamination and outgassing by polymers, adhesives and construction materials is an important design constraint.
Sight Glass Example - Beyond Inspection Window
Internal system inspection by viewing or sighting within vessels is another common application. One may qualitatively assess position, color, process completion, functionality, etc. by looking through a sight glass window. But quantitative measure necessitates a sealed optic.
The required optical element might be made of calcium fluoride (CaF2) for example. This hyperspectral material can pass not only the vis but uv and ir information across the boundary wall. Hence, as the preceding examples demonstrate, window transmission is a key design consideration for all sealed optics applications.
Sight Level Sensing – What about Spectroscopy?
A somewhat mundane application of sight glass windows is fluid level inspection. Whereas fluid levels are not important for a vacuum sight glass window, for many industrial vessels, monitoring process levels are of paramount importance.
For high-temperature processing chambers, high-temperature sight glass windows are commonly used to evaluate the fluid level and temperature. For more complex behaviors like phase change, optical pyrometry or spectroscopy, high-temperature, sealed optical components become necessary.
Pressure Vessel Viewports – Pressure Sight Glass
Viewports are frequently called upon to work in systems employing high internal pressure – pressure vessels. The careful design of these optical assemblies requires optimization of optical performance with material strength and structural support.
The bonding strength, unsupported window diameter, pressure difference and the optical element’s transmission and thickness are all factors in pressure vessel sight glass or pressure vessel viewport design.
It is not uncommon to encounter high-pressure and high-temperature simultaneously in a single application such as down-hole, high-pressure viewport design used for petroleum exploration and drilling. Further, in these applications, careful consideration must be made for testing and design safety factors.
Exotic Sealed Optics - Laser Output Couplers
In the manufacture of gas lasers such as carbon dioxide lasers or argon ion lasers, the partially reflective optic at the front end of the laser is commonly referred to as an output coupler. These output couplers that provide end seals to the optical pumping chamber are yet another example of sealed optics.
A zinc selenide window AR coated at 10.6 microns on the external surface and a partial reflector coated at 10.6 microns on the internal lasing side is a sophisticated sealed optic with aspects of sealing, structure and optical pumping all considered.
Optical Sensor Packaging Windows – Transparent Lids and Covers
Where the industrial process world commonly uses the term sight glass; the semiconductor industry, scientific research and development milieu terms vacuum window or vacuum viewport or vacuum viewport window are synonymous.
In optical sensor packaging sector, the terms optical windows, optical lids or optical covers are used. Optical windows are a common lid or cover option. Optical sensors require transparent windows which are hermetically sealed. These optical windows are used to inspect the sensor after the lid or cover is sealed or as a means of transmitting heat and light both into and out of the hermetic assemblies.
For image quality sensors, stress birefringence that gives rise to optical strain needs be avoided. Careful CTE matching between package and window and ductile, low- temperature sealing are critical.
This is a brief collection of sealed optics applications. Whatever your industry, the term sealed optics reminds one to consider: Are both seal integrity and optical performance critical to my application?