Introduction to optical windows: Germanium and Silicon IR windows

Posted by juliabennet on July 27th, 2013

The term of optical window, like many other related to optics, can have multiple meanings associated to different fields of activity. Our point of interest however being the one of optics, we shall look into its specific significations, with a short introduction into Germanium windows and respectively Silicon windows. So here is what anyone with basic knowledge in this direction should know and understand.

The optical window is a chunk of optical, transparent material that controls the light entering no matter what type of optical instrument. Of course, things are not that simple, so we should add the fact that the window should be perfectly flat, a process that can be achieved either through optical or through mechanical interventions. Moreover, by transparency we do not necessarily mean transparent for the spectrum of the visible light, but for any certain range of wavelengths desired by the manufacturer.

Manufactured in general with its two facets parallel, in most of the situations, its outer layer is enhanced with a special, antireflection coating, particularly when it targets the visible light. Alternatively, the window may have an edged construction. The materials behind this technology vary a lot and have specific properties. From BK7 and Fused Silica to Calcium Fluoride or Magnesium Fluoriate, plus Zinc Selenide, Sapphire, Germanium or Silicon, all these can serve as base for the tinny window.

From the entire above, Germanium windows and Silicon windows are among the most popular products. So let us make a short analyze of their features for a better understanding of the popularity they enjoy. When it comes to evaluating their quality, there are certain specifications that professionals look for. These features include the thickness of the mirror together with its thickness tolerance and the diameter tolerance, the surface quality, the parallelism, the surface flatness, the clear aperture, the coating performance and the damage threshold.

For a clearer image, let us say that a Germanium window presents an 85 – 90% clear aperture, measured central on diameter while a Silicon one scores over 90% on this chapter. Moreover, they share the same surface quality of 20 – 40 scratch dig, while the thickness tolerance is of +/- 0.25 mm for the first model and of +/- 0.3 mm for the second model. As for the coating performance, it scores a little under 2% for 3 – 5 µm at Silicon and under 5% for 8 – 12 µm surface on Germanium.

The list of technical features is considerably longer than that. You do not have to know them all, but rather to understand the main differences. For example, silicon is known to be quite effective in terms of transmission and to have a low density, nearly 50% lower than the one of Germanium, which is quite an important feature in the world of optics. Moreover, they have a higher thermal conductivity and are more solid than their major opponent is.

Even so, it is not up to you deciding which of these two options is more suitable for the devices that were recently broken. Only a specialist can tell you what to opt for, and he should motivate carefully and logically his recommendations.

You don't know what to choose between Germanium windows and Silicon windows? Share your concerns with us and we give you all the right answers!