Product Announcement from Cascade Optical Corp.


Metal-Dielectric Mirror Coating-Image

Protected and enhanced gold mirrors

Our protected and enhanced gold mirrors have durability according to MIL-M-13508C, and most meet severe abrasion requirements of MIL-C-48497. As in the case of protected aluminum or silver mirrors, when specifying protected gold coating it is important to remember that protective coating usually reduces reflection of bare metal coating. The best reflection one can get out of protected gold is very close to the reflection of bare or unprotected gold. Example to the right shows reflection of bare gold from 400-1400 nm. Obviously, gold has very poor reflection below 550 nm. Reflection curve rises very fast from 500 - 600 nm. At 600 nm reflection is already greater than 90%, and at 900 nm it reaches 98%. Reflection of bare gold is higher than 98.5% for most of optical IR spectrum.

Bare gold coating if coated under optimum conditions and using ion-assisted technology can be cleaned. It does not pass moderate abrasion test of MIL-M-13508C, but it is very cleanable with tissues soaked in alcohol or acetone. Bare gold mirrors are commonly used in instruments where mirrors cannot have any polarization effects. Of course chemical and environmental durability of gold are another important reasons why gold can be used as optical mirror without any protective layer. However, if mirror has to meet either moderate or severe abrasion then gold coating has to be protected with one or more hard dielectric layers. Thickness of dielectric layers is selected to provide optimum performance over the part of spectrum where mirror is designed to operate.

Because of relatively high reflection gold mirrors are commonly used as either bare or protected coatings without any enhancement. If needed gold can be enhanced up to 99.9% in any part of the optical spectrum above 700 nm. If polarization of the reflected light has to be kept to a minimum then for spectrum above 750 nm very simple solution is bare gold mirror coating.

Protected and enhanced silver mirrors

Our protected and enhanced silver mirrors have durability according to MIL-M-13508C except the salt-spray requirement that is typical for silver as film material. Most of the protected and enhanced silver mirror designs meet severe abrasion requirements of MIL-C-48497.
When ordering protected silver coating it is important to remember that protective coating usually reduces reflection of bare silver and the best reflection one can get out of protected silver is very close to the reflection of bare or unprotected silver. Obviously, silver has very poor reflection in the UV, at 400 nm reflection increases above 85%, at 500 nm it is 95%, and at 800 nm reflection becomes greater than 98%, the reflection value that bare silver will keep over entire optical range of IR for all ? > 800 nm.

Unfortunately, bare silver coating is very soft and cannot be cleaned by any contact methods. In addition, unprotected silver oxidizes and spectral performance of the mirror degrades over time. Hence, in all optical applications silver has to be protected. Unlike aluminum and gold, silver is very stubborn film material when it comes to adherence to other films and substrates. Each coating lab has it's own "secret" technologies how to make silver adhere better. Our coating lab has developed several different technologies for protected and enhanced silver mirror coatings. Some of them use so-called adhesion or buffer layer and some utilize ion-assisted deposition technology to make silver stick directly to glass. Both buffered and un-buffered silver coatings pass adhesion tape test per MIL-M-13508C.

Because of relatively high reflection, silver mirrors are in most applications used as protected coatings without any enhancement. If needed silver can be enhanced up to 99.9% in any part of the optical spectrum above 400 nm. If polarization of the reflected light has to be kept to a minimum then the best solution is achieved with protected silver coating that provides reflection over 96% from 500 nm up to far IR for both s- and p-polarized light.

Protected and enhanced aluminum mirrors

All our protected and enhanced aluminum mirrors have durability according to MIL-M-13508C, and most of the protected and enhanced designs meet severe abrasion requirements of MIL-C-48497. When ordering protected aluminum coating it is important to remember that protective coating usually reduces reflection of bare aluminum and the best reflection one can get out of protected aluminum is reflection of bare or unprotected aluminum. Within 200 and 700 nm reflection of bare aluminum stays above 90% and then drops down to 86% around 825 nm. Reflection curve recovers back to 90% at about 920 nm and at 1400 nm becomes higher than 98%. Bare aluminum has reflection greater than 98% throughout entire optical range of IR for all λ > 1400 nm.

Unfortunately, bare aluminum coating is very soft and cannot be cleaned by any contact methods. In addition, unprotected aluminum oxidizes and spectral performance of the mirror degrades over time, particularly in the UV. Hence, in most applications aluminum is protected by hard dielectric film. If mirror operates in visible and NIR, then protective film material is SiO2. If mirror operates in UV or in IR above 5.0 ยต then MgF2 is used as material for the protective layer.

The thickness of protective SiO2 layer is optimized to keep reflection of the protected aluminum mirror as close as possible to the reflection of bare aluminum mirror within visible, 400-700 nm, range. Average reflection of this mirror is higher than 90% @ 400-700 nm. For some applications this reflection is not high enough. Alternative is to use protected silver mirror and get average reflection of 95% and if that is not enough apply multilayer dielectric coating to enhance reflection of aluminum even more.

We provide protected and enhanced aluminum mirrors optimized to operate over a number of different spectral bands centered from deep UV at 120 nm up to IR at 15 microns.

 
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