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Choosing the right optical coating

Posted by Steve Rowe on

Gold coated astronaut helmetWhen it comes to choosing the right optical coating, details matter. But what details and what drives the decisions? When you get right down to it, it’s all about the intended application—and there are a lot of variables, with a number of pros and cons for each.

Go for the gold
Take space helmets for example. You know, like they wear in the movies. You're likely familiar with the reflective gold-tone look. Well, when it comes to gear the astronauts use, it really is gold. That coating is actually a thin layer of gold applied to a polycarbonate shield. Astronauts need it for protection. Why? Because you want to reflect as much harmful infrared radiation as possible—and gold has extraordinary reflectivity. Additionally, it has excellent reliability and corrosion resistance.

We all know that exposure to sunlight is bad for our skin. UV rays can do a lot of damage. The same goes for our eyes. The gold coating on an astronaut’s helmet actually blocks electromagnetic radiation given off by the Sun—infrared (IR), visible, and ultraviolet light (UV), in particular. For more about the sun and possible eye damage, go here.

IR and UV protection
Your eyes can focus both visible and near IR light onto your retina. But the eye only has visible receptors—not IR receptors. That means when intense visible light hits these receptors, they transmit information letting you know that this is painful and will cause damage if you don’t either close them or look away. That doesn’t happen with IR. With IR, you wouldn’t realize that your eye was being “burned.” That’s why astronauts need IR protection.

About 60% of UV light is transmitted through the gold, but a polycarbonate plastic visor has excellent visible transmittance and absorbs/reflects almost all UV as shown below.

Reflectance Curves for Metallic (Mirror) Coatings Chart

Reflectance Curves for Metallic (Mirror) Coatings

Optical coatings applications
At Esco, we apply optical coatings using vacuum deposition technology. Available options include a wide range of anti-reflective designs and interference filters, as well as metal and dielectric high reflectors. Out of all metallic reflectors, gold does have the benefits of reflectivity and corrosion resistance, but cost can be prohibitive. Silver also has excellent reflectivity from the visible to the far infrared spectrum, and can be an option as well. Generally, with silver, a durable overcoat is recommended to prevent oxidation.

Standard aluminum is our most popular mirror coating for applications in the visible and near infrared spectrums. It’s one of the few metals that retains its full silvery reflectance in a finely powdered form, making it an important component for mirror coatings. We also offer several dielectric coatings as overcoats, to enhance reflectivity, durability, and/or longevity of the underlying metal. See, we’ve got this stuff covered.

Want more detailed information on coating, check out our catalog or speak with one of our optical experts, today. 

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