The faceted windows poking out of the F-35’s chin house the infrared sensors of the jet’s Electro-Optical Targeting System. However, they aren’t made out of glass, but rather from a precious gemstone – sapphire.
Sapphire has historically been so renowned for its beauty and clarity that the Bible itself said that God walked on a pavement of sapphire stone. The ancient Greeks believed that sapphire, which comes in just about every color other than red but is most famous in blue, protected you from harm and envy.
By the Middle Ages, clergy wore sapphires as symbols of heaven, and today, we’ve figured out how to effectively grow this gemstone in a lab.
The F-35’s AN/AAQ-40 Electro-Optical Targeting System (EOTS) uses advanced mid-wave infrared optics with a laser mounted on a gimbal gives pilots long-range thermal footage of targets in the air or on the ground.
Thanks to the its onboard sensor fusion capabilities, the F-35 can do everything from identifying adversary vehicles on the ground, determining their exact distance with the laser, and then using that same laser as a designator to guide air-to-ground munitions into the target.
The same sensors can also spot, track, identify, and target the unique heat signatures produced by enemy aircraft, allowing F-35s to engage even stealthy targets via their heat signatures alone.
But these sensors can’t just hang out of the aircraft as they would immediately be damaged by dust, debris, heat, and the friction which fighters are exposed to in high-speed flight. Leaving the sensors unhoused would also compromise some degree of the jet’s stealth.
Yet, glass is too fragile to be the housing material, and even the reinforced acrylic used in cockpit canopies can be problematic, especially over the jet’s lifetime.
So, Lockheed Martin turned to a firm called Exotic Electro-Optics, now part of Coherent, to effectively grow clear sapphire windows to house the sensors.
Sapphire is a perfectly suited material for these applications: high-purity sapphire is exceptionally clear, hard, and strong.
In fact, sapphire is harder than most steel and almost as hard as diamond making this material extremely scratch- and damage-resistant.
It can also withstand very high temperatures, but importantly, also handles heat-cycling well, meaning going from extreme heat to extreme cold and then back again in short periods of time.
The gemstone itself is essentially aluminum oxide that’s been cultivated into a highly ordered crystalline structure. By heating pure aluminum oxide above its melting point of around 3,720 degrees Fahrenheit and then introducing a small sapphire “seed” crystal for it to attach to, we can grow as much clear sapphire as we need.
And once you’ve grown a high-purity single-crystal sapphire, you can cut and polish it into any specific shape you need.
So, Lockheed Martin created faceted sapphire windows that align with the stealthy platform edges of the aircraft, effectively creating an armored glass house around the F-35’s infrared sensors – and maybe ward off a little harm while they’re at it.
Feature Image: An F-35 with the canopy housing its EOTS visible underneath its nose. (Lockheed Martin)
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