J-20 canopy coating speculation

taxiya

Brigadier
Registered Member
You're thinking of the FLIR? I meant the small windows distributed around the fuselage. Take a look at the EODAS side windows on the nose in these photos: View attachment 52745
View attachment 52746
Ok, I got confused when replying.

However, problem with degerming colour (therefor material) of EODAS window is that the lens is in front of a housing whose interior colour is unknown and likely to make the lens looks much darker than it really is.

I have found a similar photo of F-35 here. Compared with your second photo of J-20, you can see the same effect where the canopy has no golden hue but merely dark grey/green, while the EODAS window is purple reddish.
f-35.jpg
 

taxiya

Brigadier
Registered Member
That's a good observation. Although, I don't follow where from the yellowish tint would arise from a transparent material. I suspect rather that the tint comes from whatever additional layers are used in the composite structure of the canopy. There could very well be a gold thin film in addition to ITO. Something on the lines of that is mentioned in the article I posted about the 787 windshield design.

In this photo the contrasting colors of the F-22 and F-35 canopies are quite apparent:
View attachment 52744

This discussion, where I took the above photo from, suggests the possibility that the F-22 canopy has an additional layer of gold film:
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I agree that ITO and gold thin film play a similar role, both being conductors and gold being one of the best, after silver and copper. I am curious to know how they deal with the reflected RF. Can they introduce some structure to the film to deflect RF away from the incidence angle, or are the canopies themselves already shaped to achieve that effect?

The tumblr discussion and the 787 article seem to suggest that gold thin film brings the benefit of IR reflectivity, therefore greatly reducing the IR signature of the canopy.

IR and RF have different frequencies, so are the frequency responsiveness of gold and ITOs. So I agree with you that although they fundamentally serve same purposes, they may differ in performance in different areas.

My knowledge of their electro-magnetic characteristics is too limited to say anything, but I think the shape does achieve that effect to some level. If you compare F-22 and F-16, F-22's canopy is "flatter at angels", while F-16 is more rounder, same choice of the fuselage shaping.

[Edit]
Some reading from wiki
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The simplest optical coatings are thin layers of
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, such as
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, which are deposited on glass substrates to make mirror surfaces, a process known as
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. The metal used determines the reflection characteristics of the mirror; aluminium is the cheapest and most common coating, and yields a reflectivity of around 88%-92% over the
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. More expensive is
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, which has a reflectivity of 95%-99% even into the far
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, but suffers from decreasing reflectivity (<90%) in the blue and
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spectral regions
. Most expensive is
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, which gives excellent (98%-99%) reflectivity throughout the
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, but limited reflectivity at wavelengths shorter than 550
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, resulting in the typical gold colour.

Please remember, ITO is a composite/alloy whose characteristic is different depending on the composition. This is to say although gold is the perfect natural material for IR reflection, ITO may still be equal or even better than gold if composition or thickness or fabrication can be tuned somehow.
 
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Max Demian

Junior Member
Registered Member
@taxiya
For the EODAS, I believe it is reasonable to assume that these are sensors in the IR spectrum. Therefore, the coating should be IR transmissive, but RF reflective/attenuating.
I read that Germanium coating is a common choice for IR/FLIR systems. The colors range from purple/pink to green, depending on the viewing angle:

The problem with Germanium though is that it it transmits RF. But it could be a component in a meta-material.
 

Brumby

Major
Gold tinting of cockpit is for radiation shielding. The addition of a indium-tin-oxide layer to the coating is for RCS reduction. It is reflective to radar frequencies. The reflective coating reduces the plane's visibility to radar as ordinary canopy would let radar signals straight through where they would strike the many edges and corners inside and bounce back strongly to the source. The reflective layer dissipates these signals instead.

This was the reasoning behind the Have Glass program with the F-16.
 
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