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What is the lump/ball thing visible below and to the left of the canopy in this image of a PAK FA (which became the Sukhoi Su-57), and what is it for? It's also seen on other fighters.

enter image description here

Image source

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That is the infrared search and track (IRST). This particular model of an IRST is known as the OLS-50M in Russia and uses quantum well photodetectors.

Radar has two serious limitations, one is that advanced software can jam it, the other is that you give away your position when transmitting radar signals.

An infra-red sensor can be used to find and destroy targets without giving away the position of the aircraft and cannot be jammed. IRST is especially effective at detecting low-flying vehicles like helicopters that would otherwise be lost against a background. IRST can also be used to find, hunt down and kill ground vehicles.

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    $\begingroup$ Being on top of the plane I doubt it would be very useful to detecting vehicles under you or on the ground. $\endgroup$ – Michael Nov 21 '16 at 21:23
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    $\begingroup$ @Michael The Russians fly inverted when they want to collect a ground target with the IRST. $\endgroup$ – Tyler Durden Nov 21 '16 at 21:25
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    $\begingroup$ @Michael being where it is on most aircraft (infront of the cockpit, directly behind the nose) you would be surprised just what it can pick up, especially as those noses are built to be transparent in a lot of wavelengths, so in most IRST systems they don't block its view. $\endgroup$ – Moo Nov 21 '16 at 22:16
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    $\begingroup$ It's called "Infrared light" for a reason; and there's no way that nose radome is transparent to any frequency of light. It may be translucent in some frequencies, but that's not going to help here. $\endgroup$ – Riot Nov 22 '16 at 14:20
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    $\begingroup$ @Riot, visible, infrared, ultraviolet, X rays, γ radiation, they are all "light" in a sense. They are electromagnetic waves. I guess the term "light" may be more frequently used for the visible and nearby frequencies but I don't think that referring to X-rays as light for example is wrong. No more than referring to infrared as light. So when you say "any frequency of light", what frequencies exactly are you referring to? $\endgroup$ – ypercubeᵀᴹ Nov 22 '16 at 14:47
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That is an passive Infra Red Search and Track (IRST) sensor. Used to passively identify and track aircraft using their emitted infrared energy. The system pictured above is used solely for an air-to-air role and does not have a use for identifying and tracking ground targets.

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  • $\begingroup$ I can't speak to the use of the OLS-50M specifically on the PAK-FA, but the description page for it does say that it's used for air, ground, and water targets. It was developed for the Su-35, and on that craft it's in the exact same position, leading one to believe that it's capable of detecting ground and water targets on the T-50 as well. $\endgroup$ – Egor Nov 22 '16 at 22:12

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