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This question already has an answer here:

Most but not all Russian Su-27's (and variants like the Su-30, -32, -33) have a device that looks like a headlight in front of the cockpit. What is it?

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marked as duplicate by ymb1, Stelios Adamantidis, DeltaLima, mins, Pondlife Jan 4 '18 at 13:25

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It's the sensor of the IRST (InfraRed Search and Track) system. An IRST allow an aircraft to search and track aerial or ground targets using passive infra-red radiation only thus reducing the probability of being discovered (for example using a Radar Warning Receiver).

The Su-27 can use it's IRST to track a target at very long distance and engage it using IR homing AAMs like the R-73 "Archer" or the R-27T / R-27ET "Alamo" without using its radar.

Nowadays, many current fighter aircrafts use some sort of IRST, for example the Eurofighter Typhoon.

Typhoon's IRST sensor

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That is the IRST which can be used for tracking aircraft passively.

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