While watching a YouTube video (2.5hrs), I noticed this device on the top of most Russian Su-27 combat pilot's helmet. A google search only shows it for a Su-27 and MiG-29, but apparently no other aircraft. It was not on any Chinese, Syria, Ukraine or Russian aerobatic pilots flying the Su-27. It appears to be at the wrong angle and has a metal plate that seems to exclude it from being some type of illumination.
What is this on a Su-27 helmet?