EOTech CNVD-T3 Clip-On Night Vision Device - Thermal 3 - Manual (Page 24)

2-6
2.4.2.2 When attached, the eyecup reduces emission of
stray light from the eyepiece. To install, stretch the base of
the eyecup into place over the eyepiece assembly. Rotate
the eyecup to obtain a proper eye socket and cheek weld.