Nikon D5500 - Manual (Page 105)

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The AF-Assist Illuminator
If the subject is poorly lit, the AF-assist
illuminator will light automatically to assist
the autofocus operation when the shutter-
release button is pressed halfway (some
restrictions apply;
0 352). Note that the
illuminator may become hot when used
multiple times in quick succession and will
turn off automatically to protect the lamp
after a period of continuous use. Normal
function will resume after a brief pause.
AF-assist illuminator