What is cold source of light

LEDs (they can heat up, but they do not require heat to emit light, usually heat is an unwanted effect)

Light-emitting field-effect transistors (same consideration as for LEDs)

Fluorescent lighting and generally cold plasmas that can emit light.

Old TV and PC monitors (ie cathode ray tube screen - although the light emission itself does not require heat, the cathode itself that shoots the electrons does)

Chemo-luminescence (like fluorescent sticks)

Fireflies. LED, fluorescent light