What is cold source of light

Fireflies. LED, fluorescent 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)