Question

4 cold source of light

Answer 1

what cold sources....?I can't understand

Answer 2

Adding to David Townsend’s answer I would add indeed:

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) although that also includes some examples David mentioned.

Phosphorescence.