Single L.E.D. with keyring fitting, good quality UV LED, but tiny (will illuminate a tiny area).
To read an article about testing with UV
Note: this is a UV (ultra violet) lights, for ordinary (white light) lights click here.
Lamp: single L.E.D.
Size: 1.5 inches diameter X about 1/3 inch thick (40mm X 8mm)
Power: two button batteries, size CR2016
To buy these batteries click here
More info, more pictures
Click on the picture above, see the security markings on an old (pre-2017) twenty pound note,
The smallest UV that actually works, see UV-9.
The cheapest keyring UV (thought it doesn't really work), see UV-KR2.