Details

UV pen (security marker) with a UV light built into the cap. 

The UV light is for viewing UV ink. It works fine, it is OK for security marking, it is great fun for secret writing.

Please do bear in mind that this is a £4.90 item, it works OK in the dark, it just about works in dim light if you shade your hand, you will be viewing an area of about 10mm at a time. But (remarkably, for £4.90) it does work! 

Read all about how UV light works; see lots of photographs of what you will see with UV light,