ancient \"headlight\" on helmets

[Cacaius]

Salvete

Do you have any idea what those guys on the far left and right wears on their helmets ?
Have you seen anything similar on other sources ?

This one comes from Arch of Constantine.

Here you can see it more clearly
http://theeyegame.com/DeepZoom/ArchOfConstantine

Its on the one of panels related with Trajan's period.
http://www.rome101.com/Topics/ArchConstantine/


If its not
- a headlight
OR
- a rocket launcher

Then what ?

Any SERIOUS ideas ?