Sony Alpha 200 + HVL-F43AM
If subjects are wearing high visibility jackets with reflectors on them the trigger pulse surely will show in the pictures. If they're not close to you it might not be too bad though. To avoid this, you can either use HSS (in high speed sync mode the pulse will not be emitted while the shutter is open), or if you need slower shutter speeds you can use an infra red pass filter in front of the pop-up as your flash unit only needs the IR to work and it won't show in the pictures.
|Smile by Olymguy|
from Ultra Asian Indian Female Faces
|Space Shuttle Cockpit- by vbuhay|
from Aircraft Control Stick