The Ai Barndoor filter can be found within the spot lights filter list in the Arnold attributes

A tutorial that demonstrates how to use the Barndoor filter can be found here

The images below show the various attributes of the Ai Barndoor filter (the spotlight model is purely for illustrative purposes). Click on the images to see the animation. 

Top Left


Top Right



Top Edge


Bottom Left


Bottom Right


Bottom Edge


Left Top


Left Bottom


Left Edge


Right Top


Right Bottom


Right Edge