Optoisolator

Hi,
If a pen (for example) is passed between the two parts of an optoisolator, can we know the direction of the pen?
(whether it is passed from right-2-left or from left-2-right)
in other words, is the optoisolator unidirectional or bidirectional
Note: I am interfacing the optoisolator with a micontroller
please reply asap...

Thanks gor your help...

A single opto sensor cannot tell the direction. However, two sensors spaced less than the thickness of the pen will be able to detect the direction like an optical encoder.