Y
Z
X
Electric vector of exciting light
Exciting light
O
Consider an XYZ coordinate framework with a fluorescent solution placed at the origin, as shown below, where XZ is in the plane of the page.
In this system, the exciting light is traveling along the X direction.  If a polarizer is inserted in the beam, one can isolate a unique direction of the electric vector and obtain light polarized parallel to the Z axis which corresponds to the vertical laboratory axis.
Polarizers III