3 perfect polarizing films

[chaosprimer]

If you have 3 perfect polarizing films and you place them on top of the other, with each successive film oriented +45 degrees from the one before it (first and last oriented perpendicular to each other and middle polarizer at 45 degrees between them). If one shines a light on this polarizer stack with an intensity of 1 watt, what is the intensity of the light on the other side of the three polarizer stack?

Now when I tried to figure this out I assume that the light is collimated and that the films are perfect polarizer. But I got stuck on the actually equation... Anyone out there knows how to solve this?

[Astronuc]

Perhaps the Law of Malus applies.

For each polarizer - Itrans = Io cos2\theta, where \theta is the angle of between axis of polarizer and some reference.

The question is what is the transmission of light through the first polarizer.

Edit: See http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polcross.html

for discussion of crossed polarizers and Law of Malus.