The polarization filter does not work

I am working in Conduction phenomena Photo-conductivity Experiment. Here the experiment guide . The aim is to measure the photocurrent $$I_{Ph}$$ as a function of the voltage $$U$$ at a constant irradiance.

The problem is the photocurrent has the same value when i change the polarization angle from 0 to 30 to 60 to 90. It seems that the polarization filters does not work or does not change the amount of the irradiance. Knowing that i turn off the light while doing this experiment, but there is the sun light coming from high window that i can't cover.

So anyone can help me in that?

Here the result that i should have:

And here my result:

there is no different in the photocurrent of the photo-resistor when i change the the polarization angle between the two filter

• Did you check whether you are using the filters correctly by just looking at them and noticing a difference in transparency when you rotate one of them? – noah Sep 24 at 21:25
• There are different types of polarization filters. Circular polarization filters are ordinary linear polarizers on one side and quarter - wave plates on the other side. You might be using the wrong kind if filter. Test this by reversing one of your filters. – S. McGrew Sep 24 at 22:59
• If that doesn't change your results, reverse the other filter. And, try reversing both of them. – S. McGrew Sep 25 at 1:34
• thank you all i am going to try these and comeback – Hana Sep 25 at 7:56