Today we actually did some readings on the DPF (Dual Polarized Filter, more in post "Day 10"). The last time we tried this we measured an interference pattern which was unexpected. Today we found, after a close inspection, that the needle in the DPF was slightly off center allowing light to pass through with out being polarized and also allowing a single polarization of light to go around both sides of the needle. We realized we should also check that the needle is centered and covers the seam between the two polarizations.
Here in this chart you can see a comparison between the DPF (Pink) no interference, and the needle (blue) interference.