# How does a circular polarizer affect the “stars” from strong lights at night?

When taking night photos at narrower apertures strong light sources such as streetlights demonstrate a star effect based on the number of blades in the lens' aperture. What effect would a polarizer rotated to its most effective orientation have on these "stars"?

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## 1 Answer

Unless you put a polarizing sheet on the lamps, no effect (other than the global ND effect inherent in these filters), except if there is a reflection on the ground from them, then the reflection will be attenuated.

The polarizer removes the reflections from non-metallic surfaces and the stars are direct light from emitting sources.

Since the stars appear due to diffraction caused by narrow aperture slits and they have a discrete amount of angles, causing double that amount star lines (even number of blades overlap), I don't see how the polariser could affect it.

One thing is theory, another is practice. I set up an ikea lamp in my vision lab to make the star effect and then took an image with and without polariser:

Indeed, theory is shown to be true. We only get an ND effect. Rotating the polarizer had no effect at all.

If we put a polarizer behind the aperture, like this:

Then we get this (1. without filter 0.5x exposure, 2. with pol, 3. with pol rotated 90d):

The small differences I will blame the fingerprints on the filter that I tried to wipe off, but there is still some left :)

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The light from the lamps are direct, but the stars are the result of the light's interaction with the aperture blades that occurs behind a front mounted polarizer. – Michael Clark Feb 22 '13 at 9:57
and how does polarity affect the angle of the aperture slit, in turn affecting its diffraction? – Michael Nielsen Feb 22 '13 at 10:02
Since the aperture slits are more or less evenly distributed around a 360 degree circle, if we had an eight blade diaphragm we could rotate the polarizer until some of the slits would be parallel to the direction of polarization and some would be perpendicular. I'm wondering what, if any, effect this might have on the shape of the "star". – Michael Clark Feb 22 '13 at 10:28
added practical example. – Michael Nielsen Feb 22 '13 at 10:33
No star came up with freelensing so instead I cut a piece out of the polsheet and mounted it inside. It doesnt affect the stars neither. Btw, the best part of my vision lab is the white chair on top of the rack! :) – Michael Nielsen Feb 22 '13 at 12:13