How to find the orientation of polarization of a polarizer

[boxfullofvacuumtubes]

If you have a new linear polarizer, how do you find its orientation of polarization? In other words, how do you orient it so it would, let say, polarize light vertically if there is no marking for the 0 degrees angle?

 

[Orodruin]

The easiest way would be using a known linear polariser and place them in series or use a known polarised source of light such as the sky or an angled reflection.

Edit: Or the light from an LCD screen. A few years back I noted in delight that all LCDs in the bus stations where I lived had the correct polarisation orientation to be seen with polarised sunglasses (which are always oriented the same way to reduce reflections on horizontal surfaces) assuming you keep your head vertical. Whether by happy coincidence or intelligent design I cannot say.

 

[boxfullofvacuumtubes]

If I don't have another known linear polarizer to check the orientation against, is there any way to use a quarter wave plate with a known fast axis for this purpose? I just can't think of any, but smart people around here might.

Orodruin, I like your idea to use an LCD screen as a source of light with known polarization.

 

[nasu]

The polarization of one of the LCD displays I have here is oriented at 45 degrees relative to the edges.
But you can always look at the light reflected by a shiny floor, at some angle. The reflected light is partially polarized parallel to the floor.
When the floor looks the darkest through the polarizer the direction of the polarizer is vertical (perpendicular to the floor).

 

[xavier60]

I recently made the same observation as mentioned in the thread "Flipping a polarizer 180deg. changes polarization" with some film I purchased on ebay to repair the LCD displays on my tractor's instrument cluster.
I eventually realized that the polarization axis is offset by 45°. Flipping the film about its H or V axis is equivalent to rotating by 90°.
I determined the film's polarization axis by noting the effect of viewing reflections from water surfaces.

 

[A.T.]

Orodruin, I like your idea to use an LCD screen as a source of light with known polarization.

Have you tried your phone screen? If it's polarized then it's along the longer side.

 

[sophiecentaur]

In the absence of anything else, you can look at oblique reflections of a clean sheet of glass.
If you have a simple polariser with unknown polarisation plane, you could check results by finding nulls for light in both directions.
As far as I can make out, the 'true' polarisation will correspond to the bisector between the two results.