What's going on with Cir. Polarizer?

[MBement]

The other evening, I had a Nikon 77mm Circular Polarizer in my hand, and randomly held it up in front of my two Apple 23" HD LCD monitors. As I rotated the polarizer around, at a certain point, no light from the monitors was getting through...they were black, just like if you hold up two polarizers together and rotate one. I was surprised.

Can someone explain why this happened with the LCD monitors? I understand what happens with polarizers outside, but I don't know what is going on indoors with the monitors.

Cheers,

M

[Martin Aherron]

Two properties of liquid crystals explain what you are seeing.
1) As used in an LCD the crystals twist the plane of polarization of light passing through them.

2) This twisting effect disappears when you subject the crystal to an electric field.

So if you sandwich a layer of liquid crystals between two polarizers with their plane of polarization at right angles you have an electrically controlled light valve. That is the basis of an LCD.

All light coming from your LCD is polarized.

Check out these links.

Liquid Crystals
How do liquid crystal displays (LCDs) work?

[David_Buzzard]

Wow, you learn something every day...

David Buzzard Photography

[LarryLetzer]

Martin, Thanks for that explanation. 50 years ago I learned the hard way that one does not use polarizing filters to hide glare on metal cars as this does not work. This topic came up a few days ago on another forum. I was so happy to lend something as in digital I am the learner most of the time.

[MBement]

Martin:

Many thanks for your explanation and the links. Very informative and interesting.

Mike

[MarkAlsop]

It appears that this effect only occurs when looking at the LCD monitor through the polarizer from the lens side.

Turn the polarizer around and you only see a warming or cooling of the light from the monitor, depending on the rotation of the polarizer. At least that was my observation with circular polarizers from two different manufacturers (Tiffen and Hoya) and two different monitors (a ViewSonic VP2030b and a LaCie 320), both rather good monitors.

So, a circular polarizer is effective only when light is passing through it from the ‘front’ to the ‘back’ and not the other way around. My basic understanding of how polarizers work is based on linear polarizers and that ‘understanding’ does not account for this behavior.

[Martin Aherron]

I had never dug into circular polarization either. A quick check yielded this information.

1) The fact that you need circular polarization for thru the lens metering. From Luminous Landscape: Polarizers

From the article:

If that light is linearly polarized it renders either the metering or the autofocus ineffective.

2) A "suggestion" that modern filters first linearly polarize the light then use a quarter wave plate to convert to circularly polarized light. This would "seem" explaing the directionality of the filters that you have. But I am still not convinced it explains the behavior you saw.

Polarizer

From the article :

A circular has an additional quarter-wave plate or scrambler behind the (still linear) polarizing foil. Although not scientifically correct, it more or less restores the natural 50/50 vertical/horizontal balance of polarization, without affecting the initial pictorial result.

3) More info on polarization
good hyperlinks for digging into the subject Classification of Polarization

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