# Can looking at a computer screen show the difference between linear and circular polarizing filters?

About the difference between linear and circular polarizing filters (quite well developed on this Q&R): What is the effect of placing those filters in front of a computer screen? I have seen two behaviors :

1. The computer screen (through the polarizer) varies from completely black (or almost depending on the quality of the polarizer) to almost completely transparent.
2. The color of the computer screen (through the polarizer) varies from bluer to more orange.

Is this difference related to the difference between a linear or circular polarizing filters? Or is this difference due to something else? In that case, what?

PS: I just bought a polarizing filter that behaves like the second case. And some friends have theirs behaving like mine when for some other friends, their polarizer behave like the first case. All of them believe they have a circular one.

The difference is in the depolarizer filter that is on the back side of a circular polarizer filter, and which side you turn towards the screen.

A linear polalizer filter shows the behaviour number 1 regardless of which side you have towards the screen. It works the same in both directions.

A circular polarizer filter shows the behaviour number 1 when you turn the front side towards the screen, and behaviour number 2 when you turn the back side towards the screen.

With the back towards the screen, the polarized light from the screen first hits the depolarizer on the back of the circular polarizer filter, then the light passes through the linear polarizer filter mostly unchanged. The depolarizer filter is not perfect, not all colors are present at all angles, that's why you get a color shift when the light passes thought the polarizer filter.

• cristal clear explanation!
– Jav
Commented May 15, 2014 at 12:08

The screen has cropped linear polarizers so all the white light will be blocked. The first one lets one direction light pass, but the second would only pass the light that was blocked. Unless the crystal layers sandwiched between them is induced with electrons to turn the polarization of the passed light, so it can pass the 2nd one. It can then turn R G and B independently and to varying degree, so you get shades of grey and colours. This is obviously unlinear as it is about turning the polarization based in a current.

So then you put a 3rd polarizer in front. If it is linear than you can align it with the 2nd polarizer and you pass the same light as the 2nd one did, so its still white (with a slight ND effect). Then you align it with the 1st pol filter and then you block all the light that was turned to pass the 2nd one and it is black.

Then you take a circular polarizer in this "linear Rectified" light. It is less predictable which effect you get. The cir-pol has a quarterwave on one side and a linear polarizer on the other side. if I hold it up with the quarterwave towards the screen, it turns the linear polarized light into circular polarized light, with 2 directions with a phase shift between the two. blue is passed more turned one way , and green and red another way. See C1 and C2 graphs. Blue vs Yellow/Orange is opposites. Orange is yellow with more red. yellow is Red + Green in the additive world. So it is basically opposite from blue, but with more red passed light, as spectrums of polarizers often show is true. So we do have some opposing effects here.

If you hold the quarterwave away from the screen, you get black and white, because how the linear polarizer on the cir-pol filter comes first and the passed light hits the quarterwave and becomes circular polarized instead.

I have both types of pol-filters here and I just tested with both. It is true that linear makes Black/white and the circular makes it blue/yellow when you face the part that is facing the camera towards the screen. If you face that part towards yourself, it acts like the linear polarizer

• sorry but I don't really understand your answer... If I refer to your experience, my polarizing filter is a circular one? I updated my "ps" because all of my friens claim to have a circular polarizing filter.
– Jav
Commented May 15, 2014 at 9:42
• ah, it does make sense that it could be orange and not yellow, as some filters give a warmer tone, as red pass better than green. lenstip.com/… Commented May 15, 2014 at 10:57
• ok I get it now. Commented May 15, 2014 at 11:06
• read my updated answer Commented May 15, 2014 at 11:13
• I better understand, thank you. I will do the screen test again (with the two sides of the polarizing filters) and give my results here.
– Jav
Commented May 15, 2014 at 11:17

Objects are seen via the colors they reflected and the colors they absorbed. A secondary reflection often occurs that is polarized and this light may appear different as to its coloration. A polarizing filter transmits light that is vibrating in in only one plane (direction). Unpolarized light vibrates in every possible plane.

Some modern cameras are adversely effected should a standard polarizing filter be mounted. This filter may interfere with auto-focus and or exposure determination. To avoid, we mount a circular polarizer. This is a two part filter. The part that faces the subject is an ordinary polarizer. This first part does the polarizing job. What passes is light polarized in one plane. The second part is called a retarder. This is sandwiched to the polarizer and the retarder faces the camera.

The retarder scrambles the polarized light thus it travels onward towards the camera as unpolarized light. The idea is to minimize the adverse effects that otherwise would interfere with the camera's automation while still providing the good effects of a polarizing filter (the most valuable filter in most photographers gadget bag).