# The Naked Scientists Forum

### Author Topic: Can the angle of a linear polarising filter be measured?  (Read 879 times)

#### arawiki

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##### Can the angle of a linear polarising filter be measured?
« on: 13/10/2015 22:22:19 »
Hi everyone,

i need to know if there is a possibility to measure or calculate the fixed angle of a linear polariser. With Malus law i get
I_after = I_before • cos(Θ)^2, where I is the light intensity and Θ is the angle of the linear Polariser.

But I don't know if it is realistic and possible to measure the value of the light intensity (e.g. from an LED, which I would want to use), and if this intensity value will be stable or not, when measuring it before and after the polariser.

Is there an idea how to measure or calculate this angle precisely?

the components i have: 1x LED, 1x Polarizer, BSO Crystal, Fiber,
1x Beamsplitter and 2x Photo-diode

Thanks in advance and i hope someone has an idea.
« Last Edit: 14/10/2015 08:10:43 by chris »

#### Colin2B

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##### Re: measuring of the angle of a linear polarizing filter
« Reply #1 on: 13/10/2015 23:54:22 »
Malus's law only allows you to measure Θ relative to the polarisation of the source, and with an ideal lossless polariser.
Are you saying you have a polarised LED? If you do, the polarisation should be marked and you just rotate it until you get a minimum intensity on the other side of the fixed polariser, at this point they are at 90°.
Measuring intensity on either side of the fixed polariser won't work because of losses in the polarising material.

Your setup sound like you are looking at photo refractive effect or holographic inferometry?

#### evan_au

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##### Re: Can the angle of a linear polarising filter be measured?
« Reply #2 on: 14/10/2015 21:54:46 »
Quote from: Colin2B
Are you saying you have a polarised LED?
I understand that most LEDS are non-coherent; lasers tend to be polarized.

Quote from: Arawiki
measure or calculate the fixed angle of a linear polariser
Even if your LED is unpolarized, you can generate a mostly-polarized light source by reflecting it from glass at Brewster's Angle. (Light of the opposite polarization passes straight through the glass.)

You rotate the polarized object until the photodiode output reaches a minimum. This represents the point at which the polarization reflected from the glass is opposite to the polarization permitted by the polarizer.

You can then accurately measure or mark the angle between the glass reflector and the polarized object you are measuring.

It is easier to measure the fairly sharp minimum in current as you pass the point of opposite polarization, than to identify the maximum of the fairly broad peak when the two have the same polarization.

#### Colin2B

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##### Re: Can the angle of a linear polarising filter be measured?
« Reply #3 on: 14/10/2015 23:08:39 »
Quote from: Colin2B
Are you saying you have a polarised LED?
I understand that most LEDS are non-coherent; lasers tend to be polarized.

I was reading last year of the development of LEDs with polarised output, I wondered if he was using one of these.
Interestingly, with the kit list he has I was expecting to see a laser in there.

Good idea to use Brewster's angle.

#### The Naked Scientists Forum

##### Re: Can the angle of a linear polarising filter be measured?
« Reply #3 on: 14/10/2015 23:08:39 »