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Nah! Nah!
The first polarisor gives plane polarised light that is white (all colours) the cellophane or other plastic rotates the light an amount depending on the stress and the wavelength of the light, so when you put the second polarisor in so the polarised plane is at right angles to the first, the only light allowed to pass has been turned through an angle.
Since this angle depends on wavelength and stress the colour depends on the degree of stress. The colours are attenuated depending on the angle through which the light of that colour is rotated. So the colour you see is the result of the sum of what light is passed.
Since we are dealing with eyes then the primaries of
red green and blue are the ones involved. So if red is twisted 15
degrees, green 30 degrees and blue 90 degrees then there will be a resultant
colour attenuation of cos(15)=.966 for red + cos(30)=0.866 for green and +
cos(90)= 0 for blue. I think that makes it RGB: 1, 34, 255, (
Other colours can be obtained by different degrees of stress. This property is under the heading of birefringence with an ordinary and extraordinary ray, but I cannot remember the connexion.
To calculate the transmittance directly, use the sine of the angle of rotation.
If you want to measure the stress use monochromatic light, obtained from white light passed through a diachronic filter to give (say green) then measure the angle of rotation by rotating the second polar to make the light extinct, this is then at right angles to the plane of polarisation of the light.
Chris.
Question: What happens to me when I die? Response: What happens to a cat when it dies? Answer: You throw it away. Response and answer: You are a clever cat!
:> -----Original Message----- :> From: owner-photoforum@xxxxxxxxxxxxxxxxxxxxxx [mailto:owner- :> photoforum@xxxxxxxxxxxxxxxxxxxxxx] On Behalf Of ADavidhazy :> Sent: 17 August 2005 18:22 :> To: List for Photo/Imaging Educators - Professionals - Students :> Cc: andpph@xxxxxxxxxxxxxxx :> Subject: Question about polarization :> :> I am trying to prevent making a total ninkompoop of myself (yeah, I know :> it is :> too late for that!) and so I need some advice/information on the color :> seen in :> a typical stress analysis polarization set-up. Two linear polarizers :> generally :> at right angles to each other with a birefringent material (cellophane, :> stressed plastics, etc.) in between. Under such conditions colors are :> seen. :> Explanation? Especially as concerns magenta and green. Possible :> explanation: :> :> Since the spectrum does not contain the color magenta in it, then in a :> crossed :> polarizer set up is magenta seen due to the fact that green has been :> selectively removed from the incident white light beam and we see some :> red and :> some blue passing through and thus preceive magenta? :> :> Further, if this is so, then all the colors seen in a polarization system :> are :> due to the mixture of those wavelengths that pass through the 2nd :> polarizer. In :> the case of magenta both red and blue must be passing through while when :> we see :> green then in that area green has lined up its plane of polarization with :> that :> of the 2nd polarizer and red and blue eliminated. :> :> So we see colors both due to subtraction of specific wavelengths and to :> simple :> transmission of others. :> :> Ideas? Is this correct? If not would you be so kind as to help me :> understand :> the process better? Thanks for any assistance beforehand! :> :> Andy :> :> Andrew Davidhazy, Professor :> School of Photographic Arts and Sciences/RIT :> andpph@xxxxxxx http://www.rit.edu/~andpph :>
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) this is obtained by
transmittance = 1-attentuation and multiplying transmittance by 255.