Re Film-based depolarisation and ghosting

[P3D Peter Homer <P.J.Homer@xxxxxxxxxxxxxxxxx>]

George Themelis asks about Film-based depolarisation and ghosting .  First
of all only the milky layer that he mentions as developing with time or
abrasion will strictly speaking depolarise light by scattering. Where the
film material does it itself it is actualy repolarising from plane
polarised light to elliptical or circular or even twisting plane
polarisation through 90 degrees . This is because they have a crystaline or
crystal like structure due to the polymer chains being aligned along the
length of the film strip. What I cannot understand is why all film
materials dont do this .I would expect any plastic produced in long strips
to have such an allignment either intentionaly or accidental by the initial
extrusion and then by stretching between rollers in the manafacture
.Probably deliberately in "cold drawing" to "prestretch" the material so
that it does not stretch in use when the film is wound on .
  I cant think they avoid this for a minority photographic interest like
ours so there must be some other reason and I think the situation could get
worse in the future as they change manafacturing techniques . Manafacturers
in the 50,s and 60,s would have been more likely to avoid this when stereo
was more commonplace. So to Question 1 "Does the film tend to depolarise
light as it ages ?"  I would guess that plastic film bases which are not
initialy crystaline but are more like "Super cooled liquids" such as glass
gradualy tend to crystalise as true solids with time . So yes they could
either depolarise by scattering the light if many small crystals are formed
making it milky. Or by repolarisation if a larger crystal structure forms.
I suppose this would also depend on the different manafacturing processes
used in the first place so it could be different for different films to try
to answer question 2. P.J.Homer




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