Hiyahans wrote: ↑Sun Jun 27, 2021 7:44 pmHow does it behave when you rotate it, relative to the direction of your polarizers? Have you checked the orientation of the Hoya PLs you have? I read conflicting things about whether or not the mark on the ring is supposed to be aligned accurately with the polarizer direction and some reports of them not being aligned even in new ones. I have several I bought used and they were all misaligned. In all mine the filters are held in with a circular spring clip and can rotated relative to the alignment mark without too much difficulty while pinching the filter between thumb and finger. I aligned them approximately using reflection from a glass microscope slide near Brewster's angle as a reference.
As mentioned in #113 and #115 all the pieces I have laying around act like waveplates. Unsure of absolute retardance but judging from the interference color charts probably third order or higher.
You mean if I rotate the polycarbonate between cross polarisers? The brightness varies and there is a minimum but it's not so dark as without having the polycarbonate sandwiched. I'm thinking this is maybe not so relevant though I'm a bit unsure. I do get the expected interference fringes but they just seem relatively narrow with the dark destructive band either towards the top or towards the bottom of the prism. However, I've been using two of the relatively large open frames based on Rathi's dimensions. Maybe I should go back to using a 'compact' holder for the condenser prism.
The hoya polarisers are camera types so rotatable, so I can set them at any angle relative to the prisms. I can set the polarisers to be crossed/opaque with no prisms in place and then position the two prisms. There doesn't seem to be an issue there. I can't remember if I previously set them to be both crossed and to have a minimum transmittance with unstressed polycarbonate in between, but it's something I'll check out next time. (Out of the top of my head, I think Rathi mentioned something about polarisers being at 45 deg. relative to the prisms - I presume this equates to the minimum transmittance - but might be a red herring!)
Is there someone in the USA who has had the same behaviour with their polycarbonate? Or anyone who has definitely and successfully achieved DIC? (I think @jmp said he did but not seen a post from him since). Anyway, I'm not sure whether all 'standard' polycarbonate sheet is the same? I think you can get sheets that have been hardened or coated (e.g. extra UV coating) but I presume any non-standard things would be stated. I imagine Rathi et al. just used the ordinary non-treated sheet here https://www.mcmaster.com/polycarbonate-sheet-stock/. Apart from it being measured in fractions of an inch thickness, it doesn't seem any different to my sheets.
Hans - sorry I either missed or didn't fully appreciate your previous 113, 115 posts. Are you in the States or Europe?
I'm likely to have to postpone working on this project until after July as I have a course and other things I have to do.
Thanks for everyone's replies
Louise