Photoelasticity - TDK Head Cleaner, featured[edit]
Voting period is over. Please don't add any new votes.Voting period ends on 27 Mar 2017 at 15:07:39 (UTC)
TDK Head Cleaner - Black background Polarising filter cancels out direct light from polarised light source
TDK Head Cleaner - White background Polarising filter aligned with direct light from polarised light source
Info Going further back in time than Compact Disc, here is a TDK Head Cleaner. The clear plastic is birefringent and demonstrates internal stress as coloured patterns (photoelasticity) when photographed using cross polarisation. In first photo, the polarising filter on the lens is at right-angles to the polarised light from the LCD monitor behind the cassette. This cancels out all the direct light, producing a black background. In second photo, the monitor was rotated 90°, aligning the polarised light with the filter. This lets through all the direct light, producing a white background. The colours are stronger in the first photo but also switched about (e.g., green and magenta). You can see a Juxtapose of the two images here. For reference, the cassette under normal light is shown in this photo. The pair of images demonstrate how rotation by 90° affects the interaction of polarised light with a polarised filter. All by me. -- Colin (talk) 15:07, 18 March 2017 (UTC)[reply]
Support out of pure interest, mostly, and also respect for the great focus on the subjects (the one on the left being cooler to me than the other). I'm still not quite sure I understand what you did, though. Did you produce some kind of spectroscopy? -- Ikan Kekek (talk) 15:20, 18 March 2017 (UTC)[reply]
More details on the setup/physics
Ikan, this is just the effect polarised light has on some clear plastic objects. You can see this effect yourself if you hold some clear plastic object in front of a white area on your computer monitor. I'm looking at my ruler right now and it sparkles. --cart-Talk15:27, 18 March 2017 (UTC)[reply]
(ec)No high-tech equipment required. The physics is a bit beyond me, though I've linked to the relevant articles on WP. I've got an LCD monitor in the background, showing a white page and with the brightness turned up. The room is dark, so there is no other source of light, just what shines through the clear plastic. The LCD monitor has a linear polarisation filter in it, as part of how the display works. On my camera I also have a polarising filter. Modern polarisers for SLR cameras have a linear polarising filter on the outside layer of the glass and a quarter-wave plate on the inside that converts it to circular polarised light. This extra step is necessary as the optics of exposure meters and phase-detect autofocus systems do not work well if the light is linear polarised. For the purpose here, that extra layer isn't relevant, as it is the interaction of two linear polarising filters that matters. If they are aligned, then it lets through light and you see the white LCD. If one is rotated 90° then it lets through none of the light and you see black. The clear plastic interferes with polarised light, changing its polarity in a way that is wavelength (colour) dependent. Apparently the stress in the plastic affects this so photoelasticity is used in industry to examine the stress on a model (even if one does not intend to make the final product in clear hard plastic). This effect can be seen with one's own eyes -- there's no magic going on with the camera or in Photoshop. I haven't enhanced the saturation at all. As an aside, the quarter-wave plate on a circular polarising filter means that if you position it back-to-front, then it has no effect. -- Colin (talk) 15:43, 18 March 2017 (UTC)[reply]
While Cart is right that one can see some effect with clear hard plastic held in front of one's LCD monitor, it is quite faint. One has to hold the object at just the right angle (which presumably is related to how light becomes polarised when striking a reflective surface at an angle). If I hold the cassette tape squarely in front of the monitor, I see no special colours at all. But if I twist it at an angle then colours start to appear like a weaker version of the white photo above. You need another linear polariser to get cross polarisation that demonstrates the full effect like with the black photo. If I hold the cassette tape up to the white overcast sky, then there are no colours, no matter what angle I turn it. -- Colin (talk) 15:51, 18 March 2017 (UTC)[reply]
Ikan I did physics for one year at university, and I found the WP articles tough going. I'm afraid much of the WP maths/physics articles are written for other mathematicians and physicists to read, and they resist all attempts to make them understandable by lay readers. One just has to glean what little one can. Btw, this effect only works with some clear hard plastics, but not others such as polythene. I think it is due to the way the carbon chains align and the injection-molding process used to manufature the product. -- Colin (talk) 16:22, 18 March 2017 (UTC)[reply]
TDK Head Cleaner - Black background
TDK Head Cleaner - White background
Info Going further back in time than Compact Disc, here is a TDK Head Cleaner. The clear plastic is birefringent and demonstrates internal stress as coloured patterns (photoelasticity) when photographed using cross polarisation. In first photo, the polarising filter on the lens is at right-angles to the polarised light from the LCD monitor behind the cassette. This cancels out all the direct light, producing a black background. In second photo, the monitor was rotated 90°, aligning the polarised light with the filter. This lets through all the direct light, producing a white background. The colours are stronger in the first photo but also switched about (e.g., green and magenta). You can see a Juxtapose of the two images here. For reference, the cassette under normal light is shown in this photo. The pair of images demonstrate how rotation by 90° affects the interaction of polarised light with a polarised filter. All by me. -- Colin (talk) 15:07, 18 March 2017 (UTC)
Support -- Colin (talk) 15:07, 18 March 2017 (UTC)
