Iceland Spar: the Mineral That Changed Science

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Iceland Spar is a clear variety of calcite that exhibits a property known as birefringence: light passing through the crystal is split into two beams, creating a double image. The investigation of this phenomenon led to the discovery of light polarization, which revolutionized multiple fields of science and industry and inspired many technologies still used to this day.
SOURCES:
petapixel.com/2013/12/12/icel...
web.archive.org/web/201707020...

[@Muonium1]

Because you are in North America like me, if you got that piece of spar recently and cheaply, it most likely came from Nuevo Leon or Coahuila, Mexico. This means, like the piece sitting on the table in front of me right now, that it's quite likely to be of the so called "Terlingua type calcite" and due to radiation induced crystal lattice defects accumulated over millions of years, thus have another very interesting property associated with it - photoluminescence. If you have access to a good, filtered 365nm longwave UV light, shining it on your sample will likely cause it to fluoresce a pinkish color and induce a second or two of phosphorescence after you turn the light off. If you have access to a shortwave 255nm UV light, it will excite a brilliant unearthly blue fluorescence and long lived phosphorescence that lasts at least 10 seconds in a darkened room.

[@bentboybbz]

Thanks for sharing! I can't do any of the things you suggested but I felt like someone could at least say Thank You!

[@Muonium1]

​@@bentboybbzsure! If anyone has a piece of this material but needs a cheap source of UVC light, search for something like "Homedics UV phone sterilizer" and "fluorescent minerals" and you should be able to find something for about $5 that can be repurposed into a very inexpensive shortwave mineral light!

[@devinpatterson2185]

Scientists are full of shit.

[@Aaron-zu3xn]

gonna buy a slab of it and install LEDs..will it hold up as a table?

[@Muonium1]

@@Aaron-zu3xn it won't hold up as anything. the mohs hardness is 3, basically the same as a fingernail.

[@petergerdes1094]

Finally, someone explaining things from the history of science that aren't one of the 5 standard stories every damn science popularizer tells. Great work.

[@fredblonder7850]

Once again, you’ve made a video on a topic of interest to me. Regarding the Viking Sunstone: I am a member of the Longship Company of Maryland. We own two full-scale Viking ship replicas, and operate them for fun and educational purposes, and occasionally wind up on TV. I have played around with birefringent calcite and found a method that works. First, I place an opaque mark in the center of one face of the crystal. This is difficult because the crystal is slightly slippery, like teflon, so not everything sticks to it. The blue light from the sky is most strongly polarized 90° from the sun. You can easily see this by donning a pair of polarizing glasses and rotating your head while looking at the sky. Now. picture the scene where the sun is near or just below the horizon, and obscured by clouds or haze, yet there is a patch of blue sky directly overhead. Hold the calcite crystal directly over your head, with the opaque mark on top, and look straight up at the crystal. As you rotate the crystal, the two images of the spot on the far surface will gradually increase and decrease in contrast. When they are of equal intensity, the sun will be directly in line with the fracture plane of the crystal, that is, its side-surface. There MUST be a small patch of blue sky for this to work, as the light coming through the clouds or haze is diffused and not polarized. This is the principle used by the Pfund Sky Compass. And if you want to go off on a really wild tangent, look up the sea-creature known as a Brittle Star. Its skeleton is made of birefringent calcite, and (strangely) the skeleton makes up the lenses of its eyes.

[@joshuakuehn]

What in the world

[@fredblonder7850]

@@joshuakuehn I have no idea what your comment means.

[@JacquesMare]

Say what now?

[@Spinifax]

Cool! I was curious and found this info about starfish "eyes": «They suggest that the lenses focus incoming light onto photosensitive nerve bundles in the organism that work in concert as somewhat of a compound eye. ... A scanning electron micrograph of one of the brittle star's dorsal arm plates reveals an array of calcite microlenses. Each 40- to 50-µm-diameter lens corrects for spherical aberration and for birefringence. ... "Calcite, of course, is a really well-known optical material to elementary-school students for its pronounced birefringence," Aizenberg said. Only one orientation, she explained, would enable the animal to take advantage of the optical properties of the material while avoiding the effects of birefringence. "The brittle star uses this orientation in the single-crystal lenses." Specifically, the researchers determined that the lenses are oriented along the crystallographic c-axis. ... "We can find very interesting, very complex designs to optical problems in biological systems," Aizenberg said. "The microlenses are almost perfect optical elements, on a micron scale, which is beyond our current technology."» www.photonics.com/Articles/Brittle_Star_Features_Calcite_Lenses/a10912

[@JacquesMare]

@@Spinifax Thank you for quoting the source 👍 Isn't evolution just awesome?

[@michaelimbesi2314]

This is amazing! Growing up, one of our neighbors had an interest in geology. We visited their home once and saw their rock collection, and they gave me an interesting souvenir: a clear crystal that I spent the last 18 years convinced was a large single crystal of quartz, although I’d never been able to find another similar piece of quartz. Today, I realized that it is not quartz, because it looks identical to your piece of Iceland Spar, and also produces a double image. Thank you for teaching me something new about a rock that has been decorating my windowsill for years

[@whitehat1]

I recall a story about the first Scandinavian Airlines commercial polar flights from Copenhagen to Tokyo. Since magnetic compasses are nearly useless on trips in the polar region, all ships and airplanes in that part of the world were commonly equipped with a gyrocompass. The pilot of that Scandinavian Airlines flight, in the spirit of his Viking forefathers, also carried a sunstone as an unofficial experiment. As the plane passed the polar ice cap, he was able to verify that indeed the sunstone worked as reported in the ancient sagas.

[@oliverer3]

That was a fun Wikipedia rabbit hole. Thank you!

[@Thomas..Anderson]

5 months and just 12K views. This channel is underappreciated.

[@FransBlaas1]

One month later +50,000 views…

[@gabe_owner]

sometimes the algorithm just takes its time i guess

[@noyb7920]

Anyone using Mussorgsky as intro music to a technical explanation... Great choice.

[@mattwilliams3456]

I truly love how I can never predict what topic I’ll be learning about when I get a notification that you’ve posted. Bravo as always.

[@giacatollo]

I'm ashamed to say I judged a book by its cover. You are a great host.

[@ehulbert5]

Driving across the Borrego Desert east of San Diego a few years ago I came across a bronze plaque that marked a spot where Iceland Spar was mined during the Second World War, it mentioned that the reason the mine was economically viable at the time was because the crystals were exceptionally pure, and that crystals from this mine were used as specified optical elements in Norden Bombsights. During the war, Norden Bombsights were closely guarded secrets. These were given armed military escort when installed and removed from bomber planes, and each one had its own self destruct device. After the war, the mine closed.

[@scotthannan8669]

Which apparently was a device that didn’t work all that well.

[@ehulbert5]

@@scotthannan8669 show evidence of that statement, or STFU

[@scotthannan8669]

@@ehulbert5 kzbin.info/www/bejne/i2enZqWOl7J1gdEsi=SBM6KSjZcG3whA_E There’s one. There are many others.

[@abundantYOUniverse]

Your channel is going to be huge very soon. Congrats you deserve it!

[@lohikarhu734]

Wow, you have managed to encapsulate a great amount of interesting history, discoveries, and the intersection of many famous names!

[@pepe6666]

this is top-shelf content. thank you. also +1 for wearing a suit that causes moire patterns in a video about optics. everything about this is fantastic.

[@darreno9874]

Great video, I have played with Iceland Spar but never realised quite how important to science it is. God bless

[@nicedragon815]

You are so interesting and explain everything clearly, logically and are easy to follow and understand. Big applause to you! Now I’ll spread your link to this video to others discussing the Viking sun stone. Thank’s again!

[@UniverseSinking2011]

Great videos. Very educational. One suggestion: Any way to add the names in writing of the various scientists? That would help people follow along. (As one example, where you say "HAY-guhnz" I think you mean Huygens which is pronounced more like "HOI-gunz" or sometimes like "HIGH-guhnz." Again, great videos & I've subscribed.

[@ludodg]

Lol, mentioning “hoygunz” … That’s like thinking the whole world should use the US-english prononciation. Mr Huygens was Dutch. And the Dutch prononciation is completely different. Try to respect this please.

[@defenestrated23]

​​@@ludodgyou try to get the average English speaker to pronounce ['ɦœʏχə̴̃ns]. "HOY-gens" is probably as close as most folks will get. "HOW-gens" would work too. "HAY-gens" is pretty bad though.

[@philipoakley5498]

Just had a morning fog and checked the polarisation of the fog ("sun in sky" was still visible through the cloud/fog) and did see a little bit of dimming when rotating the polaroid when viewing the fog/cloud at 90 degrees to the sun, to aid the Viking navigation. It's also worth noting that the Viking were really far north compared to all the USA (except Alaska) viewers. Iceland itself is level with Baffin island (north of Hudson Bay) and the northern half of Alaska, so the sun doesn't get that high in the sky. You could look straight up and use the polariser there to determine the sun angle. Haars and see-fogs are quite common and not that thick (in altitude), but a real navigation problem. It's possible that the archaeologists and researchers here in UK & Europe hadn't realised the importance of some small quartz-y like stone among the multiplicity of Viking sites. (I've had a family lunch in the oldest house on Europe on Papa Westray! - so lots of sea borne navigation going on)

[@Paul-ou1rx]

I love how all the classical scientists were like the stoners of the 60s and 70s. "Dude, if you think about it, what is light? Check out these freaky crystals."

[@Texsoroban]

Man i just found your channel a few days ago, but im blown away by all that you cover.

[@LiborTinka]

One correction: Although Edwin Land discovered the synthesis of quinine, it was Dr. William Bird Herapath who combined quinine with iodine, resulting in light-polarizing crystals. The patent was used by Polaroid to make adjustable glasses. (I took this form one site so please verify this to be sure)

[@brianmckerrow817]

Great for teaching and researching such a format to help other conceptualise fundamental ideas.

[@malectric]

This is absolutely fascinating. I had not heard of Iceland spar prior to watching this. Many thanks!

[@minacapella8319]

This is extremely fascinating. Thanks for your hard work in the research and explanation of your findings.

[@mattbritton6222]

You got me. Subscribed. The suit, the French cuffs, the bow tie, the content, just...magnificent.

[@rogerscottcathey]

Quartz Risley prisms were used by Rife in his universal microscope. The barrel contained two quarts orienting prisms to produce a monochromatic illumination . . . eliminating aberration

[@AutistCat]

You pronounced Reyðarfjörður much better than I would've expected, actually!

[@DutchBlackMantha]

10:56: Mostly, except RealD uses clockwise and counter-clockwise polarized light instead of vertical and horizontal. That way, you can tilt your head without messing it up.

[@genericalfishtycoon3853]

Speaking of optics, your bowtie and glassess stylistic combo brings a smile to my face.

[@ultratumba280]

Another fantastic video.

[@alluba4867]

Great video. In brewing, refractometers are used much more often, but polarimeters are also used

[@JamesAllredWriter]

I appreciate the great amount of research you put into your presentations. I remember being introduced to Iceland spar as a youth, your presentation was fascinating.

[@enscroggs]

Regarding polarized AAA gunsights, you might try the USS North Carolina Battleship Memorial, a non-profit 501c3 corporation based in Willington, NC. USS North Carolina is the best preserved WWII battleship on public display. There are other museum ships, but most were modified post-WWII such that they exist in somewhat different configurations than when they fought against the Axis. North Carolina was mothballed immediately after the war and was not modified or upgraded until she was sold as-is to the museum foundation. If any such sights are installed anywhere it would be aboard the "Showboat". USS Cod, a Gato-class submarine preserved in her WWII configuration has 20mm and 40mm AAA guns. She's berthed in Cleveland, OH. You might inquire there as well.

[@paulcrusse7800]

Thank you for all of your hard work.

[@nexpro6985]

Thank you for this video, excellent work.

[@ripj5301]

Nice choice of intro music!! Amazing piece!

[@rainerburgdorfer4017]

A great video, with an excellent and cohesive script.

[@rxotmfrxotmf8208]

Thank you for the very interesting video. I learnt about birefringence in university and had forgotten about it till now.

[@stflaherty63]

I now have a greater understanding of why the optical calcite mines located in what is now the Anza-Borrego Desert State Park were created back in the 1940’s.

[@cherrybacon9790]

Nicely collected and presented information concerning a stone I have never thought about. Thanks for entertainment!!

[@fredrichenning1367]

The sunstone was way too valuable to bury with anybody -- swords, axes, you name it, in they went, but NOT a piece of Iceland Spar.

[@dannydetonator]

Maybe because it slowly dissolves in water, so wouldn't remain in burial during hundreds of years..

[@fredrichenning1367]

Sounds logical, didn’t think of thant.

[@Resomius]

This is so Incredible interesting! Glad I found your channel!

[@BBQDad463]

Thank you for this video. Fascinating! Subscribed.

[@daviddelaney363]

Good video! Great historical info! Thanks!

[@richardlilley6274]

Thank you for sharing your research and wisdom

[@soul0360]

Small suggestion. When ever mentioning names, like the scientists in this video. Write the name, rather then just putting up a painting/image. Makes it possible for us to who are interested, in doing some more digging in to them. Great videos btw. I only found you a few days ago.. instand subscribe

[@willn851]

Love your videos! Thank you!!!!

[@higgs923]

Fascinating subject well handled. Subscribed.

[@johnmanderson2060]

Very interesting subject 👍🏻👏🏻 i learned a lot, thanks 🙏🏼

[@BoboZimbabwe]

I love finding these random channels that make me just a little bit more nerdy after watching them.

[@colinwatt9387]

Good video, learned a lot about polarization.

[@brianmckerrow817]

Great content. its full of the principles of nature and science. As it should be. Very informative. Any history of water air filters? eh!

[@timnickerson7508]

What a great channel!

[@luisestebanr6311]

Instantly suscribed. so much science, and so much elegance. this is content for gentlemans, Like it I use a device to see concetration of coolant in the cnc milling fluids, now I know how it works! thks

[@DumbSkippy]

At last... An intellectual KZbin channel which doesn't dumb it down to service an audience with a two figure IQ. Thank you so much. Liked and subscribed!

[@housellama]

Ironic username

[@matejlieskovsky9625]

I feel that "technology connections" is not dumbing it down, but less formal about the presentation. But to each according to their tastes!

[@newchannelization]

Oh wow I have learned something new and awesome. Thanks

[@psblad2667]

You must have spent a lot of time of research for your videos. Great job and intersting subjects!!

[@FireAllOfEverythingAtOnce]

Try contacting various mesuem ships from WW II scattered around the United States. Try contacting them first to see which one(s) still have them (some underwent refits & modernizations after the war for the cold war, so they may have removed them). I'd try the USS Texas & Alabama first, since they had little refits after the war. Whether or not they retained them in the many decades since becoming museum ships (& the multiple refits they've underwent to keep them from sinking), I don't know.

[@FernandoScarpelli]

holy shit this video is so well made! I had a blast

[@ericpierce3660]

Your videos are so interesting

[@brianedwards7142]

I first came across the phenomenon of birefringence when on a wiki crawl reading about magnetars which are pretty bizarre things. They make the very vacuum of space birefringent with their honking great magnetic fields.

[@pedzsan]

My brother has a photograph. The sun is rising and there is fog over part of a lake. There are two reflections of the sun coming off the water. I wonder if this is also due to the polarization perhaps by the fog.

[@charlesachurch7265]

Thankyou for a great presentation. XXX

[@maxreinsch]

Wonderful stuff - thank you!!

[@loszhor]

Fascinating!

[@PapaRocks]

Very interesting. Thanks!

[@ciprianpopa5010]

The polaroid polarizer is not necessarily better that the Nicol polarizer, but is more durable. Nicols tend to delaminate due the Canada balsam and also tend to fog due to humidity. When they are new though, they are pretty neat. I'd say better than polaroid.

[@ruperterskin2117]

Cool. Thanks for sharing.

[@colinellicott9737]

Fascinating. Thx

[@user-mx9xv1je3t]

Wow, very interesting 😊

[@philipoakley5498]

If you have polarised sunglasses, then half of the rain bow disappears. You see the two sides (where the gold is hidden;-) but not the top of the arc. This same effect happens to the 'blue sky' one side darkens and the other side has massive scatter. I'll need to cross check that 'sun in fog' issue as it's very important when travelling over snow covered terrain (and other high moors with cloud). Sounds very plausible!

[@joyboricua3721]

That intro brass is simply astounding

[@SuezWSuezW]

Mussorgsky: Pictures at an Exhibition (Orch. Ravel) - Promenade II

[@d.t.4523]

Thank you. Keep working, good luck.

[@zachreyhelmberger894]

WOw! Great stuff!!

[@lmackenzie89]

The sunstone idea is so damn cool!

[@philipoakley5498]

Also worth adding (at the split sunglasses step) the effect of slipping in a third polariser at 45 degrees between the crossed polariser sheets and getting the spooky action "quantum" effect that the light suddenly re-appears. Part of that 'effect' is also the way that what we think of normal multiplying to make [rectangular] areas starts to fall down as we have circular effects (areas of slices of circles). Another aside style point is that Maxwell's original equations were numerous (~28 IIRC) that were greatly simplified by Heaviside's use of complex numbers, and the inclusion of Gibb's vector notation (though I prefer Hamilton's Quaternions which simplifies the equations even further!)

[@kempedkemp]

I saw an excellent piece of optical calcite from Lewis County NY. There is a big variety of calcite in parts of NY.

[@user-fp1hw1yr7x]

oh hey aside from those six spots you mentioned there's also a bunch of that stuff in the talkeetna mountains of alaska, I grew up around there, there's a valley about 40 miles in with geodes just scattered around everywhere, they weather out of the softer rock real nice so they just sit on the surface, you can't go 20 paces without finding a few, you smack em open with a hammer and you get bigass quartz and calcite crystals, perfectly clear and all that, I never really took much of the calcite cause it just crumbles in a bag full of rocks when you're running/sliding down a 500 foot scree slope, and you can only carry 40 or 50 extra pounds out on foot so quartz and pyrite takes priority but they would be pretty easy to get if you had a helicopter or somethin

[@fjb4932]

That valley Must have a name, or Lat / Longitude. "...40 miles from Talkeetna..." is code for Hidden. ☆

[@user-fp1hw1yr7x]

@@fjb4932 idk the lat and long, just know how to get there. you want to go in august, once most of the snow is melted but before it comes back. there's an old mining trail about 5 miles south of cantwell, just after you cross the railroad tracks. take the trail till it hits the mouth of the first valley, take that all the way in, it's shaped like an L rotated 180 degrees. there's a nice lake with good quartz deposits but I've pretty much grabbed everything on the surface there by now. hug the left valley wall and head up, it's the only way to safely get into valley 2.take the easiest path up, it's not too steep, an walk up to the ridgeline. once you hit that (you can get internet there btw) you want to slide down the scree slope, it's pretty steep but pretty fun, just stay upright s you're going down or you're dead. (safest path, not great but better than the others, trust me on this) once you get to the bottom of valley 2, go down and across. in the middle there's a beautiful valley, nice streams and little lakes, big boulders for climbing, very nice. you're going to want to go pretty much straight across the valley to the opposite wall, to the left of the boulder slope, an follow the switchbacks of the sheep trails up to the ridge. from there, drop down and hed to the right along the valley. there's some god crystals in the upper valley, but ignore them for now. follow the trail of crystals to the ridgeline and pop over that, it's a good bit easier than the others. at that point you're in valley 4, that's the crystal valley. my da found it when he was hiking with NOLS, there are also valleys filled with piles of bones and such but those locations are family secrets (also sorry I meant 40 miles round trip, only 20 miles in)

[@user-fp1hw1yr7x]

@@fjb4932 also nah, ain't no valleys got names in alaska unless they're a major tourist thing or there's a town in them, otherwise there would be too many and you would have to name them "valley 225381-B-17" or somethin like that

[@mavhc]

RealD uses circular polarization, IMAX 3d uses linear. You can tell by wearing the glasses and looking in a mirror, then closing one eye

[@makanaokalanichong808]

These were used as navigational tools by the vikings to help determine longitude and latitude.

[@OscarFerro]

Excellent report. Too bad you didn't mention at all the great and forgotten physicist François Arago, who made a lot of discoveries and inventions with polarized light. He invented the polarimeter, and also a photometer and a micrometer, all of them based on the phenomenon of birrefringence. And pointing his polarimeter to the sky he discovered that its light it polarized, as you said.

[@gustavorad]

gustavo, from argentina. one of my cell phone has a particular polarized film. at the noon, when the sun has gone, y can see in the reflect of the sky, a violet band dividing east from west, specially when the west is cloudy.

[@jp-um2fr]

YT is so full of $$$ grabbing videos, sometimes I give up. Thanks so much for your research, out of 40 odd Subd you are always very near the top.

[@outcastoffoolgara]

There used to be an Iceland Spar mine near Grafton in Australia. Closed now and derelict.

[@markworden9169]

Good content

[@charleshotchkiss1813]

Lenses in Polaroid sunglasses are set up with the polarization in each lens at 45 degrees, not vertical or horizontal. Each lens in turn is 90 degrees to the other to defeat at least some glare off all surfaces. Also, stacking two filters with the polarization at 90 degrees filters 87%, not 100% of light (from sine squared.)

[@uncletiggermclaren7592]

This was the MOST comprehensive and interesting video I have seen this year. 1:35 Did you not get the general memo?. New Zealand is supposed to be left off maps.

[@MichaelStoko]

Thank you for a great vid! Just to note: Maxwell's equation's are not actually Maxwell's original equations. Maxwell wrote 20 equations and 20 unknowns, and used quaternions not vectors. The equations we call "Maxwell's Equation's" are actually Heaviside's vector equations. Although Heaviside's vector interpretation of Maxwell is easier to calculate in, it does not capture Faraday's phenomenology in full, which Maxwell set out to capture in his original 1864-65 theory. Also - I'm pretty sure that the aliens use Maxwell's original quaternions and not Heaviside's vectors because their stuff works and ours doesn't. Let's get after it and fix this foundational error! Lots of love y'all, it's the only way forward! 🙂

[@WhiteDwarfVR4]

Regarding the backup sights for anti-aircraft guns, you might consider contacting Ian of Forgotten weapons on KZbin. Not only is he a wealth of information regarding older and obscure weapons and accessories for them. Honestly you remind me a lot of him in the extent of detail that you cover. Not to mention your voice is similar. Haha! Further, I was drawing parallels before you posed your question regarding the reflex sight. Keep up the great work! I've really enjoyed the few videos I've watched so far.

[@dougp6664]

Or, possibly - www.youtube.com/@Drachinifel

[@glennmcgurrin8397]

Modern 3d movies typically use circular polarization to avoid issues with slight head/glases tilt causing bleed through from the other eyes image among other issues

[@clavo3352]

Very interesting video! You didn't discuss whether the light energy of the two beams were fractional amounts of the entering light. This might be used as a solar energy multiplier ; no?

[@nuguns3766]

I have my own piece of iceland spar calcite. I thought it was quartz until now it indeed creates a double image. Mine is really scratched which makes it real difficult to see thru so make sure its handled and stored with soft things

[@JCO2002]

I'm enjoying your videos and subscribed. Please, please tell me what musical piece you're using for the intro. I know I've heard it dozens of times, but can't even remember the composer.

[@dannydetonator]

kzbin.info/www/bejne/jGKloYOpdph_kMksi=wMatTYIcCjwf0p9s It's Modest Musorgsky, "Pictures in a gallery", piano opus (this was 'Promenade" theme in arrangement of symphonic orchestre) ;)

[@cwtrain]

"Yeah yeah. Polarized rocks. I know this one. 🙄" _16 minutes later_ "No no noooo! That cant be all! What else!?"

[@TomsBackyardWorkshop]

There is a source of this near where I live. Some of the crystals have been as clear as glass.

[@dogstar5572]

I live in Iceland. There’s none left. Tourists took it all. Big sign saying ‘please don’t remove anything from this mine’. It must be odd to read this and find nothing.

[@chrismofer]

fascinating

[@brianclimbs1509]

And I think the modern 3D movie glasses are circularly polarized, so don't expect them to behave like a linear polarizing filter.

[@BenHydeSPARKScience]

At 5:07, you use the standard graphic of an EM wave but say that it is TWO (2) separate waves. Did you mean to do this? Was this the only graphic you could find? And what do you mean by " a light ray" or a "ray of light"?

[@hunnybunnysheavymetalmusic6542]

I have little pieces of Iceland Spar on my land. I only found out this when I was breaking up some really big chunks of rocks to move them. Unfortunately, the ones I found [so far], they are too small, and too occluded to be useful for optical purposes. If I happen to find any in the future though, hopefully they will be optically satisfactory to do optical experiments with.

[@SianaGearz]

We're told by RealD that their glasses use a circular polarised light system, not horizontal/vertical. However some stereoscopic TVs and PC monitors of mid-late 2000s, early 2010s used HV polarised light, as did the IMAX projection system. Allegedly, HV polarisation is viewer position sensitive, which is fine for IMAX, since they built very limited seating arrangements (at least here in EU where i've been to) with the whole audience in the sweet spot, and it's fine for home gear, since only 1-3 people will be watching anyway, but RealD solves the problem for somewhat hodgepodge typical movie theatres. Is that true? Of course a lot of home gear used an active shutter system instead.