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How to Convert a Digital Watch to a Negative Display (watchuseek.com)
188 points by zenburn on April 21, 2014 | hide | past | favorite | 49 comments

Nice. I remember doing this to my scientific calculator at school.

Modern colour LCDs are still polarised, so I can only view my phone and tablet in one orientation when wearing my polarised sunglasses. On the plus side, they make for an excellent real life ad-block to those annoying video billboards (which are just portrait LCD TVs).

I wonder how practical it would be to make a private display by removing the polarising film from it and then viewing with polarised glasses.

> I wonder how practical it would be to make a private display by removing the polarising film from it and then viewing with polarised glasses.

It's been done, pretty much how you'd expect:


There are quite a few hacks around but I wondered if there was some reason these aren't more widespread or commercially available. Business idea?

You could also make something that filters all light but near infra-red so it could be seen by CCDs (on cameras or phones) but not with the human eye. Or do the reverse and flood the picture with near infra-red to prevent it being photographed.

I think you can buy this kind of thing, I've seen them in use in some offices where sensitive information is being viewed.

3M sells this.

I was really excited to read the idea about advert blocking, but when I donned my glasses, this is what I saw:



>I wonder how practical it would be to make a private display by removing the polarising film from it and then viewing with polarised glasses.

In Japan a lot of people have "privacy" screen protectors on their cells that basically make the viewing angle very small. I always assumed it was through some form of polarization though now I'm not sure.


They probably have the same effect as an LCD with a very narrow viewing angle. Modern displays have a pretty high viewing angle; older ones start to change color and become illegible when viewed at an angle. I've seen the same thing on ATMs and the like.

Anyone wishing to try this should use proper polarised glasses, though, rather than repurposing filters. They're not protective from UV, you see.

It is funny and strangely comforting that despite such a wide variety of topics, forum culture is still the same. There is always a large segment of the forum population that derive some level of pride in listing their collection of X in their signature. Here we see posters list their "casios in rotation," on the spyderco it is lists of knives and ubuntu forums it is "rig specs" and version names. The puzzling thing is that--thankfully--this does not seem to happen on mailing lists.

This is can actually be quite useful on "help me!"-style forums, like car forums. People often forget to give enough info about eg. their car year model, for others to help specifically. But if this info is in your sig, it's hard to forget (but also you forget to update it when it changes)

> The puzzling thing

I'd guess that this is because forums have forum-specific profiles, whereas mailing list are replied to from the one inbox.

Also, people can probably manage to find the signature section of a forum profile, but not their (web)mail client!

It serves a technical purpose just as much as their ego. It makes it possible to search for someone with a specific make and/or model of product in order to ask for help or recommendations, or gauge someone's expertise/seniority/subfocus within the forum's focus during a discussion.

Inkjet transparency sheets like you'd find at your local office supply store [0] are polarized; you may not have to order polarizing film.

I used to own a car that had an LCD display where the orientation of the polarization was perfectly set so if I was wearing my polarized sunglasses, the screen was blank. I cut out a little overlay made out of an overhead transparency and fixed the problem.

0: http://www.officemax.com/office-supplies/presentation-equipm...

An overlay of other polarizing material won't change the polarization unless your transparencies are somehow quarter wave plates? (http://en.wikipedia.org/wiki/Circular_polarizer#Circular_pol...)

Most thin sheets of polyester actually function as retarders (not necessarily quarter wave plates) due to the way their molecules are aligned during manufacturing. Here's one of the better cites on the topic, it includes some math to design any desired retarder:


Using a linear polarizer does change the polarization. If he puts his polarizer halfway between the incident angle and the desired angle, he'll get cos⁴(theta/2) of the original intensity in the desired polarization.

If you have two perpendicular polarizers, no light will pass through both. But, if you insert a polarizer at a 45 degree angle to both of them inbetween, some light will pass through. Am I wrong?

You are exactly right.

This is why I trawl the comment section.

I remember doing that to my calculator when I was a teenager. Thought my unique calculator was badass then, hehe.

Back then I hadn't access to the internet(1997-8?) and I was trying to figure out how this LCD screen thingy was working by dissecting the parts of my Casio clock and I found out about this "miracle" glass that looks just as normal one but inverts the screen colors(=b&w) when put in different position. I have to say, it made me a badass :)

Now-days kids could just look it up in minutes an move on. I am not sure if this is a good thing.

Nice. I flipped the LCD and added ram to my HP 48G. Reverse RPN! :)

Anyone know if the "cmoy" behind the howto mentioned in the OP is the same guy (Chu Moy) that designed the CMoy headphone amplifier circuit?

Related to the Polarization subject, when you are shopping for Polarized sunglasses, then take two copies of the sunglasses and placed them so that you are looking through both lenses, but one in 90 degree and notice if it turns opaque-black.

Many times in the past when I was looking for some cheap polarized sunglasses, I found that some were actually falsely labeled.

I was going to do it to my Casio GLX5600 but I realized it would reverse the moon phases :(

Cut a separate piece out and flip it.

According to the time difference in the before an after shots, it looks like this takes ~4 hours to do?

According to the post itself, 3 of those 4 hours were spent removing the glass and then replacing it after realising it never needed to be removed in the first place. If you skip that step it's probably a lot faster.

> It took me a little over four hours to do this, but nearly three of those were spent trying to replace the glass display that I shouldn’t have removed in the first place. There were also some other distractions along the way.

I've owned a Casio G-Shock that was negative by default (G-5600KG) and I found that the viewing angle on a negative display was much more narrow than on a positive. Most of the reviews on Amazon state the same. Anyone have an explanation for this?

In a similar vein with a TFT screen https://www.youtube.com/watch?v=xUw3u9lh9rQ

Wow! I remember doing this to my and my friends' 4-function calculators in 5th grade! This was actually how I learned how LCDs worked.

You can also remove the reflective sticker on the back of the black and white LCD, to make it transparent where it was previously reflective. That's how I made my calculator work with our classroom's overhead projector. (and some really interesting pranks)

That's a pretty neat hack. Reminds me of a DIY projector where an LCD screen was stripped down and placed on top of an overhead sheet projector.

Phosphor makes watches with E-Ink displays, so you can toggle between white-on-black and black-on-white. I've been happy with mine, which I've had for a couple years now:


I did this to my JVC car radio. It made it so much easier to read in the sunlight.

I did this to my calculator, possibly a CASIO one.

I did this a lot to LCD gaming handhelds back in the early 90s. Also using two polarising sheets to understand polarisation. Was very interesting, a pity nowadays taking apart stuff is not so easy.

I'm wearing a Newton Digital which is already negative in the default state, and it seems possible to convert it to a regular positive display. Wouldn't look as cool though.

I remember discovering polarization and how LCDs worked in grade school when my calculator came apart and I accidentally flipped the polarizing film around.

I remember changing a digital clock to anticlockwise.

BTW good work.

but.. Who hasn't done this?

LCD monitors all use the same technology...

Fun office prank?

Sounds more likely to shock the pranker than the prankee.

The "magic" explanation doesn't satisfy me. Any physicists in the crowd who can explain why rotating the film 90 degrees creates a negative display?

It's actually really simple. The display works in a way that you control the polarization of outgoing light (but the intensity is constant). Then you filter whichever polarization you want. So a black dot on a white screen could be achieved with a small vertical polarization region on a horizontal pol. background, and filter out the vertical pol., with a polarizer horizontally aligned. Align it vertically, and you swap the intensities.

Note: In this case the light whose polarization is controlled is the reflected incoming ambient light, which isn't polarized; in a computer monitor it's the back light.


This article contains helpful diagrams and an obtuse but reasonable explanation:


Perhaps this will help: http://qxwujoey.tripod.com/lcdworks.gif

Another decent source: http://www.howstuffworks.com/lcd.htm

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