polarization... or Take Me Out to the Ball Game

[Richard Z.]

My 41CV has a blemish on the LCD that only shows up if I am wearing my polarized sunglasses: On the left is normal lighting, on the right is shot through my sunglasses.

I found this out when I took it to my kid's baseball game so I could catch up on some grading during slower moments. My kid thought I was nuts pointing to a blemish that to him wasn't there.

On one hand, it makes me wonder how long the blemish has gone undetected, and what else would show up all over the place if E-bayers made it a point to ask to see similar "polarized" pictures of all the calculators they were bidding on.

Some more playing with the polarized glasses and the calculator(s) led me to discover that most (maybe all?) calculators have their LCD's polarized at 45 degrees to the glasses "horizontal". Does anyone know why LCD's seem to be made to favor this orientation?

[Dave Shaffer (Arizona)]

re: "Does anyone know why LCD's seem to be made to favor this orientation?"

I suspect it is to cut down on unwanted reflections (from some direction).

In general, if you look at most pieces of backlit plastic through a polarizer (the fancy name for your sunglasses), you will see all kinds of stress coloring. The very process of making and forming the plastic induces stresses which line up the plastic molecules on various scales which then polarize light in various directions.

You will see the same effect if you look through the side windows of your car (i.e. at the sky or some other fairly smooth, bland background). The glass is stressed so that if it breaks in a collision, it shatters into a zillion tiny little pieces. (The windshield is made differently - a glass/plastic sheet/glass sandwich - so it tends not to show the effects as much.)

[Jonathan Eisch]

Quote:

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On one hand, it makes me wonder how long the blemish has gone undetected.

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Probably since it was first manufactured. The case you show is a rather minor case, but this type of things happens almost every time a pliable (plastic) polarizer is manufactured in a process that involves any stress (screws, tabs...). This is referred to as "stress birefringence", and is caused by the plastic being stretched or compressed in one direction more than another, which alters the polarization slightly. (which is why you can see it with your sunglasses)

Quote:

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Some more playing with the polarized glasses and the calculator(s) led me to discover that most (maybe all?) calculators have their LCD's polarized at 45 degrees to the glasses "horizontal". Does anyone know why LCD's seem to be made to favor this orientation?

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If the polarization were perpendicular to the polarization of your sunglasses, the whole display would look black. The idea would be to polarize the light parallel to the polarization your glasses, to let the most light through to your eye. To simplify manufacturing, 45 degrees is often chosen, as it will block only slightly more light (the relationship is not linear).

[Chris Woodhouse]

Very interesting, I'm going to have to find the polarizing filter for my camera look at all the lcd screens I can find around the house.

Chris W

[Paul Brogger]

I once experimented with a Sharp "landscape" (or "slide-rule") format scientific. Taking it apart, I noticed that there were no electronics behind the LCD. I thought it would be a cool back-lighting effect if I cut a window in the back case, and removed the reflective sticker on the back of the LCD glass, thus letting light in from behind.

Well, imagine my shock when, upon removing the back sticker, the LCD image completely disappeared! I had created a blind, empty, glass window in place of the calculator's display.

Somehow, I stumbled across the fact that the plastic cover was polarizing the light passing through. I dug into my calculator "bone box" and found another plastic window from an old LCD four-banger. I found that if I put the second window behind the LCD, my display was restored. Furthermore, if I flipped one or both of the plastic covers (I don't remmeber the details), I could reverse the display's polarity -- with "clear" segments swimming in a black background.

This was all cool, but my back-lighting experiment actually made the calculator more difficult to use. I had to hold it off the table to let some reflected light pass in from behind -- I couldn't simply set the thing down and use it.

Eventually, I sat down at the coffee shop with the thing in my back pocket, and cracked the LCD. Bummer! But it had been fun to experiment with . . .

[Frank Boehm (Germany)]

Polarizers are necessary for LCDs (and have always been there) - just check out

http://en.wikipedia.org/wiki/LCD

Black spots are probably misaligned crystals or defective molecules - this should be irreversible

[bill platt]

Once, I was sailing in a long-distance race where the windspeed and boatspeed repeaters where so highly polarized that I had to cock my head to one side to read them!
The next time I sailed that boat, I brought along non-polarized glasses!

Edited: 20 June 2006, 9:15 a.m.

[GE]

Here are my own findings regarding polarizing filters. Was done on 'another brand', so was originally posted on another vintage calc forum...


"

I found a new method to find the current direction of the sun (when hidden by clouds of course !! Otherwise there is a simpler method...) using a TI82.

Amazingly, it needs very few program steps and is probably portable to many other TI models (or other brands).

The trick is to keep the machine off, lying flat in your hand without the display cover. Then turn the machine around a vertical axis and watch the screen turning greyish - greenish - greyish.
Obviously the polarization filter of the LCD reacts with the polarized light coming from the sun through clouds, and this allows one to find the direction of the sun (provided you have a rough estimate of where the North is so you can rule out one of the two directions).

The sun is in the direction where the screen is the darkest. Works well even in heavily cloudy situations, and is quite precise. Is this method reliable ?

"

[Garth Wilson]

Several years ago I was evaluating a wide-temperature-range LCD for one of our aircraft applications. One thing I wanted to be sure of was that the polarization would be correct for crew members using polarized sunglasses. The LCD definitely did not get an A on that one. I called the manufacturer and told them what we were up to. They said they never thought of someone using polarized sunglasses! They said the polarizer was cut in such a direction so as to get the most LCDs from a sheet, but they could cut it any direction we wanted.

[sjthomas]

For several hp (and other brand) calculator polarization images, visit Joe Horn's page at:

http://holyjoe.org/hp/rainbow.htm