CR4® - The Engineer's Place for News and Discussion®


Challenge Questions Blog

Challenge Questions

Stop in and exercise your brain. Talk about this month's Challenge from Specs & Techs or similar puzzles.

So do you have a Challenge Question that could stump the community? Then submit the question with the "correct" answer and we'll post it. If it's really good, we may even roll it up to Specs & Techs. You'll be famous!

Answers to Challenge Questions appear by the last Tuesday of the month.

Previous in Blog: Flying in the Shadow of the Moon: Newsletter Challenge (November 2017)  
Close
Close
Close
16 comments

Stumped by the Screen: Newsletter Challenge (December 2017)

Posted November 30, 2017 5:01 PM
Pathfinder Tags: challenge question lcd Screen

This month's Challenge Question: Specs & Techs from GlobalSpec:

Viewing a common LCD display from straight on, you see a figure in pink. Tilting the screen so that it is at a sharp angle in your field of vision, the figure appears to have changed to bright blue. Why has the apparent color of the figure changed?

And the answer is:

The apparent change in color of images on the screen is due to the structure of the LCD screen.

A liquid crystal display (LCD) monitor’s screen is composed of millions of elements known as pixels (picture elements). Each pixel is made up of three individual subpixels: red, green and blue. Each of these subpixels controls the amount of light that passes through them, regulating the amount of each color emitted by a pixel. The mixture of the three primary colors determines the final color that the eye perceives for a given pixel.

An LCD is comprised of layers of filters, liquid crystals, and transistors. For each subpixel, light emitted by a backlight passes through a polarizing glass filter, travels through twisted, nematic liquid crystals, and continues through another polarizing glass filter.

The first glass filter polarizes light so that all light waves except those vibrating horizontally are blocked. The light passes to the liquid crystal layer where all of the light vibrating horizontally passes through since the liquid crystals are by default twisted to the correct orientation to allow horizontally vibrating light to pass. The light then arrives at the second glass filter, which is structured to only allow light vibrating vertically to pass. Since all of the light at this point is vibrating horizontally, none of it can pass through the second filter, resulting in a dark subpixel.

To illuminate the same subpixel, all that is needed is to twist the liquid crystals to the appropriate orientation. This is accomplished through the use of transistors. Each of the subpixels is controlled by a transistor that switches on and off to control the flow of electricity through each of the liquid crystals. When liquid crystals have current flowing through them, they twist and rotate light waves passing through them by 90 degrees. Rotating the horizontally vibrating light by 90 degrees causes it to vibrate vertically, allowing it to pass through the second glass filter. The result is an illuminated subpixel. A color filter over each subpixel gives each element its color.

The reason the figure on the screen appears to change color is that by tilting the screen you are viewing light escaping before it has passed through the liquid crystals and polarizing filters in the same manner as it does from a face-on angle. Colors that would have been blocked are therefore able to leak through, resulting in the apparent change in color.

For more on how colors are displayed on computer monitors, see this article.

Reply

Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru

Join Date: Apr 2010
Posts: 5810
Good Answers: 588
#1

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

11/30/2017 6:06 PM

There are lcd cells for each primary color, blue, green, and red. These cells have polarizers on both sides, crossed so that no light normally comes through. These cells are turned on (made transparent) by applying a voltage. This voltage causes the cells to become more transparent because it rotates the plane of polarization of the light passing through. The amount of rotation is determined by the path length of the light and when viewed at an angle, this path length, and hence rotation is greater. This causes the color shift.

Reply Score 1 for Good Answer
Guru

Join Date: Apr 2010
Posts: 5810
Good Answers: 588
#4
In reply to #1

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

11/30/2017 8:41 PM

If the above is true, the effect should be symmetrical in different directions, and a little experimentation proves it is not.

I believe it has to do with the color mask and the parallax (color leakage between adjacent cells).

I think this paper explains it.

https://www.deepdyve.com/lp/wiley/p-185-color-shift-analysis-of-ltps-tft-lcd-viewing-for-large-angles-CqqKrpvSDr

Reply
Guru
Engineering Fields - Optical Engineering - Member Engineering Fields - Engineering Physics - Member Engineering Fields - Systems Engineering - Member

Join Date: Apr 2010
Location: Trantor
Posts: 5223
Good Answers: 622
#5
In reply to #4

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

12/01/2017 9:49 AM

An microscopically thin alignment layer is placed on the glass to give the liquid crystal material a pre-tilt, to help the liquid crystal take the correct orientation when power is applied.

This pre-tilt is what causes the asymmetry in the optical phenomena. Ideally the LC fluid would undergo a 90 degree rotation between the full on and full off states. In reality, without the pre-tilt (in a TN LCD) some of the molecules could rotate 90 deg 'left' and some could rotate 90 deg 'right', and the optical switching would not be as good. The pre-tilt helps ensure they all rotate the same, and that they all orient in the same direction when unpowered.

__________________
Whiskey, women -- and astrophysics. Because sometimes a problem can't be solved with just whiskey and women.
Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 19465
Good Answers: 1140
#2

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

11/30/2017 6:46 PM

The pixels and screen are designed to be viewed straight on, altering the viewing angle allows different wavelengths to be seen...

http://blogs.plos.org/mitsciwrite/2013/05/02/your-computer-screen-from-an-angle/

__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Reply
Guru
Engineering Fields - Optical Engineering - Member Engineering Fields - Engineering Physics - Member Engineering Fields - Systems Engineering - Member

Join Date: Apr 2010
Location: Trantor
Posts: 5223
Good Answers: 622
#3

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

11/30/2017 8:00 PM

Rixter's answer is sort-of close, but not exactly right.

The 'normally black' LCD he describes is generally of the 'In Plane Switching' type (there are various names for it). These all have excellent color rendition out to extreme angles. You're not going to see pink become blue for these LCDs.

The older 'TN' (twisted nematic) color LCDs are 'normally white' (white when unpowered; you apply power to get gray shades, including black). These older TN type LCDs don't have as good contrast or color at extreme angles compared to IPS type LCDs. So at extreme angles you will see color shifts. The color shift is due to two phenomena: 1, the path length difference through the LCD which affects the net cross-polarization effect, and 2, the fall-off in contrast at steep angles. I.e., the blacks are less black at steep angles.

The net result is a color shift which can turn pink into blue.

__________________
Whiskey, women -- and astrophysics. Because sometimes a problem can't be solved with just whiskey and women.
Reply
Guru

Join Date: Apr 2010
Posts: 5810
Good Answers: 588
#6
In reply to #3

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

12/01/2017 10:36 AM

I'm using my laptop screen as an example. I don't see any color shift left or right. Looking from the bottom, it appears to darken and from the top, it appears to wash out. My telephone appears to not change at all at any angle. The TV is also good at about any angle. So the laptop has a TN display and the phone and TV are probably IPS.

So why doesn't the laptop screen show the same difference when viewed from the top as from the bottom? I'm thinking that with no electric field, the molecules are all oriented parallel to the screen plane with the long axis towards the top and bottom of the screen. The molecules are polarized so that an electric field tends to attract opposite ends toward the front and back of the screen.

https://en.wikipedia.org/wiki/Twisted_nematic_field_effect

When the electric field is completely turned on, the molecules align front to back allowing the light to pass through without rotating the polarization. The light is extinguished because of the crossed polarizers front and back of the LCD. When the electric field is partially turned on, the molecules will be at an angle such that the optical path from the bottom of the screen will be looking at the molecules from the end (dark condition, no optical rotation) and the optical path from the top of the screen will be looking at the molecules from the side (light condition, more optical rotation). At extreme angles from the bottom of the screen, you are actually looking at the "bottom side" of the molecules, so the optical rotation is reversed resulting in a negative image. This would result in a color reversal on a color LCD display.

https://encyclopedia2.thefreedictionary.com/IPS+panel

Reply
Guru

Join Date: May 2006
Location: Placerville, CA (38° 45N, 120° 47'W)
Posts: 4593
Good Answers: 168
#7
In reply to #6

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

12/01/2017 11:09 PM

"I'm using my laptop screen as an example. I don't see any color shift left or right."

Agreed. My Laptop (a 15" MacBook Pro with "Retina" screen) shows virtually no change in color at any reasonable viewing angle (up to 70° or 80° from normal). There are changes in apparent brightness and contrast. but not in color.

"They" have come a long way in display quality! I've been amazed for many years how they managed to achieve uniform brightness over such large areas, especially back when they used two fluorescent tubes for illumination. I have no idea how many LEDs are used in the current backlights, but I see absolutely no grid or array of brightness.

__________________
Teaching is a great experience, but there is no better teacher than experience.
Reply
Guru

Join Date: Apr 2010
Posts: 5810
Good Answers: 588
#9
In reply to #7

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

12/02/2017 10:25 AM

This guy explains it pretty well...

Reply
Guru

Join Date: May 2006
Location: Placerville, CA (38° 45N, 120° 47'W)
Posts: 4593
Good Answers: 168
#10
In reply to #9

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

12/02/2017 11:33 AM

Thanks! This is a very different video that shows several different aspects of the technology.

__________________
Teaching is a great experience, but there is no better teacher than experience.
Reply
Guru
Engineering Fields - Optical Engineering - Member Engineering Fields - Engineering Physics - Member Engineering Fields - Systems Engineering - Member

Join Date: Apr 2010
Location: Trantor
Posts: 5223
Good Answers: 622
#8
In reply to #6

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

12/02/2017 7:20 AM

Yes, TN LCDs show 'image reversal' in one direction. Most are designed for either '6 o'clock' or '12 o'clock' use orientation.

LCD manufacturers have used various techniques to minimize the image/color reversal. Typically they use polarizers with 'optical retardation' layers built in. Light travelling straight through the LCD is linearly polarized and yields the desired full on / full off/ or intermediate gray shade desired.

But light travelling through the LCD at an angle becomes elliptically polarized and does not fully 'switch' on/off. To compensate for this, the retardation provides a 'negative' amount of elliptical polarization which then results in the light being linearly polarized once it has fully traversed the LCD. The LCD cell gap is 'tuned' for green light (due to various properties if the human visual system), so the technique works best for green. The bi-refringence of the retardation layer is color dependent, so the net result is that some color shift still occurs. And again, the pre-tilt and other asymmetries of the LCD structure cannot be fully overcome.

__________________
Whiskey, women -- and astrophysics. Because sometimes a problem can't be solved with just whiskey and women.
Reply
Guru

Join Date: Oct 2014
Location: Hemet, Land of milk and honey.
Posts: 1262
Good Answers: 23
#11

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

12/02/2017 1:55 PM

This might be a dumb response: where is the figure in pink at ? My cell phone has a liquid crystal display, when I look at it straight on, I can see an image or writing. When i view from an angle, I can still see the writing or the image, there is no change in color.

If the figure in pink is only visible on a blank screen & the figure changes to blue when the screen is tilted, that is neato, but why would I want to look at a blank screen ?

Reply
Guru

Join Date: Apr 2010
Posts: 5810
Good Answers: 588
#12
In reply to #11

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

12/02/2017 3:40 PM

Your phone, like mine, most likely has an IPS (In-plane switching) display and you don't see any color change. With IPS displays, the liquid crystal molecules rotate in a plane parallel to the display surface.

The color change with viewing angle is only a "feature" of TN displays (twisted nematic). TN displays are cheaper, needing only 1 transistor per pixel versus 2 transistors for IPS displays, but obviously IPS displays are much superior for color rendition.

Here is a description of how the IPS LCD display works...

"Implementation[edit]

In this case, both linear polarizing filters P and A have their axes of transmission in the same direction. To obtain the 90 degree twisted nematic structure of the LC layer between the two glass plates without an applied electric field (OFFstate), the inner surfaces of the glass plates are treated to align the bordering LC molecules at a right angle. This molecular structure is practically the same as in TN LCDs. However, the arrangement of the electrodes e1 and e2 is different. Because they are in the same plane and on a single glass plate, they generate an electric field essentially parallel to this plate. The diagram is not to scale: the LC layer is only a few micrometers thick and so is very small compared with the distance between the electrodes.

The LC molecules have a positive dielectric anisotropy and align themselves with their long axis parallel to an applied electrical field. In the OFF state (shown on the left), entering light L1 becomes linearly polarized by polarizer P. The twisted nematic LC layer rotates the polarization axis of the passing light by 90 degrees, so that ideally no light passes through polarizer A. In the ON state, a sufficient voltage is applied between electrodes and a corresponding electrical field E is generated that realigns the LC molecules as shown on the right of the diagram. Here, light L2 can pass through polarizer A.

In practice, other schemes of implementation exist with a different structure of the LC molecules - for example without any twist in the OFF state. As both electrodes are on the same substrate, they take more space than TN matrix electrodes. This also reduces contrast and brightness.[17]

Super-IPS was later introduced with better response times and colour reproduction."

https://en.wikipedia.org/wiki/IPS_panel

Reply
Participant

Join Date: Dec 2017
Posts: 1
#13

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

12/05/2017 10:57 AM

Because the human eye can't distinguish pinks/reds in peripheral vision.

Reply
Guru
Safety - Hazmat - New Member United States - US - Statue of Liberty - New Member Engineering Fields - Chemical Engineering - Old Hand

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 13859
Good Answers: 155
#14

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

12/05/2017 11:54 AM

The LCD is pink when straight on because it is not blue (but stills contains blue wavelength). The optical path to your eye at an oblique angle with respect to the display is far longer, and you see the absorbance color of the plastic over the LED display which is decidedly blue. Not only this, but the grazing angle provided a different reinforced wavelength of emitted light as viewed obliquely.

__________________
If it ain't broke, don't fix it. Just build a better one.
Reply
Power-User
Hobbies - Musician - New Member Technical Fields - Technical Writing - New Member CR4 Admins - CR4 Moderator - New Member

Join Date: Nov 2012
Posts: 305
Good Answers: 4
#15

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

12/07/2017 12:01 PM

Thanks to all users for chiming in to this thread. The answer has been posted.

__________________
"Freedom is nothing but a chance to be better." -Camus
Reply
Guru

Join Date: May 2006
Location: Placerville, CA (38° 45N, 120° 47'W)
Posts: 4593
Good Answers: 168
#16
In reply to #15

Re: Stumped by the Screen: Newsletter Challenge (December 2017)

12/07/2017 12:54 PM

"...you are viewing light escaping before it has passed through the liquid crystals and polarizing filters..."

I'm not at all sure I can accept that answer as correct. How can light from behind escape without passing through those other layers?

As JS hinted, light traveling obliquely through the liquid crystal follows a longer path than light passing through normally, so is presumably rotated more, causing a different fraction to pass through the second polarizing filter, and at sufficiently wide angles, causing the light to pass through a different sub-pixel of the color filter.

Thanks for posting that the "answer" was available.

Aside from the above, I'd like to know how different intensities of each primary color are achieved. Do different control voltages produce different amounts of polarize rotation, or is it Pulse Width Modulated, or something else?

__________________
Teaching is a great experience, but there is no better teacher than experience.
Reply
Reply to Blog Entry 16 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

dkwarner (3); Hannes (1); James Stewart (1); LongLine (1); Rixter (5); SolarEagle (1); tonyhemet (1); Usbport (3)

Previous in Blog: Flying in the Shadow of the Moon: Newsletter Challenge (November 2017)  

Advertisement