Curved TV

Curved TV?
Does the world need curved TVs?

04 May 2014 | Torben Rasmussen |

Back in 2013, we saw the first signs that curved screens might be the next "3D" as both Samsung and LG showed off curved OLED TV. In 2014 we were once again presented with this trend as we joined CES 2014, now only in a much larger scale as Samsung launched curved LCD panels.

But what justifies these curved panels? Have the TV manufacturers found a solution to a problem we didn't even know existed? And if so, what is this mysterious problem that they are trying to fix? Let's find out.

Samsung 105-inch UHD-tv
Samsung’s 105 inch curved Ultra HD TV


Where are curved screens used?

Curved screens are not new. Most cinemas use them and I bet you remember the CRT TV era, where the cathode tube also curved (although it was convex and not concave).

In cinemas the curved screen serves two purposes:
  • The picture source in cinemas is almost point-like in the form of the lens of the projector. Light from this source will propagate on a spherical shape and if the screen did not curve you would see increasing levels of distortion towards the edges of the screen. This effect is known as the "pincushion effect".
  • The typical seating distance in a cinema is so close that a curved screen is needed to increase the "field of view", which minimizes the need to turn your head when the actions flies across the screen.


The Imperial movie theatre in Copenhagen


If you have ever experienced a true IMAX Dome movie you will also know that a curved screen can offer great immersion as you are positioned under a dome, or a canvas, that almost embraces you completely.

Before we proceed let's just introduce one concept that we will be using throughout this article, namely "sweet spot". The sweet spot is the centre of the arc that e.g. a curved screen follows i.e. the spot where all points on the screen are perpendicular to your line of sight.

How do TV manufacturers promote this?

Since the appearance of the curved screens it hasn't exactly been swarming with mathematical explanations about the delights of these new panels, and so far we have been passed nothing by vague explanations related to looking at flat vs. curved screens.

Both Samsung and LG have promoted the curved screen as "providing greater immersion" and improving the 3D effect as the screen almost embraces you. Improved viewing angles also come up on occasions along with a reduced distortion of the picture, fewer reflections and an improved contrast ratio.

LG has gone through the trouble of telling people about the "horopter line", which defines equidistance between you as a viewer and the TV screen. Or in other words; the arc where you are at the centre. If the TV screen follows the horopter line your eyes won't have to change focus as you sweep the screen. Both a theoretical and an empirical model is mentioned, which aren't completely the same, and LG has chosen to adopt the empirical version, which describes a softer arc (larger radius of curvature) than the theoretical model does.


From LG's press material.


LG and Samsung also argue that a curved screen will feel bigger than it actually is. In the below illustration LG has illustrated the added width you “gain” by choosing a curved panel. Notice how close the subject is to the screen.


From LG's press material.


But what is reality?

Let’s address the pro-curved arguments one by one to see if they hold up to a closer inspection. And let’s at the same time ignore that the current screens only curve in the horizontal direction and not the vertical direction as well...

Greater immersion

First of all: Does a curved screen provide greater immersion at any level? To this the answer is simple: Yes. If a screen truly “embraces you” - as with IMAX - you will feel a much greater immersion in the movie (if you like this or not is another matter). The same goes if you sit on the front row in a movie theatre. And here the keyword is “size”! Because if you want to make a screen to “embrace” you it has to have a considerable size relative to your seating distance. It is quite hard to make a 55” TV embrace you in any way - especially if you have your couch placed at any normal viewing distance from your TV.

THX has a guideline that relates to proper seating distance, which aims for a TV to cover 40% of your field of view to achieve an “embracing experience”. To cover 40% of your field of view you need to be seated about 1,7 meters from a 55” TV, which is pretty much the same distance you would get if you evaluate the optimal distance to get the most of a Full HD resolution. At 1.7 meters you should also consider upgrading to 4K, but that is a different matter.

So what does this mean? Well, if we consider LG’s latest curved 55" OLED TV the sweet spot is actually 5 m from the screen if you believe the stuff about the horopter line. This is considerably further away than THX's recommendation. So the radius of curvature for the LG OLED TV is actually quite far from optimum if you are aiming for an embracing experience, simply because the TV is way too small to give you the proper field-of-view coverage at the sweet spot.

If we consider the same 5 meter radius of curvature you would need a screen size of 101” to achieve the full potential of an immersive movie experience. Samsung has actually shown off a curved TV of this size, but unfortunately you can’t buy it. Yet.

Improved viewing angles and contrast ratio

The next argument involves improvements of viewing angles (which are the same arguments that apply to improved contrast ratio). Do we need this? If we talk about TV’s in general the answer is not unambiguous and if we are talking OLED TV’s the answer is no.

OLED panels have no issues when it comes to viewing angles and contrast ratio - it is actually some of their greatest merits. Looking at LCD TVs instead, we definitely see room for improvement in both areas - maybe especially for Samsung when it comes to viewing angles, and for LG when it comes to contrast ratio. So what could be gained by using a curved screen then?

By curving the screen you reduce the effects that occur when sitting in front of a large screen at close distance and looking towards the corners. On a monitor this will especially be evident if you purchased a panel based on the Twisted Nematic LCD crystals (TN) in a large size. It is very common to see dramatic changes in contrast ratio towards the corners simply because of the great angles. And this is a considerable issue if you care about color reproduction.


The typical seating distance for a monitor is about 60 cm and with a screen size of 27” you will feel a gain if the screen is concave - especially as you rarely sit more than one person in front of a monitor.


When it comes to TVs matters are a little different. First of all you will very often not be alone in front of the TV and unless you are the cuddly type there can be only be one person in the sweet spot (yes, it’s Christopher Lambert). So the optimum viewing angle only exists for one person. It should be noted that a regular flat LCD screen will also only provide an optimum seating position right in front of the TV, where you have the same angle to the left and right side of the TV, so the concept of sweet spot is not isolated to curved screens. But here we are talking about advantages over the flat cousin.

But is it at least possible to benefit somewhat from a curved screen in terms of improved viewing angles if more than one person sits in a sofa? Again we need to take a look at how you might be positioned as for example a 55” TV is only about 120 cm wide so someone needs to sit in a position that is not directly in line with the TV.


As soon as you start to move away from the sweet spot you will also start to have less than 90 degrees angle to the far corner (acute angle) and more than 90 degrees angle to the close corner (obtuse angle). At the same time you could end up in a situation where one side of the screen will be parallel to your line of sight but not the other (this will be an extreme angle no matter what and you will most probably not want to watch TV from this spot anyways).

The change in viewing angle is of course also applicable to the flat version, but here the change is less dramatic when you move away from sweet spot. As the contrast ratio is dependent on the viewing angle for LCD panels you will also experience equivalent problems with contrast, black level, and color reproduction when you move out of the sweet spot.

The figure below illustrates at which positions you would experience the greatest benefit from choosing a curved screen instead of a flat one. Mind you that all the persons watching the screen must fit inside the green area to benefit from the concave front when it comes to viewing angles, black level, and color reproduction. The rule of thumb is not as sharp as the figure below illustrates as the curved panels don’t automatically become the poorer choice exactly at the point where you exit the green area.


It goes without saying that the closer you place your couch to the TV the more people you can fit within the green area. It is also obvious that a greater radius of curvature will allow more people to fit inside the area if you don’t move the couch. So while you will get improved viewing angles when sitting in the green area you will get poorer viewing angles on one half of the TV if you sit anywhere else.

Let’s just throw in a person in the mix to get a feel for how much space someone take up in front of a 55” TV. Let’s also expand the radius of curvature to something that is proportional to LG’s 5 meter example and see where you might sit. The typical viewing distance is around 3-4 meters and at that distance you will struggle to squeeze in more than one person
.


So the answer to the question “Does a curved screen improve viewing angles” is “Yes” as long as you sit in the right spot.

Ironically enough it is actually not all types of LCD panels that benefit from being watches at a perpendicular angle. Samsung's VA panels suffer from black crush when viewed head-on, so with this type of panel you might actually prefer to sit slightly off center anyway.

Less optical distortion and a more uniform focusing distance

OK, now let’s have a look at the argument about less optical distortion and the elimination of shifting focus distances. Here TV manufacturers are on thin ice. Recording equipment used today is based on flat sensors, so they are meant to be viewed on something flat.

The depth perception in a movie scene is hence projected onto something flat and by displaying it on a curved screen you are changing the way the producer of the movie intended it to be seen. We experienced this during e.g. our LG OLED TV review. One of our gripes was that it actually added optical distortion, which was very obvious when looking at the screen at off angles.

So what about focus distance? TV makers argue that your eye will relax more if everything is held in the same plane of focus. While this might be true if you change the focus distance a lot relative to the distance to object you are looking at, you will have a difference of about 6 cm from the edges of a curved 55” TV (5 m radius of curvature) to the flat version. If you sit 3-4 m away you need to call Guinness if your eyes suffer from strain when shifting focus distance by ~2%.

Everything you do during a typical day requires your eyes to change focus distance. They are built to do it. You don’t need to fix it.

Reduced reflections

How about reduced reflections then? When you change the open angle of the TV to anything smaller than 180 degrees you will also change the angle of which it can pick up reflections. So this is already an improvement.

Because of the concave shape of the screen the so-called focal point will lie at half the radius of curvature and for a 5 meter radius this means at 2.5 meter from the front of the screen (for light reflected from directly behind the screen).


What this means to you is that reflections are enlarged if you sit further away than this and effectively this also means that reflections will also get increasingly blurry the further away you sit. This then assists in reducing the hassles from reflections. Because of the enlargement of the reflected objects the light intensity you experience will also decrease the further you sit from the focal point, which further reduces the perception of the reflection.

This phenomenon is also starting to be utilized in the mobile industry where you will see an immediate benefit when you take your cellphone into brightly lit surroundings.

The screen will be perceived larger than it is

Yes, it will. But this is actually just the good old discussion about distance to the screen, because you are now moving the corners closer to you. Let’s just see how much you actually gain.

The figure below shows the screen width you gain depending on your seating position. As you can see this added width perception diminishes as you move further away.


To assess how you actually win in terms of added screen size let’s consider a 55” curved screen. The diagonal is 55” when flat (i.e. 140 cm) and the curved shape will reduce this to a projected width of 121.7 cm, where a flat 55” has a width of 122 cm. Given a radius of curvature of 5 meter this means that the screen spans an arc of 14 degrees when sitting in the sweet spot. 14 degrees angle will give you approximately 3% more perceived width compared to the flat 55” version. Not a lot. How about moving in closer then? At 3 m the angle has increased to 24 degrees from corner to corner, which will now give you an extra 9% of perceived screen width.

To the ones interested in geometry here are the basic formulas you need to calculate things on your own.



The graph below shows how perceived width depends on the angle from corner to corner with you in the middle. As you can see the effect of a curved screen is reduced as you increase the seating distance (i.e. reducing the angle):


Summary of pros and cons

Advantages compared to a flat LCD-screen Disadvantages compared to a flat LCD-screen
Reduced reflections Worsened viewing angles for people not seated correctly
The TV is perceived to be larger than it is Contrast is worsened for people not seated correctly
For monitor use it will be possible to take advantage of sweet spot, which will yield better viewing angles and contrast ratio Wall mount is less elegant (but a matter of taste)
The TV effectively gets “thicker”
More expensive to produce and hence to buy
Cannot be turned off...

Finishing remarks

The year 2014 was the year in which TV makers tried to trick us into believing that our TVs were sick if they didn’t curve towards your favorite spot in the couch. They weren't exactly generous with evidence of this illness, but as we have seen many times before the marketing machines can be wheeled into position no matter how vague an argument might be. Most people probably remember how the TV industry tried to mislead us into believing that “LED TV” was a whole new breed of TVs.

Every year something new fills the spec sheets. Predecessors have had buzzwords such as “3D”, “smart TV”, “dynamic contrast”, “x numbers of Hz” and “enhanced sharpness”. We are now facing something that stands out in one fundamental way. The “curved display” cannot be turned off. Even Philips’ Ambilight - which also to a very high degree affects the physical boundaries of the TV - can be turned off with the touch of a button. A curved display is forever - unless you get your hands on one of the prototypes that will actually straighten.

It can be hard to accept that TV manufacturers will now try to sell us something that is actually a step backwards in terms of picture quality for pretty much everyone else than the one lucky person seated perfectly in front of the TV. Bad viewing angles have previously been something the industry had to fight because of the limitations of LCD technology, but now it seems that they are destroying them on purpose. At the same time some manufacturers have decided not to launch their high-end models as flat versions, which can potentially push the market in a very wrong direction.

We hope that you, the consumer, do not get dazzled by the fancy marketing campaigns and hot air sales pitches, but can see this effort for what it is: Just another attempt to promote the next “3D”. Give us back the flat screens and leave the curved ones at the CES show floors.



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