types that are utilized; the first of which is LCD. LCD stands
for Liquid Crystal Display, and while I'm not going to go into
the complex designs of LCD panel circuits and exactly how
they work, I'll explain the different parts of an LCD display
and exactly what the crystals do.
There are four main layers to an LCD panel: there's the
outer protective layer, the polarizing layer (or layers), the
liquid crystal layer and the backlight. The outer protective
layer is basically there to protect the other components
from getting damaged, and it's usually made of clear plastic
or glass. The polarizing layers help the crystal layer deliver
the correct light, or no light when off or black, to your eyes.
The most important part is the liquid crystal layer, which is
what controls the colors passed through and ultimately the
picture displayed. When an electrical current is passed
through the crystalline layer, liquid crystal cells coupled
with filters of red, blue and green, corresponding to the
subpixels in the display, "twist" to let backlight through at
different intensities. The crystals filter the neutral back light
into certain color intensities, and combined with neighboring
crystals of different colors, the full range of millions of
colors is created.
A basic diagram of a TFT LCD panel | Image: TEAC
The backlighting layer is almost always LED backlight, and
while there are different types of LED backlighting the one
used almost always is white LED backlighting. This is
where thin and solid white light-emitting diodes (LEDs) are
placed behind the liquid crystal layer to provide a base light
for the crystals to modify. RGB LED backlighting also exists
which allows for better color reproduction, but this is more
expensive and seldom used in smartphones (as far as I
know).
Again this is a simplified explanation of how LCD panels
work, for more info I recommend checking here .
LCDs that are used in smartphones are all active matrix ,
which refers to the way the pixels are addressed, and they
are all also used TFT technology. TFT basically means thin-
film transistor and its these components that help with
more accurate color reproduction, contrast and
responsiveness. Underneath the TFT banner there are a two
different types you can get.
- Twisted Nematic (TN) LCD
Twisted Nematic is a term that is rarely used by
smartphone manufacturers, instead preferring to call their
displays simply "TFT LCD". It refers to the method in which
crystal cells are twisted in the display to reproduce the
colors, and is most commonly used in cheaper smartphone
displays due to their ease of production.
Compared to the other type of LCD, In-Plane Switching
(IPS), TN LCD panels have more limited viewing angles,
contrast and color reproduction, hence why they are
generally used in cheaper devices. That said, your computer
monitor or (older) LCD TV is most likely going to be using a
TN panel, so they are not always bad, just there is better
technology out there.
The Sony Xperia S features a standard TN TFT LCD, but it's one
of the better ones
The best type of TN LCD panel available is the Sony/
Samsung-made Super LCD , or S-LCD , which has
considerably better contrast levels and color reproduction
compared to standard TN panels. These types of displays
started appearing in the HTC Desire as a replacement for
AMOLEDs when supply was short, and has since been
superseded by Super LCD 2 displays.
2.In-Plane Switching (IPS) LCD
IPS LCD panels use a more organized method of crystal
cell twisting, which allows for a better quality picture and so
it's the preferred type of display for higher end
smartphones. The main advantages over TN panels is
significantly better viewing angles and truer color
reproduction because the way the panel works reduces off-
angle color shift. Modern generation IPS panels also
feature much better contrast ratios than TN panels, which
makes them (in some instances) comparable with AMOLED
technology.
The Super LCD 2 display on the HTC One X is an IPS-type TFT
LCD, and it looks amazing
Most IPS panels used in smartphones are technically either
Super IPS (S-IPS) or Advanced Super IPS (AS-IPS), and in
some cases proprietary technology that improves on
different aspects of IPS panels. Occasionally smartphone
manufacturers will designate their panels as "IPS LCD" or
"TFT IPS LCD", but in other cases they will use a brand
name such as those listed below.
Retina - The term used for Apple's LG-manufactured IPS
LCD panels with high pixel densities (more on that later),
used since the iPhone 4 and 3rd-gen iPad.
NOVA - LG's marketing term for their IPS LCD panels that
produce a brightness of 700 nits, which is brighter than
many other displays
Super LCD 2 - The second-generation of S-LCD panels made
by Sony that switch from using TN to IPS technology. They
have phenomenal color reproduction, great contrast,
brightness and viewing angles due to reducing the size and
spacing of the component layers, and are arguably the best
displays available.
Pros and cons to LCD panels
As one of the major two display types, it's good to know
what the good and bad things about this type of panel.
Good:
Cheap to produce
IPS panels have accurate color reproduction
Low chance of color tinting or color shift
Can be bright and clearly readable outside
Bad:
Due to the need for a backlight, huge contrast ratios
and solid black levels are hard to achieve
TN panels have bad viewing angles
In some cases they are power consuming and
physically thick
Display type #2: AMOLED
Where LCD panels are made from a variety of different
layers that all work in harmony to produce a picture, with
AMOLED displays it's much simpler. AMOLED stands for
Active-Matrix Organic Light-Emitting Diode, as the name
hints, the display actually emits colors directly from organic
diodes rather than needing polarizing filters, crystals or
backlights. As such, there are a number of benefits over
LCD technology.
The way an AMOLED display works is very simple: there is
a lower transistor layer that controls the power going to the
organic upper layer; when power is applied to the organic
diodes they emit light, the color of which corresponds to the
molecular structure of the diode. The intensity of the light
can be varied by the power sent by the transistors, which in
turn allows millions of colors just like the twisting of liquid
crystals in LCDs.
A diagram of an AMOLED panel | Image: Wikimedia Commons
As the diodes themselves emit light, they don't require any
sort of backlight for the filtering of colors. This helps not
only save power, but it also slims down the display
considerably, which is a bonus for phones that are pushing
to be the slimmest on the market. Furthermore, the lack of
a persistent backlight allows high contrast ratios, because
to display black the organic diodes simply switch off and
show nothing.
Of course there are some downsides to AMOLED displays.
As the usual red, green and blue subpixels are used to
create the full gamut of colors, different organic compounds
must be used to provide each of the three colors. The
properties for each of these compounds varies significantly,
and so it's very hard to get each diode emitting the same
intensity of light at full power with the correct wavelength.
This leads to a number of problems. If one color of diode is
too intense it can tint the display slightly; usually the blue
diodes are the culprit which is why white webpages can
often look somewhat blue. Also, while AMOLEDs are very
vibrant due to the diode intensity, color reproduction is not
as accurate as IPS LCDs, again due to the problems getting
all colors on an even playing field.
The HD Super AMOLED on the Samsung Galaxy Note can look
very vibrant
The final problem is the lifespan of the different diode
types: as each color is a different organic compound, they
will only "live" (or emit light) for so long, and this length
varies for different colors. In early AMOLED displays it was
known that the blue diodes died around twice as fast as the
green diodes, however in recent display types the
technology has evolved to make this less of an issue.
Hopefully the color accuracy issues will also be improved
as the technology evolves.
As with LCD panels there are a number of brand names
associated with implementation of technology by specific
companies:
Super AMOLED - The first-generation Samsung-made panel
that integrates the touchscreen digitizer into the display
while providing better outdoor readability
Super AMOLED Plus - The newer generation of Samsung
AMOLEDs that swaps out the old PenTile matrix to an RGB
matrix (see more below) for improved color reproduction
HD Super AMOLED - Again the "Super" denotes a Samsung
panel with an integrated digitizer, and the lack of "Plus"
means it has a PenTile matrix. The HD simply means it has
a HD resolution with good pixel density
ClearBlack AMOLED - Used by Nokia, this is an AMOLED
panel that uses a "ClearBlack" coated with an anti-glare
polarizer that helps outdoor readability.
Pros and cons to AMOLED panels
As the other of the major two display types, it's good to
know what the good and bad things about this type of
panel.
Good:
Very thin, and (sometimes) flexible
Vibrant colors and high contrast due to organic
diodes
Excellent viewing angles
Low power consumption in some situations
Bad:
Inaccurate color reproduction and mild color tinting
is sometimes present
Shorter lifespan than LCDs
Often PenTile subpixel matrices are used
3.The subpixel matrix confusion
Since the inclusion of the notorious "PenTile" subpixel
matrix in smartphone displays there has been a lot of
media talk over how this particular matrix is worse than the
traditional "RGB stripe". Sure, it's great to say the PenTile
matrix is bad, but I've seen few sites actually go on to
explain why this particular matrix delivers an inferior
experience. That's what this section is about.
As many tech-savvy readers would know, to produce a
picture a display uses a composite of pixels; each pixel
ideally being able to produce every color. However as far
as we know, there is no single material that allows for the
production of every single color, so we cheat and use a
combination of smaller fixed-color subpixels (that are too
small to see) at different intensity levels to deliver color.
This is an RGB stripe subpixel layout. Notice that each green,
blue and red subpixel forms a square, and also note the small
black specs which are the transistors. | Image: Wikimedia
Commons
Almost all computer displays use red, green and blue
colored subpixels, which are added together using the RGB
color model to deliver a huge amount of composite colors.
Each subpixel should be capable of 256 color intensity
levels, where 0 has the subpixel "off", ~128 is the color
half-on and 255 is full intensity. As there are three colored
subpixels all capable of 256 levels, this multiplies together
to give 16,777,216 possible colors per pixel.
As to produce these 16.78 million colors you need one of
each of the three RGB subpixels, the preferred method is to
have all three of these arranged in a square, and this
square becomes a pixel. This is known as the "RGB stripe"
method, and it's pretty much universally used across LCD
monitors as it provides the most accurate color
reproduction and the highest level of clarity.
With AMOLED displays as I mentioned above there are
some issues with the technology that must be overcome
such as the inconsistencies between the different subpixel
intensities and lifespans. There is also another issue:
it's currently much harder to produce a high-density
AMOLED display at a reasonable price because the
technology to create extremely small subpixels isn't there
yet, whereas with LCDs, producing tiny subpixels is much
cheaper and easier.
And so comes in Samsung's trademark PenTile subpixel
matrix. Instead of putting all three RGB subpixels into the
one pixel, the PenTile RGBG matrix pairs a green subpixel
with alternating blue and red subpixels; this means that
there are technically only two subpixels per pixel in a
PenTile RGBG matrix compared to three in a RGB stripe
layout.
This is a PenTile subpixel matrix; note that a single, square
pixel has a green subpixel but alternating red/blue subpixels. |
Image: Matthew Rollings
Due to the optics of the human eye and its different
sensitivities to different wavelengths of light, a PenTile
matrix display is still capable of delivering effectively the
same colors as the traditional RGB stripe using special
subpixel rendering. As it uses fewer subpixels per pixel, this
also allows the display to be more dense than if it were
created using the RGB stripe method, and in some
situations it uses less power. Finally, due to there being
fewer blue subpixels, the display should last longer than a
traditional layout AMOLED using the same organic blue-
light-emitting diode.
Of course people who complain about PenTile matrices do
have a point. The fact that there is only two subpixels per
pixel technically reduces the subpixel resolution of the
display: for example a 1280x720 display using the RGB
stripe layout has 2.76 million subpixels whereas a 720p
PenTile display has just 1.84 million subpixels; 0.92 million
fewer. Most of the time subpixel rendering compensates for
this, but in certain situations the difference is noticeable.
On hard edges, such as crisp text or the edge of an
interface element, the PenTile matrix sometimes has to
"borrow" subpixels from other pixels to form a picture that
is the correct color. This is most noticeable when looking at
the left edge of a white icon or text, where there appears to
be small red dots along the edge, or along high-contrast
lines, where the line either appears not crisp or - in the case
of blue/red lines compared to green lines - dotty.
Generally speaking you have to get reasonably close to the
display to notice these imperfections, but then again
comparing a PenTile display to an RGB stripe display, the
text rendering on the latter is noticeably clearer at a
comfortable reading distance. The good news though is
that PenTile displays are often nowadays only used on
devices with a PPI density (more on that later) of 250 or
above, and as you approach 300 PPI it becomes
increasingly hard to notice the problems.
On devices like the Samsung Galaxy Note and Galaxy
Nexus, which use PenTile HD Super AMOLED displays but
have high pixel densities, the PenTile problem is virtually a
non-issue. It would obviously be nicer to have a high-
density RGB stripe AMOLED, and even Samsung
acknowledges their Super AMOLED Plus displays are better,
so in the future we'll probably see technology and
components improve so they can kill off the dreaded
PenTile matrix.
The importance of pixel density
It all started with Apple's "Retina" display: a 3.5-inch IPS
LCD panel touting a 640 x 960 resolution. At this size and
resolution, the display had a pixel-per-inch (ppi) count of
326, a number seldom seen in other displays at the time
and well over the magical 300 ppi rating. So, what is pixels-
per-inch, and what does the magical 300 ppi mean?
Pixels-per-inch is a count of how many pixels in one
dimension fit along a one inch line; that is, if you put a ruler
on the screen it's how many pixels could you count along
the edge of the ruler before it reaches one inch. Due to the
fact that pixels are square, it doesn't matter whether you
count vertically or horizontally to get this number, and
thanks to the handy formula on the Wikipedia page for pixel
density, you can work out the pixels-per-inch for any
display without having to do this counting for yourself.
For a display to be good, ideally you should not be able to
make out individual pixels at a reasonable distance from
your eyes, leaving images and text to be presented at the
highest quality and crispness. As with the print rating of 300
dpi (dots-per-inch), 300 ppi is an ideal level to achieve
because at 30cm (12in) away from your eyes, the average
person will not be able to see individual pixels.
A 4.3-inch 720p display has a density of 342 ppi. Even
magnified, individual pixels are hard to determine.
At standard resolutions such as 1280 x 720 (720p HD), 960
x 540 (qHD) and 800 x 480 (WVGA), there is a limit on the
diagonal size of the display that keeps the pixel density at
or above 300 ppi. For 720p, displays can go up to 4.9" while
still managing 300 ppi, giving a huge amount of flexibility
and pretty much exceeding the comfort limits of display
sizes. qHD maxes out at 3.65", and WVGA at 3.1", which
are good limits for the smaller end of the spectrum.
When it comes to tablets achieving 300 ppi, it is less of an
issue because you will be holding the device (in most
cases) further away from your eyes, and so manufacturers
should be looking for densities of 250 ppi or above. This
does mean that 10.1 inch tablets will need to exceed 1920 x
1200 (WUXGA) as that only gives 224 ppi; however 2560 x
1600 (WQXGA) would deliver a nice 299 ppi at 10.1 inches
and remains above 250 ppi right up to 12 inches. For
tablets up to 8.9 inches, WUXGA will suffice.
As display technologies improve, especially in the AMOLED
front, it should be possible to deliver high pixel densities in
all situations. Most upcoming high-end smartphones are
utilizing a high-density display, as with some mid-range
devices, but it's still definitely something to look out for in
new tablets.
Adding touch to the mix
The final part of the whole display module in a smartphone
is the all important touchscreen, otherwise and more
correctly known as the touch digitizer layer. Luckily pretty
much all smartphones these days (except for the really
cheap and terrible ones) use capacitive touch sensors as
opposed to the resistive touch sensors used in older
devices; as such I'm not going to bother explaining resistive
touchscreens.
The capacitive sensing digitizer layer most often uses
projected capacitive touch (PCT) technology, which sees the
materials used in the detection etched into the layer as a
grid. This grid projects an electrostatic field when a voltage
is applied, and when a human finger (which is electrically
conductive) touches the area covered by this grid, the
electrostatic field is altered. A controller then determines
the position of the finger based on sensors and other
components.
As only conductive materials can alter the electrostatic field,
this is why things such as human skin work on capacitive
touchscreens but cloth and plastic do not. However,
depending on the strength of the field and sensors, and the
fact that the field is slightly three-dimensional, it is possible
to sometimes activate the touchscreen without actually
touching the glass, or through thin cloth such as gloves.
A diagram showing roughly how a capacitive touchscreen works
| Image: Telecom Circle
The main component that delivers the electrostatic field
(usually indium tin oxide) is transparent, which is why in
most touchscreens it is not possible to see the capacitive
electrode grid in the digitizer layer. Although, occasionally
you will be able to see small dots across the face of the
display when placed at a specific angle under direct light:
these are small capacitors that are at the intersections of
the grid which allow for mutual capacitance, which in turn
provides multi-touch.
With LCD displays the touch digitizer layer is placed above
the liquid crystal layer but below the final glass protecting
layer, which allows you to infrequently see some of the
components as mentioned above. With some AMOLED
displays, specifically Super AMOLEDs by Samsung, the
digitizer is actually integrated into the same layer as the
organic light-emitting diodes, making it essentially invisible
while consuming less space - one of the advantages of
AMOLED technology.
Often the protective glass (such as Gorilla Glass), digitizer
and display itself are all attached tightly together in the one
panel to reduce the chance of glare and reflections while
saving space. Due to this, it is near impossible to replace
just one of the components if, say, the glass was broken or
the digitizer stopped working. Instead, you would need to
shell out more cash to replace the entire glass-digitizer-
display unit, and often they are not cheap.
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