We’ve
seen that the majority of displays on the consumer electronics market are active and looked at two kinds of
display technology: TN and IPS LCD panels. We’ve also said that these LCD panels
are turned in to active displays by
way of a backlight. Finally, we’ve briefly
touched on passive displays that
don’t require a backlight to be visible. Let’s look at backlights first, and
then at passive displays.
Backlight
types. The kind of backlight used affects a number of properties of the overall
display. For example, a brighter backlight will often mean more accurate (or
more saturated) colour reproduction, but a more responsive backlight will often
make for faster on/off times, and less eye strain for the user.
1. CCFL (Cold Cathode Fluorescent Lamp) bulbs have
typically been the mainstay of display manufacturers. Sitting behind the panel,
these bulbs are not overly bright, but can be manufactured cheaply, and take up
little space. However, they require fairly large amounts of power, which
reduces the battery life in most devices that use them significantly. A benefit
of CCFL bulbs is that they can turn on and off quickly. Why is that useful?
Because displays typically dim via something called Pulse Width Modulation (PWM). This means that electricity is fed to
the display in digital ‘pulses’, which are either on or off. Dimming your
display would lower the current provided in each pulse. Because CCFL bulbs can
turn on and off relatively quickly, they do not ‘flicker’ if the brightness of
the backlight is turned low. That means that the screen has a higher effective
‘refresh rate’ - it turns on and off very fast, far too fast for the eye to see
or the brain to detect - and so it appears fully lit to your eyes, at all times.
2. LEDs (Light Emitting Diodes) are the
other technology used in many modern displays. Most modern tablet PCs
use LED-backlit displays. Why? Because LEDs draw very little power and provide
a great deal of light. In other words, they’re very efficient. That makes for
longer battery lives on mobile products. So what’s the downside of LEDs? They
take longer than CCFL bulbs to turn on and off. This means that because the
brightness of LED backlights is usually also controlled by PWM, lower levels of
brightness can lead to a noticeable ‘flicker’ of LED displays. Even if it’s not
perceptible to the eye that has a maximum refresh rate between 30 and 60Hz, it
can cause eye strain, and severe headaches in some users. On the other hand, a
brighter backlight makes for a better-lit and more colourful display.
Passive displays.
Most famous among passive displays is
‘e-Ink’ manufactured and originally marketed by Philips Components. As passive displays have no backlight, they don’t have an effective
‘refresh rate’. This means they are typically more pleasant to read and have
higher effective contrasts (as of 2012, similar to that of a regular printed
newspaper).
The
e-Ink display panel is embedded with millions of tiny microcapsules. These capsules are filled with two kinds of substance:
one coloured black and positively charged, and one coloured white and
negatively charged. Depending on the charge on the base of the microcapsule, a
different substance will float to the top to be viewed by the reader. It uses electrophoresis, which you can read more
about in most university-level physics textbooks.
Because
of this, the e-Ink display panel uses very little electricity. Once a
microcapsule is charged, it will retain its charge, on current e-Ink devices, for
up to several months at a time. This means that an image can be displayed at
next to no electric loss to the device, rendering battery life extremely long.
Devices such as the Amazon Kindle use e-Ink technology, as it is highly suitable
for long sessions of reading.
Because
electrophoretic displays such as
e-Ink have very long response times, they are not yet suitable for watching
moving images or films. A viable
commercial solution for e-Ink in colour has also yet to be developed. However,
due to the specific construction of microcapsules and their lack of backlight, flexible
e-Ink displays can quite easily be made. The first example of this went on sale
in Europe in April 2012.
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So,
there we have it. You now know have
an expert knowledge of current display technologies, can tell your LCD from
your LED, and advise friends and family on which kind of display they’ll need
in their latest tablet. Any comments? Feel free to drop them in the
section below.
AUTHOR: Louise Miller
I was born in Berlin, Germany. I studied in London and wrote my thesis on social media marketing. Now I works for a London-based PR company and I also loves blogging about technology and social media in My spare time.
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