LCD Types
The State-of-the-Art in Display Technology
(September 2001 issue)
Transmissive LCDs In transmissive construction, all of the light seen by
the user comes through the LCD from the backlight. Most LCDs used in portable
computers today are transmissive. A transmissive LCD looks good indoors and is
typically completely black (unreadable) in direct sunlight. The reason is that
sunlight is up to 1,000 times brighter than LCD's backlight, so the reflection of
sunlight from the surface of the LCD overwhelms any light coming through the LCD.
Portable computer manufacturers such as Amrel, Intermec, Itronix, Microslate,
Panasonic, Xplore and others all claim to have "daylight-readable transmissive
LCDs" in their laptop or tablet products. Given the above, how do they do it? One
of two basic ways: (a) increase the brightness of the backlight, or (b) modify
the LCD to reduce the amount of ambient light that's reflected. See the sidebar
on "Making Transmissive TFTs Work Outdoors" for more details.
Transflective LCDs Transflective construction starts with a transmissive
LCD and adds a partially reflective mirror layer between the LCD and the
backlight. Depending on the LCD manufacturer, the mirror layer can be either a
half-silvered mirror or a full mirror with tiny holes punched in it. When a
transflective LCD is used indoors, it is illuminated by the backlight just like a
transmissive LCD, except that the mirror layer blocks some of the light. When a
transflective LCD is used outdoors, ambient light reflects off the mirror layer
and illuminates the LCD. Note that outdoors, light has to go through the LCD
twice, once on the way in and once on the way out. This tends to makes the
outdoor performance of a transflective LCD not as good as the indoor performance,
where the light only has to go through the LCD once.
The balance between outdoor and indoor performance can be adjusted during
manufacture of the LCD by varying the reflectivity of the mirror layer. For
example, the mirror layer might be set to reflect 60% and transmit 40% of the
light. This means that if the normal indoor brightness of the LCD is 130 nits,
only 52 nits (40% of 130) is transmitted from the backlight through the mirror
layer and the LCD. Outdoors, 60% of the light hitting the mirror layer is
reflected, but since the light has to go through the LCD twice, the user actually
sees less.
A transflective LCD is therefore by definition a compromise. It can never be as
bright as a transmissive LCD indoors, and it can never be as bright as a
reflective LCD outdoors. It's particularly poor at the "crossover point", where
there's enough light outdoors to overpower the backlight, but not enough to fully
illuminate the LCD by reflected light. Whether the compromise is acceptable or
not depends on how badly the user wants or needs to be able to use the LCD both
indoors and outdoors.
Reflective LCDs A reflective LCD always has a fully reflective mirror
layer. All light used to view the LCD, whether it's ambient light or from a
frontlight, goes through the LCD, bounces off the mirror layer and goes through
the LCD again. (See the sidebar on "Light Guides" for an explanation of how
frontlights work.) Since the light still has to go through the LCD (and the
frontlight) twice, even though the mirror reflectivity is 100%, the outdoor
performance of a reflective TFT with frontlight is generally slightly worse than
a transflective TFT (assuming that the same technology is used in both). Indoors,
since a frontlight is not as efficient as a backlight, and the light still has to
go through the LCD twice, the performance of a reflective TFT with frontlight is
again slightly worse than a transflective TFT (once more assuming that the same
technology is used in both). -
Based in Silicon Valley, Geoff Walker is a consultant with Walker Mobile. Geoff
has worked on the engineering and marketing of pen computers since 1990 at GRiD
Systems, Fujitsu Personal Systems (now Fujitsu PC) and Handspring. He can be contacted at geoff.walker@att.net.
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