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Special Considerations for LCD Panels
Manufacturing of LCD panels is very delicate with large panel elements being manufactured in
one unit and then cut to smaller sizes to be used in different products. In addition a panel consists
of several layers of components (back-light lamps, diffusers, electronics and color filters). There
are different technologies used with each having its advantages and disadvantages resulting in
end user products with different specifications even when taken from the same manufacturing
batch. Below are explanations to some observations that can be made and which are not
considered defects since they are merely problems inherent in the technology of LCD monitors.
Over time, the performance of the panel will also change.
Native resolution
All panels have a fixed number of pixels in
both horizontal and vertical directions. For
instance is a 19” panel normally built with
1280 pixels horizontal and 1024 vertical giving
a native resolution of 1280x1024. At this
resolution you will get the sharpest picture and
it is therefore recommended that you use the
native resolution. Thru scaling technologies it is
possible to use other resolutions but the picture
will be less sharp. Some text will appear to
have shadows.
Native color temperature
Most commonly LCD panels are manufactured
to have a color temperature of 6500K (Kelvin
degrees) for full white picture. The color
temperature can vary with shade of grey being
showed. For instance it could be 7500K at
50% grey and more than 9000 at 90% grey.
For an individual panel the color temperature
at full white can also vary by +/- 15%. Inside
the panel there is a color filter and over time
this will age and become more yellow in its
color tone. Therefore, over time, the color
temperature will gradually go lower so that at
full white it could come down to 5000K.
Typical values
In product specifications there are values for
brightness, contrast, view angles etc. The
values are given as “typical values” meaning
that actual value for any given product can vary
by up to 20% from this value. For instance,
a product specified to give 300 candela as
typical maximum value for brightness may
for individual samples vary from 240 candela
to 360 candela. The values specified are for
a new product. Due to wear of the backlight
lamps the values will change over time and
gradually become lower.
Uniformity and Mura patterns
Depending on the placement of the backlight
lamps, how many they are and the size of
the panel the brightness over the entire panel
will vary by up to 20%. Generally the highest
brightness is in the centre of the panel and
becoming gradually lower towards the outer
edges. This does not follow a linear curve
and there might also be “areas” on the screen
where there is a noticeable difference in the
uniformity. Such clouded areas are referred
to as “Mura” and are more related to the
panel itself than to the backlight. These Mura
patterns are different in size and shape and are
color and grayscale dependent since they are
a result from deterioration of the liquid crystal
alignment layer. Mura is most commonly
caused by long term operation under high
ambient temperature and is a phenomenon
that cannot be repaired.
Non-performing pixels
Each pixel on the panel actually consists of 3
sub-pixels (one for each of Red, Green and
Blue). A 19” panel with 1280x1024 therefore
has almost 4 million sub-pixels. It can happen
that a pixel can get stuck in ON status (bright
pixel defect) or in OFF status (dead pixel
defect) or in an in-between status (low bright
pixel defect). Usually such defects only affect
a sub-pixel and not an entire pixel. The defect
can therefore only be seen at certain color
settings. The ISO 13406-2 standard specifies
how many pixel defects that are acceptable
before an entire panel will be considered faulty.
