NEC AccuSync 500 Reference Manual

Defective Pixels in
Liquid Crystal Displays
Introduction
LCD technology has taken off in the past several years, with expanded availability of increasingly
cost-effective product alternatives to the traditional CRT display. LCD manufacturing yields have
improved, lowering costs. Technology innovations have enabled the production of ever-larger
screen sizes, and today's LCD monitors are brighter than comparable CRT monitors, with viewing
angles that rival the CRTs they are being groomed to replace. With their slim profile and small
footprint, LCDs require about 60% of the desk space and consume approximately one-third the
power of a comparably sized CRT. However, while LCDs offer many advantages over CRTs, and
the price gap is continually narrowing, they are still more expensive to produce than their cathode-
ray tube cousins, due to the required clean room manufacturing process and lower-than-CRT
yields. A concern for some manufacturers and consumers regarding LCDs has been the issue of
dead pixels.
Detecting & defining Dead Pixels
Visible pixel malfunction is occasionally noted in both types of displays, however, the higher cost of
LCDs is one reason this issue seems to be more prominent with LCDs than it has been with CRTs.
Pixel outage is difficult to assess during the manufacturing process of both CRTs and LCDs. Only
upon completed assembly can an individual display be assessed for defective pixels. The more
units classified as defective due to pixel malfunction, the lower the overall yield. This results in
scrapped materials and, therefore, higher production costs. With higher volume production and
lower material costs for CRT displays, this decrease in yield has a minimal effect. Because of the
lower production runs and higher material costs associated with LCDs, however, every attempt is
made to reduce the number of scrapped displays.
Active matrix TFT LCD panels achieve their beautiful images, in part, because of the individual
transistor placed at each pixel, which controls the backlight shining through a given pixel (see
Figure 1). In actuality, each pixel or dot is made up of 3 sub-pixels (one red, one green and one
blue) with each having its own transistor. Occasionally, these individual transistors will short, or
remain open, resulting in a defective pixel. There are two phenomenon which define a defective
LCD pixel: A "lit" pixel, which appears as one of several randomly placed red, blue and/or green
pixel elements on an all-black background; or a "missing" or "dead" pixel, which appears as a black
dot on all-white backgrounds. (By comparison, CRT defective pixels exhibit themselves as black
holes in an all-white raster. This is due to missing phosphor material or an obstruction in the
shadow mask.)
The "lit" pixel phenomenon, more common than "missing/dead" pixels, results when a transistor
occasionally shorts on and results in a permanently "turned-on" (red, green or blue) sub-pixel.
There are some possible corrective measures, such as "killing" a transistor using a laser, however,
this just creates black dots which would appear on a white background. Fixing the transistor itself is
not possible after assembly. Additionally, it is not possible to turn a "lit" pixel off, except for the
aforementioned laser method, which essentially just makes the transistor inoperative, thus resulting
in a black dot.