NEC 500 Guida Di Riferimento

 

 
 
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.