As phosphors age from use, they require more energy to glow as bright as when new. Phosphor burn is actually uneven phosphor wear.
When plasmas are used in high ambient lighting environments, they must have their contrast and brightness controls turned up. This demands more light output from the phosphors in the color cells. As the phosphors are driven harder, they age more quickly and gradually require more energy to produce a given amount of light.
When 4:3 images are displayed, the black bar areas one either side are not producing light, therefore, aging more slowly than the center of the screen producing the program image. If mostly 4:3 programs are displayed on the TV, the phosphors will behave differently across the screen when wide screen programs are being displayed. The black bar areas will look brighter. Those phosphors have not been driven as hard, or aged as much, and require less energy to appear as bright. The center area phosphors have been aged more, require more energy to appear as bright, and look dimmer compared to the side bar regions during wide aspect ratio programs.
New plasmas should be "seasoned" for several hundred hours with full screen images only. It is reported that this helps the phosphors resist uneven wear once the seasoning has been accomplished. Phosphor burn (uneven phosphor aging) occurs more easily when the TV is new. As Michael said, this characteristic is not as bad as in the past, but still a factor to be kept in mind throughout the life of the plasma TV.
Best regards and beautiful pictures,
G. Alan Brown, President
CinemaQuest, Inc.
A Lion AV Consultants Affiliate
"Advancing the art and science of electronic imaging"