Dark and light areas on TVs. What causes this?

[evil_ash_xero]

I have been buying some used Trinitrons, and they all have light areas. Usually in the center, and around the sides it's a little darker. You can see it when the TV is off.
On the CRTs, I was able to get new, they do not have this at all. The big 36 Trinitron is black as night, when turned off. Then I have a Sharp that is lighter. But they're not patchy.
I've had a Toshiba CRT for about 9 years now, and it also has the big lighter circle in the middle, with the edges being darker.

What exactly is this?
When I look up "burn in", I see images fried to the screen. So, I'm not sure what this is. I see it all the time though.

It tends to mess with 8 bit games' images the most. Since they are usually solid colors.

[cools]

That's burn from normal use, rather than burn from static images.

[evil_ash_xero]

Gotcha. Thanks.