Why does my screen have vertical stripes on it?

Why does my screen have vertical stripes on it?

Slowly (over the past several years), the screen on my TI-83 has developed more and more vertical stripes in the background, making it more and more difficult to see the numbers on the screen. How can I fix this? I realize that a new calculator doesn't cost very much, but this one has sentimental value, and I'd really like to repair it, if possible. Thank you.