The Afterburner used a "light guide" to illuminate the screen.
Here is a web site with some before and after pictures.
http://www.goldenshop.com.hk/AI-trad/gba/backlit.htm
And here is another web site that sells light guides for other handheld systems:
http://www.mash-mods.com/products/Fr.../frontlightkit
Nintendo did a much better job with "light guide" technology on the GBA SP.
However, "light guides" can wash out the screen and can also cause a trapezoid effect across the screen.