I encourage you to try Primal's suggestion first, but if it doesn't help and you want to check if it's auto-brightness adjust, you'll need to determine your laptop manufacturer's instructions for disabling auto-brightness adjustment. As a for instance, here's some instructions for a VAIO computer:
http://forum.notebookreview.com/sony/716567-guide-how-disable-auto-brightness-auto-contrast.html
...with a mix of some general and Sony/VAIO specific information. However, you'll need to research your particular machine for specifics.
Note that if it *is* this feature, it would affect all applications, not just Legendary Heroes.