OS X :: Screen Looses Brightness On Desktop Computer

I have a Mac OS X, Version 10.5.7, 2.4GHz Intel Core 2 Duo, 2 GB 667 MHz DDR2 SDRAM, iMac desktop computer. I have looked high and low for a way to prevent my screen from dimming after about 2 minutes and have found nothing to help. The screen doesn't go completely dark, it just loses brightness. Here's what happens: I load a guitar chords page and begin to practice the song on the computer screen. After about two minutes the screen goes dim, which is very annoying and is difficult for me to see.

Every proposed solution I've come across refers me to the Energy Saver in the System Preferences. The problem is that the Energy Saver in the System Preferences for my desktop computer does not have the same selections as an Energy Saver in the System Preferences for a laptop. As is seen in the picture below, I don't have selections for dimming preferences that a laptop has. I don't want to install some third party program (like Caffeine) if I can avoid it. How I can disable this dimming feature on my computer?