Answer
Apple’s power management system was designed for portables, and as such turns a lot of things on and off when not in use. The processor, for instance, cycles between 1GHz and 2GHz on my MacBook Pro, depending on what power source I’m using and what I’m doing. Of course, there’s the display that changes as well, because the backlight is a power hog.
So these same energy conservation bits were brought to the desktop as well so that the desktop would use less power (always a good thing) but sometimes you’re reading a long diatribe and can’t read it fast enough for the computer and the display goes into that state known as “half-dim” to conserve power.
To be entirely honest, it’s not just power it’s conserving, but life of the backlight as well. There are claims of studies that have shown that a 50% reduction in brightness can extend the life of the backlight up to three times. Hence the “half-dim” setting in the system to bring an idle display down to half brightness when not in use.
However, there are times this is unwanted. For instance, there’s the DVD preset that turns this off so you don’t lose brightness while watching a movie. Makes sense. So if you have a reason, a good one, to not want this feature enabled then you can use pmset to turn it off.
sudo pmset -c halfdim 0
Read the man page for pmset for more details, but that line says “when on AC power, don’t half-dim the display.” I’d turn it back on when you’re done with whatever needed it, myself.
I’m being geeky. There’s a setting in Energy Saver for this as well. Pick your power source from the “Settings For:” menu and then click on Options. The item called “Automatically reduce the brightness of the display before display sleep” is the halfdim setting. But that’s no fun.
While I’m talking about cold-cathode fluorescent lamp (CCFL) life (your backlight) I’ll bring up something else that’s rather important: it’s better off than on. Like any bulb, the miniature fluorescent lamps in the back of your display have a specific lifetime to them, one that’s reduced by many factors including: time used, brightness used, vibration, power cleanliness, and power ons. If you’re not going to be at your computer sometimes, it’s best to leave the Energy Saver settings such that the displays turn off when you’re gone, rather than going half-dim and displaying a screen saver.
When you’re at the display, use it. When you’re not, let it turn the backlight off or down. Five years down the road, when it’s still working and still bright, you’ll be happy you did.
In 10.4, at least, in the Energy Save preferences pane, under the Options tab, there’s a checkbox labelled “Automatically reduce the brightness of the display before display sleep”. Turning this off has the same effect — stopping the automatic half-dim.
Edit: Whoops — I could have sworn you hadn’t mentioned it already in the article.
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