A reader pointed out the questionably power-saving Blackle to me a couple of days ago; now it’s hit Slashdot.
In brief: Yes, a CRT monitor uses more power when it’s displaying light colours than when it’s displaying dark ones. But no, there isn’t likely to be any significant difference if your computer has an LCD screen.
A bit of searching (with evil white-screened Google…) turned up this page on the blog of one Mark Ontkush, who started the whole thing.
A reader of the first-mentioned post reports he actually tested a 19 inch CRT and LCD, and found, as you’d expect, that the LCD used less power in the first place but changed its power consumption not at all with the content of the image, while the CRT dropped from 83 watts to 60.
It’s conceivable that an LCD might consume slightly different amounts of power when displaying different images, since it takes power to turn on all of those zillions of thin film transistors, three per pixel, that make up the image.
It might even use less power for darker images. I used to be under the impression that when an LCD pixel was fully “on” it was black, but that actually only applies to the old twisted nematic kind of LCD. Most LCD screens these days use newer flavours of LCD technology in which a fully energised pixel is white.
Ontkush’s site also has this page, in in which a low-brightness design palette is put forward for much the same reason, and in which a (different) reader finds that the darker colours do indeed seem to reduce an LCD’s power draw… by a lousy three watts.
There are some cleverer flat panels these days that modulate their backlight brightness according to the image being displayed. I don’t know whether any computer monitors do it (I think it’s still a home theatre thing), but it makes sense, and would cause darker images to consume less power.
Most LCDs these days have such super-bright backlights that to use them in most indoor domestic situations you should turn the brightness down to minimum manually anyway. Variable-brightness backlights probably won’t be able to go any dimmer than that minimum manual brightness. LED-backlit LCDs (which are still very rare) may be able to go further, though; the minimum brightness at which a cold-cathode lamp will continue to work properly is quite a bit higher than the minimum brightness for a bank of LEDs.
The reader who brought Blackle to my attention had it brought to his attention by that internationally recognised bringer of totally reliable information, a multiply-forwarded e-mail. That particular e-mail, or one very like it, can be found all over the Web now, including the allegation that “Google created a black version of its search engine…”.
Blackle is not, of course, actually a Google project. The domain’s registered to some outfit here in Australia.
I’ve never been able to figure out how it is that these sorts of aimless fabrications get tacked onto much-forwarded messages. Somebody somewhere along the line had to make up the “Google created” factoid all by himself and add it to the text… but why?
Snopes is full of stuff like this. Sometimes it’s obviously someone just making up a story to go with a funny picture because it entertains them to start a hoax or they want to reverse the political slant of a forward they just received, but just as often there’s not even that much justification.
It’s like a model of evolution. Messages mutate randomly as they pass through different people’s hands, and the most appealing ones are then more likely to be forwarded-to-all by everyone’s dimwitted coworkers and dotty old relatives.
(Thinking you’re doing something For The Environment by darkening the palette on a Web page is also, of course, a pretty good example of slacktivism.)
UPDATE: I’ve got a power consumption meter now, and I’ve used it to see how much juice my giant Dell monitor consumes. The screen brightness setting made a big difference; what the screen was displaying made a small one. Read all about it!