iOS forums are full (see for example THIS, THIS etc.) of complaints of (mostly) bluish (or, in some cases, yellowish) screens. In addition, several articles have been published on the adverse effects of blue(ish) lights – see for example THIS and THIS (two very important and interesting articles!)
Note that this article doesn't discuss “just” darkening the screen as much as possible. See today's update in my constantly-updated Cydia Compatibility List HERE for more info on Dimmer (for jailbroken devices only).
1. If you are not jailbroken,
you have two choices:
1, under Settings > General > Accessibility, by toggling the White and Black switch, you can invert the colors on the screen. While this in no way tries to filter out unwanted wavelengths of light, it may still be better for the eyes.
2, You'll need to wait for other devs' adding support to their individual apps. I've, so far, contacted the developers of iCab Mobile (IMHO the best third-party Web browser) and GoodReader (the best all-in-one document reader). Hope they add some kind of a warm-color, switchable overlay.
2. If you are jailbroken,
as usual, you have far more (and far better) choices than with the non-jailbroken case. Your best bet will be the free(!!!) F.lux.
You may already know F.lux for all the three most important desktop operating systems (Mac OS X, Windows, Linux) - not to be mistaken for the simpler, but still great RedScreen. (If you don't, it's high time you check it (them) out: it's (they're) excellent. Both are free.) Well, the iOS version is almost the same – and it's, again, free!
The homepage of the iOS version is HERE. Note that the jailbreaking advice there is pretty much outdated. (With iOS 5.1.1, all you need to get and run is Absinthe 2.0.)
(Cydia main screen. As usual, click the thumbnail for the original, high-quality iPad 3 screenshot!)
Also, the official homepage of the iOS version contains screenshots of an older version, in which it was still not possible to set a color temperature to be used during the day. It's one of the two annotated (by an oval) settings in the following screenshot:
I've made several comparative still shots to demonstrate the tweak in action and to show the effects of all the color temperature settings, comparing a jailbroken iPad 2 and iPad 3 to a non-jailbroken iPad 1 and 3 with both a color chart (source) and the Settings screen. First, let's start with the latter.
First, a shot with “Enabled” (see the switch annotated with a rectangle in the previous screenshot) set to Off on both JB'n devices; that is, in disabled state. In the top left, the jailbroken iPad 3, in the bottom right, the also-JB'n iPad 2; in the top right, the non-JB'n iPad 3 and, finally, in the bottom left the non-JB'n iPad 1. As you can see, the default color temperature of the four iPads are pretty similar, with the iPad 1 being a little bit warmer that the rest:
By the way, I've maxxed out the brightness level (by disabling auto-brightness) in all these shots. As you can see, they're about equally bright at this level. Interestingly, the iPad 3 consumes about 2 Ampers in when the backlight is maxxed out, while the iPad 2 “only” around 900 mA. This is mostly because of the stronger filters used with the iPad 3 screens as is also explained HERE (a highly recommended article, to which I'll later refer again!) BTW, speaking of external articles, the one HERE also emphasizes the backlight of the new display uses a lot more power than that of the iPad 2 to make the color gamut / reproduction as good as possible.)
Now, let's start with the “Fluorescent” and the “Halogen” settings, which offer an increasingly (in this order) warmer color temperature. The iPad 3 (again, the one at the top left) is set to “Fluorescent” and the iPad 2 (bottom right) to “Halogen”:
As you can see, the “Fluorescent” setting is definitely cooler than “Halogen”.
Now, let's go further and examine & compare the two even warmer settings, “Incandescent” (iPad 3) and “Candle” (iPad 2), the latter being the warmest possible setting:
They're much more yellowish / reddish than the previous two presets, let alone the default setting.
Now, some other comparisons:
Both iPad 2 and 3 set to “Candle” (the warmest setting):
iPad 3 is Candle, iPad 2 Halogen:
2.1 Color chart tests
Now, let's move to discussing color validity / trueness with the color chart: after all, these tweaks do have an effect on the quality of rendered colors. (You won't want to do any color retouching, where color rendering quality is of extreme importance, while the tweak is active. Fortunately, as has already been stated, you can quickly disable the tweak in Settings.)
Note that reds and browns are far deeper (see for example the topmost cells in the third, fifth and seventh colum) and the blues / greens definitely less pinkish on the two iPad 3's screen than on the iPad 1 / 2. Yes, both articles I've previously linked to were right when stating the same.
Now, go to the warmest mode, Candle, enabled on both the JB'n iPad 3 (top left) and 2 (bottom right). I present you with two different shots:
As you can see, the color quality does suffer. There's almost no blue: most of them are rendered as greenish ones (which also means your eyes get as little high-frequency light as possible), a lot of originally yellow / green cells have become (light) brown and are indistinguishable from previously light blue / pink cells.
This also means that, basically, you need to imagine the tweak as “just” a yellowish / brownish filter on top of the screen. With truly warmer backlight, there wouldn't have been so radical color changes (blues becoming greens, for example) – then, blue cells (that is, ones that have the highest light frequencies) would have become far more darker but wouldn't have changed their hue.
2.2 Battery / CPU issues
I've thoroughly benchmarked the tweak's possible adverse effects on the iPad's performance. There's no additional CPU or battery usage if you do use the tweak.
I've tested the tweak on several different iOS 5.1.1 devices (iPad 2, 3, iPod touch 4). Everything worked just fine.
2.4 All in all,
this app is a godsend, particularly for free. If you have jailbroken your iDevice, it's one of the must-have's.