Short answer: It is a feature.
This is the effect of either Intel's "Display Power Saving Technology" or AMD's "Vari-Bright" feature, the intent is to reduce power usage by reducing brightness when you look at dark images (which ostensibly would not need to be back lit so much). This technology detects whether the screen is displaying a mostly dark or mostly light image, and adjusts the back light accordingly. Unfortunately this can be annoying if important parts of the UI are light colors (like Chrome), or when dealing with highly visual applications (Photoshop, Media, etc) where changes to brightness have a negative effect on visual fidelity.
So yes, this is a feature, unfortunately this means either dealing with the effect, or forfeiting the power savings it produces.
If you dislike this feature, it can be disabled (or adjusted) it in the settings for your graphics card.
Intel:
The exact layout of Intel's options may vary from system to system, but on my machine:
Intel system tray icon > Graphics Properties > Power > On Battery > Display Power Saving Technology
It shouldn't be too hard to find, even if it isn't in the same place on your machine. You may need to look for the properties in control panel.
AMD:
Open the Catalyst Control Center (search for "CCC" in start, choose CCC - Advanced) > Graphics > Power Play > Enable Vari-Bright [Check-box] [Source]
Hello, almost 4 years later bumping into this because I experience the same problem with a Toshiba L50-C running Intel HD 5500 and nVidia 930M. No matter what drivers I installed the issue remained! Have you found a solution to this? Did you try the registry solution proposed right below? – laertis – 2017-02-16T09:46:45.577
2What brand of laptop do you have? I know the Dell I'm using also has an adaptive brightness, regardless of the fact it does not have a light sensor. It just changes brightness of the screen depending on how much white/black is shown at that moment. Whenever I switch from a very dark to a very bright website I notice and increase/decrease in screen brightness. I'm running Windows 7 by the way. – Matthias – 2013-01-23T07:24:39.473
That is the same with me. Is this a feature? – agz – 2013-01-24T20:27:45.707
I believe it's a feature on some screen models, yes. – Matthias – 2013-01-25T14:25:49.503
1This article lists the 3 types of adaptive brightness in Windows 8 & 8.1. You should disable them one-by-one (and reboot) to totally rule-out adaptive brightness. – harrymc – 2013-11-29T19:44:52.287
You might find something useful in http://superuser.com/questions/187302/tool-to-adjust-monitors-backlight-brightness-in-windows
– LSerni – 2013-11-30T00:21:35.687There is of course a light behind the display, and the more white space on the display, the more light escapes. On monitors which attempt to use a low-power light it may be necessary to increase power when more white is displayed to achieve "even" brightness, but of course the adjustment may be imperfect. – Daniel R Hicks – 2013-11-30T02:49:18.487
Are you sure it is the brightness that’s changing and not the contrast? You could take a photo of the screen in both states and add it to the question to be sure. – Synetech – 2013-12-01T04:33:40.947
I encountered the same problem on an HP Pavilion dv6-6117dx in both Windows 8.1 and Windows 7. This thing most likely stems from the hardware itself. – gparyani – 2013-12-02T17:29:36.870