Brightness dims depending on background
You are facing a "feature", referred to by Dell as "dynamic brightness adjustment". This is actually a power saving feature of Intel graphics chips, but is also marketed that it increases your photo viewing experience. For dark pictures, the backlight intensity will be reduced, and contrast increased so you see more of the detail. Bright pictures will appear very bright in contrast with the dark content.
The battery saving is that for LED screens, displaying dark colors actually costs more power than displaying bright colors. In LED screens, there is a backlight that is continuously on. Dark pixels are created by blocking the light. That is never perfect, so that is why blacks on LED screens are not perfect. Creating dark pixels costs extra power to shield the dark pixels. For dark content, power saving is achieved by decreasing the backlight (reduces power) and at the same time, darken pixels less (reduces power).
Admittedly, most will find this a hateful feature for daily use. It is, for example, impossible to do photo editing in such conditions.
TL;DR: Fix is here: link
This is a feature of Intel Graphical chips. You are lucky that it only kicks into action when on battery. On other laptops, it is effective all the time. On some laptops, there is no way (and also for the manufacturer, no possibility) to disable it, on other laptops Windows drivers have become available to disable the feature.
You will have to find out what the possibilities are for your particular machine. It is unlikely you will be able to solve it from within linux. If you are lucky, you can disable it from within Windows after installing drivers from Dell. That will require you to (temporarily) install Windows again.
Nevertheless here is a excerpt from this Dell support manual. It is called "EcoPower":
Table 4. System setup options—Video menu
VIDEO LCD Brightness Brightness on battery power Sets the screen brightness when the >computer is running on battery power. Brightness on AC power Sets the screen brightness when the computer is >running on AC power. EcoPower Enables or disables EcoPower which increases the battery life by reducing the screen brightness when appropriate. Default: ON.
Use these manuals for your help!:
- Entering BIOS setup program
- Navigation keys
- System and setup password