Within the Window menu inside Lightroom is a section called Lights Out. This section contains 3 settings and pressing the L key will cycle through these modes. These modes are…
The Lightroom interface appears as normal.
The Lightroom interface dims while any images remain at full brightness. Although not mentioned in the video, the percentage by which the interface dims can be controlled from the Interface section inside the Lightroom Preferences.
The Lightroom interface goes completely black while any images remain at full brightness.