You may have heard the term luminosity masks? I’m not going to explain luminosity masks here, as that would be a tutorial, not a tip. Instead, I’ll show you the very foundation of luminosity masks, and that’s making selections based on the luminosity (light and dark bits) in an image. With a photo open, go to the Channels panel (Window>Channels), and Command-click (PC: Ctrl-click) the RGB channel at the top. All the light pixels in the image are now selected. If you want to select all the dark pixels, press Shift-Command-I (PC: Shift-Ctrl-I) to invert the selection. 

This tip originally published in the Photoshop Tips column by Colin Smith, in the January, 2018 issue of Photoshop User magazine.