These steps create a luminosity mask protecting the shadows:
- Open the Channels Panel in PS
- CMD+Shift+Click the RGB Channel which selects the brights.
- CMD+Shift+Click the RGB Channel again and more darker areas are selected (leaving only the shadows)
- Cmd+Option+Shift+Click selects only highlights
- Cmd+delete key fills the selection with Option+delete (FG/BG) or Shift+delete key for fill options.