We hear a lot about using luminosity masks.  I hear the term thrown around a lot, but when I dig in, folks often understand that they are great tools, but don't know where they come from, what one can do with them and how to make them.  In fact, I have encountered nice people who are paying $$ for Photoshop actions that make luminosity masks.  Ok.  I'm going to teach you to make your own as well. A luminosity mask is a mask that masks out elements of an image of a certain brightness.  We build luminosity masks to use with adjustment layers so we can edit parts of images based on the brightness.  I learned how to make luminosity masks from the amazing Dave Cross.  Mr. Cross has been teaching Photoshop literally since the product came out, and is a powerful advocate for non-destructive editing in Photoshop.  The method I will show you is EXACTLY the way I learned it from Mr. Cross so all credit for the method goes to him.

You create luminosity masks in the Channels panel, starting with the RGB channel image.  The steps are simple.

1. Do your global edits in Lightroom or Camera RAW (non-destructive) and then open in Photoshop as a Smart Object (the start of non-destructive).
2. Go to the Channels panel (usually one right of Layers panel)
3. Select the first image with a single click marked as RGB
4. While holding down OPTION-COMMAND, click the image once.  You will see a series of marching ants that identify the bright areas of the image.
5. Now create an Alpha mask by clicking the Mask icon that is second from the left on the bottom right of the Panels frame.  The mask will be created in the Channels panel below the Blue channel image and be called Alpha 1.
6. Click the image called Alpha 1 to select it.  Now hold down SHIFT-OPTION-COMMAND and click the Alpha 1 image.
7. This creates a selection of even lighter areas.  Click the Mask icon to make a second masked that gets named Alpha 2.
8. Select Alpha 2.  SHIFT-OPTION-COMMAND click on Alpha 2, then click the Mask icon to make a third mask (Alpha 3), this one holding the brightest part of the image.
9. Now don't panic.  The masks look darker as they select lighter areas. It is supposed to be this way.  You now have luminosity masks for lighter areas.  Time to make luminosity masks for darker areas.
10. Clear all selections by clicking COMMAND-D
11. Now click the RGB image again to select it.  OPTION-COMMAND click the image.
12. Before you go any further, you need to invert the selection so you are working with darker rather than lighter areas.  Go to the SELECT menu and click INVERSE.  This inverts the light selection and makes it dark.
13. Now click the Mask icon to make your first luminosity mask for the dark sections.  It will be called Alpha 4.
14. Repeat steps 6 to 8 substituting Alpha 4 for Alpha 1 and Alpha 5 for Alpha 2.
15. You now have six luminosity masks.  Time to give them useful names.
16. Double click the name of each Alpha, naming them in sequence Light, Lighter, Lightest, Dark, Darker, Darkest.
17. Now clear all the selections by clicking COMMAND-D

Now that you have the masks, you work with them using Adjustment Layers.  Simply COMMAND Click the mask that you want to use in the Channels panel, go to the Layers panel and then click the little half black half white cookie and select the Adjustment Layer type you want.  Whatever layer type you choose will have the mask applied to it.  Make your adjustments.  If you find that the whole image has a colour cast, simply click the mask section of the layer, and then click the adjustment icon to remove the colour cast.

Now use each luminosity mask as you see fit.  Once you are done, hit File Save and your PSD will be created and the layers retained for future editing.

Now be sure that if you are sending to Photoshop from Lightroom, that you configure your Preferences so Photoshop returns a PSD not a TIF otherwise you will lose all your layers.  Note as well that Photoshop's 32 Bit HDR mode HAS to return a TIF so just be ready to adjust your preferences as necessary.