Luminosity Masks, unlike Layer Masks are not built in with Photoshop (any version), but have to be created. The Luminosity Masks are to increase or decrease the light in your images. It is very easy to make Luminosity Masks. The results after applying the Mask are remarkable.
Step 1 : I will use this free stock image to demonstrate a Luminosity Mask.
Step 2 : Click the Channels palette to open it. Drag the RGB channel down to the Load channel as Selection button the arrow points to.
Step 3 : A selection of marching ants appears.
Step 4 : Go to Select>Save Selection.
Step 5 : This dialogue box appears. Give it a name. I have named it Lights.
Click OK.
Step 6 : You will find the Lights has been saved as a channel in the bottom of the Channels palette. Press CTRL+D to deselect.
Step 7 : Click back on the Layers palette.
Step 8 : Click on Edit>Load Selection.
Step 9 : This dialogue box pops up. You can see the 'lights' channel. Click OK.
Step 10 : As soon as you click the selection reappears.
Step 11 : Press CTRL+M. This will bring up the Curves dialogue box.
Step 12 : I have now dragged the diagonal in the middle to darken the figures. Click OK.
Step 13 : Press CTRL+SHIFT+I to inverse the selection.
Step 14 : Press CTRL+M. The Curves dialogue box opens again. Now drag the diagonal in the opposite direction. Click OK when satisfied. You will be able to notice the change in the image. Click OK.
Step 15 : Press CTRL+D to deselect. Now notice the difference in the two images. The grass and the fence are darker in the original image. They look nice and bright now.
That is all. Other Photoshop tutorials are here.
