Selecting Image Luminosity
Photoshop CS3 Tutorial
How to Select Image Luminosity in Photoshop
This technique works well to build detail and density in the image highlights.
Open an image in Photoshop (File > Open).
Open the Channels palette (Window > Channels).
Cmd or Ctrl click the top channel (RGB) thumbnail. A new selection is made. The lighter portions are fully selected and the darker areas are not selected.
Return to the Layers palette (Window > Layers) and click the image layer thumbnail to make sure the correct layer is selected.
Press Cmd/j (PC-Ctrl/ j) to copy your luminosity selection and paste it to a new layer above the original layer.
Set the Blending Mode of the new layer to Multiply and adjust the Opacity if necessary.