To apply tools and effects only to certain areas of a raster layer with data, you can use the Lock Transparency feature. Transparent areas remain protected when you paint, apply effects, paste selections, or make other modifications.
Important! The Lock Transparency feature applies to raster layers only. You cannot lock the transparency of vector, art media, adjustment, mask, or group layers. Background layers do not support transparency.
To lock or unlock transparent areas of a raster layer
Edit workspace
• On the Layers palette, select the layer, and click the Lock/Unlock button .
The Lock/Unlock button appears on the layer when transparency is locked. Click the button again to unlock the transparency. The icon on the layer disappears when the layer is unlocked.
You can also lock or unlock transparent areas by double-clicking the layer, marking or unmarking the Lock transparency check box in the Layer Properties dialog box, and clicking OK.