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With transparent objects the alpha normally takes into account the amount of transparency of the object and renders with varying degrees of grey - for example very dark grey for very transparent objects. It is possible to render a completely white alpha also for transparent objects with the Opaque option. This can be useful if you want to maintain the full look of the transparency from the render, instead of blending it with any other images in your composite.
From left to right: Render, Alpha with opaque option OFF, alpha with opaque option ON
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Option 1: Selecting only one of the channels in the alpha. Select one of the channels in the channels palette. Then Ctrl+A to select all, then Ctrl+C to copy.
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Option 2: Converting the RGB alpha into greyscale. Make sure the alpha image is the active document in Photoshop and go to Edit>Convert to Profile and choose sGrey from the drop down list and click OK. Then select the entire alpha image with Ctrl+A, then Ctrl+C to copy it.
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