The NIH has a free pixel counter you might try before shelling out $99 bucks. You'd just have to use a known size object, use the app to measure that object in pixels, then enter in the pixels/unit value. It's meant for biologists, but it has some nice features, like the ability to recognize objects ("particles") and 3d representation of pixel intensities.
http://rsb.info.nih.gov/ij/docs/menus/analyze.html
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The NIH has a free pixel counter you might try before shelling out $99 bucks. You'd just have to use a known size object, use the app to measure that object in pixels, then enter in the pixels/unit value. It's meant for biologists, but it has some nice features, like the ability to recognize objects ("particles") and 3d representation of pixel intensities. http://rsb.info.nih.gov/ij/docs/menus/analyze.html