Get Device Info PPI

Im aware that [lua]display.contentScaleX[/lua] will give me a percentage of the screen resolution, but i need to also get the pixels per inch from the device.

For instance iPhone 3 [lua]display.contentScaleX = 1[/lua]
while iPhone 4 [lua]display.contentScaleX = 2[/lua]
but the actual screen size for the iPhone 3 and iPhone 4 are the same.

How can i get this info!? [import]uid: 50514 topic_id: 20195 reply_id: 320195[/import]