How to access dng namespace in xmp

Hello all, i'm working with dng files and i need to access the tag

dng:OriginalImageWidth

We have namespace constants for tiff and everything but i don't see constants for the dng tags.

What i mean is, i'm currently accessing  the namespace exif like so:

xmp.getProperty(XMPConst.NS_EXIF, "PixelXDimension")

 Now i want to access the dng namespace like so (pseudo):

xmp.getProperty(XMPConst.NS_DNG, "OriginalImageWidth")

 I just want to read it, because it contains the image width without crop etc...