Extract Byte Plane
The Extract Byte Plane command is the same as the Extract Channel command but is applied to non RGB images. This command works with all supported luminance format from 8 bit integer to 64 bit real data types. See the description of the Extract Channel command for further details. The dialog shown below is in the form taken when the selected image is a luminance (i.e., non RGB) image.
Open this command from the Process > Math menu.