Practice question from ETS verbal book

Researchers trying to make it possible to trace counterfeit documents to the

printer that produced them are (i) blank the fact that the rotating drums and

mirrors inside laser printers are imperfect devices that leave unique patterns of

banding in their output. Although these patterns are (ii) blank to the naked eye,

they can be (iii) blank and analyzed by computer programs that the researchers

have spent the past year devising.

can any one explain the grammatical structure of “Although”. is Although contrast with two ideas with in the sentence like “though”, or “Although” works like “however”.