No pd=1-Pm where pm is missed detection .not probability of false alarm.However when we plot the detection probability we plot is against probability of false alarm or no of sample frequeciences
Adding to that given by Mehboob please for precise definition of different probability terms for detection please refer to the demonstration in the link:http://www.globalspec.com/reference/79944/203279/4-6-detection-and-false-alarm-probabilities