yes the formula is correct. Here, images are face images, which are correctly identified. The identification is done manually, for known answers. This is for the purpose of testing the system accuracy. We must know the answer before hand. Only, then we can test a system
As Kaya said you consider measuring the false acceptance rate (FAR) and the False Rejection at different thresholds. then you also plot the ROC curve to see the performance of the system
I think False positives , True positive, false negative FAR-,is calculated for Face Detection not for Face Recognition. Then how to calculate for Face Recognition .I have drawn ROC curve for Face Detection, Is there separate formula for Face Recognition