create test set matrix in face recognition for identifying unkmown faces

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C#

C#4.0

I have reached to a point in my face recognition project(using PCA) where i need to differentiate between the face and non-face images......
please tell me the method of how to reject unknown faces in face recognition project... i mean the method before i choose the proper threshold value(something between 0 and 1)....i googled and found that: dividing my database half into training and half into testing images,creating test matrix and so on but i am a bit confused about it.... Please describe it in detail the whole process... thanx in advance.