Face recognition using Emgu cv

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C#

C#4.0

I've been working with c# .Net 4.5 framework.I'm developing an application for login to the pc using face recognition.
I use EigenFaceRecognizer class for recognize the face.My code detect human face perfectly. But in recognition process, recognizer always recognize not only the saved faces but also the unknown faces also as the face in database and it always return the face label.
when unknown user comes in front of the PC I want to return the label as unknown.
how can I modify my code(EigenObjectRecognizer method) to get better result..
I really gratefull any one can help me.....