Abstract: After a great deal of investigations on the principle of BP model and the structure of the neural network,this paper manages to demonstrate how to increase the convergent speed of the BP neural network by optimizing the input of the network.Further more,a creative optimization technique—length extension technique,is presented here to improve the performance of neural network.After lots of experiments,this paper successfully increases the convergent speed of the BP neural network.

Key words: face recognition； K-L expansion； BP neural network； length extension technique (LET)；