An additive discussion for the validity of using the weighted information entropy to evaluate the complex degree of infrared (IR) backgrounds is given. Since small targets can be temporarily lost in actual infrared video sequences, an adaptive binarization threshold for small targets detection is presented. Experimental results show the robustness of our method. An additive discussion for the validity of using the weighted information entropy to evaluate the complex degree of infrared (IR) backgrounds is given. Since small targets can be temporarily lost in actual infrared video sequences, an adaptive binarization threshold for small targets detection is presented. Experimental results show the robustness of our method.