Detecting knives in surveillance videos are very urgent for public safety. In general, the research in identifying dangerous weapons is relatively new. Knife detection is a very challenging task because knives vary in size and shape. Besides, it easily reflects lights that reduce the visibility of knives in a video sequence. The reflection of light on the surface of the knife and the brightness on its surface makes the detection process extremely difficult, even impossible. This paper presents an adaptive technique for brightness enhancement of knife detection in surveillance systems. This technique overcomes the brightness problem that faces the steel weapons and improves the knife detection process. It suggests an automatic threshold to assess the level of frame brightness. Depending on this threshold, the proposed technique determines if the frame needs to enhance its brightness or not. Experimental results verify the efficiency of the proposed technique in detecting knives using the deep transfer learning approach. Moreover, the most four famous models of deep convolutional neural networks are tested to select the best in detecting knives. Finally, a comparison is made with the-state-of-the-art techniques, and the proposed technique proved its superiority.