To solve the problem of low efficiency and poor conversion effectiveness of traditional quantization methods for gray scale measurement images, three fast quantization methods for gray scale images are proposed according to the actual processing requirements of measurement images and different operation demands. These methods have the features of high conversion efficiency, simple algorithm, easy implementation, small computation amount and strong adaptability to image quality. They have been widely used in the processing and analysis of infrared images and good effectiveness is achieved. Thus, the gray scale quantization quality of infrared images is improved remarkably.