Abstract  This paper proposed an efficient algorithm for cont rast enhancement of natural images. The contrast of images is very important characteristics by which the quality of images can be judged as good or poor. The propo sed algorithm consists of two stages: In the first stage the poor quality of an image is processed by modified sigmoid functi on. In the second stage the output of the first stage is further processed by c ontrast limited adaptive histogram equalization to enhance contrast of images. In order to achieve better contrast enhancement of images, a novel mask based on input value together with the modified sigmoid formula that will be used as contrast enhan cer in addition to contrast limited adaptive histogram equalization. This new contrast enhancement algorithm passes over the input image which operates on its pixels one by one in spatial domain. Simulation and experimental results on benchmark te st images demonstrates that proposed algorithm provides better results as compared to other state*of*art contrast enhancement techniques. Proposed algorithm performs efficiently in different dark and bright images by adjusting their contrast very freq uently. Proposed algorithm is very simple and effic ient approach for contrast enhancement of image. This algorithm can b e used in various applications where images are suffering from different contrast problems.

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[41] Zuiderveld K., Contrast Limited Adaptive Histogram Equalization , Academic Press, Cambridge, 1994. Shyam Lal received his B.Tech. (with Hons) in electronics and communication engineering from Bundelkhand Institute of Engineering & Technology, India and M.Tech. (with Hons) in electronics and communication engineering from National Institute of Technology, India in year 2001 and 2007, respectively. He is pursuing PhD degree in the area of digital image processing from the Department of Electronics & Communication Engineering, Birla Institute of Technology, India. He has been an associate profess or in the Department of Electronics & Communication Engineering, Moradabad Institute of Technology. He has published more than 40 papers in the area of di gital image processing and wireless communication & computing at International/National Journals & conferences. He is a member of IEEE since 2011, lif e member of Indian Society of Technical Education India, of International Association of Engineers an d Life member of International Association of Compute r Science and Information Technology. He has more than 11 years of teaching experience. His area of interest includes digital image processing, digital signal processing and wireless communication. Mahesh Chandra received his BSc degree from Agra University, India in 1990 and A.M.I.E. from I.E.I., India in winter 1994. He received his M.Tech. from J.N.T.U., India in 2000 and PhD from AMU, India in 2008. He has worked as Reader & HOD in the Department of Electronic and Commune Engenineering at S.R.M.S. College of Engineering an d Technology, India from 2000 to 2005. He is presentl y working as an associate professor in the Department of ECE, BIT, India. He is a life member of ISTE, Indi a and member of IEI Kolkata, India. He has published more than 46 research papers in the area of speech, signal and image processing at international/nation al level conferences and journals. He is currently gui ding 05 PhD students in the area of speech, signal and image processing. His areas of interest are speech, signal and image processing.