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Two stochastic resonance (SR)-based techniques are introduced for enhancement of very low-contrast images. In the proposed SR-based image enhancement technique-1, an expression for optimum threshold has been derived. Gaussian noise of increasing standard deviation has been added iteratively to the low-contrast image until the quality of enhanced image reaches maximum. A quantitative parameter `distribution separation measure (DSM)` is used to measure the enhancement quality. In order to reduce the required number of iterations in the second enhancement technique the author`s have derived an expression for optimum noise standard deviation `optimum that maximises signal-to-noise ratio (SNR). Image enhancement is obtained by iterating only with few noise standard deviations around σoptimum. This reduces number of iterations drastically. Comparison with the existing methods shows the superiority of the proposed method.