Noise cancellation by averaging multiple images together

As the title suggests , I want to know how noise cancellation is achieved by adding multiple images together . I know the theory behind it , but simple adding the pixel values of 100 noisy images of the same sample would simply produce a white image . Can anybody elaborate , how it is normally done.

• You're not adding them together; you're averaging them together. You have to divide by the number of images in order to not clip at white. – endolith Aug 8 '13 at 20:00
• @endolith : You are right.I mae a mistake – motiur Aug 9 '13 at 13:37

This works simply by virtue of the central limit theorem. For a given pixel, you are given 100 observations of its true value + random noise; and if you assume that the noise has zero mean, then the average of these will have a normal distribution centered at the true pixel value, with a variance decreasing as more observations are added. The central limit theorem also tells you that this is a rather terrible denoising method: for a gain in SNR of $6dB$ (reducing noise by a factor of 2) you have to increase the number of averaged samples by a factor of 4.