How can I measure image noise

Question

I've found a few ways of reducing noise from image, but my task is to measure it.

So I am interested in algorithm that will give me some number, noise rating. That with that number I will be able to say that one image has less noise than others.

Answer 1

To solve my problem I used next approach:

My noise rating is just number of pixels that were recognized as noise. To differentiate normal pixels from noise, I just calculated the medium value of its neighbor pixels and if its value was bigger than some critical value, we say that this one is noise.

if (ABS(1 - (currentPixel.R+currentPixel.G+currentPixel.B)/(neigborsMediumValues.R + neigboursMediumValues.G + neigboursMediumValues.B))) > criticalValue)
then
{
    currentPixelIsNoise = TRUE;
}