Difference between results of SNR

Hi,

I want to measurement the SNR of imaging system. I have a image of the system that contains edge target (half image black and half white) and i don't know the signal and the noise in the image. I calculate the SNR by calculate the mean and the standard deviation of the white part and i divided the mean result in the standard deviation result.

meanWhiteGL = mean2(whitePartOfImage); stdWhiteGL = std(double(whitePartOfImage(:))); SNRWhite = meanWhiteGL / stdWhiteGL;

I did the same to the black part of the image. But the result of the white part was 61 and the result of the black part was 1.5, how cann i know which SNR is correct?

please help me, Itai