How to calculate image's noise and how to understand what noise it is ? thermal or shot noise?

Hi everyone ,im not sure that this is the right place for my question but i'll try anyway.

I have this video that i shot , it's a scene of a static view. iv'e analysed 3 pixels in the frames - a bright pixel , a dark one and a mid range pixel . iv'e looked for the statistical characters of the pixels and found the mean and the variance of it.

if im right then by finding the std of the dark pixel i'm able to find the dark noise of the image (thermal noise) , i'm not sure what the std of the bright and mid range pixel suppose to tell me.

and another thing - once iv'e found the std how can i filter the image based on it? iv'e tried median filter for example but i couldn't use the std that iv'e already found.

thx for any help.