according to 
[Estimating acquisition noise P. 2](https://classes.soe.ucsc.edu/cmpe264/Fall06/lec4.pdf)

Estimating the mean and the standard deviation for each pixel is calculated as below:

$$
 \begin{equation\*}
 i, j = 0,\dots,N-1 \\ 
 \bar{I}(i, j) = \frac{1}{N} \sum\_{k=0}^{N-1} I\_k(i,j)\\
 \sigma(i, j) = \left [ \frac{1}{N-1} \sum\_{k=0}^{N-1}(I\_k(i, j) - \bar{I}(i,j)^2) \right]^{1/2} 
 \end{equation\*}
$$

**My Question**

1. What is $I\_k$ ?
2. What should mean of each pixel be? Is it the mean of whole Image?

Thanks