Setting scale (pixel to mm)

So I have an image taken by a microscope (cel.png) it has a scale bar and the cell is gray etc. I need to find the area of the cell. My code is

clear;
clc;

prompt = 'Enter filename (include extension name): ';

str = input(prompt,'s');

A = imread(str);
image(A)

%Thresholding the image to gray-scale

level = graythresh(A);

BW = im2bw(A,level);
imshow(BW)

%Determine image dimensions
size = size(BW);                   
xpixel = size(2);                   %image dimesnions in the x axis
ypixel = size(1);                   %image dimesnion in the y axis

wpixel = length(BW(BW==1))
bpixel = length(BW(BW==0))

The threshold (refer to thshld.png) us working, its counting the white and black pixels. However, now I need to figure a way so the scale bar doesn't get counted and also I need to find out how to make matlab get pixel to mm ratio using the scale bar.

Any suggestions or do I need to use imageJ to get the pixel to mm ratio.

[Walter Roberson]

The cel.png file contains errors, apparently. At least my browser thinks so, but Preview (OS-X) seems happy with it.