Hi sadak
Welcome to PhotoShop Gurus.
First things first, what you need to do in order to extract only your white spots or light gray from the image as you mentioned, is to increase the shadows and highlights on your image, via levels "Layer>Adjustment layer> Levels", meaning you need to make the dark even darker and same thing whit whites, use the levels slider to achieve such task,
Then what you need to select or isolate those color shades via "Selec>Color range" , that way you will be able to select any color shade you need to, Once on the select by color range window, you need to sample the color shades you need to extract, using the "eye dropper" tool select your color, if you need to add another shade to your selection hold the "Alt" key, while selecting colors or drag your mouse across those color shades, if you need to subtract colors from your selection hold the "Crtl" while selecting, and hit enter.
Then, on your layers palette you can just duplicate your layer whit the active selection, creating a new layer whit those selected colors or invert your selection and press the delete key, to get rid of your selection and leaving only your original selection
And for the measurements, once you got your clean layer whit out the dark or grey shades, you can select your layer "Crtl+a" and go to "Analisys> record measurements" and easily you will determine the area, perimeter, circularity, high, width, etc. Remember that the analisys tool in this case, will only work whit active selections.
See Image below
[ATTACH]9794[/ATTACH]
1. Analisys results, measurements window.
2. Extracted only white and light grey colors.
3. Levels Adjustment Layer.
4. Some random color background to accentuate extracted shades.
Hope this helps.
