What's the principle of dye exclusion assays?

Dye exclusion assays are used to determine the number of viable cells present in a cell suspension. These assays are based on the principle that viable cells have intact cell membranes that are impermeable to certain dyes such as propidium, trypan blue, and eosin, whereas dead cells have damaged cell membranes that allow these dyes to readily penetrate and stain the cytoplasm. 

In a dye exclusion assay, a dye such as trypan blue, is incorporated into the cell suspension. On visual examination by light microscope, it is easy to differentiate the stained cells from the unstained cells. The unstained cells with clear cytoplasm represent viable cells with intact cell membranes that did not take up the dye while the nonviable cells that took up the dye will have a blue cytoplasm.