What is method use to count five differential of white blood cell ?

By the use of Laser scattering aimed through a flow cell.

Neutrophil , lymphocyte, monocyte, eosinophil and basophil are five parts of WBC are counted by light scattering method in hematology analyzer

Using laser beam to be diffracted from the wbcs and to take reflections corresponding to the cells morphology. there is a device used to do this process called 5 part hematology analyzer.