Defocusing microscopy: An approach for red blood cell optics

Thin transparent objects (phase objects) can become visible in a bright-field light microscope, if the microscope is slightly defocused. Thick transparent objects, like red blood cells (RBC), are seen because some of their parts are always out of focus. By applying our recently developed defocusing microscopy technique to RBC, we are able to interpret RBC bright-field light microscopy images, an old standing problem. From the average image contrast we obtain RBC shape, size, and refractive index. From contrast fluctuations caused by the flicker phenomenon, we obtain RBC bending modulus and cytoplasm viscosity.