Figure 1 presents the suggested close-to-common-path interferometer. As can be seen in this figure, coherent light from a laser source is spatially filtered and expanded. The area of the generated plane wave should be larger than the area of the recorded field of view on the digital camera. This plane wave is first reflected by a beamsplitter and then passes through a 4f optical system, containing two lenses and with focal lengths of and , respectively, which demagnify the illumination beam before reaching the InCh. The InCh consists of two coverslips. The front coverslip of the InCh is inclined at an angle , whereas the rear coverslip is normal to the optical axis. When hitting the front coverslip, part of the incident beam is reflected at an angle of relative to the optical axis. This reflected beam serves as the reference beam. To ensure that lens will collect the oblique reference beam, its numerical aperture (NA) should be at least , where is the refractive index of the immersion medium between lens and the sample. The beam transmitted through the front coverslip is reflected from the rear coverslip of the InCh and leaves the chamber after interacting with the transparent sample twice. This beam is the sample beam. The sample and reference beams are projected by the two lenses onto the camera, while the sample beam propagates along the optical axis and the reference beam reaches the digital camera at an incidence angle of . This angle must be adjusted so that the carrier frequency of the interferogram will not exceed the Nyquist frequency of the camera. This will happen if , where is the wavelength of the source and is the pixel size of the digital camera. For example, if and , we get . If we choose and that yield a magnification of the InCh’s front coverslip inclination angle should satisfy . If is chosen (so that ), and assuming that the immersion medium is air, should have a NA of or larger to ensure collection of the inclined reference beam by . The hologram captured by the digital camera is then digitally processed to yield the unwrapped phase profile of the sample by the off-axis WFDI digital process explained in our previous work,4 where for the InCh, the measured phase is twice the sample phase. Note that since the sample beam passes through the sample twice, the system will more accurately measure phase profiles of transparent samples that are flatly attached to the rear coverslip of the InCh. This condition is generally valid for cell cultures after several hours of incubation.