The GLIM module is connected to the imaging port of a Phase Contrast microscope, with a camera connected at its output. The microscope separates the illumination into a sample and a reference beam with a phase shear between them that pass through the sample and are collected at the imaging port. The beams pass through the GLIM module where an electro-optical system introduces four accurately controlled phase delays between them. The GLIM camera acquires an intensity image for each phase delay. The intensity images are combined by interference and a recombination algorithm outputs the quantitative OPL map of the entire field of view of the microscope objective. The OPL map is converted to specimen height/volume, dry mass, and refractive index.
GLIM rejects much of the multiple scattering contributions present in an optically thick specimen (e.g. embryos and 3D organoids): the two imaging beams are always equal in power, and suffer equal degradation (that is, the same background noise) because of multiple scattering in the sample such that they interfere with high contrast.
For more on GLIM check out this Nature paper.
GLIM Paper – Nature Communications (2017) https://www.nature.com/articles/s41467-017-00190-7