Quantitative phase microscopy (QPM) or quantitative phase imaging (QPI) measures the phase shift of light as it passes though and interacts with matter inside of a cell. This phase shift image can then be converted into a map of the distribution of mass across a cell or multicellular colony.
The image above shows a schematic of a basic QPI microscope and a sample image of a single human pluripotent stem cell colony. Many such images can be captured over time to measure the total mass of a colony/cell, or its changing mass distribution over time: