Quick primer on quantitative phase microscopy
Interested in measuring the mass of individual cells in relatively high throughput? Or do you just think optics are cool? Look no further!
Because light can be described as a wave, in addition to having a wavelength and an amplitude, light also has a phase. The light detectors that we are familiar with from day to day life (eyes & cameras) can measure wavelength (color) and amplitude (brightness), but cannot measure phase shifts of light waves.
Biological macromolecules (e.g. protein, DNA, RNA, lipids) all have a higher refractive index than water. Because the refractive index of a material relates to how quickly light can travel through that material, light traveling through biological macromolecules will be slower than the light traveling through water. As a result, the light that has gone through the sample will be phase shifted relative to the background light, and the phase shift will be proportional to the width of the cell and to the difference in index of refraction between the sample and background.
Illustration of optical phase shift of light passing through a cell with refractive index $n_{cell} > n_{0}$. [adapted from source]
Luckily, different macromolecules have very similar indices of refraction, and as a result people typically assume that the integrated optical phase shift of a cell will be proportional to the dry mass of a cell.
Despite its name, standard phase contrast microscopy cannot give you quantitative information about the optical phase shift through a sample (instead relying on the fact that biological samples will scatter light). There are many different ways that one can modify a standard microscope in order to obtain information about the quantitative phase shift, but for my project I wanted something quick, simple, and cheap.
I ended up using a computational method that relies on the principle that if you have two waves that waves that are phase shifted relative to one another will create an interference pattern that can be measured by simply looking at the amplitude or brightness.