Transport of intensity microscopy for distinguishing single and bundled microtubules

Microtubules are filamentous protein polymers about 25 nm in diameter, ubiquitous in eukaryotes and providing key functionality in cellular biology. Although microtubules may span many microns in length, their slender width places them well below the diffraction limit. Unlabeled microtubules lack any meaningful absorption of a passing wavefront, and are therefore sub-resolution phase objects.