Inside the nucleus of an intact cell, DNA is folded around histone proteins into nucleosomes and compacted into a multi-layered three-dimensional chromatin network. The nanometre spacing between nucleosomes positioned throughout this structural framework is known to locally modulate local DNA template access and regulate genome function. However, given that this structural feature occurs on a spatial scale well below the diffraction limit, real time observation of nucleosome proximity in live cells by optical microscopy has proven technically difficult, despite recent advances in live cell super resolution imaging. A promising alternative solution is to measure and spatially map Förster resonance energy transfer (FRET) between fluorescently labelled histones – the core protein of a nucleosome. In recent work, we established the phasor approach to fluorescence lifetime imaging microscopy (FLIM) of histone FRET as a robust method for the detection of nuclear wide chromatin compaction at the level of nucleosome proximity, which can also characterise the nanoscale chromatin structure associated with gene activating versus repressive epigenetic marks. Here we present these two capacities of the phasor FLIM histone FRET method, as well as recent results, which demonstrate the underlying nanoscale structure of heterochromatin is differentially regulated by heterochromatin protein 1 alpha (HP1α) self-association