Fluorescence lifetime imaging microscopy
(FLIM) may reveal subcellular
spatial lifetime maps of key molecular species. Yet, such a quantitative
picture of life necessarily demands high photon budgets at every pixel
under the current analysis paradigm, thereby increasing acquisition
time and photodamage to the sample. Motivated by recent developments
in computational statistics, we provide a direct means to update our
knowledge of the lifetime maps of species of different lifetimes from
direct photon arrivals, while accounting for experimental features
such as arbitrary forms of the instrument response function (IRF)
and exploiting information from empty laser pulses not resulting in
photon detection. Our ability to construct lifetime maps holds for
arbitrary lifetimes, from short lifetimes (comparable to the IRF)
to lifetimes exceeding interpulse times. As our method is highly data
efficient, for the same amount of data normally used to determine
lifetimes and photon ratios, working within the Bayesian paradigm,
we report direct blind unmixing of lifetimes with subnanosecond resolution
and subpixel spatial resolution using standard raster scan FLIM images.
We demonstrate our method using a wide range of simulated and experimental
data.