The single-photon timing and sensitivity performance and the imaging ability of asynchronous-readout single-photon avalanche diode (SPAD) array detectors have opened up enormous perspectives in fluorescence (lifetime) laser scanning microscopy (FLSM), such as super-resolution image scanning microscopy and high-information content fluorescence fluctuation spectroscopy.
The objective lenses in most microscopes are achromats, and best suited for imaging with green light. Green filters narrow the bandwidth of the light, and make achromat objectives reasonably effective for most routine uses.