A method for providing an image of a sample, the method comprising: providing activation radiation from a laser or LED to a sample that includes a first species and a second species of photo transformable optical labels PTOLs the PTOLs being distributed in at least a portion of the sample with a density greater than an inverse of a diffraction-limited resolution volume of imaging optics and the PTOLs being transformable from an inactivated state wherein the labels radiate little, if any, radiation at a wavelength characteristic of an activated and excited PTOL when exposed to excitation radiation, to an activated state wherein illumination by excitation radiation results in detectable emission of radiation, to activate a sparse subset of the PTOLs in the sample wherein the sparse subset includes both species (501, 502) of PTOL; controlling the activation radiation such that density of the activated PTOLs in the sparse subset is less than the inverse of the diffraction-limited resolution volume of the imaging optics; providing excitation radiation to the sparse subset of PTOLs in the sample to excite at least some of the activated PTOLs; detecting with the imaging optics fluorescence radiation emitted from activated and excited PTOLs within the sparse subset of PTOLs and determining the locations of the activated and excited PTOLs with superresolution accuracy; and deactivating the activated PTOLs and repeating the above steps to determine the locations of more PTOLs within the sample; distinguishing the first species from the second species based on at least one of emission characteristics of the first and second species (501; 502) and excitation characteristics of the first and second species (501; 502) and combining the determined locations of the PTOLs from the different images to build up a superresolution image of the sample, and wherein a density of each species (501; 502) of PTOL in the sparse subset of activated PTOLs is less than the inverse of the diffraction-limited resolution volume. providing activation radiation from a laser or LED to a sample that includes a first species and a second species of photo transformable optical labels PTOLs the PTOLs being distributed in at least a portion of the sample with a density greater than an inverse of a diffraction-limited resolution volume of imaging optics and the PTOLs being transformable from an inactivated state wherein the labels radiate little, if any, radiation at a wavelength characteristic of an activated and excited PTOL when exposed to excitation radiation, to an activated state wherein illumination by excitation radiation results in detectable emission of radiation, to activate a sparse subset of the PTOLs in the sample wherein the sparse subset includes both species (501, 502) of PTOL; controlling the activation radiation such that density of the activated PTOLs in the sparse subset is less than the inverse of the diffraction-limited resolution volume of the imaging optics; providing excitation radiation to the sparse subset of PTOLs in the sample to excite at least some of the activated PTOLs; detecting with the imaging optics fluorescence radiation emitted from activated and excited PTOLs within the sparse subset of PTOLs and determining the locations of the activated and excited PTOLs with superresolution accuracy; and deactivating the activated PTOLs and repeating the above steps to determine the locations of more PTOLs within the sample;