Optimal speckle generation. (a) In a uniform fluorescent layer, increasing γ leads initially to a spreading of the histogram, resulting in increasingly visible speckles. For γ ⪢ 1, the tendency is reversed, ultimately driving all fluorophores toward the bleached state. Labels in the bottom row correspond to laser exposure time in seconds. (b) Experimental data for the dependence of speckle amplitude on γ, obtained by performing the experiment outlined in a. Dashed curve corresponds to fitting of the model curve (Eq. 3) using the theoretically calculated baseline for the loss of contrast factor caused by imaging, Mimg = 2.12, and the instrumental factor, Minstr.\, which arises as the single-fitting parameter in the model. Both the shape and the peak position are intrinsic to the model and not dependent on fitting. The solid curve applies a γ-dependent correction to Mimg, as obtained by computer simulation to correct for subdiffraction leakage, which becomes relevant for γ ⪢ 1 (Fig. S4 d and Materials and methods).
