Photoluminescence (10 to 90 K) processes resulting from recombination centers emitting in the region of 550 nm in <III> AgBr (88%)–I (12%) tabular grains are studied. Stacking faults, dislocation defects, or double-twin planes are present in these microcrystallites. The greater the number and kind of iodide defects, the higher the low-temperature fluorescence quantum efficiency. The second-derivative photoluminescence presents additional information of competing emitting Ag°3 R-type centers emitting in the region of 562 to 571 nm. The photophysical processes leading to photoluminescent centers relate to emulsionmaking conditions involving iodide clustering, not AgI phases. It is the differences of ΔE of the rate-determining step that influence the quantum efficiency of fluorescence in the various low-temperature regions: 0.0009 and 0.0007 eV below 30 K, and 0.065 and 0.055 eV in the 50 to 70 K region for minimal defect and S-defect tabular grains. The 0.065 and 0.055 eV are shallow electron traps beneath the conduction band. The βAgI hexagonal fine grains of 36 Å diameter were assigned to the quantum particle (confinement) fluorescence at 424 nm and a βAgI large-grain 448-nm Stokes-shifted emission; γAgBr emission for microcrystals is 455 nm. The quantum model, using effective masses, describes Wannier excitons of I_m⁻ⁿ in AgBr. The structure of clusters seems to consist of bent, 3-at. iodide structures (AgI₃)_n. Also, clusters consist of 3-at. linear configurations, formed during the emulsion-making process, and emitting radiation in the region of 540 and 590 nm with cluster diameters of 12 to 14 Å, respectively. These clusters (AgI ₃)n or (+AgI⁻−I₂)_n interact closely with the AgBr point defect lattice. Recombination emission from these clusters is studied. Microcrystals containing double-twin planes have an additional excitation band coincident with the emission band at 461.5 nm (2.6860 eV), the exciton localization at the twin plane. The higher energy excitation is shown at 461.0 nm (2.6889 eV), the assigned indirect exciton band gap. Splitting of 2.9 MeV or hole-limiting binding energy to the iodide anion located at the twin plane provides an estimate of the hole lifetime of 1.41 × 10⁻¹⁰ s at 10 K. The hole and electron are capable of traversing the doublet-win plane separation of 93.3 and 100 Å.