The excited triplet state of a molecule (T₁) is one of the principal intermediate products in various photochemical processes due to its high reactivity and relatively long lifetime. The T₁ quantum yield (${\phi }_{\mathrm{T}}$) is one of the most important characteristics in the study of photochemical reactions. It is of special interest to determine the ${\phi }_{\mathrm{T}}$ of various photoactive compounds (photosensitizer, PS) used in photodynamic therapy (PDT). PDT is an effective medical technique for the treatment of serious diseases, such as cancer and bacterial, fungal and viral infections. This technique is based on the introduction of a PS to a patient’s organism and its further excitation by visible light, producing reactive oxygen species (ROS) via electron or energy transfer from the PS T₁ state to the biological substrate or molecular oxygen. Therefore, information on the ${\phi }_{\mathrm{T}}$ value is fundamental in the search for new and effective PSs. There are various experimental methods to determine ${\phi }_{\mathrm{T}}$ values; however, these methods demonstrate a high discrepancy between ${\phi }_{\mathrm{T}}$ values. This stimulates the analysis of various factors that can affect the determined ${\phi }_{\mathrm{T}}$. In this study, we analyze the effect of the intensity profile of the exciting laser pulse on the calculation of the ${\phi }_{\mathrm{T}}$ value obtained by the Laser Flash Photolysis technique. The φ_T values were determined by analyzing the variation of a sample transient absorption in the function of the exciting laser pulse intensity, in combination with the spectral and kinetic PS characteristics obtained in nonlinear optical experiments by solving the rate equations of a five-level-energy diagram. Well-studied PSs: meso-tetra(4-sulfonatophenyl) (TPPS₄) porphyrins, its zinc complex (ZnTPPS₄) and the zinc complex of meso-tetrakis(N-methylpyridinium-4-yl) (ZnTMPyP) were chosen as test compounds to evaluate the proposed model. The ${\phi }_{\mathrm{T}}$ values were determined through a comparison with the ${\phi }_{\mathrm{T},\mathrm{T}\mathrm{M}\mathrm{P}\mathrm{y}\mathrm{P}}$ = 0.82 of meso-tetrakis(N-methylpyridinium-4-yl) (TMPyP), used as a standard. The obtained results (${\phi }_{\mathrm{T},{\mathrm{T}\mathrm{P}\mathrm{P}\mathrm{S}}_4}=0.75\pm 0.02$, ${\phi }_{\mathrm{T},\mathrm{Z}\mathrm{n}\mathrm{T}\mathrm{M}\mathrm{P}\mathrm{y}\mathrm{P}}=0.90\pm 0.03)$, and ${\phi }_{\mathrm{T},{\mathrm{Z}\mathrm{n}\mathrm{T}\mathrm{P}\mathrm{P}\mathrm{S}}_4}=0.89\pm 0.03$) are highly compatible with the medium ${\phi }_{\mathrm{T}}$ values obtained using the known methods.