Self-field theory (SFT) provides deterministic eigensolutions to the Maxwell-Lorentz equations for the hydrogen atom where Planck's constant is the energy per cycle of the principal eigenstate [1]. Based on a composite (hydrogenic) photon, an analytic expression for photon mass is obtained , where ω_γ is a discrete photon transition frequency within each cycle of the electron. This expression is compatible with the fine-structure constant , where m_γ=0.396×10⁻⁵⁵ kg (0.221×10⁻¹⁹ eV). Thus there is a series of eigenstates within individual photons, like the atom, that vary with the ambient energy density or temperature. Within atoms and molecules, photon substructure introduces a previously unknown mechanism by which binding energies can vary between strong and weak structures in variation with the ambient energy density, or temperature. A range of physical and biological examples support photon substructure with its associated photon spectroscopy.