# Biophotons: A Hard Problem
## 4 External Excitation and Biophoton Emission
As a final example, let us now examine the phenomenon of delayed luminescence (DL). Several experiments carried out in recent decades have demonstrated how a previous brief illumination of a biological sample can temporarily increase its subsequent emission of biophotons by important factors, up to a few orders of magnitude [54]. Furthermore, this signal lasts longer than usual fluorescence and has been seen in virtually all biological systems studied up to now. DL emission is also very sensitive to the biological state of the system [55], and, for this reason, it is now widely used as a noninvasive method for obtaining biological information. Typically, a sample is illuminated by a pulse of light generated by a nitrogen laser equipped with a dye laser module to enable the possibility of varying the wavelength of the exciting radiation. In this way, the system generates pulses with a 5 ns width, an energy of about 150 ul/pulse, and a wavelength ranging from 360 to 710 nm. During this illumination phase, the phototube is off to avoid any damage, and the acquisition starts 10 ms after the end of the laser pulse. Data analysis consists of studying the decay of the emission after the sample was irradiated with the laser pulse, and the decay time to return to normal emission can vary from a few tens of seconds to a few minutes, i.e., a much faster time than the typical one in germination processes, which normally takes place over tens of hours. The typical decay curve takes the form of a power law of the type [56]. \[I(t)=\frac{I_{0}}{(1+\frac{t}{I_{0}})^{m}} \tag{16}\]  
and the total number of emitted photons is the integral of this function between a minimum and a maximum observation time. Hyperbolic forms such as this have already been used to describe the decay in delayed fluorescence and the temporal decay of coherent systems. The different parameters in this equation are determined from experimental data with a fitting procedure, and they characterize both the biological system under study and its physiological state [55]. The main characteristics of DL, in particular its long decay times and its existence at physiological temperatures, indicate that it can hardly be connected with delocalized states such as normal electronic bands. On the other hand, it is currently thought that DL is connected to the formation and dissociation of self-localized nonlinear electronic states of the soliton type, which could form in the low-dimensional macromolecular structures normally present within cellular structures [56; 57]. This type of model has a more general value, and it is not only confined to the DL, but, on the other hand, it is unable to explain cases of nonhyperbolic decay or the wide variability in the observed decay times. For this reason, a two-phase model was recently proposed: a stress transfer phase that transforms stress into a generation of photons, followed by a photon propagation phase [58]. In our opinion, spontaneous and stimulated emissions originate from substantially different excitation/deexcitation channels, although they probably involve the same types of molecules.