**2.2 Illumination**

Although PSs are known to possess a certain dark activity, illumination noticeably increases their cytotoxic effect [6, 14]. An example of the difference in antibacterial activity of different PSs with and without illumination is shown in **Table 1**. In this experiment, the MIC of three PSs was determined for the bacterium *S. aureus* in the dark and after 1 h of illumination. As a result of illumination, the MIC of the examined PSs decreased approximately 6-fold for MGO, 64-fold for RB, and 4-fold for Safranin O.

The main light sources used today for activation of PSs are lasers, light-emitting diodes (LED), and gas discharge lamps (GDL) [10, 31, 32]. There is no absolute advantage of one of these light sources over the others. The choice of light source depends on the specific application. Laser is a high-intensity monochromatic source. It can be easily coupled to a single optical fiber and installed on different lighting devices. LED lamps are cheaper and provide a wide emission spectrum. GDLs are also cheaper than lasers—both in acquisition and in maintenance and have a wide emission spectrum. However, GDLs transmit more heat to the illuminated area than lasers and LEDs, which can lead to tissue damage. In general, the emission spectrum and light intensity are more important for the excitation of a specific PS than the particular light source type [10, 31, 32].
