Introduction

The Janus face of photomedicine allows light to kill undesirable cells such as cancers and pathogenic infections, while at the same time protecting and stimulating injured or dysfunctional normal cells. The killing effect is usually obtained when visible light is combined with photosensitizing dyes in a process called photodynamic therapy, that produces damaging reactive oxygen species. On the other hand, the stimulating effect of light takes advantage of the absorption of red and NIR light in the mitochondria in a process called photobiomodulation, leading to activation of transcription factors.