In photobiology/phototoxicology, it is essential to understand “dose” in order to comprehend the consequences of light exposure. Perhaps the most basic property when considering electromagnetic radiation is that energy (E) is inversely proportional to wavelength or E = hc ^ X, where h equals Planck’s constant, c equals the speed of light in a vacuum, and X is the wavelength of light. Thus, when comparing lasers, for example a ruby (694 nm) to a long wavelength Nd:YAG (1064 nm), the energy of the light from the former is greater. Similarly, when considering the biological consequences of ultraviolet (UV) radiation in comparison to visible or infrared light, the energy of shorter wavelength UV is considerable greater. The net result is that the biological consequences of exposure to shorter, more energetic “doses” of light will influence the final outcome, such as a direct DNA damage produced by absorption of UV at wavelengths from 290 to 310 nm versus thermal response from vibrational energy resulting from the absorption of infrared radiation by melanin.
The unit of energy commonly used in photobiology is the Joule. The fluence, which is regularly referred to as “dose”, is Joule per unit area. The irradiance is the rate of delivery per unit area of the light source. The relationship between fluence and irradiance is: Flu – ence (J/cm2) = Irradiance (W/cm2) x time (seconds). With this simple equation and knowledge of the wavelength of light, photobiological events, adverse or otherwise, can be compared, and in some cases, predicted.
The energy associated with the lasers commonly used for nonablative purposes in dermatology lie in the infrared (IR), which is between visible (400-760 nm) and microwave (>1 mm). The portion of the IR concerned with lasers is near IR, 760-1400 nm. These divisions are quite arbitrary and based on different considerations, from optical detection to different temperature ranges used in astronomy. Regardless of this fact, the energy from IR-emitting lasers is not sufficient to promote electrons in the majority of chromophores and, as such, it is incapable of producing changes in the molecular structure [33]. The energy from IR radiation is converted to vibrational energy, which is thermal. This is clearly the energy form responsible for efficacy and adverse events.
There are excellent reviews of basic photochemistry/photobiology which are geared toward clinicians/biologists, for example Kochevar [33] and Arnt et al. [34,35].