To characterize the efficiency of light interaction with biological tissue (inducing a photochemical reaction, temperature increase, evaporation, thermal mechanical breaking, etc.) besides choosing the wavelength of light, its energetic parameters are also important. Two major parameters are typically used: energy and power. Energy is the ability of light (as well as other forms of energy, such as mechanical, thermal, electrical, chemical, and nuclear) to produce some work; energy E is measured in joules (J). Power is the rate of delivery of energy; it is normally measured in watts (W) (i. e., joules per second (J/s)). The smaller and bigger energy units are in use in biomedical optics and photomedicine to characterize light sources and delivery optics; typically they are: energy—microjoule (pJ), 10-6 J; millijoule (mJ), 10-3 J; kilojoule (kJ), 103 J; power (P)—microwatt (pW), 10-6 W; milliwatt (mW), 10-3 W; kilowatt (kW), 103 W; megawatt (mW), 106 W; and gigawatt (GW), 109W.
At light interaction with tissues, produced photophysical, photochemical, or photobiological effects depend on energy density and/or power density that was provided within the target area. Energy density or fluence is the energy of the light wave that propagates through a unit area which is perpendicular to the direction of propagation of the light wave. Fluence is measured in J/cm2 or J/m2. Power density is the power of the light wave that propagates through a unit area which is perpendicular to the direction of propagation of the light wave. Power density or intensity is measured in W/cm2 or W/m2. The relationship between fluence (f) and intensity (I) is given by: F = I-xp, where tp is the length of pulse (pulsewidth) or exposure time. In inhomogeneous light scattering media to which tissues belong, the following parameter is often used: fluence rate (or total radiant energy fluence rate), that is, the sum of the radiance over all angles at a point r; the quantity that is typically measured in irradiated tissues in units watts per square meter or centimeter (W/m2 or W/cm2).
Several measures of light are commonly known as intensity: radiant intensity is a radiometric quantity, measured in watts per steradian (W/sr); luminous intensity is a photometric quantity, measured in lumens per steradian (lm/sr), or candela (cd); radiance (irradiance) is commonly called “intensity” or “quantum flux” measured in W/m2 or W/cm2.