Biophotonics_Concepts_to_Applications

4.1.1 Optical Flux and Power

A key radiometric parameter for characterizing a light source is theflux(also known
asopticalfluxorradiantflux), which is the rate of energyflow with respect to time.
Because energy per time is power, this parameter gives theoptical poweremitted
by a source. Standard symbols that are recommended for power areΦ(upper case
Greek letter phi) or P. The letter P will be used in this book. Theflux or power
typically is measured in units of watts (W).

4.1.2 Irradiance or Exposure Rate

When illuminating a tissue area for applications such as imaging, therapy, or sur-
gery, the quantity of interest is theirradiance, which is designated by the symbol E.
Equivalently the irradiance is called theexposure ratebecause it designates the
energy incident on a tissue area as a function of time. This parameter is the power
incident from all directions in a hemisphere, or the power emitted through an optical
aperture, onto a target surface area as shown in Fig.4.2. Thus, irradiance E is
given by

E¼dP/dA ð 4 : 1 Þ

where dP is the incremental amount of power hitting the incremental area dA.
Irradiance is also known as thepower density(orflux density) and is measured in
units of power per area, for example, W/m^2 or mW/cm^2.

Example 4.1Consider an optical source that emits a highly collimated cir-

cular beam of light. Suppose the beam diameter is 5 mm and let the power

level or radiantflux be 1 W. Neglecting any divergence of the beam, compute

the irradiance.

Power emitted from

an optical aperture

Illuminated

tissue area

Fig. 4.2 Illustration of Optical aperture
irradiance: optical power
falling on a tissue area

94 4 Fundamentals of Light Sources