Spatial average intensity (Isa) is averaged over _______.

• A. The cross-sectional area of the beam
• B. Time interval
• C. The beam path length
• D. Axial depth

Answer: (A)

Spatial average intensity is the average of the beam’s instantaneous intensity across its cross-sectional area at a given moment. In other words, you take the intensity at every point within the beam’s cross-section, average those values, and divide by the cross-sectional area. This focuses on how energy is distributed across the beam’s footprint, not how it changes over time or along the propagation path. If you were averaging over time, that would be temporal average intensity; averaging along the beam’s path or axial depth would address depth- or length-based measures, not Isa.