Spectral Irradiance Posted on 21st December 20157th May 2021 by ThePD (The Project Definition) Spectral Irradiance is the most common way of characterising a light source, usually expressed in Watts per square meter-nanometre bandwidth. Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window)Click to share on LinkedIn (Opens in new window) Related Posted in Science and tagged radiation, radiology. ThePD (The Project Definition) ThePD has been developing the Preferred Project Definitions based on the actual project execution and operation experiences and knowledge with the Project Language, and sharing with you daily basis.