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The Concept of Irradiance in Photonics

Irradiance, also known as flux density, is a fundamental term in radiometry that quantifies the radiant flux received by a surface per unit area. In the International System of Units (SI), irradiance is measured in watts per square meter (W/m²). This concept is applicable to various forms of radiation, including light.

Relationship to Optical Intensity

In the realm of laser technology, optical intensity shares the same units as irradiance but represents a distinct quantity. Optical intensity measures the energy passing through a perpendicular area to the beam, whereas irradiance gauges the energy reaching a specific surface with a given orientation. When a laser beam strikes a workpiece at an angle, the irradiance is calculated as the beam intensity multiplied by a factor less than one. Multiple light sources can contribute to the overall irradiance.

Clarifying Terminology

It’s essential to differentiate optical intensity from radiant intensity, which refers to radiant flux per unit solid angle. Moreover, spectral irradiance denotes the irradiance per unit frequency or wavelength interval, expressed in units like W/(m² Hz) or W/(m² nm).

Measurement of Irradiance

Irradiance levels are typically measured using photodetectors with specific properties for accurate results:

• Responsivity with minimal wavelength dependence within the relevant spectral range.
• Angular sensitivity that remains consistent, irrespective of incident photon angles.

Exploring Further

For a comprehensive understanding of irradiance and its applications in photonics, additional exploration into related concepts like illuminance and radiance is recommended.

Source: Sun Climate – NASA
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