Irradiance

Related Concepts

• Radiance

Definition

Irradiance E is defined as the incident power of electromagnetic radiation on a surface per unit surface area. It is expressed in watt per square meter (W ⋅ m⁻²).

Background

Irradiance is a concept from radiometry, the science of measuring radiant energy transfer [1]. The equivalent concept in photometry is illuminance, with the key difference being that illuminance is adjusted to account for the varying sensitivity of the human eye to different wavelengths of light.

Theory

The irradiance at a surface point x is proportional to the radiance L(x, θ, ϕ) arriving at x from direction (θ, ϕ) with a geometric foreshortening factor $\cos \theta$. Taking into account the whole hemisphere above the surface point, the irradiance is the integral over all incoming directions

$$\displaystyle \begin{aligned} E(\boldsymbol{x}) = \int_{\theta,\phi} L(\boldsymbol{x}, \theta, \phi) \cos{\theta} \,\,\, d\theta d\phi. {} \end{aligned} $$

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