$H_{\text{er}}={\displaystyle \int {\displaystyle \int E_{\lambda }(\lambda ,t)s_{\text{er}}(\lambda )\text{d}\lambda \text{d}t}}$

where E_λ(λ, t) is spectral irradiance and s_er(λ) is the erythema spectral weighting function normalized to 1 at its maximum

Note 1 to entry: This definition is based on the assumption that an action spectrum is adopted for the actinic effect considered, and that its maximum value is 1.

Note 2 to entry: It is essential to specify which action spectrum is used, as the unit is the same for any action spectrum.

Note 3 to entry: See also "minimal erythema dose", "standard erythema dose".

Note 4 to entry: The erythemal dose is expressed in joule per square metre (J·m⁻²).