radiant exposure, H

Radiant energy, $Q$ , incident from all upward directions on an small sphere divided by the cross-sectional area of that sphere. SI unit is $J m −2$ .

Notes:

Equivalent definition: Irradiance, $E,$ integrated over the time of irradiation.
Mathematical definition: $H = d Q / d S = ∫ t E ⁢ d t$ If $Q$ is constant over the area, $H = Q / S$ . If $E,$ is constant over the time interval, $H = E t$ .
This term refers to a beam not scattered or reflected by the target or its surroundings. For a beam incident from all directions fluence ( $H o$ , $F o$ ) is an equivalent term.

Source:

PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 409