For the equation below regarding X$-$ray

$($a$)$ defining each of the variables including physical dimensional units

$($b$)$ outline the assumptions of the physical model described by the equation, and

$($c$)$ put into English what the equation is saying.

$I(x)={\displaystyle \underset{0}{\overset{}{}}}{S}_0(E^{\text{'}})E^{\text{'}}$exp$\{-{\displaystyle \underset{0}{\overset{x}{}}}(x^{\text{'}}$;$E^{\text{'}})d{x}^{\text{'}}\}d{E}^{\text{'}}$