$$\mathrm{I}=\int_{0}^{h_1}\int_{x_L}^{x_U}\Lambda(r^\prime)\frac{e^{-jkR}}{4\pi R}dx dy$$ Here, $$x_L = \hat{n}\cdot\left(\hat{h_1}\times \overrightarrow{ \ell_2} \right) \left(1-\xi\right)$$ where, $$\xi\,\mathrm{is}\,1\,\mathrm{or}\,0\,\mathrm{for}\,y=0\,\mathrm{or}\,h_1$$
How is $\hat{n}\cdot\left(\hat{h_1}\times\overrightarrow{\ell_2}\right)$ multipled with $\left(1-\xi\right)$ will be the lower limit $x_L$ of the integral $\mathrm{I}$?