The figure shows a spot of light of uniform intensity $50\;W/m^2$ and size $10\;mm\times10\;mm$ incident at the exact center of a photo-detector, comprising two identical photo-diodes $D_1$ and $D_2$. Each diode has a sensitivity of $0.4\;A/W$ and is operated in the photoconductive mode. If the spot of light is displaced upwards by $100\;\mu m$, the resulting difference between the photocurrents generated by $D_1$ and $D_2$ in micro amperes, is ____________ $\mu A.$
