As shown in the figure, three identical polaroids P$_1$, P$_2$ and P$_3$ are placed one after another. The pass axis of P$_2$ and P$_3$ are inclined at angle of 60$^\circ$ and 90$^\circ$ with respect to axis of P$_1$. The source S has an intensity of 256 $\frac{W}{m^2}$. The intensity of light at point O is ____________ $\frac{W}{m^2}$.