Consider the radiation heat exchange inside an annulus between two very long concentric cylinders. The radius of the outer cylinder is $R_o$ and that of the inner cylinder is $R_i$ . The radiation view factor of the outer cylinder onto itself is
- $1-\sqrt{\dfrac{R_i}{R_o}}\\$
- $\sqrt{1-\dfrac{R_i}{R_o}} \\$
- $1-\dfrac{R_i}{R_o}^{1/3} \\$
- $1-\dfrac{R_i}{R_o}$