Using a photonic band calculation method, we calculate the effective dielectric function of an array of carbon nanotubes which have a simple equivalent isotropic dielectric function. The results for the equivalent dielectric function and for the local graphite-like dielectric tensor are found to be almost the same. Taking a fixed outer diameter of the nanotubes to be 10 nm and a fixed distance between them to be 10.3 nm, we investigate the effect of the hollow core and found that it is important that the ratio of the inner to the outer radii is approximately larger than 0.3. By taking a large ratio of 0.58, the theoretical results are improved greatly and can describe the experimental data very well.