The photon transfer function for accretion disks around a Kerr black hole

The observed spectrum from a thin accretion disk around a black hole can be calculated with the help of a transfer function f provided the local emission in the disk plane is given. This function contains the Doppler boost due to the disk rotation, the relativistic light deflection, and the gravitational redshift. In this paper the transfer function is calculated and some of its properties are discussed. A fast and easy-to-use computer program is presented to obtain numerical values of f for any set of parameters.