Radiation modes are the building blocks for an exact analysis of radiation. We provide a simple ‘scalar’ determination of these vector modes valid on arbitrary waveguides, when the maximum core refractive index nco is slightly greater than that of the cladding ncl. The nco≈ncl radiation modes have not been previously reported for any optical waveguide. They simplify the description of radiation as the nco≈ncl bound modes do for guided power. Radiation modes are the building blocks for an exact analysis of radiation. We provide a simple ‘scalar’ determination of these vector modes valid on arbitrary waveguides, when the maximum core refractive index nco is slightly greater than that of the cladding ncl. The nco≈ncl radiation modes have not been previously reported for any optical waveguide. They simplify the description of radiation as the nco≈ncl bound modes do for guided power.