I don't understand the assumption that the heat output is going to be directly out of the poster, only orthogonal to it, like a columnated beam of heat. Clearly

I don't understand the assumption that the heat output is going to be directly out of the poster, only orthogonal to it, like a columnated beam of heat. Clearly, like any black body source, it's going to radiate in all directions. Or am I mistaken somehow?