You can also assign visibility to point and conical lights, and can select whether they cast shadows and add a projected 'gel'. The outline of the light object defines the region of the light glow. The glow is independant of brightness and is instead a material property. The preview window in the materials composer will not correctly display the glow effect unless you chose the appropriate primitive in popup under the preview icon. The preview window in the light editor will never show the glow effect.
Glow works easily for spheres.
You can edit its material and reduce its transparency to 0 to enhance the effect. Visible lights do not work on the other light sources, despite the fact you can check the option when editing their properties. It will however lose the more natural falloff with distance. These do not fall off with distance like real lights. The bottom right edit control allows you to adjust the fuzziness of the light edge.