Do you mean the light source itself? If so, be sure "Prevent black appearance" is enabled, but do note that this may not work in reflections or refractions depending on the directionality of the light (an IES pattern is just a directionality with a pattern to it). If that's the case, the solution is to create an object with a CoronaLight material on it (emit light disabled) to act as the visible / reflected / refracted light source, and make the actual light source invisible directly, to reflections, and to refractions - ie one will be seen and not emit light, one will emit light and not be seen.