Iray Ghost Light Kit is a comprehensive suite of invisible diffuse mesh lights intended as a discreet, easy to use architectural lighting solution.
A ghost light is a diffuse mesh light which is completely invisible at render time. Ghost lights do not show up in specular reflections, are incredibly cheap to render since they use single poly meshes, and can provide ambient lighting where other light sources aren't possible.
Ghost lights are most useful in cases where full scene convergence is difficult, and renders are noisy as a result, like window-lit interiors for example. Slow scene convergence can be the result of heavy path-tracing calculations, a problem that can be fixed by forcing more light into the scene. Ghost lights can do this in a discreet, cheap, and non-intrusive way.
Iray Ghost Light Kit comes with a variety of intensity presets, colour presets, artificial light presets and even time of day presets if you wish to mimic daylight. When each light is loaded, it shows as a handy checker-board with an arrow pointing in the lights direction for ease of use.
Note: Remember to apply the "Ghost Setup" material before applying presets. Otherwise the light will not be hidden from view.