How to Paint Object Source Lighting on Sci-Fi Miniatures
Object Source Lighting is one of those techniques that looks intimidating until you actually try it, and then you realise it's mostly just controlled glazing and a willingness to be patient. The basic idea: you're faking the effect of a light source on the miniature itself, so the surfaces closest to a glowing element (an eye lens, a plasma coil, a reactor vent) pick up that colour, and it fades off with distance. Done well it reads instantly, even across a table.
I paint a lot of sci-fi stuff and OSL comes up constantly. Pieces like the Hades Cultists are perfect candidates because the sculpts have those hooded, shadowed faces where a single glowing source can do dramatic work, pulling out features from the dark.
Choose Your Light Source First
Before you put a single brush to the miniature, decide where the light is coming from and what colour it is. Write it down if you have to. Cold blue-white for energy weapons, sickly green for something toxic or warp-tainted, orange-red for plasma or fire. The colour you pick needs to be something you can mix lighter (towards white) and darker (towards a deep saturated version of the same hue). Consistency across the whole model depends on committing to this before you start.
Base Coat and Zenithal Prime
Zenithal priming is your best friend here. Black from below, light grey or white from directly above. This gives you a pre-shaded surface that already reads as lit from above, and your OSL is going to fight against that somewhat, so you want the base coat finished before you start the OSL work. Get all your standard colours down, shade with Nuln Oil or Agrax as needed, and consider the OSL as a final layer pass rather than something you work in from the start.
Glazing the Light
This is the actual technique. Take your OSL colour, heavily thin it with medium or water so it's almost transparent, and start applying it to the surfaces that face your light source. Build up slowly in layers, getting more opaque as you get closer to the source itself. For something like glowing eye lenses on a helmet, you'd work outward from the lens: brightest and most saturated immediately around the lens, fading to a ghost of colour on the cheekbone and brow ridge, nothing at all on the back of the head.
Use a wet palette. It keeps your glazes workable long enough to blend properly. If you're getting hard edges between glaze layers, you're either not thinning enough or working too fast before the previous layer dries.
The Light Source Itself
Whatever is emitting the light needs to be the brightest point on the model. Work up from your base colour through increasingly lighter mixes to almost pure white at the hottest spot. A glowing plasma coil on something like the Arctodus Tank would go from a deep saturated colour at the edges of the coil, through the mid-tone, up to near-white at the centre. That contrast is what sells the illusion.
Reflected Light on Surrounding Surfaces
Think about what's physically near the light source and what angle it faces. A flat horizontal surface below a glowing weapon barrel picks up the light more than a vertical surface beside it. The Angelic Seraphim models have fantastic armour panel geometry for exactly this, lots of facing planes that catch and reflect differently depending on your source placement. Use that to your advantage rather than applying OSL uniformly.
Common Mistakes to Avoid
Going too bright too fast is the main one. You can always add another glaze but it's harder to pull back. Keep your mixes translucent until the final highlights. The other trap is forgetting to desaturate the surfaces that are in shadow relative to your OSL source. If part of the model is facing away from the glow, it should have no OSL colour whatsoever. That contrast between lit and unlit areas is what makes the effect pop.
OSL rewards patience more than almost any other technique. Take your time with the glazes, trust the process, and you'll end up with a miniature that looks genuinely alive on the table.