I am setting up lighting for a scene and I want one light to have a god ray effect, however, when I use the volume scatter on the scene it works for ALL lights. How do I isolate a spot light source for this effect or if there is any other way please let me know. Thanks.
For this answer my scene is just 3 Spot Lights pointing down at a plane with a Strength of 200w.
If you're not using world lighting, this could be as simple as a primitive cube bounding box around the light in question with a Principled Volume node.
You might already see the problem in this image, but introducing any world lighting makes it much more apparent.
Which is that any lighting will be reflected towards the camera at a certain rate by the volume.
The core idea is sound for Cycles, since we can just use a different shape for a volume object. Unfortunately, in Eevee every volume is rendered as a cube bounding box around the object. I think it would be applicable in Eevee, but it may take some additional work. I'm not that familiar with it to be honest, so someone else may have a better answer.
I will continue for the sake of a complete Cycles solution. If we replace the cube with a cylinder we can move and scale the top and bottom faces such that they envelop the effective area of the light.
You'll have to do a little adjustment to the bounding box and see how it looks. Generally the lower your density is the harder it will be to detect any cutoffs, but that depends on what you need.
To make this animation-friendly you can make the light the parent of the volume object, and any transformations you make to the light will carry over to the child.
So hopefully that gets you started. :)
In the View Layer Properties just enable in the Light Settings In
EEVEE: Light > Volume > check Light. Cycles: Light > Volume > check Indirect.
Now when you click Render you will have a combined Pass and a Light Pass/Indirect Light Pass. It looks black and white because the volumetric light is isolated.
That Means you now have to disable all lights except the light used for the godray effect.
Then you can use the rendered Light Pass (with the isolated Direct Light from the godray) to mix it in the Compositor with the rendered scene (without any Volumetrics and all lights enabled excepts for the Godray Light).