Mental ray mist and beam, plus parti volume for a volume spot light

Lume mist

Set transparency to 0.0 and color to white to get started.

It is critical that you know the distance range in max units for where you want the fog to appear, in depth. Set this up ion the top view. Use a tape helper or the camera target. My tape has a distance of 1200.

Turn off realistic and custom falloff.

Turn on linear falloff. Here I want the fog to start 100 units from the camera, and end [be 100%] at 1200 units from teh camera.

This shows adding a mrSpot with Final gather on.

Now we are adding a volume shader to the box [room] geometry. itself.

you will need to make a new material and apply it to the room.

You can turn also turn off the camera shader

If you back out you can see the mist is within the room

Beam

Remember, you must know your real distances if you are using photometric lights. Beams works with standard lights fine as well My camera is 5 feet from the lights.

Note – the brighter light is a 1.0 standard omni

Use Parti volume to get a volume spot light

Set up a plane in the back – this gets the parti volume material.

[Make the plane on the “floor” a medium gray]

“Get Material” and select your spot light:

The scatter color should be set to a range of gray – white will blast out the light, black turns it off.