How to make light cones visible


I have a problem with lightint techniques which I can't solve on my own. I want the light cones to be visible, like you see them with volumetric light. Basicly it shoud kinda look like the screenshot.

Any Ideas?

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We finaly found a solution for this.
Our solution is a shader which makes the plane transparent, except for the parts where the spot lights shine onto the plane.
That’s no volumetric lighting but it’s good enough for our game.