Here it is an extract from a discussion between Charles Blaquiere and P.Sauvageau:
PS> I have find that the best way to make visible light is to use PS> conical shaped bright objects. I use linear texture to augment PS> transparency along the y axis, so the cone is semi-transparent PS> (160-160-160) at the source and full transparent (255) at the end. CB> In addition, you could use the Ghost or Fakely textures; they CB> would give you more control over the transparency falloff. This is CB> off the top of my head, BTW; I haven't actually tried it to see if CB> it gives better/worse results than plain ole fog. PS> One thing that I have done is to use FilterNoize applied on a PS> parent axis, with "Apply to children". When I rotate the light PS> cone, I do not move the parent axis. This way, you will see the PS> light beem moving in an immobile cloud of irregular smoke or fog. PS> One of the problem I encoutered is that Filter Noise seem to PS> override the linear transparency setting, so the beem do not appear PS> to vanish as it come farther from the light source. PS> A conical lightsource is cast in the Y axis direction and the PS> linear texture extend in the Z direction, so you will have to rotate PS> the texture axis to have it's Z pointing in the same direction as PS> the light source Y axis. The transition lenght of the texture must PS> be a little shorter than the lenght of the object or the edge of the PS> cone will be visible. Make the cone object transparency to 150, and PS> the texture 255. Make the cone object bright. PS> A very interesting effect can be made with this kind of light PS> beem in an animation: I wanted the light beem to appear to be casted PS> through smoke or fog. I applied a pastella texture on the cone to PS> make it appear to be more irregular, but when the light move, the PS> effect look verry artificial. So I added a parent axis to the light. PS> This axis have the pastella texture with "apply to children" PS> property. To animate the light beam, I moved it in the cycle editor, PS> while keeping the parent axis untouched. When animated, the light PS> beem seem to move in an immobile cloud.