What type of lighting will produce the most visible beam in daylight?

Have harder light than the surrounding light like a stage play would do to someone doing a monologue?

There is no form of light that will be visible unless it hits something in the air.

@thorninmud I understand that but there is always some particulate in the air, even in a clean room. My question pertains to what type of lighting should I choose to best achieve this effect.

@auhsojsa what is “hard” light?

The well-lit room presents a problem since any light “disappears” in a well lit room. I suggest you try an entirely different approach and consider something like suspended dowels that are painted with fluorescent green paint. They will seem to “glow” in a well lit room.

@JaneraSolomon Thanks for the creative solution but objects must be able to be passed through the beam.

LEDs

any strong light source, combined with introduced extra particles, such as a smoke generator based on co2, or a fine mist of water. Make sure to use clean water and a very fine mist.

Also, try if you can dim the room.

For a tunnel…. use laser and a motorized mirror with controls.