A specially designed lens that mounts under the stage and is usually movable in the vertical direction.  It has an iris type aperture to control the diameter of the beam of light entering the lens system.  By changing the size of the iris and moving the lens toward or away from the stage, the diameter and focal point of the cone of light that goe [..]

an advanced illumination system which uses a lens under the stage that would typically move in the vertical direction. It would usually also have an adjustable iris to control the beam diameter of the [..]

A moveable lens system under the stage that can be moved up and down vertically, regulating the amount of light with an adjustable iris to control the beam diameter from the light source. It is very useful at magnifications above 400x.