The width of the light beam is controlled by the knob on the side of the lamp housing.
The height of the beam is controlled by the wheel nearest the patient on the top of the lamp housing. The wheel nearer the examiner is used to put the cobalt and green filters in the light beam.
With the Topcon unit, the collar on the vertical portion of the light source rotates the light beam. The amount of rotation is noted on the scale above this collar in degrees. This is useful, for example, in measuring the rotation of toric contact lenses. The collar above this changes the lateral angle of the light source. This is used when you want to use retro or indirect illumination when the light source needs to be off to the side of the area under examination.
With the Marco unit the whole lamp housing is rotated to rotate the slit as for measuring the rotation of a toric contact lens. The top, narrow portion of the light tower is rotated to change the angle of the beam for indirect or retro-illumination.
