What aspect of vision is the Maddox rod primarily used to assess?

The Maddox rod is primarily used to assess phorias, which are latent deviations of the eyes that occur when binocular vision is disrupted. When testing for phorias, the Maddox rod is placed in front of one eye, typically creating a line or streak of light perceived by that eye. The patient is then asked to fixate on a light source, and any misalignment of the visual axes can be quantified by observing the deviation of the perceived image of the light source, as the other eye will see the target differently due to the rod's effect.

This method is valuable because phorias can reveal underlying strabismic conditions or muscle imbalances in the eyes that may not be apparent during standard visual acuity testing. By precisely measuring the angle of deviation, practitioners can determine the extent of the phoria and adjust treatment plans accordingly.

Other options, like color vision, visual fields, and refractive errors, are assessed using entirely different techniques and instruments, which focus on different aspects of visual function.