What is a penlight sobriety test and how does it work?

If you were stopped at a roadside DUI checkpoint and an officer moved a penlight in front of your face and asked you to follow it with your eyes, this is the horizontal gaze nystagmus test, which is supposed to test for being under the influence of alcohol. The officer attempts to estimate the angle at which the eye begins to jerk (nystagmus is medical jargon for eye jerking); if this occurs sooner than 45 degrees, it theoretically indicates an excessive blood-alcohol concentration (BAC). The smoothness of how the eye tracks the penlight (or finger or pencil) is also a factor, as is the jerking of the eye when it is as far to the side as it can go.

This field sobriety test has proven to be subject to a number of different problems, not the least of which is that the officer is not medically trained and therefore does not have a strong ability to recognize nystagmus and estimate the angle of onset. Because of this, and the fact that the medical community does not accept the horizontal gaze nystagmus test, it is not admissible as evidence in many states. It continues, however, to be widely used by law enforcement.