Drunk Driving Defense, DUI Arrest - FAQs
6. When a police officer asks me to follow a penlight what does he look for?
This test is called the horizontal gaze nystagmus (HGN) test. Nystagmus is the medical term that describes a particular eye oscillation. The steadiness of the eyes while following the pen and the degree at which the eyes begin to move erratically indicate the level of alcohol in the blood of the suspect. The police officer attempts to determine if the angle of the eyes are less than 45 degrees. An angle of less than 45 generally indicates a blood-alcohol level of .05% or greater.
The horizontal gaze nystagmus test is still fairly new and has not been accepted for use in many states. Many medical professionals do not consider this test to be credible. It is also possible that some police officers performing this test are not qualified to administer the test. The untrained officer may not be able to negotiate the proper angle and hence misjudge nystagmus. Although approved in other states, this test is not accepted for use in California.