Visual Perception in Low-Light Levels

Article excerpt

Implications for Shooting Incidents

While on evening patrol, officers discovered two men lurking near a closed gas station in a high-crime area. In the confrontation that followed, the officers fired on the suspects, one of whom appeared to be holding a shotgun. The officers believed that the other man had pulled a chrome-plated handgun from his waistband. Later investigation revealed that the man was, in fact, holding a beer can. He sued the officer who shot him.

During the trial, expert testimony centered on the nature of human vision, the low level of light at the time of the incident, and the results of a research study that demonstrated the ability of healthy subjects to identify lethal versus nonlethal items under a range of low levels of light, the type of conditions officers often face when working at night.(1) The results of this study can assist investigators when determining what an officer can identify under certain environmental circumstances.

Procedure

This experiment used 12 police cadets as subjects. Prior to admission to the academy, a general physician had prescreened the cadets for corrected 20/20 distance visual acuity. Each cadet was reexamined individually for corrected 20/20 vision and measured for hidden refractive error - the cause of nearsightedness, farsightedness. and astigmatism - by observing how parallel beams of light reflect off the retina of the eye. The examination detected no eye disease among the cadets.

The cadets were taken from their classroom. which was at a standard office lighting level, and brought to the research room. A research assistant wore a black jacket, consistent with clothing often worn by crime suspects, and showed each cadet three nonlethal objects and a large-frame handgun under each of four incrementally increasing levels of low light. The black jacket served as a background for the object, and the assistant stood behind an opaque partition that was quickly shifted down for 1 second. The assistant did not point the object in the direction of the cadet but held each object in a clenched fist close to his body, similar to the physical circumstance of many shooting incidents.

Specifically, the nonlethal objects consisted of a 6-inch piece of green garden hose, an 8-inch piece of black pipe, and a 6-inch chrome-plated screw driver, According to police documents, officers had misidentified similar objects as lethal during the past 10 years. Academy regulations prescribed only the use of academy-deactivated firearms in this study; therefore, a blue steel model 59 Smith & Wesson handgun was chosen as the lethal object because it has a large and distinctive shape.

The experiment used several lighting levels. These levels ranged from .04 foot-candles to .45 foot-candles. For comparison, a bright, full moon on a clear night exhibits illumination comparable to a .01 foot-candle lighting level. A person standing 30 to 40 feet from the direct beam of a vehicle's headlights at night compares to a .45 foot-candle lighting level.

Each cadet viewed each object individually for 1 second. After the presentation of the object, the cadet's attempt to identify the object was recorded.

Results

Each cadet viewed one lethal and three nonlethal objects at each lighting level. Therefore, 48 responses were recorded at each level.

At .04 foot-candles, cadets correctly identified an object only 4 times; they misidentified or said they could not identify an object 44 times. This represented a 9 percent rate of correct identification. At .10 foot-candles, cadets correctly identified an object only 8 times; they misidentified or said they could not identify an object 40 times. This represented an 18 percent rate of correct identification. At .25 foot-candles, cadets correctly identified an object only 15 times; they misidentified or said they could not identify an object 33 times. This represented a 34 percent rate of correct identification. …