Staff Views
L.E. Forum

Visual Perception in Low-Light Levels

Implications for Shooting Incidents
By Paul Michel, O.D.

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 non-lethal 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.

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 non-lethal 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 non-lethal 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 footcandle 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.

Each cadet viewed one lethal and three non-lethal 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 footcandles, 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. At .45 footcandles, cadets correctly identified an object 37 times; they misidentified or said they could not identify an object 11 times. This represented an 84 percent rate of correct identification.

Cadets most frequently identified the handgun correctly. At .25 foot-candles, 10 of the 12 cadets identified the handgun correctly, but 2 cadets still incorrectly identified it or stated they could not identify the object. The gun used in the experiment is one of the largest handguns usually encountered on the street. Had a smaller pocket handgun been used, a higher rate of incorrect identification might have occurred in the low-lighting levels due to the smaller size. Cadets most frequently misidentified the 6-inch piece of green garden hose. Even at the .45-foot-candle level, cadets most frequently identified the hose as a gun. Only one cadet identified the hose as a pipe or cylindrical object.

This study demonstrated the difficulty officers have distinguishing between lethal and non-lethal objects under low-lighting levels. Most of the cadets voiced uncertainty about their responses, even when they were correct. When asked to make a determination of the object in less than .25 foot-candles of light, cadets most frequently responded “I cannot tell.” During an exit interview, 80 percent stated they relied upon the positioning of the assistant’s hands to help make their determination of the objects. Yet, the assistant purposely had held the object in a neutral manner.

Practical Applications
Officer-involved shootings rarely occur exactly as constructed in this experiment. The time frame involved from when an officer perceives danger to the time deadly force is considered necessary is more realistically only a fraction of a second and not the full second allotted in this experiment. Additionally, the suspect or the officer is frequently in motion before and during the shooting. This movement decreases visual ability. The psychological and physiological effects of fear also decrease the level of visual functioning. The human body instantly undergoes profound physiological changes in response to perceived threatening circumstances. Visual functioning dramatically decreases in response to hormones secreted during acute fear. Without sufficient lighting, the retina of the eye cannot form an accurate image of the external environment. An ambiguous image is created at the retina and transmitted to the brain. The brain then integrates this ambiguity with cognitive, memory, and emotional elements to form a perception. The officer uses this perception to evaluate the suspect’s actions and to react.

Based on the factors that affect officers’ visual perceptions during confrontations in low-light levels, officers need at least 2.5 to 3 foot-candles of light to accurately identify an object. Shining a halogen flashlight on an object from a distance of 20 feet would create the level necessary for proper visual functioning. Furthermore, lighting conditions that officers face prior to an incident may significantly affect their ability to discern shapes and details in a darker environment. During the 20 minutes prior to the incident, if an officer is exposed to lighting higher than when the incident occurs, a transient disability affects the officer’s vision. The retina experiences chemical and neurological changes as lighting levels change. After 40 minutes, a person’s eyes adapt to low-level lighting conditions. If that lighting condition changes to a higher level of light for even a fraction of a second, the dark adaptation is lost. For example, if an officer seated in a darkened patrol car uses the car’s interior light to check an address, the lighting level immediately changes and the officer loses the dark adaptation. An overwhelming majority of officer-involved shooting incidents have demonstrated this often-overlooked disability. A thorough history of the officer’s activities prior to the incident will aid a complete understanding of the visual environment under which the incident occurred.

This research study demonstrates that police officers have difficulty differentiating between lethal and non-lethal objects illuminated by less than .50 foot-candles of light, the level frequently encountered during routine police working conditions at night. Officers should recognize this disadvantage and adapt their procedures accordingly, by increasing lighting levels using their car’s headlights and flashlights with halogen bulbs. A weapon-mounted lighting device, attached to the muzzle of an officer’s weapon, can increase visual ability in low-level lighting conditions. This device is designed for short-barrel defensive shotguns, uses a small halogen bulb, and has an on/off switch. In addition, officers should train in low-level lighting conditions.

Finally, during the investigation of an officer-involved shooting, police administrators should document the lighting level at the time of the incident with an illuminometer and consult a vision expert with experience in this area.

The investigation of an officer-involved shooting is never an easy undertaking. A number of variables complicate the process, including the officer’s visual perceptions prior to and during the incident. Prior shooting incidents and this research study indicate that officers have difficulty differentiating between lethal and non-lethal objects in low levels of light. By understanding the nature of human vision and the implications of this research, administrators and officers alike can prepare for the inevitable encounter with the suspect in the dark alley. Perhaps more important, by properly documenting shooting incidents, conducting thorough investigations, and preparing expert testimony on the nature of these encounters, law enforcement agencies can avoid legal liability while reminding the public of the dangers associated with protecting the community.

The author of this article conducted the study and testified in court.

Dr. Michel, a board-certified therapeutic optometrist, serves as a specialist reserve police officer for the Los Angeles, California, Police Department’s officer-involved shooting investigations unit.