Police officers sometimes shoot unarmed suspects in error. The majority, about 71%, occur at night under poor visibility conditions. In one common scenario, a suspect suddenly produces an object in his hand. The officer, believing the object to be a weapon, fires at the suspect. The officer then approaches, only to see that the firearm was actually something as harmless as a driver's license or wallet. The subsequent inquiry must judge whether the officer acted reasonably. One component of reasonableness is the officer's perception; should he have correctly recognized that the object was not a gun? The visibility conditions, especially light levels, usually become a central issue. Investigators often attempt to assess visibility from witness statements such as "It was bright," "I could see three blocks down the street," "There was enough light to read my notebook," etc. In reality, these statements reveal little about visibility conditions at the time of the shooting. A realistic assessment requires an in-depth analysis of specific situational factors that determined perception. Roughly speaking, these factors can be divided into two categories, sensory and cognitive. Sensory processing is the initial stage, where light enters the eye and forms an image. When people speak of visibility conditions, they are usually talking about light levels and other factors that affect registration of the image information.
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