Surviving the Nightshift™

By Thomas J. Aveni, MSFP

 & Edward Godnig, OD
The Police Policy Studies Council 

Part 3: Low Light Visual Adaptation: Facts & Misconceptions

Vision & Perception


Vision is learned by our personal and dynamic experiences of interacting with light energy that is absorbed, refracted or reflected from objects within our environment. Humans must integrate light information with sensory information collected from other organs in order to make sense of a world full of changing light intensities, colors, shapes, smells, textures, sounds and movements. As we mature, we are able to use vision as the dominant system to direct our attention on specific areas that we decide need further understanding and interpretation. When light becomes diminished or absent, our visual system reacts in many different ways that change the way we interpret and understand our surroundings.

It might be safe to say that most law enforcement officers have had cursory training pertinent to occupational safety under low light conditions. It’s also likely that their training included some degree of exposure regarding how the photoreceptors in the eyes adapt to changing light conditions. Unfortunately, much of whatever training law enforcement officers may have had regarding the light adaptive process has either been inaccurate, misleading, or both.

Photopic, Mesopic and Scotopic Vision

To understand how the eye functions under varied light conditions we should first discuss the three operational modes of vision; photopic, mesopic and scotopic. Photopic vision occurs at high light levels and is characterized by (1) heavy dependence upon cone photoreceptors, (2) low light sensitivity, (3) high visual acuity and (4) color vision. Scotopic vision occurs at very low light levels and exhibits 1) use of cone photoreceptors, 2) high light sensitivity, 3) poor acuity and 4) no color vision.

In urban environments, there is often enough ambient light available at night to make true scotopic vision a less frequent occurrence. However, unlit alleys, basements, abandoned buildings, etc. are ubiquitous enough to provide ample opportunity for scotopic vision if the officer finds him/her self unprepared for the occasion.

Under conditions in which street lights, car headlights, store and residential lighting is ever-present, the eye operates in mesopic vision, which is a state of photoreception where the bottom of cone and top of the rod operating levels overlap. Mesopic vision is therefore a more complex visual process than that of photopic or scotopic vision. It shouldn’t be surprising that most night-time accidents occur when the viewer is operating in the mesopic mode rather than in the scotopic visual mode.

Throughout the day we are constantly exposed to different levels of illumination entering our eyes. The size of the pupil helps regulate the amount of light entering our eyes, and the level of dark adaptation allows us to function under diminishing exposures of light. Dark adaptation is the process of changing from cone dominated vision to rod dominated vision. A person becomes dark adapted over a period of 10 to 60 minutes. Although dark adaptation begins slowly as we enter a dark environment, for the first 7 minutes after someone enters the dark the fovea is still the most sensitive region of the eye. After this, the rods begin to control visual sensitivity and within 30 minutes, the eye is estimated to be virtually completed dark adapted. Between 30 minutes and 60 minutes there may be a small degree additional dark adaptation. When rods are at their highest dark-adapted state, they are highly sensitive to low levels of ambient light. However, even though the rods set the lower sensitivity boundary, the cones will still respond to a sufficiently bright light. A driver on a dark rural road, for example, may be in a scotopic modality but could still see the color of warning light well ahead if it were intense enough to stimulate cones.

Conversely, light adaptation is an extremely fast event. If you are dark-adapted and are exposed to bright light, light adaptation begins as soon as the luminance levels are high enough to stimulate the cones and begin bleaching the rods. If a person remains in a high luminance environment, full light adaptation usually is complete within a minute. During the first minute of light adaptation, contrast sensitivity gradually improves. The appearance of images begins to change from the earliest seconds of light adapted images and colors looking “washed out”, to full color saturation and contrast of fully light adapted images. As light adaptation continues, the contrast between the light and dark areas becomes more evident and easier to see. Both rods and cones participate in light adaptation, showing a change in sensitivity to lights superimposed upon a dimmer background light environment.

In a given tour of duty, the eyes might be forced to adapt to changing light levels many dozens of times. This process is an important but frequently overlooked issue in many officer-involved shooting cases. An officer’s visual and perceptual ability is often determined not only by the scene that he/she was viewing at the time of the incident, but also by what the officer had been viewing previously. For example, an officer transitioning from time expended looking at the bright screen of a cruiser’s MDT would likely encounter difficulty if threatened from a darkened area adjacent to the exterior of the patrol car.

Along with color vision loss, when the eye is in the state of scotopia there is a degradation of visual acuity and the ability to see fine detailed images. Visual acuity drops to the range of 20/100 to 20/200. To put this into perspective, a person with daytime visual acuity of 20/20 can identify a certain sized target object at 200 feet away. If this same person loses visual acuity and now has 20/200 visual acuity, this same person must now move to 20 feet to see the same sized target object. To be more specific, the numerator represents the testing distance measured in feet, so 20/20 and 20/200 both correspond to a 20- foot testing distance. The denominator represents the size of a target that subtends a five-minute arc at a specific viewing distance. Therefore at 20/20 visual acuity, the denominator 20 is the size of the target that subtends a five- minute arc at 20 feet. If the visual acuity is 20/200, 200 is the size of the target that subtends a five-minute arc at 200 feet.

Related to visual acuity degradation during scotopia is the reduction of contrast sensitivity for all spatial frequencies. Since high spatial frequency contrast sensitivity is lowered, it becomes quite difficult to discriminate between light and dark contoured lines. Appreciating where gray-toned borders begin and end in dark environments requires adequately functioning contrast sensitivity skills. Scotopic vision weakens the ability of humans to distinguish a figure from its background because the contrast sensitivity is inefficient in low levels of light. Of course, once it becomes confusing as to where the figure ends and the background begins, locating and searching for targets becomes tentative. Background surfaces begin to lose their texture gradient identification characteristics, and this in turn further confuses the separation of figure from ground.

Specific Operational Issues

If light levels become low enough so that there is not a target stimuli bright enough to elicit a specific accommodative response, then the eyes begin to show ‘night myopia’ (also called ‘dark focus’). Although there exists a wide range of individual variance as to the degree of night myopia, most studies show the average degree of night myopia causes the eyes to focus at about one yard in front of the eyes. This means that in darkness, the eyes will not maintain focus at far distances, and tend to focus very close to the observer. If you are trying to see something at a distance more than a few feet in front of you in darkness, your eye’s focusing system will not cooperate. This involuntary loss of far focusing control further degrades visual confidence as to what you are observing in darkness.

If you combine some of the visual changes that take place during scotopia, particularly loss of color, reduced visual acuity, and loss of contrast sensitivity and accommodative control, it is easy to imagine that having confidence as to where an object is located, what details define an object, and how an object may be changing, is easily lost. Scotopic vision in comparison to photopic vision presents visual physiological and visual perceptual differences that may be confusing and lead to visual uncertainties.

‘Dark focus’, sometimes called ‘night myopia’, is a visual phenomenon that occurs if the eye focusing system has no specific target to focus upon in darkness. While functioning in this dark environment, the eyes tend to focus at a close range within a few feet from the eyes. Any threat identification beyond a few feet away will be blurred because the eyes may be unable to focus at far distances. Understanding the concept of ‘night myopia’ adds more understanding as to why threat identification is compromised at night.

Low levels of light can lead to other types of visual illusions and physiological changes. These illusions occur because of the lack of visual cues necessary to judge spatial relationships. The so-called, “autokinetic effect” is the visual sensation of perceived movement of a dim light observed while staring at the dim light in a dark environment. The dim light appears to move even though it is stationary or barely moving. The illusion usually disappears when the eyes can view multiple lights. This illusion can be reduced by increasing the brightness of the dim light, or by moving your eyes to different positions of gaze. This effect is often seen while driving at night, particularly while fatigued, as taillights begin to jump around in random motion.

Viewing an unlighted terrain from an elevated vantage point can result in a ‘black hole illusion’. This illusion is exaggerated when the horizon is not easily seen. With a scarcity of visual cues necessary for accurate spatial orientation, the observation is similar to looking into an unlit hole. This illusion makes judging distances problematic. Deciding on how far an assailant may be located from an observer is at best a random possibility if the observer is experiencing the ‘black hole illusion’. In addition to the obvious forensic implications, there are also many compelling training and equipment concerns embedded within these issues.

In our next installment, we’ll be examining very problematic “Threat Location & Identification” issues, and the alarming regularity in which police training seems to have contributed to some of the most egregious problems encountered on the street.