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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.
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