Shedding light on light
Surviving the Nightshift™
By Thomas J. Aveni, MSFP
The Police Policy Studies Council
Sponsored by ITT Night Vision
Related article: Introduction to Policing at Night
As we established in the first installment of this series, linked above, without light there can be no (unaided) human vision. Light is the enabler of the complex visual/cognitive process. But, how much do we need to know about light? The most concise answer might be that we need to learn enough about the nature of light to be able to accentuate our visual strengths and compensate for our visual limitations in operational settings.
In delving into this topical area it’s important to understand that some degree of technical knowledge is desirable to acquire for a number of substantive reasons. Pursuant to affording officers the most “usable” information, technical information will be limited in nature to what is essential for proper equipment selection and critical threat identification purposes.
What, When or How Do We Define Darkness?
To illustrate this point, attempt to read a newspaper or magazine directly under a 75-watt light bulb. At a distance of 19 inches, under a bare bulb (no lampshade or bulb enclosure), the 25 foot-candles afforded by that bulb at that distance will enable most people to read the text on a typical newspaper page. However, at a distance of just 7 feet, the light measured from that same 75-watt bulb is now just 2.5 foot-candles, and many people will experience difficulty reading text at that light level. At 36 feet, the measured intensity of light from that bulb is just .055 foot-candles, and even large, bold text would be impossible for most to read at that light level. The occupational safety implications of this phenomenon are many, and most tend to be significant.
Illumination follows an “inverse square” law. For example, for any given reading, if the light meter is held twice as far away from the light, the meter will read only one fourth as much light intensity; if the light meter is held half as far away from the light, the meter will read four times as much.
When does diminished lighting begin to have serious occupational consequences? To some extent, that determination is dependent upon an individual’s age, diet, and overall health traits. Having said this, we’ll find occupationally significant visual impairment at light levels that most officers wouldn’t expect to be problematic. How much we can’t “see” at lower light levels isn’t always the source of flawed decision-making. The diminished quality of what we do see is often what has critical task significance.
Understanding Light Energy
As you undoubtedly learned in grade school, light has a finite velocity but travels at a speed difficult to comprehend. In a vacuum, the speed of light is 186,282 miles per second! As a frame of reference, it takes less than three seconds for a radio transmission traveling at the speed of light to travel to the moon and back.
Light is often referred to as electromagnetic radiation. Okay, that’s not a common phrase in police lexicon. However, too often when we use the word "light" our intent is to suggest "optical light," which roughly approximates the radiation visible to our eyes. Visible light is a small fraction of a much larger electromagnetic spectrum. For the sake of convenience, we can divide this spectrum up into different categories; gamma-rays, X-rays, ultraviolet, optical, infrared, and radio. However, remember, that they are all just light. There are no discernable breaks or hard boundaries in the electromagnetic spectrum. It is merely a continuous range of energy.
Infra-Red (IR) light falls below the lower end of the visible light spectrum for the human eye and is therefore not visible to the naked eye. Infra-Red spectrum light can be utilized through the use of Night Vision Devices (NVD). If used in conjunction with IR illuminators, NVDs even can be employed in scenarios involving indoor operations or those involving exceptionally overcast night skies. We’ll examine NVDs in a later segment of this series to gain a better appreciation of their operational value.
Luminance & Illuminance
Luminance is a measure of light coming from an object’s surface (in contrast to illuminance, or light output). Unlike illuminance, luminance does not follow a square law, but the area of measurement must be defined. Using a wall as an example, the luminance of the wall is the same whether measured from four feet away or six feet away. Moving closer to or further away from the wall does not change its apparent brightness. The luminance of a particular surface is usually referred to as its "brightness" because it is the quality of brightness that we perceive.
Light Measurement: Addressing Terms of Confusion
Over the last decade, many officers have been faced with confusing photometric terms when attempting to select flashlights for purchase. Many have been rated in terms of “candlepower” while others have been rated in “lumens.” In addition, manufacturer’s light output ratings were often, shall we say….. overly optimistic?
Candlepower is an old English rating of light output which is measured at the source based on the amount of light emitted by a “church candle.” This became a universal standard of light measurement that is still widely used today. It’s important to remember that foot-candles are a measurement of light at an illuminated object. Candlepower is a way of measuring how much light is produced by a light bulb, or LED. It is NOT a measure of how much light falls upon an object at a distance.
The term candela is more commonly used today than is candlepower. But, as with the term candlepower, candela is a measure of how much light the bulb produces, measured at the bulb, rather than how much falls upon the thing you want to light up. Evaluating the efficiency of a flashlight rated in candlepower or candela is further complicated by beam focus characteristics. For instance, a flashlight’s candlepower is generally enhanced by the degree of focus attributed to its reflector and lens assembly. By using an internal reflector, the flashlight projects all of its bulb’s intensity at a given spot, giving it more light intensity and a higher illuminance output rating.
You should also be aware of the fact that a candlepower unit of measure is NOT the same as a foot-candle. Candlepower is a measure of light taken at the source-not at the target. Foot-candles quantify how much of that light is directed at an object we wish to illuminate.
Be mindful of the fact that flashlight efficiency is influenced greatly by the design and quality of the reflector within it. Since the total output of the system does not take into account the focusing efficiency of the reflector, there is no way cleanly to convert values of peak beam candlepower to lumens.
Operational Light Imperatives
As has been previously noted, a major source of concern under diminished light conditions is that of threat identification. Threat discrimination problems begin to become manifest when ambient light levels fall below 3 foot-candles. Before determining whether your duty flashlight will pass muster at extended distances, consider the limitations of your most potent light platform; your police cruiser. In recent research conducted by the Police Policy Studies Council, high beam (headlights) output of new 2006 Crown Victoria police cars was measured out to 50 feet. Low beam and take-down light intensity was also quantified but will not be addressed directly at this time.
At a distance of 30 feet, high-beam output was measured at 13 foot-candles. This value will vary somewhat from headlight to headlight but represents a good working reference point. Ordinarily, this level of light is adequate light for reliable threat discrimination under more ideal conditions. However, when subjects with dark outer garments were placed directly before the headlights, it quickly became apparent that recognition of a dark (Glock) handgun was extremely tenuous due to the lack of contrast between the dark handgun and the dark (garment) background. The degree of difficulty was as apparent with the handgun tucked in the subject’s waistband as it was when the subject was prompted to completely withdraw the gun from his waistband and hold it against his (dark) sweatshirt. Beyond 30’ the task of identifying the handgun became unrealistically difficult.
What were the street implications of this research project? Our consensus was that many vehicular stops typically involve a distance (as measured from the cruiser’s headlights to the driver-side door of the subject’s car) of roughly 30 or more feet. We also tend to see a great deal of frequency in police assailants utilizing both dark clothing and “black” handguns. In essence, while we take some comfort in suggesting that threat identification problems arise at ambient light levels below 3 foot-candles, we must recognize that the “street equation” can be very different.
Another critical issue these findings impinge upon is police vehicle-stop tactics. Contrary to mainstream thinking, vehicle positioning by night should reflect a substantive departure from daylight protocols. The universally-taught 10-15° (police) vehicle cant (toward the center of the roadway, ostensibly for engine block cover) tends to diminish night-time light intensity by 50% or more when measured at the suspect vehicle’s driver-side door.
A critical yet often overlooked post-incident consideration is the documentation of ambient light at a night-time shooting scene. Agencies must make every effort to quantify accurately the ambient light prevalent in any officer-involved shooting that has occurred under low-light conditions. In doing so, agencies might more reliably determine whether an officer’s mistaken belief that a mobile phone was a handgun was an objectively reasonable error under existing light conditions.
In the next installment of this series we’ll examine critical facts and misconceptions pertinent to visual adaptation under low light conditions.