Protecting Night Vision to Create Effective Work Conditions and Safety

Police Fleet Professional Magazine

The paths of science and engineering cross at SoundOff Signal when it comes to lighting solutions that protect night vision for increased human performance in low-light scenarios. The company’s focus is on safety, and while this usually refers to external vehicle signaling lights, the type and amount of internal vehicle lighting is just as important.

Proper lighting in an enclosed workspace enables adaptive vision, allowing people to perform their jobs effectively and react quickly to unfolding emergency situations. SoundOff Signal provides many types of lighting equipment to hundreds of municipalities and commercial institutions across the country and works with their specialists on the proper resolutions for each task.

Doug Baker, Senior Vice President of Technology and Chief Innovation Officer at SoundOff Signal, reveals the science behind the company’s lightning solutions inside a vehicle at night, and how to best preserve night vision, also known as Scotopic vision. And he breaks it down quite simply, “a low level of red light is used to protect night vision.”

Human eyes have a slow adjustment immediately following exposure to white or other high levels of visible light. This can make it challenging to view objects and information when the lighting conditions change rapidly. However, if we are exposed to low levels of red light, human eyes can adjust quickly to dark condition viewing, allowing emergency personnel and civilians to operate easily and safely when moving from a properly lighted workspace to a low-light area. Some examples of these scenarios include cockpit lighting in an aircraft and interior vehicle lighting, such as a police cruiser or utility service truck. Each one requires someone’s vision to be able to work on a task inside the vehicle, such as reading gauges or filling out paperwork while quickly adapting to the outside environment’s low light.

There is a quick anatomy lesson to explain this science. “Our vision uses a combination of chromatic (color) discernment and light/dark discernment ‘sensors’ in our eyes. These are typically referred to as cones and rods. Our brains automatically process all of this information together, even though it comes from different types of optical sensors,” Baker explains.

Humans have the ability to see color through a combination of different cones, which are generally red, green, and blue sensitive. The light that we can see is measured as 400 to 700 nanometers (nm) wavelength range (Figure 1), which is a small slice of the energy across the wider electromagnetic spectrum. Under daylight conditions, the brain focuses on signals from the cone sensors to provide high-resolution color vision and ignores the saturated signal from the ultra-light-sensitive rods. In low-light conditions, only signals from the ultrasensitive rods are processed to provide grayscale visual sensation. We experience this at night by seeing objects but not with much color discernment.

Daytime (cones) and nighttime (rods) visual response

Figure 1: Response of the eye in daylight (blue, green, and red cones) and nighttime (rod), also called Scotopic vision. 

With the understanding that our rods are the critical sensor for night vision, we start to see the strategy to “save” those sensors for when we need to see in the dark without waiting for a photo-chemical adaptation delay in the eye. Any sudden overload to the rod sensor will temporarily reduce the responsiveness (low-light night vision) due to the highly stimulated or saturated rod sensor,” Baker inserts. This is exacerbated by the fact our Scotopic vision is most sensitive from low to mid wavelengths (from blue to green wavelengths). For example, flipping the lights off in a room will cause a slow response in your vision as your eyes adjust to the darkness and the same holds true for the opposite when suddenly going from the dark to lights on.

Because of this effect, small amounts (i.e., dim) of red light do not stimulate the rod very much, shown graphically in Figure 2 by the small amount of overlap between the energy in a red LED and the rod sensitivity curve. In contrast, “white” light contains energy in all the blue, green, and red regions. White light, even in low amounts, does stimulate the rod sensor. A perfect scenario for this problem is working at night under the typical white dome light in a vehicle, then straining to see outside into low-light areas. This is the reason so many special service vehicles are equipped with a red dome light option, because it preserves the person’s rod sensor and thus improves their rapid “night vision.”

Night time visual response (rods) function plus typical white LED and SAEJ578 compliant red LED spectra

Figure 2: SoundOff Signal’s red LEDs produce a peak of 640 nm making it easier for the eye to discern, as you can see in the graph. As a contrast, blue light, which is commonly associated with cell phone screens, is in the 400–500 nm range, which if you’ve ever looked at your cell phone in the dark of night, it adversely affects your ability to adjust quickly to the natural low-light environment. Fans of military-based movies will recognize green imagery in night vision goggles and that wavelength falls between 500–600 nm on the scale. The green light is less impactful on the Scotopic scale than blue, but not nearly as favorable as red.

Using science to drive its engineering, SoundOff Signal’s lighting solutions include controllers, allowing for custom management of the LED lights. These purpose-driven tools are designed to help everyone get their job done effectively and safely.


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