By Paul Mazurek
On September 1, 2009, the NTSB published a synopsis of its hearings held in February. Within this were recommendations specifically addressed to the FAA, public HEMS operators, the Federal Interagency Committee on Emergency Medical Services, and the Department of Health and Human Services’ Centers for Medicare and Medicaid Services. A specific recommendation to the FAA and public HEMS operators reads, “Require helicopter emergency medical services operators to install night vision imaging systems and require pilots to be trained in their use during night operations1.”
Technology That Will Save Lives
It should not be a surprise to anyone that a large portion of HEMS accidents occur at night, and usually involve objects invisible to the naked eye, treacherous terrain, controlled operations into unseen terrain, or any combination thereof. Taking the handicap of darkness out of the equation and allowing crews to safely perform night operations by allowing enhancement of vision (a most important sense during visual flight rule (VFR) operations) is a dramatic step towards a significant reduction in the number of devastating HEMS crashes. Night vision goggles (NVG) is one tool that can significantly reduce HEMS crashes.
While still relatively new to the air medical industry, night vision enhancement devices have been utilized and improved upon for the better part of 40 years, primarily by the military. Personal equipment involves a specially designed set of goggles, which can correct visual acuity to near 20/20 depending on the amount of ambient light (unaided visual acuity is in the 20/200 range), a mounting bracket for the helmet and a power supply pack (typically housing two sets of AA batteries). Cockpit instrumentation and all interior lights require a “blue-green filter” to prevent glare on the windscreen.
When examining how these devices work to enhance night time vision, it is important to understand how the job gets done. Enhancing available light is the name of the game, so working with light is how these systems function. Light along the electromagnetic spectrum is broken down in to three simplified subcategories: infrared light, visible light, and ultraviolet light.
On opposing ends of the visible light spectrum are violet, which contains the most energy, and red, which contains the least amount. “Infrared light” is closest to the visible light spectrum. NVGs are particularly sensitive to the orange-red part of the visible light spectrum and the portion of the infrared (IR) spectrum that is closest to visible light (the area just below red on an electromagnetic spectrum chart). Infrared is not visible to the unaided eye but is an excellent source of light for NVGs. This is particularly useful for the military applications for which NVGs were originally designed. NVGs are not, however, particularly good at picking up light at the blue/green end of the spectrum.
Night vision technology utilizes either thermal imaging using the infrared spectrum or “light intensification.” Intensification or “amplification” is the basis for NVG technology2. The system utilizes the small amount of light that’s in the environment (e.g., from the moon or from stars), and converts the light energy into electrical energy.
Night vision goggles utilizes a special tube known as an “image intensifier tube” that assembles and intensifies both available visible and infrared light. This light moves through the image intensifier tube via an “objective lens,” which is merely a standard lens. A “photocathode” within the image intensifier tube converts light energy (aka photons) to electrical energy (aka electrons) by utilizing external electrical energy. The electrons exit the photocathode and pass through a “microchannel plate (MCP).” This thin glass disc has millions of microscopic channels (i.e., holes) and is used to multiply the number of electrons in the tube. It accomplishes this by causing entering electrons to hit or “bounce” against the channel walls.
When this happens, an additional number of electrons (somewhere in the area of several thousand fold more) are “released into circulation” so to speak. At the end of the image intensifier tube, the electrons then strike a screen coated with phosphors. The electrons stay in their original position, providing a perfect image. The electrons cause the phosphors to reach an excited state, releasing high energy photons. These phosphors create the characteristic green image seen through NVGs. This image is viewed through a lens on the other side of the image intensifier tube, known as the ocular lens, or “eye piece,” which intensifies the light. The eye piece lens has a diopter adjustment to give the user a clear, crisp image.
Nothing is Perfect
NVGs have many signifcant limitations. First and formeost is that the goggles only allow a 40-degree field of view. It is easy to become overly confident now that you can “see in the dark,” but you are only seeing about 25 percent of your normal 180-degree field of view. By scanning (moving your head from side to side), you can compensate for most of the loss of field of view.
Although the 40-degree field of view is one of the most obvious shortcomings of night vision goggles, there are others. The monochromatic image (everything is green) can make it more difficult to identify objects and navigate. The poor sensativity to blue-green lights can make operations at airports more difficult (taxiway lights are blue).
It is also important to understand that if an air medical crewmember is going to ride an empty leg (as is often the case to assist the pilot in being an “extra set of eyes”), you do not have to look through the goggles to see the instruments and cockpit indications. Modifications to the cockpit that allow for the use of NVGs are better visualized with the naked eye. Therefore, looking underneath the lower NVG field of view is required to effectively see the instruments.
Getting Ready to “Go Green”
NVGs are not a “ready out of the box” or “no assembly required” piece of equipment. Part 135 operators must purchase the goggles and send their aircraft off for cockpit remodification. Additionally, an approved training program must be established for anyone who will be using NVGs, including pilots and medical crew. Such a training program typically consists of a ground school in which the participants learn about the equipment, operation and troubleshooting, and a hands-on, operational component consisting of a number of live takeoffs and landings3. Additionally, the maintenance department must submit an NVG maintenance program to the FAA for approval4.
The Comfort of Green
In the skies at night, there is no other color that I would rather have in my field of vision. While I have only been using NVGs myself for a short period of time, I can say with great certainty that they absolutely make a difference during night operations and make me ask the question, “Why didn’t we get these sooner?” Anticipated future mandates will have more people asking this.
References
1. National Transportation Safety Board. Public Meeting of September 1, 2009. Accessed at http://ntsb.gov/Publictn/2009/AB09-HEMS.htm.
2. ITT Defense Electronics and Services. How Night Vision Works. Accessed at http://www.nightvision.com/night_vision/how_nv_works.html.
3. Menga D. Extra Sets of Eyes. Vertical 911. AMTC 2009. pp. 36-40.
4. Bergan B. The Eyes of the Night (part I). Vertical 911. ALEA 2008. pp 60-67.
