Whether you are driving a vehicle on the ground or flying an aircraft in the air, you can normally avoid obstacles if you can see them. Obstacles are a bigger problem if you can’t see them. Night vision goggles (NVGs) help us see obstacles normally not visible to the unaided eye and make flying at night a lot safer. Let’s take a look at how they work, and we’ll finish up by emphasizing the importance of a thorough landing zone (LZ) briefing.
NVGs produce an image by gathering and intensifying available reflected energy. That is why NVGs will not function in total darkness, and why they do not perform as well on very dark nights, such as during overcast conditions when starlight and moonlight are hidden. Reflected energy includes visible light and near infrared that is not visible to the unaided eye. (need Pam to confirm this sentence is accurate when she views the layout)
The intent of this article is not to teach the anatomy of the eye, but a short discussion of cone and rod cells is helpful when comparing day and night vision. Cone cells are responsible for providing color vision and maximizing visual acuity (how well you can see). There are approximately 7 million cone cells distributed throughout the retina, but most are located near or within the fovea, where visual acuity is maximized. For the cone cells to work, there must be enough energy to stimulate them (daylight). However, any artificial light, such as a reading lamp or the NVG image, will stimulate the cones. There are approximately 120 million rod cells, which allow greater coverage throughout the retina (except for the fovea, where there are essentially no rod cells). Each rod cell is roughly 10,000 times more sensitive to light than a cone cell is, and this increased sensitivity allows for some reaction to the weak energy levels present in the night environment. The rod cells help maximize night vision, but “typical” unaided night vision (20/400) is considerably worse than “normal” day vision (20/20). Flight crew view the NVG image with cone cells, and properly focused NVGs can produce a visual acuity between 20/25 and 20/40, which is about five times better than unaided night vision. We can almost “see in the dark.”
Enough about anatomy, let’s talk about technology. So, how do the NVGs make it easier for the flight crew to see at night? The heart of the NVGs is the image intensifier. Light enters the objective lens, which focuses energy on the photocathode. During this process, the image is inverted and reversed. The photocathode is made of a material that releases electrons when struck by light. The original image is replaced by an equivalent electron image, which travels toward the microchannel plate (MCP). Electrons enter the MCP where they strike the inner walls of tiny microtubules. The MCP contains approximately 6 million tubules and is about the size of a nickel. More electrons are released during these collisions, and this process is repeated down the length of the microtubule. The electrons, increased in number and intensified in energy, leave the MCP, strike a phosphor screen and cause it to glow. Photons are released in the same pattern (image) as the one that entered the intensifier. The image is inverted back by a fiber-optic bundle and focused onto the human eye by a lens.
So how does this apply to one of the many firefighters, paramedics, law enforcement officers, first responders, and others who interact with helicopter crews equipped with NVGs? Most importantly, avoid shining white or red lights toward the helicopter. Bright lights can overload the NVGs or can even have a momentary blinding effect on flight crew members wearing them. In general, blue or green lights are not visible with NVGs and therefore don’t work well for marking the LZ. Another critical concept to remember is the NVGs require at least a small amount of light energy to be effective. Shadows can “mask” or hide obstacles, making it difficult for flight crew members to see them. That’s why a thorough LZ briefing is crucial, especially at night. Don’t take it personally if the pilot or another flight crew member asks about potential hazards after you’ve provided your stellar LZ description. We simply want to make sure all the possible obstacles have been detected by ground personnel to help us make a well-informed assessment of LZ suitability. On occasion, the flight crew may ask the LZ team to move to another area that appears to be safer.
In closing, know the ThedaStar flight team is very grateful for the support we receive from all our partners throughout our operating area on a day-to-day and night-to-night basis. We couldn’t do our job as safely without you!
By Dan Unruh, pilot