"Dark as a pocket" is how an old lobsterman I knew described the situation I chose for testing a FLIR M-636L thermal-imaging camera temporarily installed atop Gizmo's antenna mast. It was a moonless night, with low clouds obscuring even star shine, and the uninhabited islands I cruised amongst offered no comforting shore lights. It was really dark. But the video image coming from the FLIR to an MFD set at near minimal screen brightness showed those dark shores in detail, plus the water's surface-lobster pot buoys included- and even the clouds overhead, almost as if all were bathed in daylight. Well, make that a dull, monochrome daylight with some odd characteristics.
Thermal imaging, as offered by FLIR and other manufacturers, is brilliantly valuable to boaters in the dark, but it's important to understand just where it fits in the wide category of "night vision." Visible light occupies a tiny slice of the total electromagnetic radiation spectrum, with "infrared" an unusually wide slice just up from red in terms of increasing wave length. Therein lies confusion. Thermal cameras are often-and not incorrectly-called infrared cameras, but so too are significantly different vision aids like the Raymarine CAM100 that was also mounted on Gizmo that night. The latter is actually a conventional color video sensor that's sensitive enough to image the particular infrared light that's just beyond visible, plus it has a ring of special LEDs that can shine this so-called near-IR light on objects within close range. The result-aside from a terrific color daytime view into the distance-is a monochrome look at Gizmo's cockpit and swim platform but not many feet farther. Keep in mind, an intruder wouldn't know he was being seen, as those LEDs shine invisibly.
The infrared-assisted Cam100 ($700), and the many like it, are darn useful around a boat, but they can't hold a candle, so to speak, to infrared thermal cameras like the FLIR M-636L ($20,000) and its brethren. The longwave- thermal portion of the infrared-wave band is totally unrelated to visible light or the digital cameras that can see it. Thermal infrared is instead related to the internal molecular heat of an object and is emitted by everything, including icebergs. The sensor that can feel it, called the core, is a very exotic bit of tech, and thus somewhat crude even at this price. In fact, the M-636L's 640-by-480-pixel microbolometer core smashed a price barrier as other marine cameras in this range, and higher, had typically offered 320-by-240-pixel image resolution. Many of those competitor products have caught up now, often using a FLIR core. The quadruple resolution improvement is definitely noticeable, and increases the range at which a danger can be noticed and identified. But 640-by-480 is still low-res in modern video terms, and that's one reason that many marine night-vision systems like the M-636L pair a conventional low-light video camera with the thermal sensor.
Another reason is that thermal cameras have trouble with water in the air, which absorbs thermal radiation and thus masks objects beyond. But the effect varies a great deal with the density of fog or rain. In my testing, the thermal camera revealed a coastline otherwise hidden by fog burning off late one morning. In heavy twilight rain, it lost an island half a mile away that I could make out with my eye and see quite well with the low-light camera.