There's more than one way to see in the dark.
You wouldn’t think much of a piece of boat gear that stopped working or whose performance was severely reduced for half of the time. But in the case of our eyes, that is what we have learned to live with: In darkness, we become almost completely blind. Of course, our eyes do adapt to low light to some extent, but it’s a two-stage process that can take half an hour or so to complete. And even then, our night vision is only one-sixth as good as that of your average house cat.
Over thousands of years, we’ve learned to cope with our night blindness, mainly by using artificial light to make up for the loss of daylight. But it is still not a complete solution: The dim glow of an instrument panel can be enough to prevent our eyes from fully adapting to the darkness outside the wheelhouse, and even a brief glimpse of a bright light is enough to undo whatever adaptation to the darkness we may have achieved.
A different and better approach is to make the most of the available light with the use of “night glasses,” binoculars that mimic the eyes of nocturnal animals by using big lenses that gather as much light as possible. That’s why “marine” binoculars often have large (50mm) lenses despite their relatively low (7x) magnification.
But over the past few years, several more effective and exciting night-vision technologies have appeared that offer the real prospect of seeing in the dark. These include low-light cameras, image intensifiers, and most recently and most highly touted, thermal imaging.
Ordinary digital cameras use a system of lenses to project an image onto a light-sensitive screen called a charge-coupled device or CCD. In effect, it’s rather like an array of miniature solar panels packed into a space the size of a postage stamp with each tiny panel producing electricity when light falls on it.
A low-light (or “day and night”) camera is similar but with a vital difference. Where a regular camera has a multi-colored filter between the lens and the CCD in order to create a colored image, a low-light camera doesn’t. Doing without the filter not only lets more light through to the CCD, it also allows the camera to make use of the “invisible light” portion of the spectrum, near infrared (see next page).
This makes low-light cameras particularly good for short-range use, such as for security cameras. They are robust and can tolerate sunlight, and their relatively limited ability to see in the dark can be dramatically improved by using invisible infrared lighting. But this need for supplementary lighting means that they are of limited value at long range in darkness.
An image intensifier does exactly what the name suggests: In near-darkness, such as a starlit night, it magnifies the ambient light to produce an image that’s bright enough to be visible to the human eye. It does so by using a system of conventional lenses to form a dim image on a “detector plate.” Every time the detector plate is struck by a photon of light energy, it releases an electron. At the eyepiece end, the electrons hit a phosphor-coated screen—a miniature version of the kind of thing that was at the heart of the old green-screen radars—to produce a copy of the original dim image that can be seen through the eyepiece or copied by a CCD and sent to a separate display.
The reason for doing all this is the fact that a flow of photons can’t be amplified but a flow of electrons can, so the image at the eyepiece can be made brighter than the image at the objective. There are several ways of amplifying the electron flow, and it is these that account for the main differences between first-, second-, and third-generation image intensifiers, with both price and performance increasing from one generation to the next. Handheld gen-one devices, for instance, can be purchased for a couple hundred dollars. They do work but the edges of their image are so distorted that the effective field of view may be quite limited.
Handheld gen-two devices are typically priced at around a couple thousand dollars and offer a much brighter and less-distorted image, but like gen-one devices, are easily dazzled by bright lights and can be permanently damaged if they are switched on in daylight. An enhancement called “autogating” reduces both problems by controlling the amount of amplification, but it’s a sophisticated and expensive feature and so is found only on the more up-market devices.
The third night-vision technology is known as thermal imaging. Unlike the other two, which use visible light and near-infrared, thermal imaging operates in the mysterious part of the spectrum called the far-infrared. The beauty of this approach is that it doesn’t rely on light from an outside source, because everything—even ice—emits energy in the far-infrared part of the electromagnetic spectrum—that is, everything has a heat signature.
The overall structure of a thermal camera is very much like that of a regular digital camera: It uses a system of lenses to create an image on a screen that converts the infrared “light” into an electronic image. One big difference is that glass is opaque to far-infrared, so the lenses have to be made of a rare material called germanium. The other is that instead of using a light-sensitive CCD to create the digital image, a thermal camera uses a heat-sensitive device called a microbolometer.
The resolution and update rates of civilian thermal-imaging devices are limited by the U.S. government for security reasons as well as by price and the available technology, so the resolution of thermal images is never as good as those derived from a visible-light camera. This is particularly significant because it is resolution above everything else that determines how far you can see while using a thermal camera. But the images produced by the current crop of thermal imagers are all still perfectly acceptable. A 640x480 image, for instance, is quite capable of showing a small boat a mile away or a person in the water at several hundred yards.
But there are some things that thermal imaging can’t do. It can’t see through glass, so a hand-held thermal camera is of no use if it is being used from inside a wheelhouse, for instance. It also can’t see lights, so while you may be able to see a ship, you won’t see her navigation lights and you certainly won’t be able pick out the range lights that guide you into harbor—although you will see the buoys on which they are mounted. And because the thermal camera sees heat rather than light, you can’t use it to read names or notices that would be clearly legible through a $500 low-light camera—or maybe even with a $5 flashlight.
That, perhaps, is the key to choosing night-vision equipment. Although, simplistically, one might say thermal is best, it isn’t the complete solution. The best solution would be to use a combination of technologies that can give the best of both worlds—or rather the best of all three.
This article originally appeared in the January 2011 issue of Power & Motoryacht magazine.