|From Ocean Navigator #61 |
One of the best tools for navigation is the human eye. Unfortunately, when sailing at night, the utility of the eye is obviously reduced.
Due to extensive military development of night vision technology in the past decade, remarkably capable night vision devices are now being sold in the marine market. Night vision units give the navigator a nighttime seeing capability that enhances navigation and safety.
Although not as effective in low-light conditions as the eyes of many animals, the human eye is remarkable in its ability to discern color and to function over a wide range of brightness levels. When fully dark-adapted, the eye is 10,000 times more sensitive to light than during daylight conditions. However, even at this level of sensitivity, the eye is often incapable of seeing unless aided with some type of light gathering or light amplification device.
When the light level is low, the first thing to do is dark adapt. Dark adaptation requires 30 minutes to an hour, during which time the eyes should not be exposed to any bright light source (other than monochromatic red).
The second approach to low light conditions is to pick up the best pair of 7 x 50 binoculars available. During World War II, considerable work was done to develop devices that would provide enhanced low-light visual ability. One result of this was better binoculars. A pair of night glasses can aid night vision because the 50 mm (diameter) objective lenses of binoculars gather far more light photons than can an unaided human pupil. A quality pair of 7 x 50 night glasses can make a remarkable difference in our ability to see in low light conditions.
Electronic image intensifiers depend on the photoelectric effect, discovered by Heinrich Hertz in 1887 and explained by Einstein in 1905. Light energy comes in quanta, or discrete packets of energy. When a suitable material, called a photocathode (a cathode is negatively charged), is exposed to light radiation, the molecules of the photocathode will absorb the energy of some of the light photons and emit electrons.
Additionally, the photocathode can be tailored to respond to only specific frequencies of light. This is because the absorption process only occurs when an arriving photon has a sufficient energy level. And this level varies inversely with wavelength, so light of a wavelength longer than some critical value will have insufficient energy to displace electrons.
Electrons liberated from the photocathode are accelerated by an electric field that attracts the negatively-charged electron. After sufficient acceleration, the electron impacts on an anode (positively charged) that is coated with a fluorescent material. The interaction of an electron with the fluorescent material produces a photon – the reverse of the process at the photocathode. Since the energy of the photon arriving at the input of the image intensifier has been increased by acceleration, the brightness of the light emitted from the fluorescent anode will be many times greater than that of the incident light. In some devices, the electrons emitted by the photocathode can also be made to liberate additional electrons while traveling from the photocathode to the phosphor-coated anode. This latter process is used to great advantage in a special form of light intensifier tube: the channel plate amplifier (see accompanying sidebar). Different sensitivities
Image intensifiers have been built that respond to many levels or frequencies of energy. Some are sensitive to what we call visible light – a very small part of the overall spectrum of electromagnetic radiation. Photocathodes are available that respond to the longer wavelengths of infrared light and others that are sensitive to the shorter ultraviolet wavelengths. Image intensifiers are also produced whose photocathodes respond to X ray and Gamma ray wavelengths. Most modern medical diagnostic X ray systems and many industrial X ray inspection systems include X ray image intensifiers.
The most frequently advertised light intensifiers for marine use are those offered by Fujinon, ITT, and Vetus-Den Ouden. All three are monoculars, although the ITT unit provides binocular viewing of a monocular output screen. Other units, generally of Russian origin, are also available in the market. This article primarily applies to the Fujinon, ITT, and Vetus products.
The light intensifiers use optical lenses to capture the available light from the scene of interest. Since the light level is, by definition, low, a large diameter lens that can capture more of the available light is needed. The standard Fujinon unit, the Starscope PS-910-II, uses an objective lens rated at f/1.2. The ITT Night Mariner objective lens is rated at f/1.4. The f rating of the Vetus lens is not stated in its accompanying literature. Fujinon also offers a unit with interchangeable lenses, allowing – c mount – camera lenses to be used when desired. As expected, the optics in all three units are carefully coated to reduce reflection and increase light transmission.
The magnification and field of view designed into the image intensifier is critical in determining its usefulness on a moving vessel. The 1:1 magnification view is most useful. The ITT and Fujinon products normally provide a 1:1 view. ITT offers an optional 3x magnifying lens attachment. As noted, the Fujinon C model permits use of different lenses, allowing any desired magnification (the Fujinon C model is not waterproof). The Vetus unit’s standard lens provides 5x magnification. A relatively wide field of view is desirable for maritime night vision image intensifiers. The ITT and Fujinon units provide a 40° field of view. For comparison, the field of view for most 7 x 50 binoculars is 7.5 degrees.
As in quality binoculars designed for marine use, the design of the exit lens (ocular) of the optical system is important. The lens must be large enough to transfer maximum light to the eye and allow sufficient eye relief-the distance from the eye to the lens, to allow for the use of glasses. The Fujinon unit exit ocular is 8 mm in diameter, that of the ITT is 15 mm. The Fujinon provides +/- 4 dioptor adjustment to accommodate differences in the vision of the user, while the ITT unit provides individual adjustment for each eye over a range of +2 to -6 diopters.
As for power, all night vision units use batteries. The Fujinon will operate continuously for 60 hours from two AA size alkaline cells, 35 hours if powered with zinc chloride heavy duty cells. The ITT Night Mariner obtains its power from a 6-volt lithium battery that will provide 13 to 25 hours of continuous operation. The 13-hour battery life quoted assumes substantial use of the unit’s motor-driven focus adjustment system. The battery life in the Vetus unit will likely match that of the Fujinon since it uses a similar image intensifier tube. Close focusing abilities
The degree to which careful focus for distance is required may be surprising at first. The Fujinon and ITT units have large aperture lenses, which translates into limited depth of field (the range distance in which objects will be in sharp focus). As a consequence, when using one of these light amplifiers to aid navigation, it is likely that considerable refocusing will be needed. The motor-driven focus system in the ITT unit is clearly superior in this respect. Should it become important to view something close at hand, the Fujinon can focus as close as two meters (6.6 feet), the ITT focuses as close as one foot (30.5 cm). The closest focus of the Vetus unit is 10 meters.
Since the light intensifier operates as two separate optical systems-one for light gathered by the objective lens and focused on the photocathode, and the other focused on the photoemissive output screen-it is necessary to independently focus the objective lens and the ocular (or output) lens. The ocular needs focusing only once for each user since the distance between that lens and the output screen is fixed. In the ITT binocular viewing system, independent focus adjustment is provided for each eye. Interocular adjustment is provided to accommodate the variation in spacing between the eyes of different users. The ITT Night Mariner resembles the shape of a 7 x 50 binocular, while the Fujinon and Vetus units resemble monoculars.
Fujinon states that the Starscope with the built-in lens is waterproof, although no maximum depth of immersion is quoted. Waterproof capability is not claimed for the c-mount unit. The ITT Night Mariner claims water-resistant construction and that the unit will float. When questioned, ITT did not claim waterproof construction since the unit is not designed for diving or other underwater use. Regardless of the makers’ claims, given the cost of the devices, keeping them out of the drink is surely a good idea. Since they will inevitably be splashed with salt water when in use, rinsing them off under a gentle flow of fresh water is a good idea.
Owner’s manuals for all units caution against exposing the devices to high temperatures. Exposure to direct sunlight or leaving a unit in an automobile in the sun are specifically cautioned against. The manuals also note that since battery output decreases at low temperatures, it may be desirable to warm the batteries before using them. This is not likely to be a problem in yachting applications since little pleasure sailing is done at very low temperatures. Night vision units are expensive devices and, like cameras, they require proper care.
These units allow a user to see objects that are illuminated by the very lowest levels of light, such as that from stars on an overcast, moonless night. Overall light amplification or gain is one measure of the ability of the unit to perform its intended function. Gain alone does not determine the performance of the device. Noise level, as evidenced by random light emission over the surface of the viewing area is also important. (This second characteristic is not specified by manufacturers and is best judged by comparing the units side by side.) The operating gain of the intensifier, which determines the brightness of the viewed field, must be limited to prevent excessive gain when a scene is brightly illuminated. Gain control in the Fujinon unit is fixed. The ITT Night Mariner provides a very useful manual brightness control. Light gain
Each of the manufacturers specifies the light gain for the image intensifier used in their unit. Fujinon, which uses a generation 1 intensifier (light intensifier tubes are classified by the industry as generation 0, 1, or 2) made by DEP (Delft Electronische Producten in Roden, Holland) claims a gain of more than 1,000 for its intensifier tube. ITT, which employs a generation 2 intensifier, specifies a gain of up to 20,000. Comments have appeared in the press indicating that night vision devices using generation 2 intensifiers require a government license when taken outside the U.S. This is not true. At present, an export license is required for direct sale of units such as the ITT Night Mariner. However, an individual may carry and use the unit anywhere. The Vetus Night Glass, which uses an intensifier tube similar to the Fujinon unit, quotes a light gain of 250 to 600. Since the Night Glass lens provides a 5x magnification, it necessarily gathers less light than the 1x magnification designs of the other two.
The usefulness of any optical system is often limited by the ability of the device to show fine detail. This ability is normally expressed as resolution, in line pairs per millimeter. The DEP tube in the Fujinon unit specifies 95 line pairs per millimeter at the viewing screen. The ITT unit provides 45 line pairs per millimeter.
Light intensifier devices require care on the part of the operator. When turned on, they should not be exposed to bright light sources. Scene illumination from a full moon or low levels of artificial light is not harmful, however. Typical lifetimes for the image intensifier tubes used in these devices often exceed 2,000 hours-they eventually wear out as the gain of the photocathode gradually decreases. Outright failure is rare and is generally covered by the manufacturer,s warranty since it is most likely to occur early in the life of the device.
These night viewing devices use a photocathode with a frequency sensitivity to light energy that does not match that of the human eye. Due to this difference in frequency response, objects may appear different when seen through the light intensifier than with the unaided eye. Objects whose color is toward the red end of the spectrum may appear brighter, and therefore larger, than blue objects. These photocathodes may also provide some sensitivity to infrared light energy. This infrared responsiveness is enough to see low temperature heat sources, such as people, engine exhaust outlets, and the like. Very hot metal objects, such as the turbocharger on an engine, will be visible due to their radiated infrared energy. Objects that efficiently reflect infrared energy will appear brighter than those that do not. It is important to remember the picture on the output screen is in false color, usually in shades of green. It is impossible to differentiate between red and green lights or colors on daymarks or buoys.
The latest Fujinon units incorporate a small, infrared light emitting diode (LED) in the top housing. This built-in infrared LED may prove useful if one has to find an object dropped on the cabin sole (provided the object is within the focusing range of the unit) and one doesn’t want to turn on any visible light. Its useful range is quite limited and its value in searching for an elusive channel marker will likely be negligible. Interestingly, the infrared LEDs used in TV remote controls are clearly visible with these night scopes. As with radar, practice in using the device and in learning how to interpret what is presented on its screen will pay big dividends. Reducing dark adaptation
The use of a light intensifier is complimentary to the use of other sensing devices: the unaided eye, binoculars, and radar. Ironically enough, a possible problem of using a radar and a night vision device, either separately or together, is loss of one’s dark adaptation. Should an observer’s eyes be dark-adapted prior to the use of a night vision unit, the light from the device will be bright enough to decrease the eye’s sensitivity. A solution for this problem would be for the manufacturers to produce night vision units with monochromatic red, a wavelength that does least harm to the dark adaptation of the eye. This approach is not currently taken because the most efficient conversion of electrons into photons in the visible range of the eye occurs when a green output phosphor is used. This same conversion efficiency characteristic is the basis of the use of green for the phosphor in the CRTs used in radar sets. The light emitted by the output phosphor is quite monochromatic, therefore, adding a red filter to the light path will not work. Perhaps, as the use of night vision devices on boats increases, some manufacturers may consider making intensifier tubes that produce pure red light. This would help a sailor maintain full dark adaptation. The same could be done for the CRT in the radar, and for all other navigation displays.
Overall, there are impressive benefits to improving night navigation using night vision devices. It gives one the ability to use a range of resources: unaided full peripheral vision, vision aided by good quality night glasses, enhanced vision using a night vision device, plus the input from electronic aids such as radar, loran, GPS, and sonar. The cost for night vision devices is still quite high. If the cost/performance history of the other electronic navigation devices is any guide, however, night vision systems will become more and more common as the cost declines with increasing production and the application of new technologies. At some time in the future, it might be practical for a mariner to wear night vision goggles of a type similar to those used by the military. If they were worn full time in really dark conditions, and the other navigation aid displays were suitably matched to the sensitivity of the goggles, navigation in the dark would be no more difficult than it is in daylight.
Contributing editor Chuck Husick is a sailor, pilot, and Ocean Navigator staff instructor.