Today's binoculars, spotting scopes, and telescopic sights are the best that have ever existed. Various factors account for this, but not the least of which is an intense competition among optical manufacturers vying for market share. Most significant, however, has been the development and implementation of newer and better optical coatings. The broad categories include: anti-reflection (AR) coatings, reflective coatings, phase-correction (P) coatings, hydrophobic (anti-fog) coatings, and abrasion-resistant coatings. All are important but foremost among these are the various anti-reflection coatings, which have improved over the decades to the point where they have nearly doubled the light transmission of sports optics. Light transmission denotes, as a percentage, the amount of usable light that passes through an optical instrument, relative to the amount that entered.
The spoilers that have bedeviled optics since the invention of Galileo's first telescope in 1610 are absorption and reflections, which dramatically reduce the amount of usable light that reaches the viewer's eyes. Each optical element (individual lens, prism, or mirror) inevitably absorbs some of the light that passes through it. Far more significant, however, is the fact that a small percentage of the light is reflected from each air-to-glass surface. For uncoated optics, this "reflective loss" varies between 4 percent and 6 percent per surface, instruments have anywhere from 10 to 16 such surfaces. The net result can be a light loss of as much as 50 percent, which is particularly troublesome under low-light conditions.
More seriously, however, is the fact that the reflected light doesn't just disappear, leaving a dimmer image. Instead, with some of the light from these second, third, and fourth reflections eventually coming out through the instrument's exit pupils and into the viewer's eyes. Such scattered light is called "flare," and is defined as "non-image-forming light, concentrated or diffuse, that is transmitted through the optical system." The result is a veiling glare or haziness that obscures image details and reduces contrast. In extreme cases, it may even cause ghost images. An extreme example would be if you were trying to glass game on the shady side of a low ridge with bright sunlight streaming over the top and into the instrument's objective lens.
Single-Layer Anti-Reflection Coatings
The long awaited solution to the problem of reflective light loss came in the mid 1930s when Alexandar Smakula, a Carl Zeiss engineer, developed and patented the "Zeiss non-reflecting lens coating system" (now called anti-reflection or AR coatings), which was heralded as "the most important development of the century in optical science." Soon thereafter the military needs of World War II accelerated the development of the coating, which was used by both the Allied and Axis forces in optical instruments ranging from field glasses (binoculars) to bombsights.
The theory behind AR coatings is a very complicated scientific concept. In application it consists of a transparent film, usually of magnesium fluoride MgF2, one-quarter of a wave-length of light (about six millionths of an inch) thick, deposited, by molecular bombardment, on a clean glass surface. Developing a method for applying such microscopically thin film, which is done in vacuum chambers, was a great technological triumph. This single-layer anti-reflection coatings reduced the reflective light loss from between 4 percent to 6 percent for uncoated surfaces to about 1.5 to 2 percent for coated surfaces, thus, increasing overall light transmission for fully coated instruments of about 70 percent, which, considering the accompanying reduction in image-degrading flare, was a remarkable improvement.
Multi-Layer Anti-Reflection Coatings
A major shortcoming of single-layer coatings, which are still widely used, is that they work perfectly well only for the specific wavelength (color) of light where the thickness of the coating is equal to one-quarter of the wavelength. This deficiency eventually led to the development of multi-layer broadband coatings capable of efficiently reducing reflective light loss over a wide-range of wavelengths. Today's best multi-layer coatings can reduce reflective light loss to as little as two-tenths of one percent at each air-to-glass surface.
My introduction to multi-layer coatings came in 1971 when Pentax began using it's "Super Multicoating" on camera lenses, where it nearly eliminated flaring and ghost images when photographing brightly backlit subjects. Sports optics manufacturers were a bit slow getting on the bandwagon, and it wasn't until 1979 that Carl Zeiss introduced its "T*" Multicoating, which boosted the light transmission of Zeiss binoculars to slightly over 90 percent, while simultaneously improving image contrast. The reason it took so long getting from the first single-layer coatings to today's multi-layer broadband coatings was because the latter, though based on the same scientific principles, are incredibly complicated, involving several thin layers of various fluorides, oxides, dioxides, etc. As you might expect, computers play major roles in the formulations and applications of such coatings.
Though overall light transmission continues to improve slightly, the highest levels with which I am currently familiar are about 92 percent for binoculars and 95 percent for riflescopes, which are well above the averages for such instruments. The primary reason why riflescopes tend to have slightly better light transmissions than binoculars is because they use simple erector lenses rather than complicated prisms for image erection.
Likewise, Porro prism binoculars tend to have better light transmission than roof prism binoculars of similar optical quality. Notable exceptions are the Carl Zeiss binoculars that use Abbe-Koenig roof prisms instead of the widely used Pechan-type roof prisms, which have one mirrored (usually aluminized or silvered) surface where between 4 and 6 percent of the available light is lost during internal reflection. (In a process called "total internal reflection," Porro prisms and Abbe-Koenig roof sprisms get 100 percent reflection on all their internal surfaces, without having any coatings.) Some leading manufacturers' solution to the Pechan-prism problem are special multi-layer reflective coatings that get 99.5 percent reflection on the mirrored surfaces.
The caveat here is that one shouldn't get too carried away in their quest for a few extra percentage points of light transmission. Consider, for example, that a 5 percent gain in light transmission in a high-performance optical intrument is roughly equal to a 150 fps gain in the muzzle velocity in a .300 magnum rifle - you'll never notice the difference.
Many believe that the quality of AR coatings can be determined by the color of light reflected from the surfaces. Perhaps, but to do so with any certainty requires considerable expertise. The color seen is not that of the coating material itself, which si colorless, but the reflective color or combined reflective colors of the wavelengths of light for which the coating is least effective. For example, a coating that is most effective in the red and blue wavelengths will produce a green reflection. Conversely, if the coating is most effective in the green wavelengths, the reflection will be some combination red and blue, such as magenta. The reflections coming from single-layer coatings of magnesium fluoride usually range from pale blue to dark purple. While the colors reflecting from the latest multi-layer coatings can be almost any color of the rainbow, with different colors showing on various optical surfaces throughout the system, a bright white (colorless) reflection usually indicates an uncoated surface.
Though unscientific, the following do-it-yourself test for evaluating AR coatings is both educational and informative. The only tool needed is a small flashlight or, lacking that, an overhead light. The trick is to shine the light into the instrument's objective lens so that when looking along the beam you can see images of the light reflecting off the various air-to-glass surfaces within the instrument. (Note: Reflection will be coming from both the near and far sides of lenses and prisms.) Now, based on the above information, regarding color, you'll get some idea concerning the types of coatings used and, more importantly, whether some surfaces are uncoated.
Other Types of Coatings
Phase-correction (P) coatings: Developed by Carl Zeiss and introduces as "P-coating" in 1988, phase-correction coating is second in importance only to anti-reflection coating in roof prism instruments. The problem (non-existent in Porro prisms) is that light waves reflecting off opposite roof surfaces become elliptically polarized so as to be one-half wavelength out of phase with each other. This results in destructive interference and a subsequent deterioration of image quality. The P-coatings correct the problem by eliminating the destructive phase shifts.
Reflection coatings: These mirror-like coatings - which often owe their effectiveness to constructive interference - are used more often in sports optics than one might think. Examples include: most laser rangefinders and the few riflescopes that employ beam-splitters; red-dot sights where a wavelength-specific coating is used to reflect the image of the dot back to the shooter's eye; and, as previously discussed, in roof prism instruments with Pechan prisms.
Hydrophobic (water repellent) coatings: The archetype for water-repellent coating is Bushnell's Rainguard coating that sheds water and resists external fogging. I extensively tested Rainguard coating in cold climates where inadvertently breathing on a scope's eyepiece lens would have obscured one's view of the target. The results were that, even when I intentionally breathed on both the objective and eyepiece lenses causing them to either fog or frost over, I could still see targets well enough to shoot.
Abrasion-resistant coatings: A persistent shortcoming with some anti-reflection coatings is that they tend to be soft and, therefore, scratch easily. Thankfully, today's "tough" coatings, though still not universally used, are greatly improving the durability of outdoor optics ranging from eye-glasses to riflescopes. The toughest coating, by far, that I have tested is on the T-Plated external lens surfaces of Burris' Black Diamond 30 mm Titanium riflescopes. I couldn't scratch it, even with the cutting edge of a razor-sharp pocketknife. The latter is not recommended.
The following terms are often used by optics manufacturers to describe the extent to which their instruments are protected by AR coatings.
- Coated optics (C) means that one or more lenses have been coated.
- Fully coated (FC) means that all air-to-glass surfaces have received at least a single layer of anti-reflection coating, which is good.
- Multicoated (MC) means that one or more lenses have received an AR coating consisting of two or more layers. When used by reputable manufacturers, this designation usually implies that one or both of the exterior lens surfaces are multicoated and that the interior surfaces probably have single-layer coatings.
- Fully multicoated (FMC) means that all air-to-glass surfaces should have received multi-layer anti-reflection coatings, which is best.
Unfortunately, not all AR coatings of a given type are created equal, and some may even be bogus. Lovely as they are to behold, I am very skeptical regarding the value of the so-called "ruby" coatings, which reflect a dazzling amount of red light, making viewed objects appear ghastly green. When leading manufacturers, such as Carl Zeiss, Leica, Nikon, and Swarovski, start using ruby or other offbeat coatings, I'll start believing in them. The first line of defense against inferior and bogus coatings is to buy from a manufacturer with a proven track record for honesty. That isn't to say that even the best companies are above hyping their proprietary coating. It's usually the advertising people who get carried away.
You'll notice that most new binoculars no longer have a dog-legging in the barrels because most new binos are slim and made with roof prisms (a feature once found on only premium binoculars).
Binoculars are classified with two numbers (8x32, 10x40, etc.) The first number refers to the magnification - how many times closer an object appears. For most of us, seven or eight power is perfect; serious naturalists or hunters may want ten. The second number is the diameter (in millimeters) of the front, or objective, lens. The bigger the lens, the wider their field or view - and the more the binos weigh.
What size lenses do you need? Compact binoculars have Objective lenses in the 18-to-25-millimeter range. Midsize binoculars are 30 to 35 millimeters, great for all around use. Full-size binos have 40-to-50 millimeter lenses, for the brightest views.
The diopter control corrects any vision difference between your eyes. Look at an object, close your right eye, and focus the glasses normally. Then repeat for your right eye, using the diopter control.
To maximize light transmission, the best binoculars have coating on both surfaces of all their lenses. Such glasses are "fully multicoated." Better models are also sealed against rain, and the best are nitrogen-filled to eliminate internal fogging. Check the specs for "phase corrected prisms" - they have a coating to enhance contrast and color accuracy.
If you must wear glasses while using binoculars, look for a pair with at least 15 millimeters of eye relief - the distance you can hold the eyecups away from your face and still have a full view.
The US military is currently using binoculars equipped with USB and Ethernet adapters and receivers that are capable of beaming secure voice and video information. The LightSpeed system exploits free-space optics, the ability to pass data between two points via an optical beam. The method usually involves lasers, but this system uses eye-safe infrared LEDs, similar to those used in TV remotes. The binocular has an attachment that fits over the ocular side and produces a beam that comes out of the right eye of the binocular. The receiver is on the left side. The binoculars' communication is limited to line of site, and range is determined by the strength of the optics. Unlike radio-wave transmissions, data transfer through the LED beam is harder to detect.
Many hunters solve the problem of deciding on the "right" magnification by using a variable power scope. To change the power of such a scope to any setting between, say, 2x and 7x or 3x and 9x, you turn a ring near the eyepiece. This ring moves a series of internal lenses. If you plan to do several kinds of shooting with one rifle, or wish to use one scope on several rifles for different purposes, a variable makes sense. However, most models are relatively expensive and -because some light is lost in the extra series of lenses- they are not quite as good optically as fixed power models.
Special scopes have been developed with extra long eye relief, so that you can sight through them when you hold your gun at arm's length. However, long eye relief impedes magnification, which means that these scopes are low-powered- 1x, 1.3x, and 2.6x in currently available models. While you may find that you do slightly better with a 2.6x instrument than with the others, great magnification isn't important. Even a 1x scope can boost your range by clarifying your target and replacing the waver prone front and rear sights on your handgun with crosshairs.
Whether your shooting is primarily done with a rifle, handgun or shotgun, you may be wondering about the answer to the old one eye or two eye question. The best answer I can give is that the large majority of accomplished gunners aim best with both eyes open; that way they have increased depth perception and better peripheral vision. But there are exceptional shooters who close one eye, because doing so sharpens the sight outlines for them, or because they learned to shoot that way and years of practice have made them extremely good at it.
Operators are also exploiting the latest in image intensification (I2) night vision technologies.
The AN/PVS-14 MNVD provides a representative example. As a replacement for some single-tube/twin eyepiece night vision binocular designs (like the AN/PVS-7 series), the monocular device has become a ubiquitous presence in many SOF organizations. In addition to its physical design advantages over earlier systems, the PVS-14s now entering the field incorporate significant advances in "third generation" night vision I2 technologies.
The same advanced night vision technologies that allow SOF to see through the night can also be integrated into a range of scopes and sighting systems. An example can be found in the AN/PVS-17 Miniature Night Vision Sight from Northrop Grumman Corp.'s Electro-Optical Systems business unit. Selected as the dedicated sight for the U.S. Special Operations Command (USSOCOM) Special Operations Peculiar Modification (SOPMOD) M4A1 carbine program, the PVS-17 utilizes the latest image-intensifier tube technology pioneered by Northrop Grumman, including an auto-gated power supply that provides extended dawn and dusk operation in addition to exceptional low-light night performance. The design combines an internally projected, bore-sighted, red dot aim point and a unique dual momentary on/off control that saves battery power (In late 2001 the sight was also selected for the U.S. Marine Corps Miniature Night Sight (MNS) program).
SOF operators from all services were quick to recognize the tactical synergies of coupling I2 vision technologies with infrared laser illuminators/pointers. In a nutshell, the combination allows users to shine a battlefield flashlight that is invisible to opposing forces not equipped with night vision capabilities. When mounted to a weapon the light can be used for both illumination and aiming purposes. Examples of these systems fielded in recent years include the AN/PAQ-4C and the AN/PEQ-2A aiming light/illuminators.
SOF exploitation of new range finding and battlefield observation and integrated GPS technologies was evident in the early days of Operation Enduring FreedomAmerica's war on terror. Two new systems that saw a limited application by Army special operations elements were the Long Range Advanced Scout Surveillance System (LRAS3) and the Viper (Vector IVBinocular Laser Range Finder). The Vector IV package provides target range, azimuth and vertical angle measurements to the operator. Coupling that data with the PLGR provides self-position data while allowing calculation of target location. Finally, the PVS-14 MNVD provides the user with a night vision capability.
Thermal imaging systems also play an important role in special operations aiming and targeting applications. An example of the former capability can be found in the AN/PAS-13 thermal weapon sight (TWS). Downsizing that tactical capability is the goal of another new program under development by NVEC. Currently designated internally as the "MX3," the new thermal imager is being designed for multi-role applications. The new system, which the company hopes to begin marketing in the mid-summer time frame, will measure just 5 3/4 inches by 4 1/2 inches by 2 1/2 inches and weighs 16 ounces. without batteries (it's powered by two Lithium AAs).
SOF elements mandate smaller and lighter systems to identify their locations and equipment to friendly forces. "We have cloth 'thermal panels,' not the metal ones," Stamey explained. "They're either tan or green on one side with orange on the backside. You can use the orange side for daytime visual identification and the flip side has the thermal panel on it. What that does is to provide a 'cold' square for thermal imagery on helicopters, fixed wing aircraft or the thermal sights on armored vehicles." NVEC's thermal identification panels (TIP) come in two sizes: large 4 feet by 4 feet panels and smaller 2 by 2 feet "TIP-LITES." The smaller panels can be draped over rucksacks or joined together via edge grommets and attachment strips.
Another ID marking option involves the use of infrared-visible reflective "tape" known as "Glint Tape." The tape option has been used to identify friendly forces for several years with some of the early lessons learned dating back to Operation Just Cause (Panama).
Unfortunately, not all early lessons were positive. In one such case, an AC-130 crew over Panama found their infrared sight degraded by smoke from preparatory fires on a target. Switching to a thermal sight provided better vision for the AC-130, but eliminated the friendly location feedback provided by the tape on friendly personnel helmets and vehicles. In the confusion, the AC-130 engaged elements of the ground unit.
So it will be great for the SOF guys doing strategic reconnaissance missions. [It will be] small enough to put in your cargo pocket and we're keeping it at a price that's a lot cheaper than a lot of the other thermals out there so you can have two per team. That way, if they go split team operations at least one member will have it. And with the mount that we have on it with one mount to fix it to an M4 and another mount on the side where you can fix an IR laser pointer on it one person on the team can spot stuff and mark it so the rest of the guys on the team can see what he's looking at with their night vision.
The SOF community is taking a lead in the "fusion" of image intensification and thermal technologies into a single night eliminating image. For more information read:
Weather Effects on Optics
Common weather elements are atmospheric pressure, clouds, dew point temperature, humidity, precipitation, temperature, visibility, and wind speed/direction. Any one of these may of may not affect your optics, but certain combinations could create weather phenomena that could render your optics useless. Weather phenomena such as thunderstorms, hurricanes, snow, blizzards, and dust storms will affect range of visibility for up to days at a time.
Sometimes other factors can be combined with a weather element to create a weather phenomenon. For example, when temperatures are below -30C and water vapor is released into the cold air by internal combustion engines, artillery fires, or launching of self-propelled munitions, visibility can be reduced to zero when the moisture freezes instantly and changes into ice fog. Ice fog can restrict visibility across a whole valley and linger for hours.
Systems that use available light such as binoculars, scopes, and NVDs are the easiest to defeat obscurants such as haze, smoke, dust, and precipitation because visible light has a short wavelength and can be more easily attenuated. We attempt to mimic natural elements in combat using smoke and artificial light.
Weather Effects on Visible Light Electro-Optics
Night Vision Devices (NVDs) are an example of passive image intensifiers. They use extremely low-level-light (LLL) sources (starlight), and amplify that light so that objectives are visible. These systems operate in the visual and near-infrared wavelengths.
LLL Television (TV) is a passive system and is capable of picking up targets at light levels below those usable to the human eye. This TV electronically enhances the video signal and makes it visible to the operator. Night sights, heat seeker munitions, and infrared detector munitions are examples of passive infrared imaging systems.
Natural light is critical in planning operations where NVDs are used or in operations timed to use only available light. Natural light values vary as a function of the position of the sun, moon, stars, and clouds. Variables such as altitude, cloud cover, terrain-produced shadows, visibility, and direction of vehicle or aircraft movement in relation to the sun or the moon can also affect light level availability.
Although the level of light affects all devices operating in the visible light spectrum, image intensifiers are influenced the most. Too much or too little light adversely affects the use of NVD. On relatively clear nights with a near full moon, you can normally operate without the aid of NVD. With less than a full moon, there may still be too much light. Too much light, when amplified by an NVD, saturates the viewing area as seen through the device and makes the device unusable because light and dark contrasts are no longer possible. When illumination is limited, NVDs must be used. For partial or heavy overcast skies with little moonlight, even these light levels may be too low to use an NVD.
Additionally, terrain influences on available illumination must be considered. Even though illumination may be adequate to support the use of NVD, flying in a valley with shaded areas may end disastrously.
Weather Effects on Infrared (Thermal) Electro-Optics
Electro-optics (EO) exploit portions of the electro-magnetic spectrum to acquire, image, and/or target objects of interest. The EO spectrum covers the ultra-violet, visible, and infrared (wavelength range of 0.01 to 1,000 microns). The amount of infrared energy is determined principally by the object's temperature, its surface reflectivity, and its structural properties. Natural infrared energy is produced when objects absorb sunlight.
The performance of EO systems depends on three basic factors:
- EO characteristics of the target and its background on the battlefield.
- The atmosphere between the EO system and the target and its background.
- The sensitivity of the EO detector system (to include human operator performance).
The most fundamental environmental conditions inhibiting EO signals are:
- Attenuation or reduction of the signal by atmospheric moisture such as clouds, precipitation, fog, and high humidity.
- Temperature affecting atmospheric refraction near the surface.
- Temperature contrast between the surrounding environment and the target.
- Winds kicking up dust and sand.
- Low-level-light (LLL).
EO systems are classified as active (overt) and passive (covert). Active systems emit a detectable wavelength signal; while a passive system senses emitted or radiated energy. EO systems include image intensifiers, infrared imagers, laser designators, and low-level-light (LLL) television (TV). To fully understand how weather impacts these systems, we need to know their basic operating principles.
These systems are characterized as near infrared (short wavelength) or far infrared (long wavelength). For infrared imagers to function properly there must be a temperature or thermal contrast between target and background area (signature). They can tell the difference between target "hot spots" and the rest of the target itself, like a warm engine in a cold truck.
Laser designators are active systems. They are used with smart munitions capable of receiving reflected laser light. The designator "pings" a target with a laser beam at a specified wavelength. The receiver in the munitions recognizes the reflected beam and homes in on the designated target. These designators do not emit light in the visible spectrum and, therefore, cannot be easily seen or detected.
When combined with EO guided munitions, the transmitted laser beam reflecting off the target greatly enhances the delivery accuracy over conventional delivery techniques. However, the EO guided munitions must receive the reflected beam in time to make course changes prior to hitting the target. If the beam is not reflected off the target or transmitted, or reception is disrupted, a miss will probably result.
A target may be acquired with a thermal imager only if the amount of infrared energy of the target is sufficiently different from that of the background. This difference, called thermal contrast, is the difference between the temperatures of the target and its background.
Wind, rain, snow, humidity, and clouds reduce the temperature contrast between target and its background and even cause thermal reversal where instead of a "hot" target against a "cold" background, you find a cold target against a hot background. This can occur in the early morning and late afternoon when a thermal sight encounters a condition where some inactive targets without an internal heat source will warm up or cool off to the same temperature as the background.
If the difference in temperature is not enough to be detected, the thermal devices will not be able to see targets. This phenomenon is called "thermal crossover." Thermal crossover may occur zero, one, or two times per day. Thermal devices can be rendered useless by thermal crossover from anywhere between 1-15 minutes. The sensitivity of the thermal device to the difference in temperature and the rate at which a target is heating or cooling will determine how long thermal crossover will occur.
Most metal targets heat up or cool off faster than the ground and vegetation in the background. At night, a metal target appears to be colder than the background, but with the sun shining on it, the target appears to be hot compared to the relative coolness of a background of trees or bushes.
In bright sunshine, the thermal crossover period may be just a few seconds.
On cloudy days, however, the thermal crossover period may be a number of minutes. When this happens, optical sightings must then replace the infrared devices. Temperature, wind, and precipitation have a major influence on your ability to pick out a target from the background in the infrared spectrum. They also affect seismic (sound and acoustic) signatures. Detection of objects in the infrared spectrum depends on a temperature contrast between the object and its surrounding environment.
Listed below are weather elements affecting thermal contrast at crossover time:
- Clouds: Clouds will reduce the thermal contrast. Lower and thicker clouds have a stronger influence than higher or thinner clouds.
- Surface Wind: Wind causes the temperatures of both the target and background to become closer to the air temperature, and as a result, closer to each other.
- Humidity: Moist air does not enhance the rate of cooling as much as dry air. With high humidity and a moist background, the thermal contrast would be minimal between target and background. If the air was dry, the cooling influence on the moist background would cause a greater thermal contrast.
- Precipitation: Falling rain and snow have cooling effects that bring target and background temperatures closer together. In the case of operating vehicles, the temperature contrast may be increased since the precipitation will have little effect on the heat generated in the engine compartment and the exhaust.
- Surface Wind: Wind causes the temperatures of both the target and background to become closer to the air temperature, and as a result, closer to each other.
The relationship between rain and EO systems is very complex since the result is a function of the precipitation particle size and the wavelength of the EO system. Table 1 can be used to categorize the weather effect into one of two categories, severe or moderate degradation, based on the weather and your EO system.
|Severe Degradation||Moderate Degradation|
Aerosols (smoke, dust, sand)
Thermal crossover is computed for a specific target using a defined sensor (and its capabilities) with a "known" background. For example, an AH64 with a Hellfire missile is targeting a T-72 tank. The tank's background consists of sand and small rocks with little moisture content. The weather is providing 25% cloud coverage, 15% humidity, and 84C temperature. The aircraft altitude is 100' on a heading of 020. Computer programs take all that information (and more) to compute a "thermal crossover" for that specific target (the tank).
Here is an infantry situation: You have a thermal imager mounted on a shoulder fired weapon. As the person holds the weapon and uses the thermal scope to acquire the target, the background of that target is not uniform. So as the person tracks the target, the target moves to a background with a different emissivity, changing the "crossover" for that target. If the target is a vehicle moving through a populated area and moving through shadows with a person tracking it with a thermal scope, the sensor "heading" changes also. So as the scope moves, so does the crossover.
If the target is a person, the thermal crossover is almost impossible to compute due to the fact that the human body is a heat source and emits a thermal pattern of its own with the temperature varying immensely with height, weight, and the type of clothing worn. To make a long story short, it would be impossible to give with any accuracy the thermal crossover for a thermal sight mounted to a rifle. Software is generally not designed for that type of mission. A crossover time can be "guesstimated," but it would be a guess with no promise of any accuracy.
EO devices are also affected by atmospheric refraction. Basically, the sun's heating of the surface air creates sufficient vertical motion or turbulence to cause this effect. A mirage is caused by this heating and can make a building appear to move or even cause a target to disappear altogether. Such apparent displacements can lead to target misses.
Although these refractive conditions are associated with periods of high heat, this condition has been observed over a snow cover when the air temperature was -32C. The higher you are from the surface, the less likely you will encounter mirages.
As the wavelength of the spectrum used by an EO system increases, the less it is affected by obscurants. However, the long wavelength EO system provides less target resolution. Near infrared systems like the handheld thermal viewer can penetrate some light fog oil and diesel oil smokes. Far-infrared systems, such as vehicle mounted thermal sights, use longer wavelengths of the spectrum and can penetrate low densities obscurants that defeat both visible and near infrared.
For more information read the Naval Meteorology and Oceanography Professional Development Detachment's report on atmospheric effects on EO sensors and systems:
EO.pdf (44kb Nov 2004)