Why are binoculars filled with Argon?

Hiking binoculars

Source: Pawel Janiak


Binoculars are not just the ultimate aid for hunting game in the woods, but they are equally popular among bird-watchers, campers, and plane-spotters too. However, they need to be handled with care and like most delicate instruments, such as your digital camera, they will start to malfunction if precautionary measures are not taken into account during use. 

Optical instruments like binoculars and other scopes are susceptible to moisture contamination because they are more likely to be used in places with high humidity and frequent climatic variations, such as rainforests, or the sea. Once moisture is trapped inside the binocular housing, water vapors start to collect on the internal surface of the objective and eye-piece lenses disabling the observer from viewing anything clearly, just like how a pair of eye-glasses fog if you enter a room with air-conditioning on.  

It is not difficult to wipe a little mist on the external lens surface, but when the moisture is inside the barrels, it is not a do-it-yourself cleaning job for everyone, due to the inaccessibility of binocular housing components. Binoculars with fogging problems are seldom sent for repairs and are usually thrown away for recycling once they start to show signs of blurry vision. Although, moisture only can be daunting enough for your beloved companion, dust and numerous other particles carried by air, such as bacteria and pollen grains can reduce the optical performance of your instrument in a similar manner. Also, the air has a 21% Oxygen concentration, which is high enough to potentially interact with materials used in the optics housing under extreme temperatures and support fungal growth inside the barrels. 

Source: Alessandro Vallainc

Experts in the field of optical sciences came up with a solution for this nuisance in the 1970s, which involved sealing the binoculars shut by removing the air inside with the help of Nitrogen-purging, a process which uses compressed Nitrogen gas to replace and drive out the air present inside the housing under pressure, along with all impurities. Since there is no moisture content due to the absence of air, Nitrogen-purged binoculars are effectively fog-proof and can be used in the wilderness with an improved sense of freedom. 

Continued scientific developments over the years since the first Nitrogen-purged binocular came out have made it possible for adventurous people from all types of societies to proudly own fog-proof binoculars. Though almost all binoculars are fog-proof rated, there are still some knock-offs available in the market which lack this essential feature. People who believe in the myth that not everyone needs Nitrogen-purged binoculars often regret buying the cheap ones later. Moisture contamination is no joke and must be taken seriously when making a decision between different binocular models 

More recently, Argon gas has been replacing Nitrogen for fog-proofing high-end binoculars and telescopes in spite of both performing the same function, which is keeping the internal components of the binoculars free from moisture. The transition is likely due to the larger size of Argon atoms and its absolutely inert nature, compared to Nitrogen. Since air comprises only 0.8% of Argon, the process to harvest it directly from the air is a bit complicated, unlike Nitrogen, which is why Argon-purged optical devices are pricier than their Nitrogen-filled counterparts.


Advantages of binoculars filled with Argon

There are a number of advantages of Argon-filled binoculars over conventional ones in addition to just a prolonged life, but how does a gas help achieve this? Some pros of using Argon in binoculars are discussed below: 

Completely dry internal components of binoculars:

Air always carries a small percentage of moisture wherever it goes and binocular housing is no exception. As soon as there is even a small amount of moisture content inside the binocular, chances are it will deposit on the lens’s surfaces, effectively disrupting the view through them. One of the advantages of purging binocular barrels with Argon under high pressure is that internal components remain completely dry due to the absence of air and water vapor. Since all joints of binoculars are sealed shut with the help of rubber seals, nothing can enter inside the barrels as well.  

Sealed binoculars give a profound sense of assurance to the user so that he is able to concentrate on his objects without having to obsess about lens fogging coming in the path of their enjoyment.  

Larger molecules  

A party balloon is an excellent example of a container filled with a gas under pressure. The gas used to fill a balloon is air at a slightly higher pressure (14.76psi) than standard atmospheric pressure (14.69psi), but it can give us a hint of how a gas behaves inside a sealed container.  

Even though the pressure is almost identical to the outside, the air molecules inside a balloon begin to slowly leak through after a bit of time since their inflation. This phenomenon, which is imminent, is known as ‘Diffusion due to pressure difference, that is, between the air inside and outside the binocular housing. 

Similarly, Argon is filled inside the barrels of binoculars, but under a pressure of 22.7psi which is 7psi higher than the standard atmospheric pressure. The pressure difference acts as a barrier to keep the outside air and water vapor from entering the binocular housing. However, just like a balloon, Argon atoms will try to leak through the several joints in a binocular assembly after an undetermined time period. Since Argon has a higher atomic number (18) than Nitrogen (7), Argon atoms are significantly larger than Nitrogen molecules due to an additional outer electron shellNow since leakage is inevitableArgon takes more time to completely leak through the seals and, therefore, keeps your binocular fog-proof for a longer time compared to Nitrogen 

Argon Gas

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Argon is a member of the noble gas family in the periodic table. Members of this family are least chemically reactive of all elements because even at critical temperatures and pressures they rarely form compounds. This is because of the completely filled outermost valence shell in all noble gases, which makes them totally inert than any other element. 

This property is of the utmost importance when it comes to binoculars. The air inside conventional un-sealed optical devices is known to interact with the materials used in their housing. It also retains water vapor, which is not favorable for the optical performance of binoculars, as discussed above. Now Argon, although not the least reactive of noble gases, are virtually unreactive and therefore, does not interact with the plastics, metals, and lens coatings used inside a binocular.  

Moisture traps heat 

Some gases have the property of trapping heat energy for a long time, such as water vapor and carbon dioxide, both are naturally found in the air. They are known as greenhouse gases because they keep the atmosphere warm enough for the growth of vegetables and fruit in greenhouses or hothouses around the world.  

Since air is present in conventional non-purged binoculars, these gases have been known to contain heat in the confined binocular housing during prolonged use in tropical climates and causing substantial damage to the lenses as well as the housing itself. Argon does not have an affinity for water molecules, which is why it is completely dry. Additionally, it does not behave like a greenhouse gas, therefore, unwarranted wear and tear by excessive heat is not a possibility in Argon-filled binoculars. This is one of the reasons why Argon is being favored over Nitrogen for optical purging purposes. 

Prevention from fungal growth

Lens fog is not the only problem that is associated with trapped moisture inside the binocular. A more grim issue is the growth of fungus inside the barrels and around lenses, which usually requires a low-light, confined space with lots of oxygen and water vapor, for example, all non-purged optical devices. Since the development of fungus and mold can occur in as little as a few hours, a little careless attitude during the handling of binocular can contribute towards permanent binocular damage.  

fungus on binoculars

Source: DIYPhotography

Although cleaning fungus from affected areas is possible with a number of chemical mixtures, Fungi are notorious for using various enzymes to chemically modify the surfaces around them, causing irreparable damage to both the objective and eyepiece lenses in the case of binoculars. Enzymatic chemicals are capable enough to scar the internal lens surface until the binocular can no longer be useful for viewing purposes. Some retailers offer repolishing services for lenses but they rarely fully recover from the damage. 

Since Argon neither retains water nor reacts with it, fungal growth is never a possibility with Argon-purged optical devices. However, this doesn’t hold true in the case of excessive gas leakage. 

Fog-proof binoculars will also be waterproof

Almost all fog-proof binoculars available nowadays also feature a waterproof rating. This is possible because Argon is filled under a pressure of 22.7psi which is significantly higher than standard atmospheric pressure. The positive pressure difference helps keep unwanted air and other particles from entering the optical housing, thus making the binocular waterproof and weatherproof. 

If a binocular packaging says that a particular device is only fog-proof, it means that the air pressure at which it was purged is not suitable enough. Buying such binoculars is not recommended by experts as by spending a little more money, you can get something worthwhile that will last for a long time.


Source: Photoxels

Optical Advantage

Most people believe that optical instruments, such as rifle-scopes and binoculars that are purged with Nitrogen or Argon have an optical advantage over regular non-purged binoculars, however, studies have shown that this is not a fact. Both Nitrogen and Argon are transparent gases and have a similar value of the refractive index as that of air, and therefore, are incapable of improving or reducing the optical performance. 

Even at elevated temperatures, such as in the tropics, there is no apparent optical benefit of having pressurized gas in a sealed binocular. One thing that is undeniable is that due to an absence of air and subsequent water vapor condensation, the observer’s view will not be obstructed by even the slightest presence of fog, which in some situations even transparent to the naked eye.



Binoculars are the main companion of campers, ship’s crew as well as enthusiasts, such as bird-watchers and airplane spotters, but owners of binoculars and telescopes have been baffled for years with the issue of blurred view through their devices due to constant lens fogging. Having to deal with water vapor inside their beloved devices has led to the scientific development of a process known as Fog-proofing to eliminate chances of moisture contamination 

Lens fogging occurs because of the condensation of excess water vapor present in the air on the surface of lenses due to changes in temperature and pressure. Since reaching the inside of binocular to clean mist from the internal surfaces of lenses is not something that everyone can do on their own, the moisture content continually increases every time the binocular is used, up until a point when the observer can no longer see through it clearly.  

Fogging is not the only problem owners had to deal with in the past. Moisture accumulation also promotes the growth of fungus inside the lens, damaging binocular internals in the process. The only way to prevent this is by keeping the binocular’s components free from moisture at all times. To achieve this, modern binoculars are purged with pressurized Nitrogen to drive out the air in the lens compartment and then sealed shut. This was first witnessed in the 1970s by German scientists working for Steiner Optics. Initially, Nitrogen-purged binoculars were targeted towards the armed forces for their consistent performance, but they soon became famous among the masses. 

Mass production started in the early 1990s, however, the fog-proof binoculars of that era were not as efficient as the binoculars of today. More and more optics OEMs have started to consider Argon in place of Nitrogen in their premium line of devices due to two reasons. One is the larger size of Argon atoms in spite of its mono-atomic nature compared to Nitrogen molecules, which some say, is the key to a longer fog-proofing capability. Secondly, both Nitrogen and Argon are inert gases at room temperatures, but Argon takes it a step further. Due to its completely filled outermost shell, it is virtually nonreactive to all other elements, which makes it an even better filler gas.  

Nevertheless, there has not been an official statement by a recognized institution regarding the matter. Argon is also a little hard to harvest from the air using liquefaction, because of a low overall concentration, which is why it is used in only high-end products for now. Nevertheless, fog-proof binoculars are recommended, regardless of the purging gas, to have the best viewing experience and a long-lasting device. 

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