This article is intended to help you choose the right kind of night vision device (NVD) for your application, budget and specific needs.   There is a lot of confusion when it comes to night vision devices.  Although there are night vision rifle scopes, monoculars and binoculars, a lot of the time, any of these devices will be called “goggles” which is incorrect and night vision goggles may actually not be the right kind of night vision equipment for your intended application.  This article should clear up some of the common misconceptions in regard to night vision devices.

First, let’s define what a night vision device is and what it is not.  A night vision device is an electro-optical instrument allowing its user to see in dark environments.  Night vision devices can be analog or digital.  A night vision device is NOT a thermal vision device; it does not allow its user to see heat coming from objects. 

How night vision devices work at a glance and why they are different from thermal vision devices.  Most of the night vision devices used today are analog.  Analog night vision devices have some important advantages over digital night vision devices, these advantages will be explained a little later in the article. 
An analog night vision device achieves its night vision capability by essentially doing two things. The first thing that an analog NVD does is gathering whatever little light is available (from the stars, Moon, other distant light sources) and amplifying this light many thousand times with high voltage to produce a bright image that the user can see through the eyepiece.  The second thing that an analog NVD does is taking in infrared radiation (usually from a built-in infrared flashlight) and applying the same high voltage to it so that infrared radiation is converted into visible light that the user can see through the eyepiece.  This is particularly important in pitch-black darkness when there are no stars, Moon or any other light sources for the NVD to amplify.  In other words, almost every night vision device has its own infrared light source which is invisible to the naked eye but is visible through the night vision device.
Digital night vision devices also convert infrared light from a built-in infrared illuminator to visible and some are also able to amplify faint light by accumulating it in a CMOS sensor.  Digital night vision devices have advantages and disadvantages over analog night vision devices.  We will look in more detail into how digital devices operate later in the article.
Thermal vision devices do not deal with visible light at all.  They also do not have built-in infrared light sources.  A thermal vision device senses the heat given off by an object; it does not “see” any light that’s reflected off the surface of an object, animal or a human being.  All objects and living beings give off a small amount if infrared radiation the frequency of which depends on the temperature of the object.  A thermal vision device senses this radiation and outputs it onto an LCD screen.   In practice, a thermal vision device would allow the user to see the outline of an object with little detail while a night vision device would produce a detailed image.  If the object being observed is the same temperature as its surroundings, it would be invisible through a thermal vision device.  We will mention thermal vision device in the article any further as they are not night vision devices and should not be called as such.

Different types of night vision devices and their applications.  There are essentially five different types of night vision devices (not to be confused with four generations which we will mention later in the article).  Here are the four different types and their applications:

Night Vision Monocular – a handheld night vision device with one objective and one eyepiece.  Monoculars are used for various tasks that do not require completely hands-free operation.  Monoculars are the most affordable and simple night vision devices and are suitable for recreational activities such as nature observation, hiking or just playing around in the backyard.  Here is an example of a night vision monocular:
Figure 1 A Night Vision Monocular

Night Vision Binoculars – handheld night vision devices with two objectives and two eyepieces.  Night vision binoculars produce two distinct images – one for each eye, and are used whenever a 3- dimensional image is preferred.  Applications include night time navigation, target detection and surveillance.  Night vision binoculars are essentially two monoculars put together and have twice the parts of a monocular, hence they are usually double the price.  Night vision devices with two eyepieces but one magnifying objective are called Night Vision Bi-Oculars or Pseudo-Binoculars.  Night vision bi-oculars provide magnification for long distance viewing but unlike night vision binoculars, they don’t provide any depth perception.  Here’s an example of real night vision binoculars:
Figure 2 Night Vision Bi-Oculars         Figure 3 Night Vision Binoculars

Night Vision Goggles – are 100% hands-free devices which usually come with a head-mount.  Night vision goggles can be divided into three different subgroups; dual night vision goggles, night vision pseudo-goggles and night vision mono-goggles.  Regardless of the subtype, night vision goggles have no magnification.  Magnifying lenses found on night vision monoculars and binoculars allow the user to see farther away but do not allow seeing anything within the user’s immediate surroundings.   Because the user wears the goggles on his/her face, it is essential that the immediate surroundings are visible, otherwise it would be impossible to walk while wearing night vision goggles.  Therefore, night vision goggles cannot have any magnifying lenses installed.  Some night vision goggles can be fitted with magnifying lenses and essentially turned into night vision binoculars; such modification however, makes it impossible to wear the goggles on a head-mount.  Here’s an explanation of the different subtypes of night vision goggles:
Dual Night Vision Goggles – have two objectives and two eyepieces.  These goggles produce two distinct images, one for each eye.  Dual night vision goggles are well suited for nighttime driving, boat navigation, aircraft piloting, hunting, police and military operations.  Here’s an example:
Figure 4 Dual Night Vision Goggles

Night Vision Pseudo-Goggles – also called Night Vision Bi-Ocular Goggles are hands-free night vision devices with one objective and two eyepieces.  These devices can be used for anything that dual night vision goggles are used for except situations where distance estimation and 3-dimensional perception are required.  Pseudo-goggles split a single image into two feeds, one for each eye.  Although the user sees the image with his/her two eyes, the images are exactly the same and therefore the user’s brain will not be able to estimate the distance to an object.  Pseudo-goggles cannot be used for nighttime driving, boat navigation or flying an aircraft.  You can see an  example of this device here:
Figure 5 Night Vision Pseudo-Goggles

Night Vision Mono-Goggles – hands-free night vision devices which have one objective and one eyepiece.  Night vision mono-goggles cover only one of the user’s eyes.  This type of night vision goggles is the most affordable one due to the least amount of parts.  Night vision mono-goggles are well suited for military and police applications, hiking and recreational use.  Since the user is only using one of his/her eyes, distance estimation is impossible and the field of view is narrower than with any other types of night vision goggles.  Besides their price advantage, night vision mono-goggles have one more advantage which could sometimes be very important; one of the user’s eyes stays constantly exposed to the dark and the user’s natural night vision capability in that eye is preserved.  If the goggles suddenly cease to operate or the user needs to quickly take them off, the one eye that was looking through the goggles will go blind for a few seconds while the one that staid exposed to the dark will not.  With dual night vision goggles as well as with pseudo-goggles, the user will go completely blind for a few seconds while his/her eyes adjust to the dark if the goggles are suddenly taken off or cease to operate.  In situations when it is possible to suddenly encounter a bright source of light (such as vehicle headlights, or a brightly lit room), the mono-goggle type is also preferred since the bright light may either flood the image or shut down the goggles so the user will have to rely on the eye that is not covered by the ocular of the goggles.  This could be a life and death situation during military and police operations.  Here’s an example of night vision mono-goggles:

Description: PULSAR Challenger GS Super 1x24 Night Vision Goggles
Figure 6 Night Vision Mono-Goggles

Night Vision Rifle Scopes – also called Night Vision Weapon Sights or simply Night Vision Sights.  These are night vision devices are specifically designed to be mounted on a weapon for precise aiming and shooting.  Night vision rifle scopes are used by the military, police and hunters throughout the world.  A common misconception is that night vision scopes can be used during the day.  There are no night vision rifle scopes that can be used during the day and most cannot be even turned on (without the protective cap) in daylight.  Some digital night vision rifle scopes can be safely turned on during the day safely but they would still be virtually unusable in daylight compared to regular daylight rifle scopes.  This is a typical analog night vision rifle scope:

Figure 7 Night Vision Rifle Scope

The chart below summarizes the various types of night vision devices.
night vision
Comparison Between Night Vision and Daylight Optical Devices.
There is virtually no limit on how large the magnification of a daylight optical device can be.  Daylight binoculars and rifle scopes are available with magnifications as high as 60 or even 100 times.  Daylight optical devices are also often available with variable magnification (zoom).  Night vision devices on the other hand are rarely available with magnifications higher than 5x and are never available with zoom capability.  Because the amount of light a night vision device takes in is very small compared to a daylight optical device, increasing the magnification to more than 5x would drop the brightness of the image to a point where the device would virtually be unusable.  The specifications of all optical devices, including night vision ones can be expressed with a pair of numbers, such as 4x50 for example.  The first number tells the magnification of the device.  In the case of 4x50, the device magnifies 4 times.  The second number is the diameter of the objective lens.  A larger objective lens takes in more light and therefore allows for a brighter image.  The field of view (the size of the area seen through the night vision device) will also be larger with a larger lens.

Night Vision Devices in Daylight.
Analog night vision devices can be damaged by exposure to bright light and should not be used in daytime.  Protective caps are usually included with any night vision device, this cap has s tiny hole in the center allowing a little light to get in.  An analog night vision device can be turned on safely in daylight provided that the protective cap is on the objective.  Analog night vision devices are absolutely useless in daytime and should only be used at night. 
Digital night vision devices are less susceptible to damage by bright light.  However, digital night vision devices are also virtually useless in brightly lit environments.
It is much more useful to have a dedicated night vision device and a dedicated daylight device rather than trying to use a night vision device in daylight.
Since rifle scopes must be calibrated for precise aiming, a night vision rifle scope cannot be simply taken off the weapon and replaced with a daylight scope for use during the day.  For this reason, a lot of users have two separate weapons, one with a permanently mounted night vision scope and another one with permanently mounted daylight scope.  It is also possible to purchase a night vision scope with a quick release mount and a daylight scope with a quick release mount.  A quick release mount allows removing the scope off the weapon and placing it back on without having to zero it in all over again.  Here’s an example of a night vision weapon sight with an optional quick release mount:

Night Vision Generations and How They Work.
Analog night vision devices are divided into five generations.  The generation of a night vision device is defined by the type of the Image Intensifier Tube (IIT) installed in the device.  One and the same night vision device may be available with different generations of image intensifiers.  The image intensifier tube is the heart of an analog night vision device.  The IIT collects photons from faint light sources or the built-in infrared illuminator onto a photocathode screen.  These photons knock out electrons on the other side of the photocathode screen.  These electrons are accelerated with high voltage after which they hit a phosphor screen on the back of the IIT.  The accelerated electrons knock out a greater number of photons from the second screen than was originally received by the first screen and therefore the second screen produces a bright image that the user sees through the eyepiece.  Contrary to a popular misconception, the four generations of night vision devices are 0, 1, 2 and 3 and 3+ and not 1, 2 3 and 4 and 5.  The price of a night vision device goes up with its generation.  For example, the price of a Generation 2 night vision device is about 10 times that of the same device in Generation 1 version.  Here is a detailed look into the generations and how they differ from one another;

Generation 0
Developed by Dr. Zworykin during the World War 2.  Generation 0 device were not able to amplify ambient light and solely converted infrared light from a built-in infrared flashlight into visible light.  These devices are no longer commercially available.
night vision
Generation 1
Developed during the Vietnam War, this is an improvement on Generation 0 technology.  A Generation 1 IIT is comprised of the same parts as a Generation 0 IIT on the above diagram.  However, it is able to amplify ambient light about 1000 times it does not require an infrared illuminator if enough ambient light is available.

Generation 2
Adds a micro-channel plate which is basically an electron multiplier.  The micro-channel plate greatly increases the number of electrons and allows the IIT to multiply ambient light by about 20,000 times.
night vision

Generation 3
Generation 3 image intensifier tubes are comprised of the same parts as the Generation 2 ITT’s on the above diagram.  However, Generation 3 devices use a gallium arsenide photocathode which allows them to multiply ambient light by as much as 50,000 times.  The micro-channel plate in Generation 3 devices is coated with a special ion barrier which increases the life of the tube but has a negative effect on its performance.

Generation 3 Plus (sometimes called Generation 4)
Generation 3 Plus IIT tubes add an auto-gated power supply which automatically adjusts the voltage fed into the tube depending on light conditions.  Generation 3 Plus devices don’t have the ion barrier coating on the micro-channel plate or have much thinner coating which has a negative effect on the tube’s life but allows for better performance.

Digital night vision devices work by accumulating photons in a low light CMOS sensor.  The photons are gathered in the sensor over time.  Once the sensor gathers enough photons to produce an image, a signal is sent from the sensor to an LCD screen which the user sees through the eyepiece.  Digital night vision devices operate with a slight lag because the sensor needs some time to gather enough photons.  The lag is usually not very noticeable but may be an issue in life and death situations (such as military operations).  Digital night vision devices are not able to amplify ambient light as well as analog night vision devices and therefore rely heavily on the built-in infrared which results in increased power consumption.  Digital night vision devices typically require many more batteries than analog ones and also consume the batteries a lot quicker.

So which generation to choose?  The higher the generation, the less you will need to rely on the built-in infrared illuminator.  Also, the infrared wavelengths for higher generation devices are longer and therefore more difficult to detect with lower generation night vision devices.  If the possibility of detection by another night vision device is an issue, it is advisable to use a Generation 3 or higher night vision device.  For nature observation and recreational activities, Generation 1 is usually sufficient.  Serious hunters may consider at least a Generation 2 night vision device.  The US military currently uses Generation 3 and Generation 3 Plus devices.