How Does it Work ? - Night Vision, Binoculars & Monoculars

The image intensifier is a vacuum-tube based device that converts visible light from an image so that a dimly lit scene can be viewed by a camera or the naked eye. While many believe the light is 'amplified,' it is not. When IR light strikes a charged photocathode plate electrons are emitted through a vacuum tube that strike the microchannel plate that cause the image screen to illuminate with a picture in the same pattern as the IR light that strikes the photocathode, and is on a frequency that the human eye can see. This is much like a CRT television, but instead of colour guns the photocathode does the emitting. The image is said to become 'intensified' because the output visible light is brighter than the incoming IR light, and this effect directly relates to the difference in passive and active night vision goggles. Amplification of light is the result of LASERs (Light Amplification through Stimulated Emission of Radiation). Lasers are quite intense and can destroy an imagetube in a fraction of a second. Currently, the most popular image intensifier is the drop-in ANVIS module.

Night vision goggles refer to an IR or thermal night vision device with dual eyepieces; the device can utilize either one intensifier tube with the same image sent to both eyes, or binocular eyepieces with an image intensifier tube for each eye. This is opposed to a monocular night vision device which uses only one eyepiece, such as a weapon mounted sight glass. Binocular night vision has the advantage of individually focusing the image to each eye to maintain depth perception, which is not possible with monocular night vision.

An image intensifier is a device that amplifies visible and near-infrared light from an image so that a dimly lit scene can be viewed by a camera or by eye. Unlike a thermographic camera, an image intensifier does not work in the total absence of visible (or near infra-red) light. It does, however, create a more realistic image, because the intensities it shows are related to true optical intensity and not to temperature. This realism makes it more suitable for use by untrained operators and can be used to view objects not visible by a difference in temperature alone. Image intensifiers are also much less expensive than thermal imaging.

HOW IT WORKS
Image intensifiers work by having an objective lens focusing an image into a vacuum tube with a photocathode at one end that releases electrons by the photoelectric effect on the incidence of incoming photons. From there, the photoelectron is accelerated through around 5000 volts into a tilted microchannel plate. The high energy electron releases some micro channel plate electrons, which further release other electrons, in a process called secondary cascaded emission. The MCP is tilted to encourage more electron collisions, thus increasing the rate of emission of secondary electrons.
The electrons all move together due to the potential difference across the tube, and where one or two electrons entered, thousands may emerge. A separate (lower) charge differential in the tube accelerates the secondary electrons until they hit a phosphor screen at the other end, releasing a photon for every electron. Provided the electric field is uniform, the electrons have a linear path, so correspond to the exact incident image. The image is focused by conventional optics using an ocular lens. The only multiplicative stage is in the secondary cascaded emission. The phosphor is usually green, as the human eye is more sensitive to green than other colours (hence the soldiers' nickname 'green TV' for image intensification devices).

UNDERSTANDING BINOCULAR TERMINOLOGY

A binocular consists of two optical systems that are joined by a hinge and (typically) share a common focusing mechanism. The ability to create an image for both eyes simultaneously provides a realistic perception of depth. Binoculars are available in a great variety of sizes, magnifying powers and features to suit any purpose or preference.
The best binocular technology has always been developed and used by the military. With the end of the cold war, much of the top binocular technology is now available in the market place at falling prices. High power magnification scopes and micro binoculars once the domain of the military establishment and spy agencies can now be obtained. As much better binoculars at less costly pricing is in the marketplace, it's important for you to understand the optic technology behind this professional surveillance gear and understand the difference between cheap low-grade optics and the optics that you want for professional use.

• Monocular: A monocular is a one piece viewing scope whereas a binocular is a two piece viewing scope for both eyes. The monocular certainly has advantages in covert surveillance in that they can be stored in pants pockets and easily palmed in your hand for quick covert viewing.

• Binoculars: Binoculars are identified by simple code. It is usually two numbers separated by an 'X', for example, 10X30. The first number represents the power of magnification of the binocular which means that the object or subject appears to be this much closer than when viewed with the naked eye. The second number is the diameter of the objective (front) lens. The larger the number, the more light the binocular gathers and the brighter the image will be. Good binoculars are always a combination of magnification, objective lens diameter, image quality, size, weight, durability and quality. The quality of a binocular is not always apparent at first sight. Its robustness, such as extreme heat, cold, moisture and jarring.

• Central Focusing Wheel: The central focusing wheel of a binocular enables you to focus both eyepieces of a binocular at the same time. At the same time it is possible to compensate for unequal vision by adjusting of dioptric correction on the right-hand eyepiece.

• Coating: Done in order to reduce reflections and increase light transmission by evaporation of extremely thin layers of substances onto polished lenses and prisms of optical instruments.

• Exit Pupil: The exit pupil is that circle of light which can be seen i the eyepiece from a distance of about 25 cm. It is a mathematical value which size is determined by dividing the diameter of the objective lens by the magnification. The larger the exit pupil, the brighter the image will be in low light conditions.

• Field Of View: The field of view is a measurement of the viewing field or subject area which is expressed by width in the feet of the area you can see at 1000 yards. The wider the angle, the wider the area seen. A wide field of view is better for following fast moving action or scanning for wildlife.

• Focusing: The adjustment of an optical instrument in order to make the image sharply visible varying distances.

• Inversion System: If you look through an astronomical telescope you will see everything not only in reverse, but also upside down. The inversion systems in binoculars make it possible to see things 'normally'.

• Magnification: It indicates how many items closer an object or subject appears compared to naked eye viewing.

• Objectives: These are the large lenses at the front end of the unit that comprise a system of two or more individual lenses made of different types of glass.

• Resolving Power: Optical's's instrument capability to sharply reproduce, point by point, details of the object viewed.

	MAGNIFICATION (POWER) Binoculars are often referred to by two numbers separated with an 'X'. For example; 8x32. The first number is the power or magnification of the binocular. With an 8x32 binocular, the object being viewed appears to be eight times closer than you would see it with the unaided eye.
	OBJECTIVE LENS SIZE The second number in the formula (8x32) is the diameter of the objective or front lens. The larger the objective lens, the more light that enters the binocular, and the brighter the image.

Why Binoculars?

Newcomers to astronomy often rush to buy the first telescope they see in a department store, and this is perfectly understandable. All the beautiful pictures of celestial objects seen in magazines or in books have this effect on most people. Unfortunately, after setting the telescope in the backyard and taking the first look at the night sky all the magic is gone. The instrument's narrow field of view and confusing image orientation will soon cause more frustration than fun.

The obvious alternative is to invest in binoculars, which have the advantages of being relatively inexpensive, highly portable and easy to use. The only disadvantage is that the magnification is generally fixed. Zoom models with variable magnification are also available, but for reasons you will find later in this article it is probably better to accept the fixed- power limitation.
Binoculars magnifying 5 to 8 times are ideal for finding the planets, scanning Milky Way star fields, studying star clusters such as the Pleiades and Hyades, and viewing bright comets. The Moon will also prove a constant source of enjoyment; the mountains, valleys and craters are beautifully brought out, and it takes only a few minutes to learn the most prominent features.

How Binoculars Work

To understand the working principle of binoculars, first you need to know a little about telescopes. In fact, this is exactly what binoculars are, two identical telescopes placed next to each other.
At the front of each telescope is a lens, called the objective. Its role is to gather light from whatever it is you're looking at and bring it to a focus in the eyepiece, where the light is formed into a visible image and magnified to take up a large portion of the retina. The magnification depends on the focal length of the eyepiece, and for binoculars it is usually between 5x and 10x.

The image produced by this telescope will be upside down and backwards, but for astronomical viewing this is not a major inconvenient. In space there is no up and down or left and right. However, for watching birds or following the action at a baseball game a right-side-up picture is essential. This is why binoculars use corrective elements between the objective and the eyepiece, called prisms. Prisms used in binoculars are blocks of glass that function as mirrors, but without a mirror's reflective backing. They come in two models and use different types of glass, and we will discuss about this later in the article. For now let's just mention their role, that is to bring the light beams From the objective closer together by means of internal reflection, and also turn the image right-side up and orient the view properly left to right To better understand the working principles take a look at the image above. It shows the path of the light as it enters the objectives, passes through a set of prisms that turn the image right side up, and finally leaves the eyepieces to enter the observer's eyes. This applies to all binoculars, no matter what model or size.

Magnification

All binoculars are described by using a pair of numbers, such as 7x50 or 8x30. The first number, including the x, represents magnification or 'power'. This tells the degree to which the object observed is enlarged. For example, a 7x binocular makes an object appear seven times closer than when viewed by the naked eye.
There are some models of binoculars that offer variable magnification, usually in the range of 5x to 8x. They are called zoom binoculars, and in most cases are not very suited for astronomical observations because of the inferior optical quality and fragile mechanics. The best thing to do is to avoid them and stick with the usual fixed-power binoculars.
Magnification is not that important, and in most cases comes within 7x to 12x. If the magnification exceeds these figures, most likely you won't be able to hold the binoculars steady enough and the images will be blurry and in constant movement. This is especially frustrating when observing faint objects like galaxies and nebulae. A tripod mount or image-stabilized binoculars will get you rid of this problem, but we will talk about this later in the article.

Aperture

The second number in the two- number code is aperture (from the Latin aperire - 'to open'), the most important specification of binoculars if you plan to use them for astronomical observations. It represents the diameter of each of the objective lenses (the lenses furthest from your eye), given in millimeters. Therefore, 7x50 binoculars have objective lenses 50 mm in diameter.

Aperture is so important because it determines the light gathering ability of your binoculars. Most celestial objects glow very dimly, so a large aperture becomes much more important in low light conditions. For example, 35 mm binoculars will do great when you watch a baseball game on a sunny day, but when used to observe the night sky you will find that they are pretty useless compared to typical 50 mm binoculars. The 15 millimeters difference in lens diameter does not sound that huge, but we need to compare the surface area, not the diameter. The area of a circle is proportional to the length of the diameter times itself, so a small increase in diameter makes a big difference in area. This is why our 7x50 binocular has twice the light gathering ability of the 7x35 binocular. After all these said, it is clear that when it comes to aperture bigger is better. The larger the light gathering area, the brighter the images will appear. Compared to the naked-eye a 50 mm binocular gathers from 50 to 100 times as much light, translating into a difference of five stellar magnitudes. Therefore, if from your observing site you can see stars to magnitude 5.5 with the naked eye, the binocular will show many more stars down to magnitude 9.5 or even dimmer.

Exit Pupil

If you divide the objective lens diameter by the magnification, you will get a number approximately between 4 and 8. This number is called the exit pupil, and represents the diameter of the beam of light that leaves the eyepiece when you hold a binocular with the objective pointed towards a light source. For example, a 7x50 binocular has an exit pupil of 50 divided by 7, just over 7 millimeters in diameter.
Ideally, the exit pupil of your binocular should be equal or slightly smaller than the pupil of your dark-adapted eye. In this way the binocular delivers the maximum amount of light and produces the brightest possible images for its aperture.
Average young adults under dark night conditions have pupils that are open to about 7 millimeters. This means that any instrument with an exit pupil larger than 7 millimeters will only waste light, as only the centre of the light beam could enter the eyes. As we get older our eyes dilate less, so the exit pupil size we need decreases to around 5 millimeters.

Field of View

The field of view is the area of sky or land seen through your binoculars, determined by the design of the instrument's optics. It is expressed in two ways; as the width in feet at 1,000 yards, or in degrees of field. When expressed in feet the field is called linear, and when expressed in degrees it is called angular. Don't let the terms confuse you, the conversion is easy - divide the liner field by 52.35 and you get the angular field.

In most cases the field is indicated on the outside of the binocular, in degrees. Average values are between 5 and 10 degrees, or roughly 260 to 520 feet. To get an actual idea of how wide this field is, think that in five degrees you can fit almost 10 Full Moon diameters! For astronomy, a wide field of view is desirable because if offers a more pleasant viewing experience, and you can see more of the sky at a better edge performance compared to a narrower field. However, when increasing the field beyond a certain point images start to exhibit signs of distortion, especially near the edges of the field. Also remember that field of view is related to magnification; the higher the power of your binoculars, the smaller the field will be.

Eye Relief

Eye relief is the distance behind the eypice lenses at which the image is in focus, and indicates how far the binoculars can be held from your eyes and still allow you to see the entire apparent field of view. In general, the longer the focal length of an eyepiece, the greater is the eye relief. Standard binoculars have eye relief ranging from only a few millimeters to 25 millimeters or more. Long eye relief is especially necessary for eyeglass wearers, because glasses increase the distance between the lens and your eye. In case your eyeglasses correct only for near or farsightedness, you can simply take them off and refocus the binocular to compensate.

Interpupillary Distance

Interpupillary distance is the distance between each of the adult person's eyes. Most binoculars are adjustable to accommodate different interpupillary distances, typically within a range of 60 mm to 72 mm. Many children and some women have interpupillary distances too short for standard binoculars, so the only solution is to choose compact binoculars.

Antireflection coatings

Because when using binoculars to observe the night sky you want the biggest possible amount of light to reach your eyes and not be reflected back into space from the objectives, modern lenses have antireflection optical coatings on at least one of the air-to-glass surfaces. Very good models will have all glass surfaces coated, but tend to be more expensive.
The most used and least expensive coating is a single-layer of magnesium fluoride (MgF), but there are also modern broadband multicoatings. To save money, some optics manufacturers coat only some of the air-to-glass surfaces, and basically we can talk about four levels of coatings used on binoculars.
Coated lenses are the lowest quality, with a single-layer MgF coating on some of the optical surfaces. Fully coated means that all air-to-glass surfaces are coated with a single-layer MgF coating. Multicoated lenses have multi-layer coatings applied on some surfaces, and finally, fully multicoated lenses have multi-layer coatings applied to all of the surfaces.
Recently the market has been invaded with binoculars that have so- called 'ruby' coatings intended to reduce glare in bright light and improve the contrast between brown and green objects. Avoid any binocular that uses these coatings, it will perform poorly for astronomical use.

Prisms

All binoculars are built with prisms that serve as mirrors to reflect the incoming light between the widely spaced objectives and the narrowly spaced eyepieces. They also have the role of inverting the image that the objective lenses project, in a right-side up and not reversed left to right view. Prisms come in two types: roof and porro prisms.

Roof prisms are in line inside the optical tubes, allowing binoculars to be made small and light. This is a great advantage for hikers and birders, but for astronomical use roof-prism binoculars prove to be poor performers. In roof prism binocular design the light beam is split in two parts, then recombined. This process leads to 'phase shifting', meaning that less light is transmitted in the eyepiece and contrast is decreased. Besides, roof-prism binoculars are generally much more expensive than porro-prism binoculars of equal quality. Porro-prism binoculars align the objective lenses and eyepieces in an offset arrangement, with the objective lenses farther apart than the eyepieces. They offer a wide field of view and are very affordable. However, porro prisms have a minor drawback: they are easier to knock out of alignment than roof prisms.

Prisms are made of two types of glass, BK-7 borosilicate flint glass and BaK-4 barium crown glass. For most designs, prisms made of BaK- 4 are preferred over the standard BK-7 because they have a higher refractive index and give brighter and well defined images.
To check for yourself the type of prisms in your binocular, hold it pointed towards a light source and take a look at the exit pupils. If the prisms are made of BaK-4 glass the exit pupils will be round and evenly illuminated. If the prisms are of BK-7 glass you will notice squarish, gray edges in the exit pupils.

Focusing Mechanism

Binoculars come with two types of focusing mechanisms. Most people opt for the center-focus model, which uses a centrally mounted wheel to adjust both eyepieces at once. There is also a separate adjustment for the right eyepiece, which helps to correct for any difference in near or farsightedness between your eyes.
Because focusing requires only one quick adjustment, this model is the most popular, and best suited if you want to use your binoculars for other purposes except astronomy.

The second focusing system uses individually focused eyepieces and has no centrally located focusing mechanism. Even though focusing is slower compared to the previous model, binoculars that use individual focus tend to be more rugged and less prone to moisture infiltrations.
These models are the the best choice if you plan to use your binoculars for night sky observing, because celestial objects don't change their distance and frequent refocusing is not required.

Waterproofing Because cleaning binoculars can be very expensive, you might consider buying waterproof models. They are designed to resist repeated changes in temperature and humidity, making it impossible for condensation to damage internal parts of the instrument. While waterproofing might not seem very useful for astronomy, it is a must if you plan to use your binoculars for boating, hunting, hiking, or other outdoor uses.

Baffling

Baffling within the binocular has the role of shielding against stray light and internal reflections, and can dramatically improve image contrast. To check if your binocular has quality internal baffling, point it at a bright surface and examine the filed of view. It should be surrounded by a black background, without additional light or shiny reflections.

Collimation

Because binoculars are basically two small telescopes mounted side by side, an error in collimation (optical and mechanical alignment) can lead to numerous problems including eyestrain and double-images.
Most cheap binoculars are shipped out from the factory with collimation problems, and even quality models come out of alignment as they age and get bumped around. The only solution to this problem is to buy binoculars with quality mechanics, which will last longer before collimation errors become a nuisance.

Stabilizing the View

For any binoculars to give their best, they need to be held steady. This eliminates the constant jiggling associated with hand-holding, and allows you to see small details and objects fainter than you might think possible.
The traditional and least expensive mount for binoculars is a simple camera tripod. Most binoculars come with a threaded mounting hole and an L-shaped adapter that screws into this hole and onto the tripod. If you don't receive the adapter when you buy your binoculars you can purchase it separately, or even make your own.

Unfortunately, this type of mounting is difficult to use and tiring for the neck, especially when observing objects near the zenith. The solution is to purchase a commercial mount especially designed for astronomical use, or go for image-stabilized binoculars if your budget allows it. Image-stabilized binoculars have an active built in optical system that compensates for the movement associated with hand-holding. All you need to do is press a button on the top of the binocular housing, and you will get tripod-like stability with the convenience of hand-holding. The only draw- backs of these models are their small apertures, and prices several times greater than their non stabilized equivalents.

Choosing Binoculars

When choosing binoculars, it can be very confusing when people start referring to the exit pupil, field of view, objective lens and so on.
The key to choosing binoculars is understanding binocular terms and how each should affect your decision in selecting the binoculars that are right for you.
For most people the price will be the prime consideration when choosing binoculars but first of all, lets explain the numbers and terminology. Whatever your budget, you still need to know what to look for when choosing binoculars, monoculars or spotting scopes. Terms apply to all 3 types.

All binoculars will have two numbers, separated by an ‘x‘, usually termed the designation. For example, 7 x 42 or 10 x 25.
The first number is the magnification. So, a 7 x 42 pair of binoculars has a magnification of 7 – it will make the object you are viewing look to be 7 times larger, or 7 times closer. To put it another way, an object 70 yards away will appear to be only 10 yards away. The second number is the size (diameter) of the objective lens (in millimetres). The objective lens is the front lens of the binoculars.

Why is this important when choosing binoculars? Because it will affect the brightness of the image. The larger the front lens (objective lens) of the binoculars, the more light is gathered, so the image will be brighter and clearer. Remember though, that bigger objective lenses mean bigger and heavier binoculars.

So now we know that a 10 x 50 pair of binoculars will magnify the image 10 times and will have a front lens 50 mm in diameter. Similarly, an 8 x 21 monocular will magnify the image 8 times and will have a front lens 21 mm in diameter.

The Exit Pupil is the size, in millimetres, of the beam of light as it leaves the eyepiece of the binoculars. It is very easy to work out. Just divide the size of the objective lens by the magnification.
So, a 10 x 50 binocular will have an exit pupil of 5mm.

If you are choosing binoculars for general-purpose use, an exit pupil of between 3 mm and 5 mm is good. More than 5 mm would be good if they were going to be used mainly at very low light levels.
What about the Field of View, sometimes referred to as FOV?
This is usually measured as an angle in degrees, but can also be given in feet. It tells you how much of a scene you will be able to see.
If you are choosing binoculars for close-quarter use (such as in heavy woods), or when the subject or the user is moving, a larger FOV is desirable. The Depth of Field can be an important consideration when choosing binoculars. This refers to how much of a scene will be in focus – i.e. the distance from the nearest object in focus to the furthest object in focus.
The smaller the depth of field, the harder it will be to focus. The higher the magnification, the shallower the depth of field will be.
Also, the depth of field gets smaller, as the distance to the subject gets shorter.

Eye Relief can be an important consideration when choosing binoculars, especially for spectacle wearers, or if you want to be able to use them while wearing sunglasses. This can affect the field of view, so you will have to decide which is more important.

Lens coating is very important. There is a lot of confusion about the different colours of lens coatings, but basically, all of them reduce reflection and so increase the amount of light captured by the binoculars. This, in turn, increases the brightness and clarity of the image.

For many people, one of the most important considerations when choosing binoculars is the price. So what is the difference between a pair of binoculars costing, say, £50, and a pair with the same designation costing £100 or even £1000 ?
Generally speaking, you get what you pay for, and the more expensive binoculars will be made from higher quality materials, giving higher quality images – sharper and brighter.

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