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Night Vision Performance Evaluation

Night Vision Performance Evaluation

The very need for a night vision capability necessarily focuses on performance as the most important factor.

What subjects do you need to see?

At what distance?

And in what lighting conditions?

The darker the conditions and the smaller or more distant the subject, the tougher the job gets for a night vision system.

If you need to see fine details, such as car number plates/recognising an individual, especially at fairly long range, then you need to ensure that the system you are considering is capable of this.

Most Gen 1 night vision equipment available today will provide an adequate image for most non-professional requirements under higher night light conditions, such as moonlight.

But, if you need to see under truly dark conditions, such as starlight, need longer range or need to see finer details then you will have to consider Super Gen 1+ or Gen 2+/SuperGen/HyperGen systems.

It is difficult to choose a night vision system by simply reading technical specifications.

Not only are the performance specifica¬tions tough for most people to understand, but they are even tougher to relate to real-world use.

Comparison of specifications is also only valid if test instrumentation and methodology are consistent and reliable - unfortunately, this is unlikely.

The best way to evaluate a night scope is to test it in real-world conditions.

Ideally, you should conduct your evaluation under the same conditions in which you intend to use the scope.

Night Vision Performance Factors to Consider

Photosensitivity

The ability of a night vision system to detect light energy and convert it to an electron image is reflected in the image intensifier’s photosensitivity.

Usually, the higher the value, the better the ability to ‘see’ under darker and darker conditions. However, be aware that at night there is more light energy available in the near-infrared region than in the visible region.

Therefore, if a device claims a high photosensitivity, make sure to find out where in the spectrum this is measured.

A high photosensitivity in the blue, or visible, region may not perform as well as another system with a lower overall photosensitivity, but a higher value in the near-infrared region.

Gain

This tends to be a confusing parameter when evaluating night vision devices.

The darker the conditions, the harder it is for the system to render a clear image with reasonable contrast.

Additional gain is required as conditions grow darker and for longer range.

The most important gain measurement is the system gain.

Very high gain values for an image tube are not especially significant - military devices can have tube gains ranging anywhere from 20,000 to 80,000.

Look for the system gain.

Military sys¬tems operate at gains in the region of 2,000 to 3,000.

The higher the value the better the ability of the device to amplify the light it detects.

However, if a system does not possess a good photosensitivity and SNR, a very high gain value simply means that you will make a poor image brighter, not better.

Populated areas always have an atmospheric glow from artificial lighting. A high-gain device might be required in remote areas on overcast nights, but it would not be necessary for use in urban or suburban condi¬tions.

Even in isolated locations without man-made light, a high-quality, affordable Gen 1 monocular/binocular will provide good imaging with a half-moon and clear skies - this can be further enhanced with the use of an IR illuminator.

The very best test is field evaluation in real-time conditions.

Range

Can you see your subject 75m away?

Effective range is a balanced function of the system’s gain, resolution, image magnification and the amount of ambient light available.

While a powerful lens will provide more image magnification, it will also reduce the amount of available light captured.

The best effective range with most intensifiers is achieved with a high-speed lens that has minimal magnification (<3x).

Higher levels of ambient light dramatically increase any device’s range capability. Just as more gain is required for longer range in darker conditions, less gain is required as conditions grow brighter.

Most applications are satisfied with systems offering image magnification of 1x to 3x. Goggles, for instance, are specifically designed for near-field viewing and use 1x image magnification to keep the wearer from becoming disoriented.

For marine use, a good quality 1x magnification monocular is ideal because it gives a wide field of view - if you also need higher magnification, buy a model with interchangeable lenses.

High image magnifications mean a narrow field of view and the possibility of missing a subject you are searching for.

Image Quality

High resolution, high contrast and a lack of distortion and noise contribute to a premium Night Vision image.

With higher resolution you might identify someone you know at 60m as opposed to simply recognizing a human figure.

Superior contrast allows you to see dark subjects against darker backgrounds.

Lower distortion renders a flatter, less rounded im¬age with crisper details.

Always try and choose a model that offers glass optics - these will give you the sharpest, clearest optical quality.

Units that use plastic optics tend to offer lower optical quality and higher distortion.

Resolution

Usually this is measured as tube resolution (lp/mm) or system resolution (cy/mr).

The higher the value, the better the ability to present a sharp picture. Gen 1 devices may produce a reasonably clear image in the centre of the viewing area, but sharpness drops off noticeably toward the periphery.

Target Characteristics

System performance is not only limited by intensifier/system parameters and light level, but also by the target characteristics and atmospheric conditions.

To the right is a graph showing the spectral reflectance of various elements - a man in drab clothing, for instance, would stand out dark against green foliage, which is more reflective, especially in the near-infrared spectrum.

The degree of contrast between the various elements visible has a lot of influence on the detectability of a subject.

Magnification and Field Of View (FOV)

Consider the size of your target, the distance you wish to see over and the overall area you are observing or searching.

For most surveillance or search applications, the higher the magnification, the narrower the FOV & the greater the number of times you need to scan an area to avoid missing important objects or events.

Usually a 1:1 lens with a wide FOV provides optimal performance.

For longer range observation or weaponsight applications, the amount of magnification needed will vary; take into account that, as the magnification increases so does the lens ƒ-number while the FOV decreases - this reduces the amount of light captured.

How versatile is the device you are considering?

Do you need, and does it offer, interchangeable lenses?

Ergonomics

Size, weight and ease of use are important considerations.

If you plan to carry your scope in your pocket or briefcase, then smaller systems will suit you better.

Lightweight devices are more comfortable during extended viewing.

Since you will be using the device in the dark, the switches and controls should be positioned logically and be easy to use.

Distortion

Gen O, Gen 1, and 25-mm Gen 2 electrostatically inverted image tubes produce a certain amount of geo¬metric distortion in the image.

In Gen 2 and Gen 3 systems, geometric distortion is normally eliminated through the use of an MCP, although it is possible to encounter some perceptible S and shear distortion.

Especially when the application involves photography, video work, or weaponsights, the distortion and peripheral resolution can be critical.

Extended Viewing

If you intend to look through a device for extended periods of time (20+ minutes) you may find that a binocular or biocular better suits your requirements.

Using a monocular continuously for this period of time will mean that your eyes will adjust individually to their lighting conditions - one to the scene through the night scope and the other to the ambient lighting. This could lead to eye strain.

If you use the device for extended periods, a tripod socket may be a useful feature.

Weather Resistance

The ability of a night vision system to operate under adverse environmental conditions.

Most devices will withstand a light shower, but if you expect to regularly use the device in inclement weather or a marine environment, choose a model that has protection against such conditions.

If you want full waterproofing, ie can be fully submersed, choose a system built to military specs or that is guaranteed as such.

Practicality

Is the battery a size and model that is commonly available? This is important, especially if you use your system in the field.

All Cobra Optics/Yukon scopes use batteries that are readily available from photographic/electrical/high street retailers.

Reliability

With proper care, modern Night Vision designs are relatively trouble-free.

However, please be aware that intensifier tubes have a finite duty cycle (they will eventually wear out).

Thomas Jacks test every single device that it sells. Every unit has to pass stringent quality checks before it is boxed for resale.

Thomas Jacks Ltd sells only new Night Vision systems that incorporate protection circuitry which turns the intensifier down or off when exposed to bright light. Their intensifier tubes are conservatively rated at 1,000 hours of use under normal conditions.

It should be noted, however, that any tube will degrade and eventually fail under repeated, prolonged or excessive exposure to bright light.

We recommend you always cover the objective lens and store the scope properly when not using it.

The image intensifier is a vacuum sealed glass tube - it can be easily damaged if the device is knocked or dropped.

Ensure that you protect your scope well from such possibilities. The intensifier tube is by far the most expensive component to replace in a night scope, on average accounting for some 60-70% of the total cost.

Secondhand or Reconditioned Devices

As stated above, intensifier tubes have a finite duty cycle (they will eventually wear out). Knocks, abuse or exposure to bright light will further compromise a tube’s performance and life.

We would not recommend the purchase of such devices unless you can be absolutely certain of their usage/performance. While the price may be attractive, be aware that the intensifier tube cannot be restored to ‘new’ condition.

‘Reconditioned’ usually means the system has a new or repaired power supply, but the tube photosensitivity, SNR and the remaining life cannot be improved and will be noticeably lower than from an equivalent new device.

Some second hand or reconditioned units may be operating at below acceptable minimums and few companies possess the necessary test equipment to evaluate the tube’s level of performance.

Service & Warranty

Is the product supplied by a reputable, branded supplier, protected by a warranty that will be honoured and is technical service available?