Night Vision:
Evaluating the Performance

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 or recognising an individual, especially at fairly long range, then you need to ensure that the system you are considering is capable of this.

Most Generation 1 night vision equipment available today will provide an adequate image for most non-professional requirements under higher night light conditions, such as full moon. 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 Gen 1 + or Gen 2/SuperGen systems.

It is difficult to choose a night vision system by simply reading technical specifications. Not only are the performance specifications 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.

Factors to consider include:

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 systems 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 conditions. 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 (up to 3x). Higher levels of ambient light dramatically increase any night vision 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.

Night Vision 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 image 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. Generation 1 Night Vision 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.

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 or 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 weapon sight applications, the amount of magnification needed will vary; take into account that, as the magnification increases so does the lens f-number while the FOV decreases - this reduces the amount of light captured.

How versatile is the night vision device you are considering? Do you need, and does it offer, interchangeable lenses? How easily and quickly can the magnification be changed?

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:

Generation 0, Generation 1, and 25-mm Generation 2 electrostatically inverted image tubes produce a certain amount of geometric distortion in the image. In Generation 2 and Generation 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 weapon sights, 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 will adjust to the scene through the night scope and the other to the ambient lighting. This could lead to eyestrain.

If you intend to 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 should be considered. 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 (that is, a device that can be fully submersed), choose a system built to military specifications.

Practicality:

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

Reliability:

With proper care, modern Night Vision designs are relatively trouble-free. However, you should be aware that intensifier tubes have a finite duty cycle - they will eventually wear out.

Does the device incorporate protection circuitry to turn the intensifier down or off when exposed to bright light?

Second-hand 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 do 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:

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

For further information about Night Vision visit the web site of Thomas Jacks Ltd, one of the country's leading Night Vision suppliers.