Night vision basis
How to buy a nightvision equipment (some recommendations to the customer)
The task of choosing a high quality night vision unit that satisfies your requirements and budget is never easy. Stores and catalogues offer several types of night vision units made by different manufacturers. Simple reading of the registration certificates may not give sufficient understanding of the available selection because sometimes manufactures exaggerate real specifications of the equipment (sometimes in 5–100 times) and aggressively advertise their product without a clear explanation of all technical data to the customer.
Not claming the completeness of this article, we hope that it will help you to choose a nightvision device of the most suitable type.
Every nightvision device (NV) works on the principle of multiplying the brightness of the image in the wavelength of visible and near-visible infrared light. The unit contains an objective lens, light amplification tube with the high voltage power supply, and the eye -glass. Light (starlight for example) reflects from the object to be viewed, passes though the objective lens and creates an image on the photo cathode of the light amplification tube. Inside of the light amplification tube, the light is electronically amplified and projected as a yellowish green glow on a screen that is then seen by the viewer. The two most important characteristics for the NV are the characteristics of the NV tube and optics of the unit. Internationally accepted terminology classifies the NV tubes in I, II, or III generation (with certain intermediate levels of I+ and II+ gen.).
NV Tube Generation I
The Light Amplification tube for Gen. I employs a vacuum tube with the photo cathode sensitivity 120–250 mA/lm. The light amplification is 120–900, resolution in the center is 25–35 lines per millimeter.
The NV products manufactured using these tubes are popular because of their low price (under 0). One of the characteristics of these units is that the image is most clear in the middle of the screen while becoming somewhat distorted and less clear along the edges. Bright light sources (street lights, lit windows etc.) may spoil a view by exposing it to light.
Even within the limits of a single class, Gen. I NV tubes may be quite different in their performance. As a part of quality control during manufacturing the tubes are evaluated by quality and are sold for significantly different prices depending on their amplification and image sharpness. Sometimes this difference in cost is reflected on prices of the NV units from different manufacturers. «Dedal-Handy» and «Dedal- 160», for example, use tubes that are in 1,5–2 times more expensive than those found in competitive products. Tube selection is based on the sensitivity of the photo cathode, light amplification, resolution and the clearness of the field of view. Small black specks on the screen, as a rule, do not distort the viewing in the dark and would not be a reason to reject the NV. However, bright specs (star like) and light spots in the view field detect that unit is invalid.
A unit, which lacks image contrast, is of a poor quality. Buying an NV, the purchaser can check the clarity and resolution, whereas the sensitivity can only be tested by professional using special testing equipment or by comparison of a few units in the dark.
Due to low sensitivity, single-stage Gen. I tubes are very demanding with regard to light transmission of the optics. Only scopes with special light gathering optics (magnification power not more than 1,5) may provide satisfactory results when viewing at a light conditions equal to the light of the moon. When light is lower, an I.R. illuminator is needed. Unlike many other products in this class, all «Dedal» products with Gen. I tubes are equipped with electronic overload flash protection system to prevent damage in case of temporary exposure to bright light.
Multistage NV tubes Generation I
To achieve higher light amplification, some manufacturers connect light amplification units in a tandem arrangement of two or three stages. This allows to increase light amplification to 20 000–50 000 times, but at the cost of increasingly poorer resolution, especially at the edges. Units made with multistage Gen. I tubes have greater weight and size characteristically. Modern units using Gen I+ or II+ technology have similar performance characteristics with few of the drawbacks of the multistage amplifiers and have proved to be more popular.
Generation I+ (Super I+)
This is the next modification of the Gen. I NV tube with a fiber optics plate being installed at the front or at the back of a tube. This modification allows for an improvement of the image resolution and prevents distortion from other light sources in a field of view. Tubes of Gen. I+ have light amplification of approx. 1 000 times, while the photocathode sensitivity increases to min. 280 mA/lm. Resolution in the center is 45–50 lp/mm.
NV units built with the Gen I+ tubes differ from the Gen. I units by increased image quality, lower distortions and better viewing distance in passive or active mode (with an IR illuminator). These scopes work well in an urban environment. In the open space, the units are effective with the ambient light levels down to ¼ of a moon. When light conditions are lower, an IR illuminator is needed. The cost of the Gen. I+ tubes is in 4–9 times higher than the cost of the regular Gen. I. Our company manufactures units «Dedal- 220» and sight scope «Dedal- 210» using the Gen. I+ tubes.
Note. The Buyer should take into consideration that some producers use the designation Gen. I+ for their devices Gen I. It often misleads buyers because in fact tubes Gen. I have a resolution not higher than 38–40 pl/mm. We use the designation Super Gen I+ in our English advertising to make clear for the buyers peculiarities of the tubes Gen I+ with high resolution.
The construction of the Gen. II tubes differs from the Gen. I+ by using a special electron amplifier — the micro channel plate (MCP).
The performance characteristics of such NV tubes are: a higher photo cathode sensitivity of 240 mA/lm, with the resolution of 32–38 lp/mm. The lifetime of these tubes is designed to be not less than 1 000–3 000 hours of operation. Two types of NV tubes with micro channel plates (MCP) are currently available: the 25mm and 18mm ones. From the users prospective the larger diameter is more efficient but also requires larger NV unit. In this category, «Dedal» manufactures a multifunctional NV Scope — the «Dedal- 45» and series of rifle scopes «D- 300». All of the above mentioned units are equipped with an additional manual brightness control that allows the viewer to adjust the scope for the optimal gain/noise ratio.
With a light gain of 25 000–35 000 the Gen II+ tubes have photo cathode sensitivity of 240–600 mA/lm and are sensitive to light in the infrared spectrum. Resolution is 39–45 lp/mm. Work capasity of the tube is estimated to be 1 000–3 000 hours. Since the Gen. II+ tubes are smaller (they do not have a speed up chamber) they have a slightly lower light amplification than the Gen. II. Yet due to the especially high sensitivity in the IR spectrum the Gen. II+ units have better viewing distance in the open space. If the main task for the unit is photo or video recording, the Gen II NV unit should be chosen due to its higher light gain. NV units with the Gen. II+ tubes are equipped with automatic gain control, flash protection, protection from the side light distortions and feature a high quality image throughout the entire field of view. Units equipped with Gen. II+ tubes are used by the armed forces of many countries and considered to be of professional quality by the experts.
Gen. III differs from the Gen. II+ in use of a photo cathode based on Gallium Arsenate with even deeper sensitivity in the IR spectrum. The photo cathode sensitivity is 900–1600 mA/lm, resolution is 32–64 lp/mm and capacity for work is 10,000 hours. This is 3 times longer than with the Gen. II tube. The NV units using the Gen. III tubes work very well in the low light environment. The image comes out clean and with excellent contrast. The only drawback of Gen. III units is their vulnerability to the side light distortions due to lack of the fiber optical plate on the front of the tube. Until recently Gen. II+ and III units where used mostly by the military, these units became more widely available not long ago.
Optical part of a night vision device consists of an objective and an ocular. The main demand on the objective is a high light transmittance in visible and near IR diapason. It is expressed with geometric light power from the line (F-number) 1; 1,4; 2,0; 2,8; 4,0 and so on. The objective with increased number (in a step) makes light transmission twice less. High light power is very important for night vision devices especially for devices Gen. I, I+. By light power degradation till 2,4–2,8 a naked eye can see better than with night vision devices Gen I in passive regime. Elaboration and producing of high light power optic with focal ratio less than 1,5 is a difficult task. Not every manufacturer is able to produce it. Of course cost of light power optic of high quality has an influence on the final cost of night vision devices. A lot of manufacturers equip their night vision devices with objectives of larger focus distance and low light power to get a bigger magnification (3,5–5,0). If you prefer to choose between two devices (of the same Generation and dimension but with greater magnification) the distance and the image you get is going to be worser than in items with lesser magnification and greater light power. It is very important especially for night vision riflescopes for hunting.
All «Dedal» devices series are completed with objectives of a lightweight design and good light power F/1.2 or F/1.5. Sometimes night vision devices are completed with mirror lens objectives. They guarantee smaller overall pivotal dimension but have a decamouflage effect and have by the same conditions the worse light power. That is why they refuse to produce night vision devices with mirror lens objectives in Russia and abroad.
Infra Red Illuminator
For civil NV systems an Infra Red (IR) illuminator can be a welcome enhancement that provides additional light on the object to be observed. This is particularly valuable when the ambient illumination is not bright enough for passive (starlight) mode or the object is in shadows. IR illuminators are designed with IR lasers or IR diodes and special light bulbs. One should keep in mind that laser illuminators might be harmful to eyes. Because of this, they are outlawed for civil usage in many technologically developed countries.
Illuminators with IR diodes are harmless with the added benefit that they provide more even illumination than their laser counterparts. Presently most of the manufacturers are producing the IR illuminators with diode IR components that deliver an output of 3–12 mW. Our company equips all it’s NV units with the most powerful IR source in Russia — 75 mW. It increases the viewing distance in 1,5–2 times.
A built-in IR illuminator it can be purchased separately as an accessory.
The physical characteristics of NV units are usually designed with three factors in mind, compactness, weight and observation distance. Unfortunately sometimes these characteristics are in conflict with one another. For example, greater observation distances can be provided by a unit with a larger lens with the inherent disadvantage that the device will be bigger in size and weight. The customer has to decide which characteristic is a priority and buy accordingly.
One factor, which is especially important, is the construction of NV rifle scope — it must be extremely rugged to withstand the shock of up to 500 Gs along the optical axis due to recoil. In this harsh environment the cross hairs, sighting posts and targeting adjustments must remain unmoved and be visible in all types of viewing conditions. Many NV rifles copes currently offered on the market are not designed to compensate for these stresses (cal. .365H&H, .416Rirby) and may have weak mounting and zeroing-in systems. This will require repeated readjustments to maintain accuracy. This is a complicated subject that cannot be discussed properly in a short article. We encourage you to contact our company for a special consultation on the subject.
Distance of observation and identification
The buyer should take into account that the distance of observation and identification, which night vision devices provide, depends on the level of night illumination, clarity of atmosphere and contrast between an object of observation and background. When illumination increases the distance of identification becomes higher. When illumination reduces it becomes lower.
The table of average distance of night observation and identification of an object is given below (based on the domestic and foreign data).
|Full moon 0,1 lux (meters)||Half moon 0,05 lux (meters)||Quater moon 0,1 lux (meters)||Starlight 0,001 lux (meters)||Overcast moon 0,1 lux (meters)|
|Without nightvision device||230||130||45||_||_|
To summarize, we suggest you use the following simple guidelines when selecting an NV unit: If image quality and distance to target are not vital for your applications we recommend you to choose your NV by price and its size/comfort.
- When your image requirements are higher — select the NV (particularly Gen I.) unit with the optical system that has a light transmission co-efficient not less than 1,5
- When selecting your NV choose a manufacturer that has been established in the NV business and has a proven track record. Make sure you are purchasing from a company that sold on the competitive international markets and has developed the reliable customer service/product support system. Do not buy without warranty — select a manufacturer who has an established record for honoring their warranty.
- Try to compare several units, as the technical specs in advertisements and manuals of many companies may be inaccurate and inflated.
- Make sure that the unit works and everything is functioning properly.
Congratulations on your successful purchase!