Night Vision Guide - Let's talk IIT

What are the parameters to look for when comparing IITs?

  • Figure of Merit (FOM) – Image intensifier tube (IIT) specification designation, calculated on line pair per mm x signal to noise. (FOM=lp/mm * SNR).
  • Gain – IIT work by collecting low levels of light, amplifying them to levels that can be easily observed. The tube gain (which is also referred to as brightness gain or luminance gain) is the magnitude, or number of times the image intensifier amplifies the light input.
  • Resolution -The ability of an image intensifier or night vision system to distinguish between objects close together. Image intensifier resolution is measured in line pairs per millimeter (lp/mm). Typically, the higher the line pair, the better the image resolution.
  • Black Spots – These are cosmetic blemishes in the image intensifier or can be dirt or debris between the lenses. Black spots that are in the image intensifier do not affect the performance or reliability of a night vision device and are inherent in the manufacturing processes.
  • EBI (Equivalent Background Illumination) is the background noise of the Image Intensifier and mainly comes from the thermal emission of the photocathode. The EBI adds noise speckles to the image. The EBI increases rapidly with temperature and can become a significant source of noise when the IIT is at elevated temperature. Tube manufacturers normally specify EBI at temperature of 21C.
  • Photocathode sensitivity is a measure of how well the image intensifier tube converts light into an electronic signal so it can be amplified. The measuring units of photocathode sensitivity are micro- amps/lumen (?A/lm) or microamperes per lumen. Even though it is a very important attribute, Photonis unfortunately does not display the value on its specs. 
  • Signal-to-Noise Ratio (SNR) – A measure of the light signal reaching the eye divided by the perceived noise as seen by the eye. A tube’s SNR determines the low light resolution of the image tube; therefore, the higher the SNR, the better the ability of the tube to resolve objects with good contrast under low-light conditions. Because SNR is directly related to the photocathode’s sensitivity and also accounts for phosphor efficiency and MCP operating voltage, it is the best single indicator of an image intensifier’s performance.
  • Autogated  – Autogating offers the tube better protection and preventing resolution loss in high light conditions.

In most cases you can find this information in generic IIT specification (except black spots). However, what you need to keep in mind, is that every IIT has min and someties max range set for every attribute. Therefore, you actually will never know what IIT you will be getting unless you are looking at the specs of that exact tube.

Since the difference between units can be pretty big (both FOM and the existence of black spots), we have decided to make public units specification and picture of the tube (to see black spots). Price depends on the quality of the tube. Please be aware that not all spots are visible on those pictures and in real life the spots appear to be bigger than on a picture. Spots score is our own metric to compare visibility of spots between tubes.

About those generations

When people discuss NVD generations, then we mean the generation of IITs. We learned that you could not compare tubes anymore based on their generation. We need to keep in mind that Gen3 for created in the late 1980s, and the technology has advanced a lot since then – that goes both for the units that use multi-alkaline and GaAs photocathode. Therefore, we remain from assigning generation based on chemical complaints to IITs that have greatly improved the technology (like Photonis ECHO and 4G).

Gen 1+

Gen1 has small light intensifying capability and is bulky. It needs a high-powered IR illuminator to see something at all. By flashing an IR light, you will be giving away your position to everyone else on a field who has some sort of NVD capability.


Truth to be told, there are no cheap options for NVDs. If you have money only for Gen1, we rather recommend investing this money into a good weapon light.

Standard Gen 2+

Everyone knows that Gen2+ IIT outperforms Gen1 without breaking a sweat. This is due to the fact that in devices starting from Gen2+ you have MCP (Micro-channel plate) which amplifies electrons, a lot, in addition to having better optics than Gen1.

By Standard Gen2+ we consider regular Gen2+ tubes. There are a lot of vendors who provide devices with standard tubes like Flir, ATN, etc.

Usually, you will get a decent device with resolution 45-55 lp/mm, sensitivity around 450-550 ua/lm, SN ratio between 20-22, and EBI around 0.15.

By the way – those are the 4 main parameters to look for. Even though when buying Gen2+, you usually never know those parameters. In most cases you will not get specs of the concrete tube you are purchasing before the deal.

Standard Gen2+ devices are not Autogated.

IIT specifications for Standard Gen2+

Luminance sensitivity 2850 K.350500/
Resolution center3250/
Signal to noise ratio1220/
Equiv. Background illumin./0.15/

Main cons of Standard Gen2+ devices

The main problem with most Gen2+ devices is that you almost never know what you will get. The minimum specs are very low (even with minimum specs it still beats the crap out of Gen1+). 


Due to high variety of Gen2+ devices it’s hard to pin-point best field of use. If the specs are there and decent, go for it. It’s a decent entry-level NVD. It will be best preforming in an environments with some light – light noise from the city, full moon etc. 

If you are looking for a device that is more reliable, would suggest to look into Commercial or ECHO as it introduces autogating. 

Some examples of different tubes (and 1x 4G):

Standard Gen2+ tube GP

Photonis Commercial GP

Photonis Commercial WP

Photonis 4G

Photonis Commercial

Photonis Commercial, we mean XX1441 and XW1441 tubes. 

Even though we have never seen a Commercial IIT that has FOM below 1100, due to low minimal requirements (min fom 380), you can never be sure. If certainty is important, then you should consider ECHO. If budget is limited, going for Photonis Commercial you can always be sure that this devices is enough for Airsoft and is better than Standard Gen2+. 

Usually the resolution is round 55-65 lp/mm, sensitivity around 500-650 ua/lm, SN ratio between 20-22 and EBI around 0.15. 

Currently we dont have Photonis Commercial units in our shop, but that might change at some point. 

Photonis Commercial devices are not Autogated. 

IIT specifications for Photonis Commercial:

Luminance sensitivity 2850 K.270570/
Resolution center3260/
Signal to noise ratio1221/
Equiv. Background illumin./0.15/


Photonis Commercial is an entry-level NVD if you want decent performance, but ECHO is out of reach. It main cons are its very low min requiremnets and absent autogating. However, we have never seen Photonis Commercial IIT with FOM below 1100. This devis will be enough for any airsoft or other mil-sim game player in the field. However, in CQB in may require extra IR light to see clearly enough. Just be careful with exposing it to the light – remember, no autogating. 

Photonis 4G vs Photonis Commercial
Photonis 4G WP = ~6500€
Photonis Commercial WP = 2 550€

Photonis ECHO and ECHO+

 Photonis ECHO

With Photonis ECHO tubes, you can already be sure about their high quality. Minimum FOM is 1600.

Photonis ECHO devices come from the 4G production line. The main difference between ECHO and 4G is that ECHO did not meet the high-quality standards that 4G has. Please read more about it here:

We currently provide Photonis ECHO in PVS-14, THE-14, DTNVS housings, the last two are new lightweight housing produced by ACT. ACT is the leading Night Vision monocular and binocular producer in Europe. 

Usually, the resolution is around 65-70 lp/mm, SN ratio between 24-28, and EBI around 0.12.

Photonis ECHO devices are Autogated.

    IIT: Photonis ECHO

    Luminance gain (at 20 μlx)8000/12000
    Resolution center576574
    Signal to noise ratio2325/
    Equiv. Background illumin./0.120.40


    Photonis ECHO has became one of the most sold IIT’s within monoculars and it’s cleary understandable why – it has good (predictible) specs, autogating and some black-spots that don’t really bother the user. 

    ECHO products available

      ECHO WP (FOM 1780) vs Harder Digital Gen3 WP (FOM 2100+)

       Photonis ECHO+

      Compared to ECHO IIT’s ECHO+ as the name says is one step up. Their minimum FOM starts with 2000. 

      Similarily to Photonis ECHO devices, ECHO+  come from 4G production line. . 

      We currently provide Photonis ECHO+ in THE-14 and DTNVS housings, last two are new lightweight housing produced by ACT. ACT is leading Night Vision monocular and binocular producer in Europe. 

      Usually the resolution is round 65-70 lp/mm, SN ratio between 27-31 and EBI around 0.12.

      Photonis ECHO+ devices are Autogated.

        IIT: Photonis ECHO+

        Luminance gain (at 20 μlx)8000/12000
        Resolution center57//
        Signal to noise ratio25//
        Equiv. Background illumin./0.120.40


        If you are looking for more high-end NVD IIT with higher FOM and dont need a mil-spec IIT – ECHO+ is the one to go. They see what others do not see. Echo+ tubes allow for more spots, but at a great value and performance.

         Photonis 4G IIT’s FOM starts from 1800 and is 2000 on average. Compared to ECHO series the 4G has higher requirements almost for every attribute which are required for a tube to be considered mil-spec. In addition to attributes that are visible on a specification, there are some very important attributes that are not displayed on a data sheet (like sensitivity, bandwidth, halo).

        4G has one of the best Detection/ Recognition/ Identification (DRI) range at various light levels. The further distance the user can see, the better he can react as he will be the first to see.

        4G Night Vision standards is defined by:

        • An Extended Bandwidth of photon collection to deliver a consistently high image quality in all environments (from below 400nm to above 1.000nm)
        • A FOM above 1 800
        • A Resolution always superior to 57 lp/mm even in the most polluted light environments such as urban areas or when entering a building where the light is suddenly switched on
        • A typical 0.75mm halo around the brightest objects seen in the image to provide the highest details around the light sources

        We provide Photonis 4G in THE-14 & DTNVS housings produced by ACT. ACT is leading Night Vision monocular and binocular producer in Europe. 

        Usually the resolution is round 55-70 lp/mm, SN ratio between 28-30 and EBI around 0.15.

        Photonis 4G devices are Autogated.

          IIT: Photonis 4G

          Luminance gain (at 20 μlx)7 000/14 000
          FOM18002 000/
          Resolution center64//
          Signal to noise ratio2730/
          Equiv. Background illumin./0.150.25


          4G takes us to the mil spec. Wide spectrum image intensifier. Exceptionally high SNR and resolution. A much wider range of the light can be used with these tubes enabling them to beat Generation 3 in most environments.

          4G Products available

          GEN 3


          When looking into Gen3 devices, there are significant differences between EU and USA-produced tubes. Within the EU leading producer of Gen 3 IITs is Harder Digital, while in USA main producers include L3Harris and Elbit. While the primary technology is similar,  the result is quite different. 

          The main difference between IITs based on Gen2 technology vs. Gen3 is the material used for the photocathodes (multi alkali vs. GaAs/GaAsP coating). Gen3 coating is deemed more sensitive. However, to protect from positive ion poisoning, producers may use thin film. Hence, Gen3 devices can also be categorized into different groups: filmed,  thin-filmed or filmless. 

          In real life difference between Harder Gen3 and high-end multi-alkali IIT like 4G can be seen with the naked eye, mainly in a very dark environment with low light levels. For example, CQB. In reality, when using NVD outdoors, you will always have some additional light, whether it’s light from the moon and stars, close-by city lights, etc. 

          We are providing 1800+ and 2000+ FOM tubes.

          Harder 1800+ Alpha

          Luminance Sensitivity18002000/
          Luminance gain (at 20 μlx)10 000/20 000
          Resolution center64//
          Signal to noise ratio25//
          Equiv. Background illumin.//0.25

          Harder 1800+ Alpha products available

          Harder 2100+

          Parameter Min Average Max
          Luminance Sensitivity 1800 2100 /
          Luminance gain (at 20 μlx) 10 000 / 20 000
          FOM 2100 / /
          Resolution center 70 / /
          Signal to noise ratio 30 / /
          Equiv. Background illumin. / / 0.25

          Harder 2100+ products available

          Currently not available

          In Conclusion

          Although IIT is only one component of the NVD, it’s undoubtedly the main one. When looking at IIT’s from different generations try to keep in mind the end-use of the product. 

          When you want the most bang for the buck, go for Photonis ECHO in either PVS-14 (make sure the optics is high quality),  THE-14 or DTNVS housing. After ECHO, the cost goes up a lot, but you dont get as much value per euro, as with ECHO. For airsoft, hunting, hiking, tactical training etc, the ECHO has plenty of performance.

          When ECHO seems too expensive, then go for Photonis Commercial (XX1441 or XW1441 tube) in PVS-14 housing. Again, make sure that housing optics produced in the US. If Photonis Commercial is also too expensive, then better buy a flashlight or save your money. The performance below Photonis Commercial is not worth your time nor money. 

          Let´s say your are in the mood and have enough cash on hand to get a premium product. Go for ECHO+, 4G or Harder Gen3 FOM 1800/2100+

          Questions? Something does not make sense?

          Please feel free to contact via Facebook Chat below. If some of your questions remain answered, please ask. Based on your questions we will update the guide and help everyone to get better understanding about Night Vision.
          Krister Lukka
          Nightvision fan, just like You

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