Until now, in our comparison blogs we have compared ECHO to a GEN3 and other GEN2s. You can read all about it here

However, we have never compared ECHO vs ECHO to answer a question  “how big is the difference between FOM 1600 and FOM 1900?“. Now, we will try to cover that question with examples.


Specification of Test Units

Let us start by comparing specification sheets of the selected test units. As you can see in table below one unit is with FOM 1661 and another is FOM 1979.

A000743

Spots score*8/10
FOM1661
Resolution67
SN24,79
Gain10022
EBI0,05
Image alignment0,04

A000745

Spots score*7/10
FOM1979
Resolution67
SN29,54
Gain10831
EBI0,07
Image alignment0,31

Let´s talk about FOM

Official definitionFOM or the Figure of Merit, is a number calculated by multiplying the signal-to-noise ratio (SNR) by the resolution of a night vision image intensifier. FOM number allows quickly estimate general performance level of the specific night vision image intensifier, and consequently the night vision device built around it. 

Basically:

 FOM = SNR x Resolution.

How much does FOM actually affect performance?

Since resolution for ECHO tubes is pretty much the same across different devices (usually around 65-68), then the main difference in FOM comes for the Signal‐to‐Noise Ratio. 

What is  Signal‐to‐Noise Ratio (SNR)?

Sigal-to-noise Ratio definition by Phtonis:

LThe Signal‐to‐Noise Ratio (SNR) is the key parameter to predict the performances of an I2 tube in low light conditions. In the lowest light conditions, the image quality is highly dependent upon the light level. When it is very dark (below highly overcast starlight), even with the best tubes there is almost no image and mainly noise scintillation. The SNR is a function of the light level; in order to compare tubes, the world standard is to measure the SNR at a given fixed light level of 108 μlx (in International Units) or 10‐5 foot candle (in American Units).

Hence, the higher is the figure of SNR, the less noise / more signal you get in low light environments.

Let’s put that to test!

Below you can see pictures taken with test-devices in different low-light environments. So you can judge results yourself. 

As you see we must agree that there is slightly more noise on the left pictures where device has lower SNR. 

On this picture above we can already notice more significant difference in image quality in low light conditions. In the picture on the right side you can almost see different objects on the background, but on the left side it’s more blurry. But let’s see one more shot …

What has happened here? Why lower FOM unit has more clear picture? It almost seems that unit with higher FOM is unfocused. The answer is that the unit on the right has also has higher image allignment. The lower the allignment, the more focus you can get from your device. 

A Comparison Video of Test Devices (FOM 1661 vs FOM 1979)

Conclusion

Based on the video and the pictures above, we can all agree that higher FOM unit performs slightly better in low light environments. But, the difference is not big, even when comparing devises with such different FOM scale. To be hones, unless you compare units side-by-side, you would probably not even notice the difference. The difference in image quality is not as dramatic as comparing standard GEN2 to ECHO.

In addition, we have observed so far that the higher FOM ECHO units are usually a bit more spotty or have some other non-desirable attribute higher (EBI, Image Alignment). Why? Because if the unit goes above FOM 1800+, then firstly this unit is considered to be 4G. If during analysis some the attribute does not meet the 4G requirements, then it is classified as ECHO device. 

In our case with the test-device with FOM 1979 – the image alignment is a bit too high for the unit to be considered for 4G. So, it was classified as ECHO instead.

Questions? Something does not make sense? Please feel free to contact us in 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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Categories: Nightvision