Tube Selection and Specs - A Beginner's Guide

As part of our Hand-Select process, we typically go through a consultative process to understand each end user's experience level, requirements, and application. That said, we ultimately provide a refined selection of intensifier specifications along with each intensifier's through tube photo, based on our understanding of each user's requirements. 

This short article is to assist users in understanding the visual impact of each performance specification to aid in making an informed decision. 

For convenience, we've highlighted the takeaways in each section in bold text for those of you who prefer to gloss over the details.

FOM (Figure of Merit)

Figure of Merit, most often referred to as FOM, is the result of multiplying an intensifier's rated centre resolution (RES) and signal-to-noise ratio (SNR). For example, if a Photonis Echo intensifier has a Res of 67 lp/mm (line pairs per millimetre - we'll get to this in the Resolution section below) and SNR of 25, then the FOM would be 1675. As Photonis Echos are between 1600-2000 FOM, and FOM can be a product of Res and SNR, sometimes you may have two intensifiers with the same FOM, but different resolutions. Example: two intensifiers both 1800 FOM, one is 66lp/mm and 27.27 SNR and another that is 73 lp/mm and 24.65 SNR. The intensifier with the higher resolution will visually "look" more clear but will have a noisier image than the intensifier with the lower resolution. FOM is simply a way to quickly determine a good intensifier from a great intensifier - example: Photonis Echo+ at 2000+ FOM will outperform Echos at 1600-2000 FOM. You will get either a higher Resolution or a higher SNR, and those are both good things. But FOM alone is not important when other specifications are present.

RES (Center Resolution)

Resolution is an intensifier's ability to show contrast and generally equates to what users experience as "clarity" or "sharpness". Generally speaking, the higher the resolution is, the more clear it will appear to you. Some may refer to this as "crispy".

Resolution is measured in Line Pairs per Millimetre. A Line Pair is a black line followed by a white line 

A typical resolution chart will have increasing line pairs on a scale and this chart is observed through the device. The measurement is taken where the line pairs effective blur together and become no longer distinguishable.

Each intensifier will have a limiting resolution and most modern night vision will measure between 64lp/mm to 81lp/mm with Photonis Echos coming in around 66-73 lp/mm. Generally speaking most users will not be able to determine the difference between a 1-2 lp/mm difference but those with good eye sight should be able to discern a small difference between the extremes of 66-73lp/mm. One should also consider their own personal eyesight - often users prioritize resolution but their own eyesight is not 20/20 which essentially "wastes" the resolution boost.

It should also be noted that most night vision intensifiers' system resolution will drop as light levels increase (and is especially true for Gen3 systems; with Gen2+ systems like Photonis being much less affected due to superior autogating) and there is a "sweet spot" of lighting conditions where the resolution will appear best. 

SNR (Signal to Noise Ratio)

Signal to Noise Ratio is often misunderstood as the holy grail of intensifier performance. Understanding the signal-to-noise ratio is easier when we better understand what signal and noise are separately. Signal is the true light being picked up by the photocathode within the tube; this is the image you see when the tube intensifies light. Noise is the "noise" or "static" in the image that you sometimes see when light levels are extremely low. This is analogous to images captured at high ISO levels on professional cameras, or static in a broadcast TV image when there is some interference. Noise can be considered a false positive. The higher the SNR, the less false positives or noise there will be in an image at low light levels.

Note that differences in SNR can generally be detectable in extremely low light levels (eg: dark room with a faint light source). Comparing two intensifiers in a room that is lit by a computer monitor, as an example, should not be a valid test of performance as the light levels in the realm of image intensification is quite high. This brings us to why SNR alone should not be as heavily weighed as it typically is. In a moderate lighting situation, SNR is not as much of a factor as there would be sufficient amount of light to generate a useable signal. Therefore, users should think about ambient lighting conditions in their primary operating environment. As an example - SNR is less important in moderately-lit urban environments. On the flip side, SNR is very important in deep heavily-forested areas.

Generally speaking, with all else being equal, most users typically cannot discern a 2 point difference in SNR in a practical environment. The difference becomes more apparent when the difference is 3 or more, and even then it would only be visible in an very low light environment.

EBI (Equivalent Background Illumination)

EBI (Equivalent Background Illumination) is essentially the amount of light through a device when there is no light on the photocathode. The higher the EBI, the more likely the device will put out its own light and provide you a false positive (the "background illumination"). In aggregate, this has the effect of giving you a "muddy" image in VERY low light conditions (near total darkness). EBI level determines the lowest light level at which an image can be detected, below that threshold, objects will be masked by EBI. EBI is also affected by temperature, the warmer it is, the higher the background illumination and vice versa. That's why your tube will likely appear more "clear" when it's cold. 

A lower EBI is generally considered better.

That being said, in most head mounted, ground-based systems, the number of instances where there is absolutely zero light reaching the photocathode is extremely low and therefore EBI should not be weighted heavily in an intensifier selection decision. EBI is more of a factor for applications such as astrophotography where false positives are very undesirable as high EBI will appear as stars through the intensifier.

For an example of an EBI comparison, click on this link HERE.

In the image shown below, even with a "great" EBI figure of 0.2, because the ambient lighting conditions are so low, the image is quite grainy and supplementary illumination would typically be used, thereby negating the EBI advantage.

Halo

Halo is the ring of light that forms around concentrated light sources when using night vision. Halo is prevalent with streetlights, headlights, or other concentrated light sources. The lower the number, the less prevalent this effect will be, so for this specification, lower is ideal. High halo values may sometimes obstruct the target you are trying to see as it become a photonic barrier.

High Halo value on the left; low Halo value on the right

Photonis Echo spec sheets do not come with Halo values. Retailers who offer halo values for Echos are generally guessing.

Gain

Gain is how much the intensifier is able to amplify a light source. The higher the Gain number the more contrast your image will have and the more you will be able to visually render an image. Some would perceive this as brightness but high Gain with a low Max Output Brightness will still resolve a darker-looking intensifier. Also, a high gain but low SNR intensifier will yield a bright but noisy image. Because human eyes have a natural iris and dark-adapting ability that machines do not have, variances of 1000-1500 cd/m^2/lx should not be used as a determining factor in an intensifier selection. 

Gain is typically measured in cd/m^2/lx on Photonis spec sheets and average between 9500 and 10500 on Photonis Echo white phosphor intensifiers and between 12000 to 16000 on green phosphor intensifiers. 

Max Output Brightness

Max Output Brightness (MOB) is how bright the brightest signal will appear in the intensifier. This is equivalent to increasing the brightness on your phone, as an example, but not the same as increasing contrast (gain). Generally, MOB hovers around 5.6 to 6.2 on Photonis Echo intensifiers but again, because most human eyes have the ability to adapt to various lighting conditions, this should not be a determining factor in intensifier selection.

Conclusion
While we are committed to providing full transparency in the tube selection process, intensifier specifications can be very confusing. Most new users using night vision in a practical mixed-lighting setting will generally struggle to visually differentiate between the low and high ends of each intensifier bracket (eg: 1600 vs 2000 FOM; or 2000 vs 2300 FOM). The difference only becomes more transparent as you move more than 3 SNR points and/or 5-6 lp/mm in resolution (effectively when you jump from NNVT to Echo; or from Echo to Echo+), or if operating in challenging, extremely low-light environments. Differences in EBI and Gain are even more difficult to differentiate without tightly controlled lab-grade conditions or with specialty camera equipment. As always - think about your own unique operating environment and method of usage to help you determine what your requirements are. 

From an absolute perspective, high SNR, high Gain, high SNR, and low EBI are always universally desirable, but as part of our Hand-Select process, we typically only provide the highest levels of performance for your given application for your review. Armed with the knowledge above, and this in mind, when you receive your top 3-5 intensifiers to choose from, the specification ranges are typically a 1-point SNR, and 2lp/mm difference in specs between choices, so therefore, we recommend new night vision users to pick between this subset based on screen quality and size/location of factory dark spots.