Questions about NV equipment performance, are some of the most frequently asked in the night vision world, most specifically related to tube specifications, or “specs”, so in this post we are going to briefly go over what specs are and how they affect the performance and use and use of night vision devices.
Q: What the hell are specs?
Image intensifier tube specifications, commonly referred to as “specs”, are quantifiable metrics, from which we can derive an idea of how well our device will perform in various conditions.
In the following guide you will learn how to read a “spec sheet”, sometimes also referred as data sheet or data record , and how to choose your night vision device based on your performance needs.
Resolution
Resolution is a metric that helps us understand the extent to which our
tube is able to resolve details. It is measured using “line pairs per millimeters” or Lp/mm. Generally image intensifiers devices will have a resolution of 64 Lp/mm or more, but from our experience testing those devices, even 60 Lp/mm tubes are capable enough for headborne use, as the unaided human eye is in most cases not able to appreciate the difference, while for NV clip-on applications, or generally anything that involves magnification greater than 1x, having more resolution is a noticeable improvement.Signal to noise ratio (SNR)
SNR is a very important factor when choosing our device.
It’s the metric that tells us how good our tube will be at processing light and creating a usable image to our eyes. Higher SNR values will result in a less noisy, or grainy image.
This is because when amplifying light, just like when amplifying sounds, noise may be generated, and that is a task that the image intensifier tube needs to work around.
As such, high SNR tubes are better at generating clean and sharp images, than low SNR ones. For Gen 2+ tubes, anything above 21 can be considered enough to operate in most conditions, and anything more than that is always welcome.
Figure of merit (FOM)
FOM is the easiest way to tell tubes apart at a glance and categorize them in “Tiers”, without having to get to check the whole spec sheet. This is convenient for retailers and customers to have a general idea of how good a tube will be.
FOM is given by multiplying resolutions by SNR. So for example an image intensifier with a resolution of 64 Lp/mm and a SNR value of 22, will have a Figure of Merit of 1408.
So briefly speaking: FOM = Res x SNR
We offer a wide variety of tubes on our website, with different FOM ratings to match each of our customer’s needs, like our NVT line of tubes.
Luminance Gain
Luminance gain is one of the more important factors when it comes to
choosing a device over another.Gain is a metric by which we know how good our tube is at amplifying light.
In layman’s terms, gain is similar volume in speakers, or horsepower in cars. Higher gain will result in brighter images, but the downside to this, will be increased noise; This is very important, as SNR plays a big role here, as the higher our SNR is, the more Gain we can afford having.
Manual gain devices have the ability to allow the user to dial in the brightness and subsequently the amount of noise in their image. Our PVS-14’s (link) and MH-1’s (link) for example have such feature.
Equivalent Background Illumination
Equivalent Background Illumination (EBI)
EBI is a metric measured in ulx, which helps us understand how good our intensifier tube will perform in extremely dark environments. The lower this value is, the better it will be, as low EBI tubes will give us an image with sharper contrast compared to higher EBI ones.
From our experience a value of 0.25 ulx or optimally less, will be enough to navigate and perform tasks with ease. To put this onto perspective, military tubes meant for aviation have a 0.25 ulx threshold, so if that is good enough for a jet pilot to fly a multimillion dollars machine, it will definitely be good enough for you to larp in your dad’s basement.
Halo
Halo is in our opinion the most underrepresented spec in the whole book, and it’s not very much talked about by most NV content creators and high profile individuals in the community.
Halo helps us measure how big hueing effects around light sources will be, and just like EBI, the lower the value, the better. Although many manufacturers don’t list it in their spec sheets, we recommend going for values equal to or less than 0.85 whenever possible.
Keep in mind that this factor is more important for urban applications than for rural exploration
Photocathode sensitivity
Photocathode sensitivity is one of the lesser known specs in image intensifiers. Photocathode sensitivity helps us understand how effective our PC is at detecting light and converting it into electrons which we can then multiply in order to amplify light. From our experience 700 uA x 10^-1 is a sufficient amount for Gen 2+ whereas Gen 3 will typically range in the 1800-2500 uA x 10^-1. PS is directly proportional to Luminance Gain, so by increasing it, so will gain