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Glossary and FAQ

Night vision and thermal glossary + FAQ

FOM, SNR, EBI, halo, shroud, NETD, pixel pitch, ITAR and more explained in plain language – plus answers to common questions.

Author: Night Gear Solutions Updated: Checked: 9 Nov 2026

In short: A glossary of the most common night vision and thermal terms – from FOM, SNR and EBI through helmet mounting to NETD, pixel pitch and ITAR. Answers to frequently asked questions are at the end.

Terms are grouped by topic. Where we have a dedicated article on a term, you’ll find a link.

Night vision tubes

  • Image intensifier tube (IIT) – the heart of analogue night vision. It collects very faint light, amplifies it and shows it on a phosphor screen. Its condition and specs decide image quality.
  • Photocathode – the tube’s input layer, which converts incoming photons into electrons. Its sensitivity is given in µA/lm.
  • MCP (microchannel plate) – a plate with a very large number of microscopic channels in which electrons from the photocathode multiply in an avalanche.
  • Phosphor screen – the tube’s output layer, which converts the electron avalanche from the MCP back into light – the image you see.
  • P43 – green phosphor (the classic “green” image).
  • P45 – white phosphor (a black-and-white image). Exosens (Photonis) states there are no lab performance differences between P43 and P45 and that colour is often a personal preference. More: White vs. green phosphor.
  • Gain – how much brighter the output image is than the input light. Some devices offer manual gain control.
  • Autogating – fast automatic regulation of the tube’s power supply as light changes, helping keep a usable image and limit tube overload. It is not full protection from bright light.
  • Resolution (lp/mm) – how many line pairs per millimetre the tube can resolve. Higher = finer detail.
  • SNR (signal-to-noise ratio) – how “clean” the image is in low light. Higher = less grainy.
  • FOM (figure of merit) – a summary tube quality figure = SNR × resolution (lp/mm). More: How to read a tube datasheet.
  • EBI (equivalent background illumination) – the background brightness a tube produces even with no light on the photocathode. Lower = a darker, more contrasty background in very low light.
  • Halo – the bright ring around an intense point light source in the image. Given in mm; a smaller halo means less “bloomed” lights.
  • Blemishes – permanent black or bright spots in the tube image, judged by size and position. More: Tube blemishes.
  • Chicken wire – a honeycomb (hexagonal) pattern visible in the image, mostly in brighter scenes. Treated as a cosmetic tube defect.
  • Generation (Gen 2+, Gen 3…) – the technology generation of a tube. More: Tube generations.
  • Datasheet – a test report with the measured specs of a specific tube and its serial number. Without the serial it’s worthless.
  • High-light cut-off – a feature that switches the device off after a time in daylight or bright room light (for the PVS-14, 70 ± 30 s according to ATN and AGM manuals). According to ATN, single point sources don’t trigger it.

Optics and ergonomics

  • Diopter – eyepiece focus adjusted to your eye so you see a sharp image without glasses.
  • Objective focus – focus adjusted for the distance you’re looking at.
  • Eye relief – the distance between your eye and the eyepiece at which you see the full field of view.
  • IPD (interpupillary distance) – the distance between the centres of your pupils. On a binocular you set it by sliding the eyepieces until each eye sees the full image.
  • FOV (field of view) – the angle the device covers, in degrees. For thermal, it follows from the sensor and focal length.
  • Demist shield – an anti-fog shield on the eyepiece. Never wipe it with wet tissue.
  • Lens cap / daylight filter – protects the tube in daylight and when the device isn’t in use.
  • IR illuminator – an infrared light that illuminates the scene for night vision when there is very little light.

Helmet mounting

  • Shroud – a flat or contoured plate permanently attached to the front of the helmet, into which the mount locks.
  • Helmet mount – the articulating arm that locks into the shroud and provides height, tilt and fore/aft adjustment.
  • J-arm – a smaller articulating link that connects a monocular (e.g. PVS-14) to the mount’s dovetail shoe.
  • Dovetail – a dovetail interface between mount and device. Dovetail and bayonet systems are not cross-compatible.
  • Bayonet – another type of mount interface; not compatible with dovetail.
  • Counterweight – a weight on the back of the helmet that balances the device in front. More: Helmet NVG setup.
  • Stow / auto shut-off – flipping the device up into the stowed position; some mounts switch a PVS-14 off when stowed.

Thermal imaging

  • Microbolometer – the most common uncooled thermal sensor; it responds to heat radiation through a change in electrical resistance.
  • LWIR – the long-wave infrared band (roughly 8–14 µm) used by most civilian thermal devices. Ordinary glass blocks it.
  • Germanium – the most common lens material for thermal optics – it passes the IR radiation that glass blocks.
  • Sensor resolution – the pixel count of a thermal sensor, e.g. 384×288, 640×512, 1280×1024.
  • Pixel pitch – the centre-to-centre distance between adjacent pixels in µm (e.g. 12 µm, 17 µm).
  • NETD – the smallest temperature difference the sensor can distinguish from noise, in mK. Lower = more sensitive. More: Thermal specs explained.
  • Focal length (F19, F35, F50…) – lens focal length in mm; a longer lens gives a narrower field of view and more detail at distance.
  • Refresh rate (Hz) – how many times per second the image updates; higher = smoother motion.
  • Calibration (shutter) – evening out the sensor background; an internal shutter briefly covers the microbolometer and the image freezes for a moment.
  • DRI – detection – recognition – identification. Under the Johnson criteria, detection needs at least 1.5 pixels across the target, recognition 6 and identification 12. Try our range calculator.
  • LRF (laser rangefinder) – a built-in laser rangefinder.
  • Palettes (white hot, black hot…) – display modes for temperature – for example, warm shown as white or as black.
  • Clip-on – a thermal module that mounts in front of a day scope or night vision objective. More: Clip-on vs. handheld vs. fusion.
  • Fusion – thermal and tube-based night vision combined in one image.

Law and export

  • ITAR – the US International Traffic in Arms Regulations. The US Munitions List, Category XII, covers among other things certain third- and higher-generation tubes and devices using them. US ITAR devices are therefore not commonly available to civilians in the EU.
  • ITAR free – a manufacturer’s statement that a product is not subject to ITAR (for example Photonis 4G tubes).
  • Dual-use – goods with both civilian and military uses. In the EU, their export is governed by Regulation (EU) 2021/821; its Annex I includes certain image intensifier tubes and cameras.
  • Observation vs. aiming – rules for observation devices and for weapon-mounted devices can differ by country. Overview: Night vision laws in Europe.

The information on this page is general, is not legal advice, and laws change. We cite the source and the date we checked it. Before buying or using a device, verify the rules with the relevant authority or hunting association. Last checked: 9 Nov 2026.

Frequently asked questions

What’s the difference between night vision and thermal?

Tube-based night vision amplifies residual light and shows the scene much as your eye would, only brighter. Thermal captures heat radiation and shows temperature differences, so it works in total darkness and warm objects stand out from their surroundings.

Can thermal see through glass?

No. Ordinary glass blocks infrared radiation in the 8 to 14 µm band, so through a window you only see the temperature of the glass itself.

Is white phosphor better than green?

Exosens (Photonis) states that there are no lab performance differences between white (P45) and green (P43) phosphor and that colour is often a personal preference.

What is FOM and how should I compare tubes?

FOM is SNR multiplied by resolution in lp/mm. It is a useful summary figure, but also look at EBI, halo and blemishes. Our tube datasheet article explains how.

Does daylight damage night vision?

The tube is built for very low light. Keep the lens cap on in daylight, don’t test the device in light for long (AGM states no more than 10 minutes, even with the cap on) and don’t point it at strong light sources. Autogating and automatic cut-off reduce the risk but are not full protection.

Does a thermal clip-on need zeroing?

Clip-on manufacturers such as Pulsar (Krypton series) state that the clip-on itself is not zeroed – you zero the day scope – provided it is installed according to the manual.

What does NETD mean?

NETD is the smallest temperature difference the sensor can distinguish from its own noise. It is given in mK, and a lower number means a more sensitive sensor.

Why does the thermal image freeze for a moment?

The device is calibrating: an internal shutter covers the sensor so the device can even out its background. This is normal.

Why aren’t US Gen 3 devices commonly sold in the EU?

US devices with third-generation tubes are subject to the US ITAR regulations, so they are not commonly available to civilians in the EU.

Is night vision or thermal legal for hunting?

It depends on the country and on whether the device is for observation or for use on a firearm. See our page on laws in Europe, and always check the rules with the relevant authority or hunting association.

Which batteries should I use in the cold?

Follow your device’s manual. For example, the AGM PVS-14 manual advises against alkaline AA cells below minus 20 °C and recommends lithium AA (L91).

How do I clean the lenses?

Start with dry lens tissue and, if needed, moisten it with water. Don’t touch the surfaces with your fingers and never wipe a wet demist shield.

What should I check when buying a used device?

Ask for the tube serial number and the original datasheet, look through the device in the dark, and if you are unsure, have it inspected before you pay.

Can’t find a term or an answer? Contact us – we keep adding to this glossary.

Sources

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