Introduction
At Nocturnal Optics Australia, we believe end-users should have a thorough understanding of both the capabilities and limitations of their night vision equipment. Image Intensifier Tubes (IITs), the core component of any night vision device, are highly sophisticated electro-optical items that can develop various cosmetic blemishes, performance issues, or damages throughout their operational lifespan.
Understanding the different types of image intensifier tube defects not only assists with accurate diagnosis and troubleshooting, but also helps users make informed decisions when purchasing, operating, inspecting, or maintaining their equipment. While some blemishes are simply cosmetic characteristics of the manufacturing process and have little to no impact on performance, others may indicate physical damage, improper handling, excessive light exposure, environmental contamination, or component degradation.
The following sections outline some of the most common forms of IIT blemishes and damage, their likely causes, how they present within the image, and their potential impact on overall system performance.
A set of Argus PVS-31 binocular night vision goggles staged on a gear room table in preparation for a local field training event. Photo Credit: Nocturnal Optics Europe
Black Spots/ Factory Spots
Black spots, often referred to simply as spots or factory spots, are the most common cosmetic blemishes encountered in image intensifier tubes. These appear as small, dark, stationary marks within the image and do not move when the scene or device is repositioned.
In the vast majority of cases, black spots are a normal by-product of the image intensifier manufacturing process and are not indicative of damage or a defective tube.
Factory spots typically originate from microscopic imperfections, contamination, or material inconsistencies within the photocathode, microchannel plate (MCP), fibre-optic components, or phosphor screen during production. Because image intensifier tubes are manufactured to extremely fine tolerances, the complete elimination of all cosmetic blemishes is generally not possible.
The size, quantity, and location of these spots are commonly used by manufacturers to grade and classify tubes. Higher-grade tubes are generally selected based on stricter cosmetic standards and fewer visible blemishes, particularly within the central viewing area.
Most manufacturers divide the image into three viewing zones:
- Zone 1: Central viewing area (most critical)
- Zone 2: Intermediate viewing area
- Zone 3: Outer peripheral viewing area
Spots located within Zone 1 are generally considered less desirable than those located within Zones 2 or 3 due to their increased visibility during normal use.
While tube grading standards vary between manufacturers, premium commercial and military-grade tubes are typically selected based on stricter cosmetic acceptance criteria, resulting in cleaner screens and fewer visible blemishes.
Importantly, small factory spots generally have little to no impact on the operational performance of the image intensifier tube and are considered a normal characteristic of the technology rather than a defect.
(Image Source: TFB - https://cdn-fastly.thefirearmblog.com/media/2024/05/19/14694199/friday-night-lights-night-vision-blemishes-and-how-to-fix-them.jpg?size=720x845&nocrop=1)
Damage Spots & Blemishes
While many spots observed within an image intensifier tube are harmless manufacturing artefacts, blemishes can also develop as a result of physical damage, misuse, ageing, or excessive exposure to intense light sources.
Damage spots
Damage spots are permanent blemishes that develop after the tube has left the factory and are generally indicative of degradation or damage to one or more internal components. Unlike factory spots, these defects were not present during original testing and typically result from operational or environmental factors.
Common causes include:
- Laser exposure (visible or infrared)
- Prolonged exposure to intense light sources
- Improper storage or handling
- Electrical damage
- Physical shock or impact
- Internal component degradation
Damage spots often appear as dark marks, irregular shapes, discoloured regions, or areas of reduced image performance that remain fixed within the image.
(Image Source: TFB - https://www.thefirearmblog.com/blog/2021/09/03/night-vision-blemishes)
Blemishes
The term blemish is generally used to describe any visible imperfection within the image that is attributable to damage, wear, contamination, or degradation rather than normal manufacturing characteristics.
Unlike factory spots, blemishes may indicate that an image intensifier tube has been subjected to conditions outside its intended operating parameters. These defects can arise from both physical and photonic mechanisms, including:
- Laser strikes
- Excessive light exposure (i.e: Sunlight, Floodlights/ Spotlights, etc.)
- Localised photocathode damage
- Microchannel plate degradation
- Mechanical shock or impact
Depending on the cause, blemishes may appear as:
- Dark spots
- Discoloured regions
- Burn marks
- Streaks
- Halo-like artefacts
- Areas of reduced brightness or contrast
While minor blemishes may have little practical impact on performance, significant blemishes can reduce image quality, obstruct target identification, and indicate permanent damage to the image intensifier tube.
Severe sunlight damage to an image intensifier tube caused by carrying night vision on a helmet without day caps fitted. Even when powered off, the objective lens can focus sunlight directly onto the photocathode, resulting in permanent streak blemishes and irreversible tube damage. Photo Credit: The Firearm Blog (TFB)
Photocathode Burn-In (Temporary vs. Permanent)
Burn-in occurs when an image intensifier tube is exposed to bright light sources or high-contrast imagery for extended periods. This can result in a visible residual image, shadow, or "ghost" remaining within the field of view after the original light source has been removed.
Temporary Burn-In (Image Retention)
Temporary burn-in appears as a faint afterimage or ghost pattern within the image. This is generally caused by localized phosphor persistence or temporary photocathode effects and will typically fade over time, particularly when the device is powered off and stored in complete darkness.
Historically, older phosphor formulations such as P22 phosphor were more susceptible to temporary image retention than modern phosphor technologies.
Permanent Burn-In
If exposure to intense light sources continues for a prolonged period, temporary image retention may develop into permanent burn-in. This results in irreversible damage to the photocathode, microchannel plate, or phosphor screen, leaving a visible ghost image permanently embedded within the tube.
Permanent burn-in can reduce image clarity, contrast, and overall performance, and cannot be repaired. Common causes include prolonged exposure to bright artificial lighting, weapon-mounted displays, laser sources, vehicle lighting, or other high-intensity light sources concentrated on a specific area of the tube.
(Image Source: TFB - https://www.thefirearmblog.com/blog/2021/09/03/night-vision-blemishes)
Fixed Pattern Noise (FPN) / "Chicken Wire"
Fixed Pattern Noise (FPN), commonly referred to as "chicken wire", is a faint grid-like or honeycomb pattern that may become visible under certain operating conditions. It is most commonly observed in very bright environments, during high-gain operation, or in image intensifier tubes with exceptionally high Signal-to-Noise Ratio (SNR) performance, particularly modern filmless and high-performance Gen III designs.
FPN is caused by the microscopic structure of the Microchannel Plate (MCP) and is an inherent characteristic of image intensifier technology. As a result, some degree of fixed pattern noise exists in virtually all intensifier tubes, although its visibility varies between manufacturers, tube designs, and operating conditions.
Importantly, the presence of FPN does not indicate damage, degradation, or tube failure. Rather, it is a normal artefact of the technology and is generally only noticeable when specifically looking for it or under particular lighting conditions.
(Image Source: CloudyNights -https://www.cloudynights.com/uploads/gallery/album_8407/gallery_139776_8407_197546.jpg)
Recoil Damage
Image Intensifier Tubes (IITs) are highly sensitive electro-optical components and can be damaged by excessive shock, vibration, or mechanical impact. While many modern night vision systems are designed to withstand normal field use, exposure to forces beyond their intended operating parameters can result in permanent performance degradation or complete tube failure.
Common causes of recoil or impact-related damage include:
- Dropping the night vision device.
- Significant impacts during transport or field use.
- Mounting a standard helmet-mounted or handheld night vision device (PVS-14, ACTinBlack THE14, etc.) to a weapon platform not designed for recoil-rated optics.
- Using a Night Vision Clip-On System on a weapon chambered in a calibre exceeding the manufacturer's recoil rating (e.g. 7.62 NATO, .300 Win Mag, .338 Lapua Magnum, .50 BMG, etc.).
(Image Source: TFB - https://cdn-fastly.thefirearmblog.com/media/2024/05/19/14694211/friday-night-lights-night-vision-blemishes-and-how-to-fix-them.jpg?size=720x845&nocrop=1)
It is important to note that not all night vision devices are designed to withstand weapon recoil. Standard observation devices such as helmet-mounted monoculars and binoculars are generally intended for navigation and observation purposes only and should not be mounted directly to weapon systems unless specifically approved by the manufacturer.
Only certain systems, such as dedicated night vision weapon sights and recoil-rated clip-on devices, are engineered to withstand repeated recoil forces reliably. Examples include the AN/PVS-22 Universal Night Sight, L3Harris CNVD-LR (AN/PVS-24LR), etc. and other purpose-built clip-on weapon sights. These systems incorporate reinforced housings, shock-resistant optical assemblies, and image intensifier mounting architectures specifically designed to withstand the forces generated during repeated weapon firing.
A precision-configured Accuracy International rifle equipped with a ZCO riflescope, L3Harris CNVD-LR (AN/PVS-24LR) clip-on night vision device, Wilcox RAPTAR rangefinder, Wilcox RAID laser aiming module, KGM R30T suppressor, and bipod. Photo Credit: User 'labsupervisor', Reddit
Bright Spots & Emission Points
Not all bright defects observed within a night vision image are the same.
Bright Spots
Bright spots appear as small, luminous points visible during normal operation. These are often caused by temporary electron activity within the image intensifier and will typically disappear when the device is placed in complete darkness or when operating conditions change.
Temporary bright spots are relatively common and do not necessarily indicate a defective tube.
Emission Points
Emission points are a more serious defect. Unlike ordinary bright spots, they remain visible even when the photocathode receives little or no light input. Emission points are generally caused by unwanted electron emission or internal electrical leakage within the tube, often occurring between the photocathode, microchannel plate (MCP), or other internal components.
Over time, emission points may increase in size or number, degrading image quality and potentially leading to complete tube failure if the condition continues to worsen.
While isolated emission points may initially have minimal impact on performance, progressive emission activity is generally considered an indicator of a deteriorating image intensifier tube.
(Image Source: TFB - https://www.thefirearmblog.com/blog/2021/09/03/night-vision-blemishes)
“Going to Air” (Loss of Tube Vacuum)
When an image intensifier tube loses its internal vacuum seal, it is commonly described as "going to air." This is a catastrophic failure that allows atmospheric gases and contaminants to enter the tube, permanently damaging the internal components.
Because image intensifier tubes are manufactured as sealed vacuum devices, a loss of vacuum is considered a terminal failure and is not repairable.
Common indicators of a tube that has gone to air include:
- Dark shadowing around the edges of the image.
- Distorted or irregular circular field of view.
- Severe image degradation or loss of brightness.
- Unusual discolouration or image artefacts.
Once a tube has gone to air, replacement of the image intensifier tube is typically the only viable remedy.
Final Thoughts
Every image intensifier tube has a finite service life, and understanding the various types of blemishes, artefacts, and failure modes allows users to distinguish between normal manufacturing characteristics, cosmetic imperfections, and genuine performance-limiting defects.
Many common phenomena, such as factory spots, fixed pattern noise (FPN), or temporary image retention, are often misunderstood and may have little to no practical impact on operational performance. Conversely, issues such as emission points, photocathode burns, recoil damage, or loss of vacuum can indicate serious degradation that may require tube replacement.
At Nocturnal Optics Australia, we believe that informed users make better decisions. Beyond supplying, assembling, and servicing night vision systems, we are committed to educating end-users on the operation, maintenance, and preservation of image intensifier technology. Proper handling, storage, and understanding of tube characteristics can significantly extend the lifespan of a night vision device and help prevent costly damage.
A member of Nocturnal Optics Europe preparing to conduct a CQB room entry and clearance within a local Military Operations in Urban Terrain (MOUT) facility as part of ongoing night vision system testing, evaluation, and user familiarisation activities. Photo Credit: Nocturnal Optics Europe
