Part 3 · Lesson 9

Fixed-Pattern Noise

Why a stationary mesh, honeycomb, or other repeating structure can appear across the image—and how to separate it from random noise, ordinary blemishes, optical contamination, and damage.

Why a stationary mesh, honeycomb, or other repeating structure can appear across the image—and how to separate it from random noise, ordinary blemishes, optical contamination, and damage.

Reading time

9–11 minutes

Difficulty

Intermediate

Focus

Screen structure and image uniformity

What Fixed-Pattern Noise Is

Fixed-pattern noise is a stationary structure or nonuniform pattern that remains registered to the tube's image while the outside scene moves.

It may appear as:

  • A faint honeycomb
  • A hexagonal grid
  • A mesh or "chicken-wire" structure
  • Repeating lines
  • Stationary shading
  • A consistent texture across part of the image

The word fixed matters. Unlike scintillation, the pattern does not randomly sparkle, crawl, or change from moment to moment.

It remains in the same location and orientation relative to the tube.

"If the texture dances around, it is probably scintillation. If it sits there like wallpaper no matter where you look, now we are discussing a fixed pattern."

Why the Hexagonal Pattern Appears

The microchannel plate is constructed from millions of microscopic channels that multiply electrons.

L3Harris explains that the visible hexagonal grid can arise from the MCP manufacturing process. Slight differences at the boundaries of MCP fiber bundles can create a repeating structure that becomes visible in the final image.

The basic process is:

  1. The photocathode converts the outside scene into an electron image.
  2. The electron image reaches the MCP.
  3. Millions of microscopic channels multiply the electrons.
  4. Small structural differences within MCP bundle boundaries affect uniformity.
  5. A faint repeating grid or mesh may appear on the phosphor screen.

Not every stationary image artifact comes from the MCP. "Fixed-pattern noise" is a broader category, while the honeycomb or mesh is a common recognizable example.

When the Pattern Becomes Visible

A fixed pattern may be difficult to notice during normal use and obvious under another condition.

L3Harris states that its hexagonal MCP pattern may become more noticeable at low gain and under higher scene illumination.

Visibility can also depend on:

  • A uniform background such as open sky
  • Smooth walls or large low-texture surfaces
  • Overall scene brightness
  • Manual-gain setting
  • Tube gain and construction
  • Objective and eyepiece focus
  • Observer attention
  • Camera exposure and sharpening
  • Whether the scene contains enough detail to hide the pattern

A faint mesh visible against a uniform surface may nearly disappear in wooded terrain or another visually complex scene.

That does not mean the pattern physically appeared and disappeared. The scene changed how easily the observer could detect it.

Fixed Pattern Versus Scintillation

Fixed-Pattern Noise
  • Remains stationary
  • Repeats in a recognizable structure
  • May be more visible against uniform backgrounds
  • Can be associated with MCP structure or image nonuniformity
  • Does not randomly move frame to frame
Scintillation
  • Appears as random sparkling or grain
  • Changes continuously
  • Becomes more pronounced as available light decreases
  • Is a normal characteristic of MCP image intensifiers
  • Relates directly to image noise and SNR

A tube can display both at the same time.

What It Is Not

Do not automatically label every visible artifact as fixed-pattern noise.

Dark Spot
A localized dark blemish or piece of debris rather than a repeating pattern.
Emission Point
A steady bright pinpoint that remains in one location.
Halo
Diffuse brightness surrounding an intense light source.
Optical Contamination
Dust, moisture, fingerprints, scratches, or debris within the optical path.
Photonic Barrier
A bright foreground or reflection overpowering the darker scene beyond it.
Image Retention or Streaking
A temporary artifact following bright exposure.
Burn or Permanent Damage
A persistent mark caused by damaging exposure or another failure.
Camera Artifact
Moiré, sharpening, denoising, compression, or sensor pattern added by the recording device.

The correct diagnosis depends on whether the artifact moves, repeats, responds to focus, follows a bright source, or appears only in photographs.

Is Fixed-Pattern Noise a Defect?

Not every visible MCP pattern means the tube is defective, damaged, or unusable.

Evaluation depends on:

  • Pattern severity
  • Location within the image
  • Conditions required to see it
  • Interference with useful detail
  • Applicable manufacturer acceptance criteria
  • Contract or grade requirements
  • Whether the pattern is stable or newly appeared
  • Intended application

A faint pattern visible only while examining a smooth wall is different from a dominant structure that obscures detail during ordinary use.

Some manufacturers may address fixed-pattern noise through acceptance testing or image-quality criteria rather than listing a simple standalone number on the customer data sheet.

A blank field does not mean zero. It may mean the characteristic was addressed elsewhere in the applicable inspection process.

FOM Does Not Include Screen Uniformity

FOM is calculated using:

SNR × Center Resolution

Fixed-pattern noise is not included.

A tube can have:

  • High FOM
  • Strong SNR
  • High center resolution
  • A visible stationary pattern

The mathematical performance can be excellent while the screen remains cosmetically or structurally distracting to a particular user.

Likewise, a clean-looking screen can belong to a tube with lower measured SNR or resolution.

Read performance and image quality together.

Safe Evaluation Process

Use a safe, evenly illuminated nighttime scene. Never use direct sunlight, high-powered lasers, or another hazardous bright source.

Evaluation steps:

  1. Confirm the objective and eyepiece are clean.
  2. Set the objective focus and diopter correctly.
  3. View a safe, uniform background.
  4. Compare the artifact against a detailed natural scene.
  5. Adjust manual gain within the normal operating range.
  6. Determine whether the pattern remains fixed relative to the tube.
  7. Check each binocular channel separately.
  8. Compare the observation with approved reference imagery.
  9. Review the matching tube data sheets and inspection notes.
  10. Contact the manufacturer or qualified service provider if the pattern is severe, new, or questionable.

Do not disassemble a purged system to chase an artifact. That is an excellent way to convert a minor concern into an actual repair.

Phone Cameras Can Lie

A phone camera can exaggerate, suppress, or create an apparent pattern through:

  • Digital sharpening
  • Noise reduction
  • Automatic exposure
  • Compression
  • Sensor-grid interference
  • Moiré
  • Misalignment behind the eyepiece
  • Focus errors
  • Frame stacking

A pattern visible only in the photograph may belong to the camera rather than the tube.

A pattern visible to the eye but absent from the photograph may have been removed by processing.

Do not use an uncontrolled phone image as the sole evidence that a tube passes or fails an image-quality requirement.

Field Scenarios

Scenario 1 — Honeycomb Against the Sky

Situation: A faint hexagonal grid appears while viewing a uniform night sky but becomes difficult to notice in wooded terrain.

Assessment: The uniform background made the stationary MCP structure easier to see.

Scenario 2 — Moving Sparkles

Situation: The image contains grain that changes constantly as the scene gets darker.

Assessment: That is scintillation, not fixed-pattern noise.

Scenario 3 — Single Dark Circle

Situation: A dark circular mark remains near the center of the image.

Assessment: Inspect for a tube blemish or optical contamination. A single spot is not a repeating fixed pattern.

Scenario 4 — Binocular Channel Difference

Situation: One channel shows more visible mesh than the other against the same smooth background.

Assessment: Evaluate each channel separately and review both tube sheets. Matching FOM does not guarantee identical screen structure.

Scenario 5 — Pattern Only on Camera

Situation: A honeycomb appears in a phone photograph but cannot be seen directly.

Assessment: Camera moiré or processing may be responsible. Repeat the evaluation without relying on the photograph.

Scenario 6 — New Artifact After Bright Exposure

Situation: A streak or pattern appears after exposure to an intense source.

Assessment: Do not automatically call it manufacturing-related fixed-pattern noise. Stop deliberate testing and contact a qualified service provider if it persists.

Scenario 7 — High-FOM Tube

Situation: A tube has excellent FOM but a distracting stationary pattern.

Assessment: FOM does not include fixed-pattern noise or overall screen uniformity.

Common Myths

Myth: Every visible pattern means the tube is damaged.

Reality: A faint MCP structure can be a manufacturing-related image characteristic.

Myth: Fixed-pattern noise is the same as scintillation.

Reality: Fixed patterns remain stationary. Scintillation changes continuously.

Myth: High FOM guarantees a perfectly uniform screen.

Reality: FOM does not include fixed-pattern noise or cosmetics.

Myth: A phone photograph proves the tube has chicken wire.

Reality: The camera can create moiré and other false patterns.

Myth: A single dark spot is fixed-pattern noise.

Reality: A localized spot and a repeating structure are different image characteristics.

Myth: If the pattern disappears in a detailed scene, the tube repaired itself.

Reality: The scene simply made the pattern harder to perceive.

Myth: A user can remove MCP patterning by cleaning the outside lenses.

Reality: Cleaning helps only when the artifact is optical contamination.

Lesson Takeaway

Fixed-pattern noise is a stationary structure that remains registered to the tube while the outside scene moves.

A faint hexagonal or honeycomb pattern may come from MCP manufacturing structure and become more visible under specific lighting, gain, or background conditions.

Separate it from scintillation, spots, contamination, camera artifacts, and damage before deciding what it means.

Lesson Complete

You can now identify fixed-pattern noise, explain why MCP mesh may appear, separate stationary structure from scintillation and spots, and evaluate a questionable pattern without relying on an uncontrolled phone photograph.

Next lesson preview

Part 3, Lesson 10 explains how to compare SNR, resolution, FOM, EBI, halo, gain, photocathode response, and screen quality together instead of ranking tubes by one isolated number.