Halo

The diffuse ring surrounding a bright source, what creates it inside the tube, and how it can erase useful detail from the area immediately around that light.

What Halo Is

Halo is a diffuse ring or area of added brightness surrounding a strong light source in the night-vision image.

Common sources include:

• Vehicle headlights
• Streetlights
• Building lights
• Infrared illuminators
• Infrared beacons
• Reflections from signs or windows
• Muzzle flash
• Strobes and other brief high-intensity sources

The halo can cover or reduce contrast in the area immediately surrounding the source. A person, doorway, vehicle edge, road shoulder, or other object near that light may become harder to distinguish.

Lower halo is generally preferred because it preserves more usable detail close to bright sources.

"The light is obvious. Halo decides how much of the neighborhood around it gets swallowed too."

What Causes Halo

L3Harris explains that halo begins when primary electrons interact at the input surface of the microchannel plate.

That interaction produces secondary scattered electrons. Instead of remaining perfectly confined to the original bright point, some of that electron activity spreads outward and produces a diffuse area around the source.

The basic sequence is:

1. A bright point enters the objective lens.
2. The photocathode converts that light into electrons.
3. Electrons reach the MCP input surface.
4. Some electron scatter occurs around the bright point.
5. The phosphor screen displays the central light and surrounding halo.

L3Harris states that an anti-scatter layer on the MCP input surface can reduce the effect.

Reading Halo on the Data Sheet

Halo may appear on the tube sheet as:

• An actual measured value
• A maximum allowable value
• Both an actual value and acceptance limit

Lower is generally preferred.

Before comparing two sheets, confirm:

• Both sheets match the tube serial numbers
• Both values are actual measurements or both are maximum limits
• Both manufacturers use compatible test methods
• The units and decimal placement are read exactly as reported
• Neither value has been rounded or rewritten by a seller

Do not compare one tube's measured halo against another tube's maximum allowable halo and treat them as equal forms of data.

The data-sheet value comes from a controlled test. It is not a promise that every light encountered in the field will produce a ring of that exact visible size.

What Changes the Visible Effect

The halo specification belongs to the tube, but the visible field effect also depends on the scene and complete system.

Source Intensity

A stronger light generally produces a more dominant response.

Contrast

Halo is especially noticeable when a bright point sits against a dark background.

Distance and Angle

Moving closer, farther away, or off-axis can change how the source appears.

Atmosphere

Fog, rain, smoke, dust, and humidity can scatter additional light.

Optics

Dirty, scratched, fogged, or lower-quality lenses may add flare around the source.

Focus

Incorrect focus can make the entire bright area appear larger and softer.

Camera Exposure

A phone camera can exaggerate or suppress the apparent halo.

Automatic Tube Response

Autogating, automatic brightness control, and bright-source protection can change the overall image response without making the tube's inherent halo specification disappear.

Halo Is Not FOM

FOM is calculated from:

Signal-to-Noise Ratio × Center Resolution

Halo is not included.

Two tubes can have the same FOM while producing different results around bright lights.

Example:

Tube A and Tube B both calculate to 2,304 FOM.

• Tube A has the lower measured halo.
• Tube B has the higher measured halo.

Their general SNR and resolution rating may match, but Tube A may preserve more detail immediately beside a streetlight or headlight.

This does not make Tube A universally superior. EBI, gain, photocathode response, cosmetics, optics, and application still matter.

Halo, Blooming, Flare, and Photonic Barriers

These terms are often mixed together, but they are not identical.

Halo

The diffuse brightness surrounding a concentrated light source, associated with electron scatter at the MCP.

Blooming

A broader loss of image detail or expansion of brightness caused by a strong light response. The term is often used casually, but it is not necessarily the tube's reported halo value.

Optical Flare

Reflection or scatter produced by the objective, eyepiece, protective lens, dirt, moisture, or another optical surface.

Photonic Barrier

A bright foreground or strongly illuminated obstruction that overpowers the darker area beyond it. Halo may contribute, but the barrier involves the broader lighting and scene geometry.

Temporary Streaking or Image Retention

A transient artifact that can remain after certain bright exposures. This is not the normal halo surrounding a source while it remains in view.

Permanent Damage

A persistent burn, streak, or defect that remains after the source is gone. Halo itself is not proof that damage occurred.

Visible Light and Infrared Light

The tube can produce halo around visible or infrared sources that fall within its sensitivity range.

Examples include:

Visible Sources

Headlights, streetlights, illuminated windows, flashlights, and warning strobes.

Infrared Sources

IR illuminators, aiming lasers, beacons, reflected IR, and another user's active equipment.

The unaided eye may not recognize an infrared source as bright, but the tube can.

An excessive IR illuminator setting can create a large bright area, reflection, or photonic barrier. More power is not always more useful.

Use the lowest practical illumination level, control spill, and avoid directing concentrated IR energy toward nearby reflective surfaces.

Field Scenarios

Safe Evaluation

Do not test halo by exposing a tube to:

• Direct sunlight
• High-powered lasers
• Concentrated IR lasers
• Welding arcs
• Magnified bright sources
• Deliberately prolonged high-intensity exposure

L3Harris warns that direct sunlight and high-intensity lasers can damage tubes rapidly. Autogating and bright-source protection reduce risk; they do not make a tube indestructible.

Use manufacturer-provided test data and approved comparison imagery.

If evaluating two complete systems in the field:

1. Use the same source and distance.
2. Match objective focus and diopter.
3. Keep gain settings consistent.
4. Use the same optics and protective lenses when possible.
5. Observe for the shortest practical time.
6. Never use a high-power laser as a test source.
7. Do not rely on automatic phone-camera exposure.

Common Myths

Lesson Takeaway

Halo is the diffuse brightness surrounding an intense visible or infrared source. Lower measured halo generally helps preserve detail near that source.

Read halo separately from FOM, compare actual values under compatible test methods, and remember that optics, atmosphere, focus, source intensity, and camera exposure can change the visible result.