Part 2 · Lesson 4

Phosphor Screen

The surface that converts amplified electrons back into a visible image.

The photocathode converts photons into electrons.

The microchannel plate multiplies those electrons.

The phosphor screen turns them back into visible light.

It is the final conversion stage inside an analog image-intensifier tube—and the surface responsible for the green or white image the user sees through the eyepiece.

The photocathode captures the scene. The MCP amplifies it. The phosphor screen displays the result.

Where the Phosphor Screen Sits

The phosphor screen is positioned after the microchannel plate at the output side of the image-intensifier tube.

The complete signal path is:

  1. The objective lens focuses the outside scene onto the tube.
  2. The photocathode converts incoming photons into electrons.
  3. The microchannel plate multiplies the electron signal.
  4. High voltage accelerates the amplified electrons toward the phosphor screen.
  5. The phosphor emits visible light when struck by those electrons.
  6. The output window transfers the visible image toward the eyepiece.
  7. The eyepiece focuses the displayed image for the user.

L3Harris describes the amplified electrons as being accelerated into the phosphor screen by the tube's high-voltage electronics, producing visible light for the user to view through the eyepiece. See the L3Harris explanation of Gen III image-intensification technology.

Photocathode → MCP → Phosphor Screen → Output Window → Eyepiece

Electrons Back into Visible Light

The phosphor screen contains material that emits light when energized by the amplified electrons leaving the MCP.

This emission process is commonly called cathodoluminescence.

A portion of the phosphor screen receiving more electron energy emits more visible light. A portion receiving fewer electrons emits less light.

The brightness pattern produced by the phosphor therefore follows the amplified electronic image.

That pattern becomes the visible night-vision image.

1 Amplified Electrons Arrive

The microchannel plate sends an amplified electron pattern toward the output side of the tube.

2 The Phosphor Is Energized

The electrons strike the phosphor material and transfer energy to it.

3 Visible Light Is Emitted

The phosphor releases that energy as visible light that can be viewed through the eyepiece.

The Screen Produces a Monochrome Image

An analog image-intensifier tube does not preserve the original colors of the scene.

The photocathode responds differently to different wavelengths, but the phosphor screen converts the amplified electronic image into one selected output spectrum.

The result is a monochrome image.

Objects are displayed as different levels of brightness rather than their original daylight colors.

This is why:

  • A red object does not appear red.
  • A blue object does not appear blue.
  • A warm object does not automatically appear brighter.
  • Two similarly colored fabrics may appear very different.
  • Different visible and near-infrared reflectivity becomes a difference in displayed brightness.

The phosphor screen shows signal intensity, not conventional color.

The Phosphor Determines Output Color

The phosphor formulation determines the visible color of the displayed image.

Two common examples are:

P-43 Green Phosphor

Produces the traditional yellow-green image associated with many legacy and current night-vision systems.

P-45 White Phosphor

Produces a pale monochrome image that is generally perceived as white, blue-white, or gray-blue.

NVD identifies P-43 as the conventional green-phosphor output and P-45 as the white-phosphor output used in modern systems. See the NVD image-intensifier reference.

The output color does not identify every other performance characteristic of the tube.

White phosphor does not automatically mean:

  • Higher FOM
  • Better signal-to-noise ratio
  • Higher resolution
  • Lower EBI
  • Smaller halo
  • Unfilmed construction
  • Autogating
  • A particular tube manufacturer

Green and white tubes can both be manufactured across different performance grades.

Do not turn this section into the full color comparison. White versus green phosphor receives its own dedicated lesson next.

The Phosphor Screen and Output Window

The phosphor screen is commonly deposited on the tube's fiber-optic output window.

The output window transfers the visible image from the phosphor screen toward the eyepiece while limiting unnecessary spreading of the light.

Depending on the tube format, the fiber-optic output may:

  • Invert the image
  • Preserve the image orientation
  • Match a particular system's optical design
  • Affect tube compatibility

The eyepiece then magnifies and focuses that output image for the user.

The phosphor screen is inside the sealed tube. Cleaning the eyepiece does not clean the phosphor screen.

If a spot remains fixed after the external optics are properly cleaned, it may originate somewhere inside the tube—but not every internal spot is necessarily on the phosphor screen.

Screen Efficiency and Brightness

The phosphor screen must convert electron energy into visible light efficiently.

Output brightness depends on more than the phosphor alone.

The final image is influenced by:

  • Photocathode signal
  • MCP amplification
  • Voltage applied within the tube
  • Automatic brightness control
  • Manual-gain setting
  • Phosphor efficiency
  • Output window
  • Eyepiece transmission
  • Available ambient light
  • Bright sources within the scene

A brighter screen does not automatically contain more detail.

A weak scene can be displayed brightly while still containing heavy noise and limited information.

Brightness is what your eye sees. Image quality depends on what useful information survives the entire signal chain.

Screen Persistence

The phosphor does not stop emitting light at the exact instant an electron impact ends.

Its light output fades over a short period. This behavior is called persistence or decay time.

Persistence must be balanced.

If Persistence Is Too Long

Moving objects and rapid head movement can leave visible trails, smear, or ghosting.

If Persistence Is Very Short

The screen provides less temporal smoothing of a weak, rapidly changing electron signal.

Modern phosphor formulations are selected to provide usable brightness while allowing the image to update quickly enough for movement and observation.

Do not publish a specific decay-time value unless it comes from a verified specification for the exact phosphor being discussed.

Screen Brightness and Automatic Control

Modern tubes regulate their output as scene brightness changes.

Automatic brightness control can reduce tube gain when the phosphor screen's output becomes too bright.

Depending on the system, the user may also be able to reduce displayed brightness through manual gain control.

These controls help:

  • Maintain a manageable output level
  • Reduce excessive glare to the user
  • Preserve contrast in changing conditions
  • Make extended viewing more comfortable
  • Limit stress on the tube

They do not make the device safe for intentional exposure to direct sunlight or high-intensity lasers.

Autogating and manual versus automatic gain receive dedicated lessons later in Part 2.

Temporary Afterimages

After viewing a concentrated light source or strong contrast pattern, the user may notice a temporary image that remains visible after the device is moved.

Temporary effects may result from several parts of the tube and are not always caused exclusively by the phosphor screen.

A temporary afterimage may:

  • Fade after the source is removed
  • Become less noticeable against a detailed scene
  • Recover after the system is safely operated under normal conditions
  • Appear more dramatic in a camera than to the unaided eye

Temporary behavior should not automatically be diagnosed as permanent screen damage.

However, repeated or severe exposure must not be dismissed.

Permanent Burn-In and Light Damage

Prolonged exposure to bright, concentrated, or static sources can permanently damage an image-intensifier tube.

Potentially harmful sources include:

  • Direct sunlight
  • High-intensity visible lasers
  • High-intensity infrared lasers
  • Welding arcs
  • Bright static displays
  • Prolonged exposure to intense artificial lights
  • Concentrated reflections

Permanent damage may appear as:

  • Dark spots
  • Bright spots
  • Fixed streaks
  • Persistent shadows
  • A recognizable static image
  • Areas of permanently reduced sensitivity

Not every permanent mark is physically located on the phosphor screen. Damage may involve the photocathode, MCP, phosphor screen, or another internal component.

Do not intentionally create a burn-in demonstration.

Use only manufacturer-provided or already documented examples.

Output Glow and Light Back Splash

The phosphor screen produces visible light inside the eyepiece.

Some of that light can illuminate the user's eye socket or reflect from the face, especially when the output is bright or the device is positioned farther from the eye.

This may be called:

  • Eye glow
  • Light splash
  • Back splash
  • Eyepiece glow

Its visibility depends on:

  • Phosphor output
  • Gain setting
  • Eyepiece design
  • Eye relief
  • Use of eye cups or filters
  • Ambient darkness
  • Viewing angle

This glow does not mean the device is projecting visible light forward through the objective lens. It originates at the output side facing the user.

Cameras Do Not See the Screen Exactly Like Your Eye

Through-the-tube photographs can be useful, but they can also be misleading.

A camera may alter:

  • Apparent color
  • Brightness
  • Contrast
  • Noise
  • Halo size
  • Screen tint
  • Sharpness
  • Edge distortion
  • Visible phosphor grain
  • Apparent blemish size

Automatic exposure and phone-camera processing can make one tube look dramatically better or worse than it appears to the human eye.

Every Academy through-the-tube photograph should identify:

  • Camera or phone used
  • Exposure method
  • Whether automatic processing was active
  • Night-vision system
  • Tube type
  • Phosphor type
  • Lighting conditions
  • Whether supplemental IR was used
  • Whether the image was cropped or color-corrected

Do not use through-the-tube photographs as scientific comparisons unless the capture method is controlled.

Real-World Phosphor-Screen Scenarios

Walking Through Wooded Terrain

The screen must update quickly as branches, terrain, and the user's head move. Excessive persistence would make movement appear smeared.

Looking Toward Headlights

The concentrated electron signal creates a bright output at the corresponding area of the phosphor screen. Halo, blooming, and automatic brightness reduction may affect surrounding detail.

Entering a Dark Building

The displayed image may become brighter as tube gain increases, but it may also become noisier because the starting signal contains fewer photons.

Viewing a Uniform Wall

A plain surface can make fixed spots, phosphor variations, and other internal cosmetics easier to notice than they are in a detailed outdoor scene.

Using Manual Gain

Reducing gain lowers the apparent output brightness. It does not change the phosphor color or create new scene detail.

Photographing the Screen with a Phone

The phone may change exposure, tint, contrast, and sharpening. The saved image may not accurately represent what the user saw.

The phosphor screen can only display the signal delivered to it. It cannot repair poor focus, replace missing photons, remove MCP noise, or decide which part of the scene matters.

Do Not Confuse the Output Screen with a Digital Display

A phosphor screen is not an LCD, OLED, or digital microdisplay.

It does not:

  • Contain digital pixels
  • Run image-processing software
  • Apply digital sharpening
  • Record video
  • Create artificial color
  • Store an image file
  • Display menus or electronic overlays by itself

It emits visible light directly in response to the amplified electron pattern.

That direct analog conversion is one reason traditional image-intensified night vision can present motion with extremely low perceptible delay.

BigRed's Short Version

The photocathode turns light into electrons.

The MCP turns a few electrons into a lot of electrons.

The phosphor screen turns those electrons back into visible light.

That screen determines whether the output appears green or white, but the color alone does not tell you the tube's FOM, resolution, SNR, film construction, or overall quality.

The phosphor screen is the display—not the sensor and not the amplifier.

And no, white phosphor does not sprinkle additional performance dust inside the tube.

Next, we will compare white and green phosphor properly.