CETC/NNVT Tubes with Diffraction Grating Technology — What Is "Rainbow Sim"?

CETC/NNVT Tubes with Diffraction Grating Technology — What Is "Rainbow Sim"?

Diffraction Grating in Night Vision Tubes: What It Is, How It Works, and Why It Matters

If you have been shopping for newer CETC or NNVT night vision tubes, you may have come across a specification that is still relatively uncommon: Diffraction Grating.

Unlike terms such as White Phosphor, Autogating, Autogain, Gen 2+, or Gen 3, diffraction grating is not a new "generation" of night vision. Instead, it is a technology incorporated into the photocathode side of the image intensifier to improve how efficiently incoming light can interact with the photosensitive material.

And that matters because the photocathode is where the entire night-vision process begins.

A more efficient photocathode can provide the rest of the image intensifier with more useful signal to work with.

How an Image Intensifier Works

To understand why diffraction grating can make a difference, it helps to understand what happens inside an image intensifier.

A night vision tube essentially performs four major steps:

Light → Electrons → Amplification → Light

First, photons from the environment enter the image intensifier through the objective lens.

Those photons strike the photocathode, which converts incoming light into electrons through the photoelectric effect.

Those electrons then enter the microchannel plate (MCP). The MCP contains millions of microscopic channels that multiply the electron signal.

Finally, the amplified electron pattern strikes the phosphor screen, producing the visible image you see through the eyepiece.

The important part for diffraction grating is the very first conversion:

Photons → Electrons

The more efficiently that conversion occurs, the more useful information the rest of the tube has available to amplify.

What Is Diffraction Grating?

A diffraction grating is a microscopic, precisely engineered structure that interacts with incoming light.

When incorporated into a photocathode design, the grating can manipulate the incoming optical energy and increase the interaction between light and the photosensitive material.

Instead of simply allowing light to pass through or reflect from the photocathode surface, the microscopic structure can cause specific wavelengths of light to interact with the photocathode more effectively.

One important mechanism involves diffraction and resonant optical effects that can increase the optical field within or near the photosensitive layer.

In simple terms:

The grating helps the photocathode make better use of the light that reaches it.

That can increase the probability that incoming photons will contribute to the photoelectric process and become useful electrons.

This is particularly interesting in the near-infrared (NIR) portion of the spectrum.

Why Does Diffraction Grating Improve Night Vision?

Night vision performance ultimately depends on how effectively an image intensifier can turn extremely small amounts of available light into a useful image.

Imagine two tubes looking at exactly the same dark scene.

If Tube A converts a greater percentage of the available light into useful electrons, it begins the amplification process with more signal.

Tube B may be looking at exactly the same scene, but if fewer photons are converted into photoelectrons, there is less information available for the MCP to amplify.

This is a fundamental point:

The MCP can amplify electrons, but it cannot amplify photons that were never converted into electrons.

This is why the photocathode is so important.

Improving the efficiency of the photocathode can potentially improve the amount of useful signal available throughout the rest of the image intensifier.

Quantum Efficiency: One of the Most Important Concepts

One of the key measurements when discussing photocathodes is Quantum Efficiency (QE).

QE describes how efficiently incoming photons are converted into photoelectrons.

A higher quantum efficiency means that, for a given amount of incoming light, the photocathode can generate more photoelectrons.

That can contribute to:

  • Better signal

  • Improved low-light performance

  • Better signal-to-noise performance

  • Improved image detail

  • Better near-infrared sensitivity

  • Improved overall image quality

Recent research on grating-based photocathodes demonstrates why this technology is attracting attention.

A 2026 study involving researchers from North Night Vision Technology examined a hexagonal nanograting structure for a Gen II+ image intensifier. In the tested design, the researchers reported measured quantum efficiency increasing from 15.90% to 22.44% in the visible band and from 12.31% to 19.15% in the near-infrared band.

The same study reported spatial resolution increasing from 64 lp/mm to 72 lp/mm, while measured halo diameter decreased from 0.78 mm to 0.69 mm.

These numbers come from a specific experimental design and should not be interpreted as a guaranteed improvement for every commercially available diffraction-grating tube.

However, they demonstrate the potential of the technology and why manufacturers are interested in using engineered grating structures in image intensifiers.

Why Do Diffraction-Grating Tubes Have a Rainbow Color?

One of the most noticeable characteristics of some diffraction-grating tubes is the rainbow or iridescent appearance that can be seen on the photocathode when the tube is viewed under normal room lighting.

You may see colors such as:

  • Blue

  • Purple

  • Green

  • Yellow

  • Gold

  • Red

Depending on the viewing angle, the appearance can change dramatically.

This is normal and is not a defect.

The color is an optical effect caused by the microscopic structure of the diffraction grating.

What Causes the Rainbow Effect?

White light is made up of many different wavelengths.

When white light interacts with a diffraction grating, the microscopic structure causes different wavelengths to be diffracted at different angles.

Our eyes therefore see different colors depending on the angle at which the light reaches us.

This is the same basic optical phenomenon responsible for the rainbow-like colors commonly seen on a:

  • CD

  • DVD

  • Holographic surface

  • Diffractive optical component

  • Other finely structured optical surfaces

The microscopic grating effectively separates portions of the incoming white light according to wavelength.

That is why the photocathode can appear to have a rainbow, oil-slick, or holographic appearance when viewed under certain lighting.

Why Does the Color Change When You Move the Tube?

The rainbow effect is angle-dependent.

Look at the photocathode from one angle and you might see mostly blue or purple.

Rotate the tube slightly and the colors may shift:

Blue → Purple → Green → Yellow → Red

Rotate it again and the colors may change or disappear.

This is completely normal for a diffractive structure.

The exact appearance will depend on the particular grating design, lighting conditions, viewing angle, and the wavelengths being diffracted.

This is also why a diffraction-grating tube may look dramatically different when viewed from different directions.

Is the Rainbow Color a Defect?

No.

The rainbow or iridescent appearance associated with a diffraction-grating photocathode is not, by itself, an indication of damage, contamination, staining, or a defective tube.

It is an optical consequence of the microscopic structure.

In fact, the visible effect can be one of the easiest ways to recognize that a tube incorporates a diffractive structure.

However, the appearance of the rainbow should not be used as a performance measurement.

A more colorful tube does not automatically mean a higher-performing tube.

The actual performance of the intensifier should be evaluated using measured specifications and the image produced by the tube.

Is the Rainbow Effect the Same as an Optical Lens Coating?

Not necessarily.

Traditional optical coatings can also produce blue, purple, green, or other reflective colors.

However, the distinctive rainbow appearance associated with a diffraction grating is caused by the interaction of light with the microscopic diffractive structure.

The two phenomena can therefore look similar while having different underlying causes.

This is an important distinction when examining a night-vision tube.

The colored appearance is not simply a cosmetic coating added to make the tube look different.

It is a visible consequence of the optical structure being used.

Why the Rainbow Is Actually Interesting

The rainbow itself is not what improves the night-vision image.

The important part is the engineering behind the microscopic grating.

The grating is designed to manipulate incoming light and improve its interaction with the photocathode.

The rainbow is simply what happens when ordinary room light interacts with that microscopic structure.

Think of it this way:

The rainbow is the visible effect.

The photocathode interaction is the reason the technology exists.

The performance advantage comes from how the grating affects the optical and photoelectric behavior of the photocathode—not from the color you see when looking at the tube.

Diffraction Grating and Near-Infrared Performance

One of the most interesting applications of this technology is improving response in the near-infrared spectrum.

Night vision is not limited to visible light.

Modern image intensifiers can respond to portions of the near-infrared spectrum, and this can be particularly useful because nighttime environments contain infrared energy and many night-vision applications involve infrared illumination.

The ability of a photocathode to efficiently respond to different wavelengths can therefore have a major impact on its practical performance.

A diffraction structure can be engineered to increase optical absorption and interaction within specific wavelength ranges.

Research into grating-based photocathodes has demonstrated enhanced response across visible and near-infrared wavelengths.

This is one of the reasons diffraction-grating technology is particularly interesting for modern night vision.

Why This Can Make a Night-Vision Image Look Better

It is easy to think of night-vision performance as simply asking:

"How bright is the image?"

Brightness is important, but it is not everything.

What matters is the amount of useful information contained in that image.

Imagine looking at a dark tree line.

A tube that produces a very bright image is not automatically producing the best image.

You want enough signal to distinguish:

  • Branches

  • Leaves

  • Shadows

  • Terrain

  • Small objects

  • Edges

  • Movement

If the photocathode can efficiently capture more of the available light, the image intensifier has more information to work with.

That can contribute to a cleaner, more detailed image rather than simply producing a brighter image.

Diffraction Grating and Resolution

Resolution is another area where this technology becomes interesting.

Spatial resolution describes how well an image intensifier can reproduce fine detail.

It is commonly measured in line pairs per millimeter (lp/mm).

Higher resolution generally means the tube is capable of resolving finer details under controlled test conditions.

The previously mentioned 2026 grating-based Gen II+ research reported an increase in measured resolution from 64 lp/mm to 72 lp/mm in the tested design.

Again, this does not mean every commercial diffraction-grating tube will automatically achieve 72 lp/mm.

Overall tube performance depends on the complete image-intensifier design, including:

  • Photocathode

  • MCP

  • Electron optics

  • Phosphor screen

  • Fiber-optic components

  • Power supply

  • Manufacturing tolerances

Diffraction grating is one part of the system.

Diffraction Grating and Halo

Another potential advantage is the reduction of halo.

Halo is the glow or spreading that can appear around bright light sources when viewed through an image intensifier.

For example, if you look toward a distant streetlight or other bright point source through night vision, you may see a glow surrounding the light.

That glow can obscure detail around the bright source.

The previously mentioned grating-based Gen II+ research reported a reduction in measured halo diameter from 0.78 mm to 0.69 mm in its tested design.

A smaller halo can help preserve useful detail around bright sources.

This can be useful in environments containing:

  • Streetlights

  • Vehicle headlights

  • Distant lights

  • Urban lighting

  • Other bright point sources

Why CETC Tubes Are Getting Attention

CETC has become increasingly relevant in the commercial night-vision market, particularly in the budget and mid-range analog night-vision segment.

Newer CETC offerings have appeared with combinations of technologies such as:

  • White Phosphor

  • Autogating

  • Autogain

  • Diffraction Grating

This is significant because diffraction grating is not simply another name for White Phosphor.

These are completely different technologies.

White Phosphor

White Phosphor describes the phosphor screen and the color appearance of the resulting image.

Autogating

Autogating refers to the way the image intensifier controls photocathode operation in changing light conditions.

Autogain

Autogain refers to automatic adjustment of tube gain to help maintain an appropriate image brightness.

Diffraction Grating

Diffraction grating refers to the engineered microscopic optical structure associated with the photocathode and how it interacts with incoming light.

A tube can therefore have all of these technologies simultaneously.

For example:

CETC Gen II+ + White Phosphor + Autogating + Autogain + Diffraction Grating

That is a combination of several different technologies working at different stages of the image-intensifier system.

Diffraction Grating Does Not Automatically Make a Tube Gen 3

This is an important distinction.

Diffraction grating is a technology, not a generation designation.

A Gen II+ tube with diffraction grating is still a Gen II+ tube.

It should not automatically be marketed as Gen 3 simply because it incorporates a more advanced photocathode structure.

Likewise, a diffraction-grating tube should not automatically be assumed to outperform every Gen 3 tube.

Night-vision performance is determined by the complete tube specification and manufacturing quality.

There are excellent and mediocre examples within virtually every tube category.

Diffraction Grating vs. Traditional Photocathodes

Traditional photocathodes are designed to convert incoming photons into electrons efficiently across their intended spectral range.

The challenge is that photocathode materials have physical limitations involving:

  • Light absorption

  • Material thickness

  • Electron escape probability

  • Spectral response

  • Surface structure

A diffraction structure provides another way to manipulate incoming optical energy.

Instead of relying entirely on changes to the photocathode material itself, the optical structure can increase the interaction between light and the photosensitive layer.

This is one reason the technology is attracting interest in the development of modern image intensifiers.

Does Diffraction Grating Replace High FOM?

No.

This is another common misconception.

FOM, or Figure of Merit, is commonly calculated using:

FOM = Resolution × SNR

A high-FOM tube can be capable of producing an excellent image, but FOM does not describe every characteristic of an image intensifier.

Two tubes with similar FOM can still have different characteristics.

For example, they can differ in:

  • Gain

  • EBI

  • Spectral response

  • Photocathode sensitivity

  • Halo

  • Autogating behavior

  • Image uniformity

  • Bright-source performance

Diffraction grating should therefore be viewed as another technology that can influence the underlying performance of the image intensifier.

It does not replace conventional tube specifications.

What Should You Look for When Buying a Diffraction-Grating Tube?

If you are considering a CETC or NNVT tube with diffraction grating, don't stop at the words "Diffraction Grating."

Look at the complete tube specification.

Resolution

Measured in lp/mm.

Higher resolution generally means the tube can resolve finer detail.

SNR

Signal-to-noise ratio is extremely important for determining how clean the image appears, particularly in low-light conditions.

FOM

FOM combines resolution and SNR into one commonly used performance metric.

Gain

Gain describes how strongly the tube amplifies the available signal.

EBI

Equivalent Background Illumination is important when evaluating very-low-light performance, particularly in warmer environments.

Spectral Response

This describes the wavelengths to which the photocathode is sensitive.

Autogating

Autogating helps the tube respond to changing illumination and can help protect the tube from excessive light exposure.

Autogain

Autogain automatically adjusts tube gain to maintain an appropriate image brightness.

Image Uniformity

Uniformity is important because an image can have excellent specifications on paper while still exhibiting unwanted variations across the viewing area.

Halo

Halo characteristics can be especially important when operating around bright light sources.

Actual Image Quality

Finally, look through the tube.

Specifications matter, but real-world image quality matters too.

Is Diffraction Grating the Future of Night Vision?

It may become an increasingly important technology.

The concept of using diffractive structures to manipulate light at the photocathode is not entirely new, but continued development is making the technology increasingly relevant to modern image intensifiers.

Recent research demonstrates measurable improvements in areas including quantum efficiency, near-infrared response, spatial resolution, and halo performance in a grating-based Gen II+ design.

This is why newer CETC and other image-intensifier technologies incorporating diffraction-grating structures are worth paying attention to.

The technology is more than a cosmetic feature.

It represents an engineering approach to improving one of the first and most important stages of image intensification:

capturing and converting available light efficiently.

The Bottom Line

Diffraction grating technology works at the beginning of the night-vision process.

A carefully engineered microscopic grating structure can manipulate incoming light so that more of it interacts effectively with the photosensitive photocathode material.

The potential benefits include:

  • Higher quantum efficiency

  • Improved light collection

  • Better near-infrared response

  • More available signal

  • Potentially improved SNR

  • Improved spatial resolution

  • Reduced halo

  • Better overall image performance

And if you see a rainbow or iridescent color on the photocathode, don't assume something is wrong.

That unusual blue, purple, green, gold, or rainbow appearance is an optical effect created when ordinary light interacts with the microscopic diffraction-grating structure.

The rainbow isn't the performance advantage—the technology producing it is.

For buyers looking at newer CETC/NNVT diffraction-grating tubes, the important takeaway is simple:

Don't think of diffraction grating as another generation of night vision. Think of it as an advanced photocathode technology designed to make better use of available light.

When combined with a quality image intensifier, White Phosphor, autogating, and autogain, diffraction grating can be another important piece of the technology behind a modern night-vision system.

At BagheeraNVG, we believe buyers should look beyond a single FOM number and understand what is actually inside the tube. The technology behind the image matters just as much as the number printed on the specification sheet.