How Are Image Intensifier Tubes Made?

The Basic Principles of Image Intensification

At the heart of any night vision device is the image intensifier tube. This vacuum-sealed tube converts photons from a dimly lit scene into a visible image by amplifying the light thousands of times. The underlying physics that make this light amplification possible are rooted in the photoelectric effect and secondary emission.

 

 

When photons from a dimly lit scene hit the photocathode material inside the tube, electrons are emitted through the photoelectric effect. These electrons are then accelerated towards a microchannel plate using high voltages. Here, each electron triggers a cascade of secondary electrons through secondary emission, amplifying the original signal. These amplified electrons hit a phosphor screen, emitting visible light and creating a green image that is far brighter than the original scene.

 

By leveraging these phenomena, image intensifier tubes can detect ambient light from the night sky, stars, and moon and convert it into a visible image for the user. But producing these complex vacuum tubes requires advanced manufacturing techniques.

 

Crafting the Photocathode

The first step in constructing an image intensifier tube is forming the photocathode - the light-sensitive component that converts photons into electrons. Photocathodes are crafted using special semiconductor materials that have a low work function, reducing the energy required to emit electrons.

 

Common photocathode materials include gallium arsenide, cesium-antimony, and cesium-tellurium. These are deposited on the input window of the tube through a process called vapor deposition. Atoms of the material are evaporated in a vacuum chamber and deposited on the window, forming a thin, uniform photocathode layer.

 

The composition and thickness of this layer impacts sensitivity, so strict controls are maintained. Cesium-antimony offers good response in the IR spectrum, while gallium arsenide extends into the near IR. Multialkali photocathodes using combinations of sodium, potassium, and antimony provide the broadest spectral range.

 

Building the Microchannel Plate

Once the photocathode is fabricated, the next key component - the microchannel plate - must be prepared. This thin disc is covered with millions of microscopic glass tubes or channels, each acting as a tiny electron multiplier.

 

The microchannel plate starts as a hollow glass boule. Thin glass fibers are stacked and fused, then sliced into wafers. Channels are etched through these wafers using an electrochemical process. The channels are treated with a hygroscopic material and then coated with a secondary emissive layer to enhance gain.

 

Two or more microchannel plates can be stacked to increase amplification. But perfectly aligning the plates is critical - even a tiny misalignment can distort the image. Precision jigging and micrometer measurements ensure proper orientation. The plates are then bonded together using an epoxy with the same thermal expansion properties as the glass to maintain alignment.

 

Phosphor Screen Deposition

The final component prepared is the phosphor screen anode. This thin film converts the amplified electron cascade into visible light. Phosphor materials like zinc sulfide doped with trace amounts of silver or aluminum are used. The phosphor composition can be tuned to emit different wavelengths, providing color options like white, green, and yellow-green.

 

The deposition process for the phosphor screen is also vacuum evaporation. Phosphor powders are vaporized and deposited on a glass disc under high vacuum. Because phosphor screen uniformity directly impacts image quality, deposition conditions are tightly controlled. Screen thickness, crystallinity, and light output are all monitored to ensure performance.

 

 

Precision Assembly

With the photocathode, microchannel plates, and phosphor screen fabricated, the components are ready for assembly. But first, each part undergoes rigorous inspection and testing to verify quality. Only components meeting stringent specifications are used.

 

The assembly process takes place in a cleanroom to prevent contamination. It begins by sealing the photocathode and input window to one end of the tube body. Next, the microchannel plates and phosphor screen are carefully inserted and aligned. Spacers separate the components at precise distances to maintainVoltage is applied sequentially to each element to assess function. If tests pass, the tube is sealed under vacuum and baked to remove any trapped gases.

 

The finished image intensifier tube integrates hundreds of components. Each one must be manufactured and integrated with extreme precision to ensure optimal night vision capability and reliability. Our expertise in these processes enables the mass production of high-performance, versatile tubes.

 

Maintaining Image Quality

Throughout manufacturing, various processes can reduce image quality in subtle ways. To counteract these effects, additional steps are taken:

 

Stray light from bright sources can reflect internally and appear in the image. Optical traps and light-absorbing baffles eliminate these artifacts.

Ion damage on the microchannel plates causes image retention or "burn-in." A "screen de-pumping" high voltage procedure corrects this.

Thermal gradients when sealing the tube can distort alignment. Fixture designs minimize mechanical stress during baking.

Contamination or outgassing degrades the photocathode over time. Precision cleaning and outgassing procedures reduce residues.

With attention to detail at each manufacturing stage, image defects can be minimized, resulting in exceptional image quality for the customer.

 

The Future of Image Intensifier Technology

As with any technology, enhancements and innovations to image intensifiers are always on the horizon. Exciting developments aimed at improving performance include:

 

New photocathode materials with higher quantum efficiency in the near-infrared spectrum. This will enable detection under darker conditions.

Smaller pixel microchannel plates. By shrinking the channel size and gap, resolution can increase.

Hybrid image intensifier-CMOS modules. These integrate the image tube directly with sensor electronics for simplified cameras.

Laser-hardened tubes. Special photocathodes and microchannel plates provide protection against damage from lasers.

Autogating tubes. Built-in circuitry eliminates high-brightness overload by momentarily shutting off gain.

By leveraging discoveries in materials science, microfabrication, and optics, the capabilities of image intensification will continue advancing. With over 50 years of innovation so far, this technology still has an exciting future ahead.

 

Conclusion

Creating the specialized vacuum tubes at the heart of night vision devices requires mastering processes at both the atomic scale and industrial scale. The marriage of numerous fields - optics, semiconductor physics, metallurgy, phosphors, vacuum technology, and precision manufacturing - all culminate in the image intensifier tube. While the underlying concepts are elegant in their simplicity, executing them is far from trivial. Yet the hard-won expertise of engineers and scientists in this field enables the routine production of these marvelous devices. After learning the careful choreography involved in their construction, I appreciate more than ever the tremendous technological achievement represented by every image intensifier tube that comes off the production line.

 

References:

https://www.sciencedirect.com/topics/engineering/image-intensifier

https://ieeexplore.ieee.org/abstract/document/6737843

https://www.alsacorp.com/support/notes/inclusionscience.html

https://nvdevices.com/image_intensifier/

https://www.photonis.com/products/image-intensifiers