Gen 3 and NVT-7 Photocathodes: Comparing GaAs and HyMa Technologies

Modern night vision technology has evolved along two distinct technological paths. In the United States, manufacturers such as L3Harris focused on maximizing performance in extremely low-light environments through the adoption of gallium arsenide (GaAs) photocathodes, laying the foundation for modern Gen 3 image intensifiers. In contrast, European manufacturer Photonis and Chinese manufacturer NNVT pursued the continuous refinement of the Gen 2 architecture, improving photocathode efficiency, microchannel plate performance, and overall tube design. This alternative development route ultimately gave rise to advanced technologies known as 4G and NVT-7—marketed as “4G” by Photonis and “NVT-7” by NNVT. 

The primary difference between GaAs (Gallium Arsenide) and HyMa (Hybrid Multi-Alkali) photocathodes lies in their spectral response range, which determines the range of light wavelengths that can be converted into electrons by the image intensifier.

A GaAs photocathode typically operates within a spectral range of approximately 500 to 900 nanometers, while a HyMa photocathode covers a significantly broader range of approximately 350 to 1100 nanometers. This means that a HyMa photocathode can respond to both shorter ultraviolet wavelengths and longer infrared wavelengths beyond the effective range of a conventional GaAs photocathode.

The wider spectral bandwidth of the HyMa photocathode has two direct consequences. First, it enables the image intensifier to utilize a broader range of available light under different environmental conditions, from environments with stronger UV and blue-light content to those dominated by red and infrared illumination. Second, the extended spectral response allows the detection of certain light sources that fall outside the operating range of a GaAs photocathode.

Addtionally, GaAs is a more fragile material and is more susceptible to degradation caused by ion feedback inside the image intensifier tube. To protect the GaAs photocathode and maintain tube lifetime, Gen 3 image intensifiers require an ion-barrier film positioned on the microchannel plate (MCP). In contrast, HyMa photocathodes do not require this protective layer, allowing Photonis's 4G and NNVT's NVT-7 image intensifiers to operate in a filmless configuration.

Generally, the use of an ion-barrier film and the characteristics of the GaAs photocathode lead to two operational consequences. The first is increased susceptibility to laser-induced damage. The cesium-coated GaAs photocathode surface is described as being more sensitive to overexposure and laser illumination than a filmless HyMa photocathode, potentially resulting in irreversible damage. The second consequence involves halo performance. Halo refers to the bright glow that appears around intense light sources in a night vision image. The source states that image intensifiers using GaAs photocathodes typically produce a halo value of approximately 1.0 mm, whereas a filmless 4G image intensifier using a HyMa photocathode typically produces a halo value of approximately 0.7 mm. The smaller halo generated by the 4G image intensifier results in a clearer and less obstructed view of targets located near bright light sources.

Source: 
North Night Vision Science & Technology Research Institute Group Co., Ltd., Product Brochure: Infrared Detectors, Low-Light-Level Image Intensifiers, OLED Micro Displays and Optical Lenses. China: NORINCO Group, n.d.
Photonis, Differences between Gen3 and 4G Image Intensification Technology
https://cdn.photonis.com/system/files/2020-10/Difference_Gen3_4G_english_version.pdf